Thursday, December 13, 2012
Thursday, March 17, 2011
Rice breeders at IRRI and other research facilities are now trying to increase yields through genetic engineering. They hope to create rice varieties that are genetically designed to require less fertilizer, resist insects and diseases, tolerate poor soil, require less irrigation, and photosynthesize more efficiently.
Scientific classification: Rice is an annual grass in the grass family, Poaceae (formerly Gramineae). Asian rice is classified as Oryza sativa and African rice as Oryza glaberrima.
Rice is used for a variety of food and nonfood products. Foods include cooked rice, breakfast cereals, desserts, and rice flour. Rice is also used in beer and in sake, a Japanese fermented brew. The inedible rice hull is used as fuel, fertilizer, and insulation, while the bran is a source of cooking oil. Straw from the leaves and stems is used as bedding for animals and for weaving roofs, hats, baskets, and sandals.
Monday, February 7, 2011
All three of these response modes (LFR, VLFR, HIR) can, and often are, involved in the control of germination and de-etiolation. Also, the role of the phytochromes in photoperiodism involves interaction with the circadian rhythms, and can take the form of either an LFR or HIR. Finally, phytochromes regulate growth and flowering in mature plants in the natural environment via a R:FR ratio response. A wide range of phenomena, including elongation growth and the rate of flowering (separately from the induction of flowering), exhibit a direct linear relationship to the proportion of Pfr established by the incident radiation (Smith, 1983, 1995, 2000). These R:FR ratio responses, which are the basis of proximity perception and shade avoidance, may indicate a quite separate response mode, or may represent a sub-set of LFR responses conditioned by acting within the environment of light-grown tissues.
Relating the different response modes to the individual phytochromes became possible with the generation of null mutants that lack functional phytochromes (for review, see Whitelam and Devlin, 1997). The conclusion from many mutant studies is that the VLFR and the FR–HIR are both mediated by phyA, whereas the LFR is mediated predominantly by phyB. Furthermore, the R:FR ratio responses are mediated predominantly by phyB, with supplementary action by phyD and phyE. These discoveries opened the way to exploiting our knowledge of the individual functions of the members of the phytochrome family by transgenic over-expression of the PHY genes. Over-expression has been important for fundamental objectives, to analyse the molecular actions of the phytochromes and to help elucidate signal transduction pathways, but increasingly transgenic methods are being applied towards the improvement of crop plant performance.
The phytochromes are a family of photoreceptors that absorb radiation across the 600–800 nm waveband. The 600–700 nm band is conventionally referred to as red light (R) while the 700–800 nm band as far-red (FR). Each phytochrome can exist in two photoconvertible conformers: Pr has an absorption maximum at ca. 665 nm and is converted to Pfr, which absorbs maximally at ca. 730 nm, being thereby converted back to Pr. The overall scheme, first worked out by Sterling Hendricks and Harry Borthwick, with their colleagues at Beltsville in the 1940s and 1950s on the basis of supremely elegant physiological experiments, is as follows:
Both Pr and Pfr have relatively broad absorption bands and these overlap below ca. 700 nm; this means that in broad-band irradiation, such as daylight, Pr and Pfr are continually being inter-converted resulting in a dynamic equilibrium, known as the phytochrome photoequilibrium, which is a quantitative function of the relative amounts of R and FR incident upon the plant. The photoequilibrium is conventionally expressed numerically as a proportion of the total phytochrome present as Pfr, or Pfr/P. The roles and mechanisms of Pr and Pfr have been investigated almost entirely by reductionist approaches, growing plants in darkness and exposing them to brief or prolonged irradiation from narrow-beam sources. For over fifty years this research has been and continues to be a tour de force of modern biology, and the knowledge accrued can now be applied to the infinitely more complex situations where plants growing in a natural environment are subject to continuous and often rapid fluctuations in light signals over a wide wavelength range.
The phytochromes are encoded by a small multi-gene family comprising five genes in Arabidopsis but possibly only three in the cereals (Mathews et al., 1996). The five Arabidopsis phytochromes are known as phytochrome A (phyA) to phytochrome E (phyE). Prior to molecular characterisation, phytochromes were classified into two main pools, based on their biochemical and kinetic characteristics as determined from in vivo and in vitro spectrophotometry. The so-called type I phytochrome is the predominant phytochrome present in etiolated seedlings, the first to be identified and still the most completely characterised. It is described as being ‘light-labile’, although it is only the Pfr form which is labile; thus type I phytochromes are also labile in the dark as long as Pfr has been previously generated.
Degradation half-lives of Pfr are typically around 20–45 minutes, and thus Pfr is essentially absent from plants exposed to more than ca. 24 hours of white light. Phytochrome A (phyA) is the only known representative of the type I phytochrome pool. In etiolated seedlings and dark-grown tissues phyA accumulates to relatively high levels in the Pr form. Exposure to light causes rapid loss of phyA, not only through degradation of Pfr
In contrast, type II phytochromes are present in low steady-state concentrations in both dark-grown and light-grown plants because the Pfr forms are relatively stable (Furuya and Schäfer, 1996). All other members of the phytochrome family, that is, phyB to phyE, have these characteristics and are thus regarded as type II phytochromes. PhyB to phyE are synthesised slowly and appear not to be under such strict transcriptional regulation as phyA. In most cases studied, phyB is the predominant phytochrome in light-grown plants. As phytochrome action is a function of the concentration of Pfr, regulation of the rates of synthesis and degradation of the individual phytochromes is clearly crucial in determining the physiological response. It follows that attempts to regulate development by the transgenic expression of phytochrome genes must take account of the rates of synthesis and degradation of the transgene products.
Wednesday, February 2, 2011
Monday, January 31, 2011
Plant Science Expansion - Presentation Transcript
- Plant Science Expansion
- Anew building to accommodate growth of federally funded research in plant sciences.
- Provide increased plant growth facilities
- Provide increased use of computation in plant science research
- Allow MSU to recruitand retain the world’s best plant scientists to solve complex problems relatedto energy, food and health.
- Located at the corner of Bogue St. and Wilson Rd.
- It will connect to the
Plant and Soil Science Building.
- A research home for faculty in…
Biochemistry & Molecular Biology, DOE Plant Research Lab,Forestry, Horticulture, Plant Biology, Plant Pathology, andCrop & Soil Sciences.
- This building will support faculty conducting the latest plant science research at MSU.
- Research funding for plant science in the College of Natural Science alone has doubled to $12M/yr in the last 5 years.
- This includes projects involving the Great Lakes Bioenergy Research Center ($50M), DOE Plant Research Lab ($4.5 M), NIH Grand Opportunity Grant for medicinal plants ($2.8M), and other NSF, NIH and DOE grants.
- The building will have four floors and 40,000 square feet.
- Three floors will be research laboratories with space for 15 research groups and…
- 144 graduate students,
- The open lab design will maximize interaction, collaboration, and
- The basement provides MSU scientists a 30% increase in plant growth chambers.
- Connections to the other buildings allows transportation of plants between growth chambers, labs and green houses without exposure to weather, disease, and insects.
- The first floor provides an entrance to the horticulture gardens.
- The first floor features a 225-seat lecture hall and interaction space.
- Budget: $43MConstruction: May 2010 - December 2011
- Architect/Engineers: SmithGroup, Detroit, Mich.Construction Managers: The Christman Company, Lansing, Mich.
- College of Natural Science
College of Agriculture and Natural Resources
Michigan Agriculture Experiment Station
Sunday, January 30, 2011
INORGANIC MEDIUM COMPONENTS
Plant tissues and organs are grown in vitro on artificial media, which supply the nutrients necessary for growth. The success of plant tissue culture as a means of plant propagation is greatly influenced by the nature of the culture medium used. For healthy and vigorous growth, intact plants need to take up from the soil:
• relatively large amounts of some inorganic
elements (the so-called major plant nutrients): ions of nitrogen (N), potassium (K), calcium (Ca),
phosphorus (P), magnesium (Mg) and sulphur (S);
• small quantities of other elements (minor plant nutrients or trace elements): iron (Fe), nickel (Ni), chlorine (Cl), manganese (Mn), zinc (Zn), boron (B), copper (Cu), and molybdenum (Mo).
According to Epstein (1971), an element can be considered to be essential for plant growth if:
1. a plant fails to complete its life cycle without it;
2. its action is specific and cannot be replaced completely by any other element;
3. its effect on the organism is direct, not indirect on the environment;
4. it is a constituent of a molecule that is known to be essential.
The elements listed above are - together with carbon (C), oxygen (O) and hydrogen (H) - the 17
essential elements. Certain others, such as cobalt (Co), aluminium (Al), sodium (Na) and iodine (I), are essential or beneficial for some species but their widespread essentiality has still to be established.
"There has been, so far as I know, up to present, no planned attempt to cultivate the vegetative cells of higher plants in suitable nutrients. Yet the results of such attempts should cast many interesting sidelights on the peculiarities and capacities which the cell, as an elementary organism, possesses: they should make possible conclusions as to the interrelations and reciprocal influences to which the cell is subjected within the multicellular organism. Without permitting myself to pose further questions, I believe, in conclusion, that I am not making to bold a prediction if I point to the possibility that, in this way, one could successfully cultivate artificial embryos from vegetative cells".
HABERLANDT did not realise that because photosynthetic cells are relatively differentiated their meristematic potential is not expressed easily and he did not know that this would require stimulating substances ie. plant growth regulators which were unknown at the time. Thus he chose to work with pallisade cells, pith cells, stamen hairs and stomatal guard cells. HABERLANDT cultured these cells in a simple organically enriched medium containing glucose under aseptic conditions and was totally unsucessful in all cases. His cells did not divide but were maintained in a living state for several weeks.
HABERLANDT failed to recognise that the meristematic cells of the plant body are basically heterotrophic and he did not know that the dedifferentiation of a cell into a meristematic state requires the presence of plant growth regulators.
Sunday, January 23, 2011
"Understanding the mechanism by which RF genes suppress the male sterile phenotype and restore fertility to plants is critical for continued improvements in hybrid technology," said Manfredo J. Seufferheld, U of I assistant professor of crop sciences.
To reach this goal, Seufferheld teamed up with post-doctoral scientists Simeon O. Kotchoni and Emma W. Gachomo of Purdue University, and Jose C. Jimenez-Lopez of the Estacion Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas (CSIC) in Granada, Spain, to develop a simplified genetic-based nomenclature that automatically catalogues the entire RF gene products into families and subfamilies.
"Up to now, there has been no unified nomenclature for naming the RF proteins," Seufferheld said. "As the systematic sequencing of new plant species has increased in recent years, naming has been simply arbitrary. We have had 'chaos' in the databases. The RF information in the databases could not be adequately handled in the context of comparative functional genomics".
This new tool will help plant breeders and researchers make decisions more quickly. Breeders can now easily match sterility in plants to male restorer mechanisms. Ultimately, growers appears to benefit sooner from new developments in plant breeding since breeders will be able to generate new hybrids at a faster pace, Jimenez-Lopez said.
"Genomic sequencing, coupled with protein modeling, allowed us to begin dismantling this complexity that has held us back in the field of science," Kotchoni said. "Now we can easily compare unknown gene functions to known and well characterized genes in order to determine their functions and family hood".
With a number of teams of scientists competing to finish this task first, Kotchoni said it has been an honor to have this model accepted as the new standard for RF protein nomenclature. This system has been developed as a building block for plant genomics.
"The nomenclature, which is designed to include new RF genes as they become available in the future, is not based on one species or another, but rather on the function of the gene itself," Seufferheld said. "This allows researchers to work with a wide range of plants and take a gene with known function(s) from one plant and transfer it into another plant to restore male fertility".
Corn growers only need to look back to the southern corn leaf blight epidemic in 1972 to see the importance of this scientific development.
In 1972, Texas-Cytoplasm Male Sterility (T-CMS) corn was heavily used in hybrid seed production because it eliminated the costly practice of hand detasseling. Nearly 85 percent of the U.S. corn crop was produced using T-CMS, which was highly susceptible to Helminthosporium maydis, the fungus that causes southern corn leaf blight.
Since then, understanding the function of RF genes in higher plants has been a priority of a number of researchers. "The first male sterility restorer ever characterized in plants was maize ALDH," Kotchoni said. "When this gene is altered, it causes male sterility".
Seufferheld said this will also be a great tool for studying plant evolution.
"We can follow how plants became domesticated," Seufferheld said. "It is easier now because we have all the structures of the RF proteins organized and can look at the evolution of these proteins in a systematic manner. If we just look at the sequence of the gene, part of the phylogenetic scene has been lost through evolution. However, the structure of protein provides more information that can go well into the past."
This public gene database will allow researchers to search using the old or new names of RF proteins, Seufferheld said.
Wednesday, January 19, 2011
The last time I featured a Parnassia on BPotD (over 5 years ago: Parnassia glauca), I wrote that the genus had been moved out of the Saxifragaceae (you"ll see that in a number of classification systems) and even out of the Saxifragales (the order containing the Saxifragaceae and related families) and into the Parnassiaceae (within the Celastrales). Many research groups have since studied the relationships between Parnassiaceae and Celastraceae; current thought provisionally places Parnassia within the Celastraceae, but it seems (after reading the Phylogeny section on the linked page) that this may yet revert to being split again.
This August photograph of Parnassia fimbriata (fringed grass-of-Parnassus or Rocky Mountain grass-of-Parnassus) was taken only meters away from a second of British Columbia"s four Parnassia species, Parnassia kotzebuei. Parnassia is another genus I am always thrilled to encounter, as it was one of the first dozen or so I learned to recognize in Manitoba.
Parnassia fimbriata is native to much of western North America, where it grows in moist sites (fens, bogs, streamside, seeps, wet meadows) at elevations ranging from lowland to alpine. It is the tallest of these herbaceous species in British Columbia, occasionally reaching 50cm in height (though more typically 15 to 30cm). Parnassia kotzebuei, by comparison, is the shortest, ranging from 6-20cm.
Parnassia is a reference to Mount Parnassus; Linnaeus applied the name to the genus based on an account in Materia Medica, a written work by the Greek doctor Dioscorides (Dioscorides called it Agrostis En Parnasso). The Plants for a Future database contains a listing of historical medicinal uses for Parnassia palustris, the species thought to have been described by Dioscorides (who also said of it: "That which grows in Cilicia (which the inhabitants call cinna) inflames rude beasts if often fed on when it is moist".
For additional photographs, see Calphotos: Parnassia fimbriata or Southwest Colorado
Green Super Rice is actually a mix of more than 250 different potential rice varieties and hybrids variously adapted to difficult growing conditions such as drought and low inputs, including no pesticide and less fertilizer, and with rapid establishment rates to out-compete weeds, thus reducing the need for herbicides. More types of Green Super Rice that combine a number of of these traits are in the pipeline.
As published in the latest issue of Rice Today, Green Super Rice is already in the hands of national agricultural agencies in key rice-growing countries for testing and development.
Green Super Rice is an example of what is needed as part of a "Greener Revolution," which is called for by rice researchers around the world and is one of the driving concepts behind the Global Rice Science Partnership (GRiSP) - a plan to improve international partnerships in rice research, its delivery, and impact that would also ensure that rice is grown in an environmentally sustainable way.
With the theme Rice for Future Generations, the 3rd International Rice Congress held in November last year was the perfect venue for the launch of GRiSP. Incredible sharing of rice research and ideas occurred, which Rice Today features in a suite of stories outlining some of the highlights and activities of the event that was attended by more than 1,900 people.
Our Grain of Truth article links Latin America in with GRiSP, highlighting the benefits of sharing expertise and experiences, while in Africa we learn about how improving the quality of rice is critical to reducing the continent's rice imports.
In our mapping section, we see how much yield and yield stability have improved since the 1960s - and also notice how much room for improvement remains.
IRRI's rodent experts, headed by Dr. Grant Singleton, take us on a journey to the northern Philippines to discover both "good" and "bad" rat species. And, we see how they are working with a local community to adopt practices to help reduce rat damage in rice crops - in 2010, rats destroyed between 30% and 50% of the rice crop there.
India is our country profile this issue and we take a look at some rice awareness-raising activities in Singapore. Meanwhile, IRRI's senior economist Dr. Samarendu Mohanty observes the recent fluctuations of rice prices and suggests that freeing up the market and creating a strategic rice reserve would help keep rice prices stable in the long term.
Finally, it is a pleasant surprise to see that nine World Food Prize laureates have had a connection with IRRI - a reminder that rice science is having an impact where it really matters.
Monday, January 17, 2011
Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?
The term “hyperaccumulator” describes a number of plants that belong to distantly related families, but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without suffering phytotoxic effects. Three basic hallmarks distinguish hyperaccumulators from related non-hyperaccumulating taxa: a strongly enhanced rate of heavy metal uptake, a faster root-to-shoot translocation and a greater ability to detoxify and sequester heavy metals in leaves. An interesting breakthrough that has emerged from comparative physiological and molecular analyses of hyperaccumulators and related non-hyperaccumulators is that most key steps of hyperaccumulation rely on different regulation and expression of genes found in both kinds of plants. In particular, a determinant role in driving the uptake, translocation to leaves and, finally, sequestration in vacuoles or cell walls of great amounts of heavy metals, is played in hyperaccumulators by constitutive overexpression of genes encoding transmembrane transporters, such as members of ZIP, HMA, MATE, YSL and MTP families. Among the hypotheses proposed to explain the function of hyperaccumulation, most evidence has supported the “elemental defence” hypothesis, which states that plants hyperaccumulate heavy metals as a defence mechanism against natural enemies, such as herbivores. According to the more recent hypothesis of “joint effects”, heavy metals can operate in concert with organic defensive compounds leading to enhanced plant defence overall.
Heavy metal contaminated soils pose an increasing problem to human and animal health. Using plants that hyperaccumulate specific metals in cleanup efforts appeared over the last 20 years. Metal accumulating species can be used for phytoremediation (removal of contaminant from soils) or phytomining (growing plants to harvest the metals). In addition, as many of the metals that can be hyperaccumulated are also essential nutrients, food fortification and phytoremediation might be considered two sides of the same coin. An overview of literature discussing the phytoremediation capacity of hyperaccumulators to clean up soils contaminated with heavy metals and the possibility of using these plants in phytomining is presented.
Sunday, July 11, 2010
Plant, any member of the plant kingdom, comprising about 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees, and various other forms that mantle the Earth and are also found in its waters. Plants range in size and complexity from small, nonvascular mosses, which depend on direct contact with surface water, to giant sequoia trees, which can draw water and minerals through their vascular systems to elevations of more than 100 m (330 ft).
Only a tiny percentage of plant species are directly used by humans for food, shelter, fiber, and drugs. At the head of the list are rice, wheat, corn, legumes, cotton, conifers, and tobacco, on which whole economies and nations depend. Of even greater importance to humans are the indirect benefits reaped from the entire plant kingdom and its more than 1 billion years of carrying out photosynthesis. Plants have laid down the fossil fuels that provide power for industrial society, and throughout their long history plants have supplied sufficient oxygen to the atmosphere to support the evolution of higher animals. Today the world's biomass is composed overwhelmingly of plants, which not only underpin almost all food webs, but also modify climates and create and hold down soil, making what would otherwise be stony, sandy masses habitable for life.
II DIFFERENTIATION FROM OTHER KINGDOMS
Plants are multicellular eukaryotes—that is, their cells contain membrane-bound structures called organelles. Plants differ from other eukaryotes because their cells are enclosed by more or less rigid cell walls composed primarily of cellulose. The most important characteristic of plants is their ability to photosynthesize. During photosynthesis, plants make their own food by converting light energy into chemical energy—a process carried out in the green cellular organelles called chloroplasts (see Chlorophyll; Chloroplast). A few plants have lost their chlorophyll and have become saprophytes or parasites—that is, they absorb their food from dead organic matter or living organic matter, respectively—but details of their structure show that they are evolved plant forms.
Fungi, also eukaryotic and long considered members of the plant kingdom, have now been placed in a separate kingdom because they lack chlorophyll and plastids and because their rigid cell walls contain chitin rather than cellulose. Unlike the majority of plants, fungi do not manufacture their own food; instead they are saprophytic, absorbing their food from either dead or living organic matter.
The various groups of algae were also formerly placed in the plant kingdom because many are eukaryotic and because most have rigid cell walls and carry out photosynthesis. Nonetheless, because of the variety of pigment types, cell wall types, and physical attributes found in the algae, they are now recognized as part of two separate kingdoms, containing a diversity of plantlike and other organisms that are not necessarily closely related. One of the phyla of algae, the green algae, is believed to have given rise to the plant kingdom, because its chlorophylls, cell walls, and other details of cellular structure are similar to those of plants.
The animal kingdom is also multicellular and eukaryotic, but its members differ from the plants in deriving nutrition from other organic matter; by ingesting food rather than absorbing it, as in the fungi; by lacking rigid cell walls; and, usually, by having sensory capabilities and being motile, at least at some stage. See Classification.
III PLANT PHYLA
The many species of organisms in the plant kingdom are divided into several phyla, or divisions, totaling about 260,000 species. The bryophytes are a diverse assemblage of three phyla of nonvascular plants, with about 16,000 species, that includes the mosses, liverworts, and hornworts. Bryophytes lack a well-developed vascular system for the internal conduction of water and nutrients and have been called nonvascular plants. It takes two generations to complete the plant life cycle (Alternation of Generations). The familiar leafy plant of bryophytes is the sexual, or gamete-producing, generation of the life cycle of these organisms. Because of the lack of a vascular system and because the gametes require a film of water for dispersal, bryophytes are generally small plants that tend to occur in moist conditions, although some attain large size under favorable circumstances and others (usually very small) are adapted to desert life.
The other phyla are collectively termed vascular plants, or tracheophytes. Vascular tissue is internal conducting tissue for the movement of water, minerals, and food. There are two types of vascular tissue: xylem, which conducts water and minerals from the ground to stems and leaves, and phloem, which conducts food produced in the leaves to the stems, roots, and storage and reproductive organs. Besides the presence of vascular tissue, tracheophytes contrast with bryophytes in that tracheophyte leafy plants are the asexual, or spore-producing, generation of their life cycle. In the evolution of tracheophytes, the spore-producing generation became much larger and more complex, whereas the gamete-producing generation became reduced and merely contained in the sporophyte tissue. This ability to evolve into larger and more diverse sporophytes, together with the ability of the vascular system to elevate water, freed tracheophytes from direct dependence on surface water. They were thus able to dominate all the terrestrial habitats of the Earth, except the higher Arctic zones, and to provide food and shelter for its diverse animal inhabitants.
IV CELL STRUCTURE AND FUNCTION
The tremendous variety of plant species is, in part, a reflection of the many distinct cell types that make up individual plants. Fundamental similarities exist among all these cell types, however, and these similarities indicate the common origin and the interrelationships of the different plant species. Each individual plant cell is at least partly self-sufficient, being isolated from its neighbors by a cell membrane, or plasma membrane, and a cell wall. The membrane and wall allow the individual cell to carry out its functions; at the same time, communication with surrounding cells is made possible through cytoplasmic connections called plasmodesmata.
A Cell Wall
The most important feature distinguishing the cells of plants from those of animals is the cell wall. In plants this wall protects the cellular contents and limits cell size. It also has important structural and physiological roles in the life of the plant, being involved in transport, absorption, and secretion.
A plant's cell wall is composed of several chemicals, of which cellulose (made up of molecules of the sugar glucose) is the most important. Cellulose molecules are united into fibrils, which form the structural framework of the wall. Other important constituents of many cell walls are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss from cells. Many plant cells produce both a primary cell wall, while the cell is growing, and a secondary cell wall, laid down inside the primary wall after growth has ceased. Plasmodesmata penetrate both primary and secondary cell walls, providing pathways for transporting substances.
Within the cell wall are the living contents of the cell, called the protoplast. These contents are bounded by a cell membrane composed of a phospholipid bi-layer. The protoplast contains the cytoplasm, which in turn contains various membrane-bound organelles and vacuoles and the nucleus, which is the hereditary unit of the cell.
Vacuoles are membrane-bound cavities filled with cell sap, which is made up mostly of water containing various dissolved sugars, salts, and other chemicals.
Plastids are types of organelles, structures that carry out specialized functions in the cell. Three kinds of plastids are important here. Chloroplasts contain chlorophylls and carotenoid pigments; they are the site of photosynthesis, the process in which light energy from the sun is fixed as chemical energy in the bonds of various carbon compounds. Leucoplasts, which contain no pigments, are involved in the synthesis of starch, oils, and proteins. Chromoplasts manufacture carotenoids.
Whereas plastids are involved in various ways in storing energy, another class of organelles, the mitochondria, are the sites of cellular respiration. This process involves the transfer of chemical energy from carbon-containing compounds to adenosine triphosphate, or ATP, the chief energy source for cells. The transfer takes place in three stages: glycolysis (in which acids are produced from carbohydrates); the Krebs cycle, also called the citric acid cycle; and electron transfer. Like plastids, mitochondria are bounded by two membranes, of which the inner one is extensively folded; the folds serve as the surfaces on which the respiratory reactions take place.
B4 Ribosomes, Golgi Apparatus, and Endoplasmic Reticulum
Two other important cellular contents are the ribosomes, the sites at which amino acids are linked together to form proteins, and the Golgi apparatus, which plays a role in the secretion of materials from cells. In addition, a complex membrane system called the endoplasmic reticulum runs through much of the cytoplasm and appears to function as a communication system; various kinds of cellular substances are channeled through it from place to place. Ribosomes are often connected to the endoplasmic reticulum, which is continuous with the double membrane surrounding the nucleus of the cell.
The nucleus controls the ongoing functions of the cell by specifying which proteins are produced. It also stores and passes on genetic information to future generations of cells during cell division. See Cell.
V TISSUE SYSTEMS
There are many variants of the generalized plant cell and its parts. Similar kinds of cells are organized into structural and functional units, or tissues, which make up the plant as a whole, and new cells (and tissues) are formed at growing points of actively dividing cells. These growing points, called meristems, are located either at the stem and root tips (apical meristems), where they are responsible for the primary growth of plants, or laterally in stems and roots (lateral meristems), where they are responsible for secondary plant growth. Three tissue systems are recognized in vascular plants: dermal, vascular, and ground (or fundamental).
A Dermal System
The dermal system consists of the epidermis, or outermost layer, of the plant body. It forms the skin of the plant, covering the leaves, flowers, roots, fruits, and seeds. Epidermal cells vary greatly in function and structure.
The epidermis may contain stomata, openings through which gases are exchanged with the atmosphere. These openings are surrounded by specialized cells called guard cells, which, through changes in their size and shape, alter the size of the stomatal openings and thus regulate the gas exchange. The epidermis is covered with a waxy coating called the cuticle, which functions as a waterproofing layer and thus reduces water loss from the plant surface through evaporation. If the plant undergoes secondary growth—growth that increases the diameter of roots and stems through the activity of lateral meristems—the epidermis is replaced by a peridermis made up of heavily waterproofed cells (mainly cork tissue) that are dead at maturity.
B Vascular System
The vascular tissue system consists of two kinds of conducting tissues: the xylem, responsible for conduction of water and dissolved mineral nutrients, and the phloem, responsible for conduction of food. The xylem also stores food and helps support the plant.
The xylem consists of two types of conducting cells: tracheids and vessels. Elongated cells, with tapered ends and secondary walls, both types lack cytoplasm and are dead at maturity. The walls have pits—areas in which secondary thickening does not occur—through which water moves from cell to cell. Vessels usually are shorter and broader than tracheids, and in addition to pits they have perforation—areas of the cell wall that lack both primary and secondary thickenings and through which water and dissolved nutrients may freely pass.
The phloem, or food-conducting tissue, consists of cells that are living at maturity. The principal cells of phloem, the sieve elements, are so called because of the clusters of pores in their walls through which the protoplasts of adjoining cells are connected. Two types of sieve elements occur: sieve cells, with narrow pores in rather uniform clusters on the cell walls, and sieve-tube members, with larger pores on some walls of the cell than on others. Although the sieve elements contain cytoplasm at maturity, the nucleus and other organelles are lacking. Associated with the sieve elements are companion cells that do contain nuclei and that are responsible for manufacturing and secreting substances into the sieve elements and removing waste products from them.
C Ground System
The ground, or fundamental, tissue systems of plants consist of three types of tissue. The first, called parenchyma, is found throughout the plant and is living and capable of cell division at maturity. Usually only primary walls are present, and these are uniformly thickened. The cells of parenchyma tissue carry out many specialized physiological functions—for example, photosynthesis, storage, secretion, and wound healing. They also occur in the xylem and phloem tissues.
Collenchyma, the second type of ground tissue, is also living at maturity and is made up of cells with unevenly thickened primary cell walls. Collenchyma tissue is pliable and functions as support tissue in young, growing portions of plants.
Sclerenchyma tissue, the third type, consists of cells that lack protoplasts at maturity and that have thick secondary walls usually containing lignin. Sclerenchyma tissue is important in supporting and strengthening those portions of plants that have finished growing.
VI PLANT ORGANS
The body of a vascular plant is organized into three general kinds of organs: roots, stems, and leaves. These organs all contain the three kinds of tissue systems mentioned above, but they differ in the way the cells are specialized to carry out different functions.
The function of roots is to anchor the plant to its substrate and to absorb water and minerals. Thus, roots are generally found underground and grow downward, or in the direction of gravity. Unlike stems, they have no leaves or nodes. The epidermis is just behind the growing tip of roots and is covered with root hairs, which are outgrowths of the epidermal cells. The root hairs increase the surface area of the roots and serve as the surface through which water and nutrients are absorbed.
Internally, roots consist largely of xylem and phloem, although many are highly modified to carry out specialized functions. Thus, some roots are important food and storage organs—for example, beets, carrots, and radishes. Such roots have an abundance of parenchyma tissue. Many tropical trees have aerial prop roots that serve to hold the stem in an upright position. Epiphytes have roots modified for quick absorption of rainwater that flows over the bark of the host plants.
Roots increase in length through the activity of apical meristems and in diameter through the activity of lateral meristems. Branch roots originate internally at some distance behind the growing tip, when certain cells become meristematic.
Stems usually are above ground, grow upward, and bear leaves, which are attached in a regular pattern at nodes along the stem. The portions of the stem between nodes are called internodes. Stems increase in length through the activity of an apical meristem at the stem tip. This growing point also gives rise to new leaves, which surround and protect the stem tip, or apical bud, before they expand. Apical buds of deciduous trees, which lose their leaves during part of the year, are usually protected by modified leaves called bud scales.
Stems are more variable in external appearance and internal structure than are roots, but they also consist of the three tissue systems and have several features in common. Vascular tissue is present in bundles that run the length of the stem, forming a continuous network with the vascular tissue in the leaves and the roots. The vascular tissue of herbaceous plants is surrounded by parenchyma tissue, whereas the stems of woody plants consist mostly of hard xylem tissue. Stems increase in diameter through the activity of lateral meristems, which produce the bark and wood in woody plants. The bark, which also contains the phloem, serves as a protective outer covering, preventing damage and water loss.
Within the plant kingdom are many modifications of the basic stem, such as the thorns of hawthorns. Climbing stems, such as the tendrils of grapes and Boston ivy, have special modifications that allow them to grow up and attach to their substrate. Many plants, such as cacti, have reduced leaves or no leaves at all, and their stems act as the photosynthetic surface. Some stems, including those of many grasses, creep along the surface of the ground and create new plants through a process called vegetative reproduction. Other stems are borne underground and serve as food-storage organs, often allowing the plant to survive through the winter; the so-called bulbs of the tulip and the crocus are examples.
The leaf is the primary photosynthetic organ of most plants. Leaves are usually flattened blades that consist, internally, mostly of parenchyma tissue called the mesophyll, which is made up of loosely arranged cells with spaces between them. The spaces are filled with air, from which the cells absorb carbon dioxide and into which they expel oxygen. The mesophyll is bounded by the upper and lower surface of the leaf blade, which is covered by epidermal tissue. A vascular network runs through the mesophyll, providing the cell walls with water and removing the food products of photosynthesis to other parts of the plants.
The leaf blade is connected to the stem through a narrowed portion called the petiole, or stalk, which consists mostly of vascular tissue. Appendages called stipules are often present at the base of the petiole.
Many specialized forms of leaves occur. Some are modified as spines, which help protect plants from predators. Insectivorous plants possess highly modified leaves that trap and digest insects to obtain needed nutrients. Some leaves are brightly colored and petal-like, serving to attract pollinators to otherwise small, unattractive flowers. Perhaps the most highly modified leaves are flowers themselves. The individual parts of flowers—carpels, stamens, petals, and sepals—are all modified leaves that have taken on reproductive functions.
VII GROWTH AND DIFFERENTIATION
The growth and differentiation of the various plant tissue and organ systems are controlled by various internal and external factors.
Plant hormones, specialized chemical substances produced by plants, are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Thus, auxin, one of the most important plant hormones, is produced by growing stem tips and transported to other areas where it may either promote growth or inhibit it. In stems, for example, auxin promotes cell elongation and the differentiation of vascular tissue, whereas in roots it inhibits growth in the main system but promotes the formation of adventitious roots. It also retards the abscission (dropping off) of flowers, fruits, and leaves.
Gibberellins are other important plant-growth hormones; more than 50 kinds are known. They control the elongation of stems, and they cause the germination of some grass seeds by initiating the production of enzymes that break down starch into sugars to nourish the plant embryo. Cytokinins promote the growth of lateral buds, acting in opposition to auxin; they also promote bud formation. In addition, plants produce the gas ethylene through the partial decomposition of certain hydrocarbons, and ethylene in turn regulates fruit maturation and abscission.
Various external factors, often acting together with hormones, are also important in plant growth and development. One important class of responses to external stimuli is that of the tropisms—responses that cause a change in the direction of a plant's growth. Examples are phototropism, the bending of a stem toward light, and geotropism, the response of a stem or root to gravity. Stems are negatively geotropic, growing away from gravity, whereas roots are positively geotropic. Photoperiodism, the response to 24-hour cycles of dark and light, is particularly important in the initiation of flowering. Some plants are short-day, flowering only when periods of light are less than a certain length (see Biological Clocks). Other variables—both internal, such as the age of the plant, and external, such as temperature—are also involved with the complex beginnings of flowering.
Rooted as they are in the ground, plants are commonly thought of as leading sedentary, vegetative, passive lives. However, a look at the ingeniously developed interactions that plants have with the other organisms in their ecosystems quickly corrects this notion.
A Cooperation and Competition
Many plant species exist as separate male and female plants, and pollen from male flowers must reach the female flowers in order for pollination and seed development to take place. The agent of pollination is sometimes the wind (a part of the physical environment), but in many cases it is an insect, bat, or bird (members of the biological environment). Plants may also rely on agents for dispersing their seed. Thus, after pollination, cherry trees develop cherries that attract birds, which ingest the fruit and excrete the cherry stones in more distant terrains.
Plants have evolved many other mutually beneficial relationships, such as the nitrogen-fixing bacteria that occur in the nodules on the roots of legumes (see Nitrogen Fixation). Many prairie grasses and other plants that flourish on open land depend on various herbivores to keep forests from closing in and shading them.
In the competition among plants for light, many species have evolved such mechanisms as leaf shape, crown shape, and increased height in order to intercept the sun's rays. In addition, many plants produce chemical substances that inhibit the germination or establishment of seeds of other species near them, thus excluding competing species from mineral resources as well as light. Walnut species, for example, use such an allelopathy, or chemical inhibition.
B The Food Web
Because plants are autotrophs—organisms that are able to manufacture their own food—they lie at the very foundation of the food web. Heterotrophs—organisms that cannot manufacture their own food—usually lead less sedentary lives than plants, but they ultimately depend on autotrophs as sources of food. Plants are first fed upon by primary consumers, or herbivores, which in turn are fed upon by secondary consumers, or carnivores. Decomposers act upon all levels of the food web. A large portion of energy is lost at each step in the food web; only about 10 percent of the energy in one level is stored by the next. Thus, most food webs contain only a few steps.
C Plants and Humans
From the prehistoric beginnings of agriculture until recent times, only a few of the total plant species have been taken from the wild and refined to become primary sources of food, fiber, shelter, and drugs. This process of plant cultivation and breeding began largely by accident, possibly as the seeds of wild fruits and vegetables, gathered near human habitations, sprouted and were crudely cultivated. Plants such as wheat, which possibly originated in the eastern Mediterranean region more than 9,000 years ago, were selected and replanted year after year for their superior food value; today many domesticated plants can scarcely be traced back to their wild ancestors or to the original plant communities in which they originated. This selective process took place with no prior knowledge of plant breeding but, rather, through the constant and close familiarity that preindustrial humans had with plants.
Today, however, the human relationship with plants is nearly reversed: An increasing majority of people have little or no contact with plant cultivation, and the farmers that do have such contact are becoming more and more specialized in single crops. The breeding process, on the other hand, has been greatly accelerated, largely through advances in genetics. Plant geneticists are now able to develop, in only a few years, such plant strains as wind-resistant corn, thus greatly increasing crop yields.
At the same time, humans have accelerated the demand for food and energy to the extent that entire species and ecosystems of plants are being destroyed before scientists can develop an understanding of which plant species have the potential to benefit humanity. Most species remain little known; those that seem to offer the greatest hope for providing new sources of food, drugs, and other useful products exist in tropical rain forests and other areas where rapidly growing human populations can quickly reduce the land to arid, sandy wastes. According to the World Conservation Union, about 34,000 species of plants are at risk of becoming extinct. This amounts to about one of every eight known species of ferns, flowering plants, and conifers and related plants. Increased knowledge of plants and attention to their survival are needed to solve many of the problems confronting the human world today. (World Food Supply.)
See also Dicots; Diseases of Plants; Fruit; Monocots; Nut; Plant Distribution; Plant Propagation; and Poisonous Plants.
Friday, July 2, 2010
In winter or early spring, Arabidopsis plants without an active DNF gene are already flowering (right). Those with the DNF gene will delay flowering until later in the year when days are longer and conditions are more favorable for survival of their seedlings (left). Credit: Dr Steve Jackson
A team of researchers from Warwick have isolated a gene responsible for regulating the expression of CONSTANS, an important inducer of flowering, in Arabidopsis.
'Being able to understand and ultimately control seasonal flowering will enable more predictable flowering, better scheduling and reduced wastage of crops', explained Dr Jackson.
Whilst the relationship between CONSTANS and flowering time in response to day length is well established, the mechanism controlling the expression of CONSTANS is still not fully understood.
The scientists present their work at the Society for Experimental Biology Annual Meeting in Prague on Wednesday 30th June 2010.
Many plants control when they flower to coincide with particular seasons by responding to the length of the day, a process known as photoperiodism. A flowering mutant of Arabidopsis, which had an altered response to photoperiod, was used in the study led by Dr Stephen Jackson.
In the study funded by the BBSRC, the team identified the defective gene in the mutant plant that caused its abnormal flowering time.
They then cloned a working version of the gene, known as DAY NEUTRAL FLOWERING (DNF), from a normal Arabidopsis plant and introduced it into the mutant plant to restore its normal flowering response to day length.
The role of DNF in normal plant flowering is to regulate the CONSTANS gene. CONSTANS is activated only in the light and the plant is triggered to flower when CONSTANS levels rise above a certain threshold level during the daytime.
In normal plants, DNF represses the levels of CONSTANS until the day length is long enough and conditions are favourable for the survival of their seedlings. In mutant plants without an active DNF gene, CONSTANS is not repressed and they are able to flower earlier in the year, when days are still short.
The presence of the DNF gene has not yet been identified in species other than Arabidopsis but the scientists believe their on-going work may prove to have a wider significance for other species.
Scientists can override complex pathways that control flowering by artificially inducing or inhibiting key flowering genes such as DNF and CONSTANS. This can already be done in the laboratory by spraying an 'inducing agent' onto plants, stimulating them to flower early.
This could be used to extend the length of the harvesting season or to co-ordinate flowering or fruit production to a specific time. Growers already regulate the flowering of a few plants such as Chrysanthemum and Poinsettia, the latter specifically for Christmas and Easter.
Unravelling the complex pathways that control plant flowering will help scientists to understand and influence flowering patterns more effectively and in many different species.
Provided by Society for Experimental Biology
Morning dew gathering on rice leaves at a farm. Researchers at a Swiss university said Thursday that they have uncovered a microscopic fungus that is able to increase the speed of rice growth by five times.
By tinkering with a type of fungus that lives in association with plant roots, researchers have found a way to increase the growth of rice by an impressive margin. The so-called mycorrhizal fungi are found in association with nearly all plants in nature, where they deliver essential nutrients—specifically phosphate—to plants in return for sugar. The findings are nevertheless a surprise, according to researchers reporting online on June 10th in Current Biology, because there has been little evidence thus far to suggest that crop plants actually respond to the fungi.
"Global reserves of phosphate are critically low, and because the demand for phosphate goes hand in hand with human population expansion, it is predicted that there will be major shortages in the next few decades," said Ian Sanders of the University of Lausanne in Switzerland. "Unfortunately, most of our important crop plants do not respond strongly, if at all, to inoculation with these fungi. This is especially so for rice, the most globally important food plant. There are no clear reports that rice benefits from inoculation with mycorrhizal fungi."
That is, until now. In fact, the researchers started with a strain of mycorrhizal fungus of the species Glomus intraradices that clearly didn't benefit rice. They then took advantage of the fungus's unusual genetics. A single fungal filament can contain genetically distinct nuclei. Those distinct nuclei can fuse together, mixing genes up in different combinations, and fungal spores can also end up with different complements of genes, the new research shows. As such, the supposedly clonal fungi maintain a degree of genetic variation that had been overlooked.
"It turns out we can very simply manipulate their genetics to produce fungi that induce up to a five-fold growth increase in this globally important food plant," Sanders said.
The genetic changes that the researchers produced in the fungi led to changes in the activity of important genes in the rice, they report. Those affected genes are known to be involved in establishing the mutually beneficial relationship between plant and fungus and in the transport of phosphate at the interface between fungus and plant.
Sanders emphasized that the genetic manipulation the researchers undertook didn't involve any insertion of new genes into the fungal genome. It rather relied on the same biological processes of genetic exchange and segregation that normally take place in the fungus. "What we have done with these fungi is not much different from what plant breeders, and farmers before them, have done to improve crops," he said. "The only difference is that the genetics of these fungi is a little bit more unusual, and no one thought it worth doing."
On a cautionary note, Sanders did emphasize that the plants they studied were grown in a greenhouse in Switzerland under conditions that only mimicked those found in the tropics. "This is clearly not at all the same environment as a rice plant growing in a real paddy field," he said. It remains to be seen whether the same growth benefits will apply in practice.
"However," Sanders said, "our study clearly shows that the potential is there to manipulate the genetics of the fungus to achieve greater crop yields."
More information: Angelard et al.: “Report: Segregation in a Mycorrhizal Fungus Alters Rice Growth and Symbiosis-Specific Gene Transcription.” Publishing in Current Biology 20, 13, July 13, 2010. DOI 10.1016/j.cub.2010.04.043
Provided by Cell Press
Researchers evaluate soybean plants within a ring of ozone in the SoyFACE facility in Urbana, Ill. Credit: Carrie Ramig, USDA-ARS & University of Illinois
University of Illinois researchers are studying the impact of elevated carbon dioxide, elevated ozone and higher atmospheric temperatures on plant diseases that could challenge crops in these changing conditions.
Darin Eastburn, U of I associate professor of crop sciences, evaluated the effects of elevated carbon dioxide and ozone on three economically important soybean diseases under natural field conditions at the soybean-free air-concentrating enrichment (SoyFACE) facility in Urbana.
The diseases downy mildew, Septoria brown spot, and sudden death syndrome were observed from 2005 to 2007 using visual surveys and digital image analysis. While changes in atmospheric composition altered disease expression, the responses of the three pathosystems varied considerably, Eastburn said.
Elevated carbon dioxide levels are more likely to have a direct effect on plant diseases through changes to the plant hosts rather than the plant pathogens.
"Plants growing in a high carbon dioxide environment tend to grow faster and larger, and they have denser canopies," Eastburn said. "These dense plant canopies favor the development of some diseases because the low light levels and reduced air circulation allow higher relative humidity levels to develop, and this promotes the growth and sporulation of many plant pathogens."
At the same time, plants grown in high carbon dioxide environments also close their stomata, pores in the leaves that allow the plant to take in carbon dioxide and release oxygen, more often. Because plant pathogens often enter the plant through the stomata, the more frequent closing of the stomata may help prevent some pathogens from getting into the plant.
In elevated ozone, plant growth is inhibited and results in shorter plants with less dense canopies. This can slow the growth and reproduction of certain pathogens. However, ozone also damages plant tissues that can help pathogens infect the plant more easily."Elevated levels of carbon dioxide and ozone can make a plant more susceptible to some diseases, but less susceptible to others," Eastburn said. "This is exactly what we've observed in our climate change experiments."
U of I's SoyFACE was the first facility to expose plants to elevated ozone under completely open-air conditions within an agricultural field.
"The SoyFACE facility allowed us to evaluate the influence of natural variability of meteorological factors such as drought and temperature in conjunction with imposed atmospheric composition (elevated carbon dioxide and ozone) on naturally occurring soybean diseases across several growing seasons," Eastburn said.
He believes rising temperatures and changes in rainfall patterns will also affect development of plant disease epidemics.
"In some cases, changes of only a few degrees have allowed plant diseases to become established earlier in the season, resulting in more severe disease epidemics," Eastburn said. "The ranges of some diseases are expanding as rising temperatures are allowing pathogens to overwinter in regions that were previously too cold for them."
For example, warmer winters may allow kudzu to expand its range northward. Because kudzu is an alternate host for the soybean rust pathogen, one result of rising temperatures may be that soybean rust arrives in Illinois earlier in the soybean growing season, Eastburn said.
"Information derived from climate change studies will help us prepare for the changes ahead by knowing which diseases are most likely to become more problematic," he said. "Now is the time for plant pathologists, plant breeders, agronomists and horticulturalists to adapt disease management strategies to the changing environment."
Eastburn's soybean research, "Elevated atmospheric carbon dioxide and ozone alter soybean diseases at SoyFACE," was recently published in Global Change Biology. Researchers also included Melissa DeGennaro and Evan DeLucia of the U of I, Orla Dermody of Pioneer Hi-Bred Switzerland, and Andrew McElrone of the University of California - Davis.