New Directions for Biosciences Research in Agriculture High-Reward Opportunities (1985) / Chapter Skim
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4. Plant Science
4. Plant Science
Pages 54-80
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From page 54... ...
A lack of basic information on the biochemistry of many metabolic and regulatory steps is delaying progress in using molecular genetics to establish the mechanisms employed in controlling plant growth. This chapter suggests ways of strengthening research that emphasizes integration of traditional biochemical and physiological 54
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From page 55... ...
the interaction of carbon and nitrogen metabolism in supporting optimal plant growth; (2) the role of plant hormones and phytochrome in regulating plant growth and development; (3)
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From page 56... ...
The information now available concerning chloroplast inheritance is spawning research toward practical applications. Investigations include manipulating chloroplast genes that confer selective resistance against specific herbicides in crop plants and designing genes that produce the enzymes involved in carbon dioxide fixation to increase the overall efficiency of the carboxylation reaction.
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From page 57... ...
This released carbon dioxide is subsequently fixed to yield phosphoglyceric acid via the C3 pathway. The formation of phosphoglyceric acid in both C3 and C4 plants is accomplished by the enzyme ribulose-1, 5-bisphosphate carboxylase-oxygenase, often called Rubisco.
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From page 58... ...
Many factors such as the developmental stage of the plant and the presence of biological and physicochemical stresses can reduce photosynthetic efficiency. In addition, each step in the photosynthetic process, from the absorption of light energy to the conversion and storage of energy in the synthesis of sugar molecules, can be affected differently by various limiting factors.
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From page 59... ...
Efforts to improve photosynthetic efficiency can be enhanced by research focused on resistance to physicochemical stress and utilization of nutrients from the soil. Although outlines of the basic steps of photosynthesis appear clear, virtually every aspect of this complex
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From page 60... ...
Additional improvements in yield and harvest index may depend on a full understanding of these factors and their interactions at the molecular and genetic level. With this information, scientists may now be able to take advantage of recombinant gene transfer methods to further improve crop quality and yields.
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From page 61... ...
Studies on the enzymatic steps involved in the biosynthesis of starch and sucrose indicate that inorganic phosphate and triose-phosphate have profound effects on regulating the rates of these interconnected biosynthetic pathways. Research focusing on a full understanding of the regulation of the storage and transport of carbohydrates has been modest.
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From page 62... ...
The nitrogen fixed biologically in the root nodules of leguminous plants is quickly assimilated into organic nitrogen compounds in the plant and is subject to very low levels of leaching and denitrification. Although some major seed and forage legumes such as soybeans and alfalfa take advantage of biological nitrogen fixation, there is potential for improving and extending the advantages of biological nitrogen fixation to other crops.
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From page 63... ...
Improving Symbiotic Nitrogen Fixation About 85 percent of legume inoculant used in the United States is applied to soybeans. Indigenous rhizobia are so dominant in most soybean fields, however, that improved rhizobia strains that are added to the soil
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From page 64... ...
Progress in the understanding of photosynthesis has been impressive, but further research is needed to define its interactions with major limiting factors in plant growth. With advances in experimental techniques and a better understanding of the fundamental metabolic steps in both photosynthesis and biological nitrogen fixation, researchers are better equipped to study the feedback relationships between these two processes.
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From page 65... ...
Pressures on fuel wood are increasing worldwide; alder, casuarina, and other comparable nitrogen-fixing plants should be investigated as alternatives to other woody species. Other Aspects of Nitrogen Metabolism Essential Amino Acids A more complete knowledge of genetic control of the synthesis of storage proteins in plants could lead to development of plant products with improved nutritional value for consumption by humans and food animals.
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From page 66... ...
In special instances, foliar application of nitrogenous fertilizers is efficient and practical. Research Status An improved understanding of photosynthesis and biological nitrogen fixation has been achieved through steady, long-term research that has included the application of new experimental methods.
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From page 67... ...
Only then can researchers take advantage of new techniques to manipulate genetic and chemical regulatory steps that favorably influence these processes. Future ARS research, with emphasis at the molecular level, should include studies of the following: · The oxygenate and carboxylase properties of the key photosynthetic enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase, to identify ways to modify the enzyme to improve the overall efficiency of photosynthesis; · Metabolic and anatomical properties of C4-type photosynthetic plants to explore possible transfer of these properties into less photosynthetically efficient C3 plants; · Chloroplast membranes and the light reactions of photosynthesis to identify opportunities for improving photosynthetic efficiency and to gain an understanding of the mechanism of action of herbicides that act on the photosynthetic systems; · Factors influencing chloroplast development and senescence, with special attention to the role of nitrogen levels; · Genetic determinants controlling the partitioning of photosynthate between the harvested and nonharvested part of the plant, including traits that determine the composition of seeds and other storage organs; · Nitrogen-fixing systems, including nonsymbiotic prokaryotes such as the azotobacter and blue-green algae, that may lead to incorporation of functioning nitrogenase genes directly into cells of crop plants; and
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From page 68... ...
With the exception of phytochrome, which is a chromophore linked to a protein, all the known plant hormones are low-molecular-weight compounds that are active biologically at very low concentrations in the micromolar range. Past studies on the plant hormones and their active chemical analogs have chronicled the types of responses obtained when one or a combination of the classes of hormones are applied to an intact plant; to plant parts such as stems, buds, roots, and other tissues; or to individual plant cells.
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From page 69... ...
Notable examples are the definition of the enzymatic steps in ethylene biosynthesis and the biosynthesis of the various active and inactive gibberellins. The study of biosynthetic pathways of plant hormones and the specific enzymes involved may ultimately lead to the development of experimental tools that will help researchers understand the regulation of plant hormones at
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From page 70... ...
Because the attached low-molecular-weight plant hormone has become a surface characteristic of the carrier protein, some of the antibodies produced might recognize and have affinity for free, unlinked hormone molecules. This approach, using antibodies against plant hormones, is in its early stages.
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From page 71... ...
An alternative approach for studying the molecular mechanisms involved in hormone-related responses is to study enzymes and other gene products that appear in response to hormone application. The effect of receptor mole ~ ~ ~ & ~ ~ 8~
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From page 72... ...
Photomorphogenesis Light serves an important regulatory role in plant growth and development in addition to providing the energy source for photosynthesis. Photomorphogenesis, the light-regulated developmental changes of a plant, is primarily under the control of a pigment called phytochrome.
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From page 73... ...
Cell Culture and Plant Regeneration Two classes of plant hormones, the auxins and the cytokinins, must be added to culture media to support plant cell proliferation in vitro. While it is relatively easy to fulfill the requirements for meristematic plant cells to continue unorganized cell proliferation in tissue culture, it is far more difficult to obtain organized growth and regeneration of plants.
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From page 74... ...
The ARS should strive to reestablish its leadership role in basic research on plant growth and development. The focus of future research efforts within the ARS should include: · Biosynthesis and degradation of plant hormones and phytochrome, with an emphasis on the regulation of genes coding for enzymes that synthesize or inactivate these substances; · A molecular understanding of the role of phytochrome and plant hormones in regulating gene expression, particularly on their effects on the regulatory sequences of genes; and o The role of regulatory substances in major yieldcontrolling processes such as flowering, fertilization, germination, and senescence.
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From page 75... ...
Factors causing such dramatic effects include extremes of temperatures and severe drought, such as occurred in the midwestern United States in 1983 when the average bushel per acre yield for corn in Illinois dropped 40 percent compared to the average yield for the state in the previous year. Less apparent are stress conditions that cause no visible injury but still retard plant growth and reduce crop yield.
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From page 76... ...
Thus, many response factors must be studied to determine the threshold of damage for different responses and to understand the relationships that might exist between the primary effect and the cascade of processes that are, in turn, affected. Drought Water stress reduces or arrests plant growth because of a variety of effects.
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From page 77... ...
Also, drought may trigger long-term developmental changes in morphology and growth pattern that limit flowering, pollination, and seed development, which can greatly reduce crop yield. Salinity The osmotic properties of high concentrations of salt ions in soil water produce the same effects as those resulting from drought.
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From page 78... ...
agriculture. Such wild plants, native to contrasting environments that are extreme in exhibiting one or a combination of stress factors, however, can provide invaluable experimental material for the identification of stress-tolerant mechanisms and genetic manipulation.
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From page 79... ...
Research must include major plant processes such as photosynthesis, nitrogen metabolism, protein synthesis, and the transport of water, ions, and other solutes that are either excluded from or concentrated in intracellular compartments such as the vacuole and other organelles. Comparative studies on plants that exhibit marked differences in their tolerance to a given stress factor provide a powerful approach toward uncovering the basic mechanisms of tolerance to physicochemical stresses.
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From page 80... ...
80 · Identification of genes and gene products associated with stress tolerance in stress-adapted genotypes and species; and O Aspects of membrane properties such as changes in the biosynthesis of major membrane constituents; temperature-related changes in lipid fluidity and membrane protein stability that affect the functional integrity of the chloroplast, mitochondrial, vacuolar, and plasma membranes; and related aspects including dehydration-induced phase transitions, freeze-induced electrical perturbations, and changes in thermomechanical properties.
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