Opportunities in Biology (1989) / Chapter Skim
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11. Plant Biology and Agriculture
11. Plant Biology and Agriculture
Pages 365-402
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From page 365... ...
Except for the polar icecaps, snow- and ice-covered mountains, and certain of the earth's deserts, all land masses are covered with vegetation. This vegetation contributes to global energy and water budgets through modification of the solar energy, water, carbon dioxide, and nitrogen exchanges at the earth's surface.
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From page 366... ...
The similarities are so great that it is generally agreed that plants were derived from green algae, and specifically from organisms that had many of the features of the multicellular, freshwater alga Coleochaete. The cellulose cell walls that are such an important feature of the adaptation of plants on land originated among the green algae, as did the ability to form starch granules within the chloroplasts rather than free in the cytoplasm, and as did certain unique features of cell division that are common to all plants.
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From page 367... ...
The characteristics of crops have been modified by selection and hybridization for at least 11,000 years and are now being modified more precisely by the techniques of genetic engineering. Cultural practices are also important in promoting crop yield.
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From page 368... ...
Thus, improvements in productivity need not rest solely on increases in genetic potential. For this reason, both the identification of the environmental forces and the manipulation of crops to express their genetic potential more fully are important research areas in plant biology.
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From page 369... ...
The expansion of cell walls during plant growth is also affected by unfavorable environmental conditions. These walls contain cellulose as reinforcing microfibrils embedded in a carbohydrate and protein matrix that can flow in a plastic manner.
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From page 370... ...
Accurate studies of plant biology demand access to controlled environments. Growth chambers and similar facilities petit the efficient evaluation of factors affecting growth of plants throughout their entire life cycles.
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From page 371... ...
Advances in Photosynthesis Research Utilize the Full Range of Modern Biological Approaches from Biophysics to Molecular Biology Tremendous slides have been made in gathering information about the catalytic components of photosynthesis at the level of atomic structure. Wideranging discoveries have created the opportunity to understand photosynthetic mechanisms at a molecular level.
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From page 372... ...
have focused attention on specific regions of the amino acid sequence, which seem to have special significance in light absorption and in charge-transfer processes. In this prokaryotic photosynthetic organism, designed alterations in the genes coding for polypeptides that make up the reaction center are possible and becoming routine.
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From page 373... ...
Much Has Been Learned About Regulatory Mechanisms in Photosynthesis, but Much Remains to Be Done With our current knowledge about the component processes of photosynthesis, it has become possible to investigate specific questions about their interdependence. The most important mechanism in the regulation of chloroplast processes is light activation, a central feature that coordinates the light-driven reactions with the so-called dark reactions of photosynthesis.
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From page 374... ...
Techniques such as kinetic absorption spectroscopy, delayed light imaging (Figure 11-2) , Now spectroscopy, flash fluorescence, and photoacoustic spectroscopy are now applied to diagnostic studies of how particular environmental conditions may influence intersystem electron transfer, adenosine triphosphate formation and consumption, enzyme activation, light regulation, photosynthetic reaction center activity, and the transfer of light energy.
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From page 375... ...
The progress that has been made in developing a genetic transformation system for cyanobacteria highlights their potential for the investigation of photosynthetic processes and events generally. Among plants, Arabidopsis and Petunia are genetically tractable, and they lend themselves particularly well to being modified through genetic engineering.
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From page 376... ...
Genetic transformation of the plant gene coding for the catalytic subunit of rubisco appears now to have gone beyond cloning in bacteria. Transfer of the chloroplast gene into the nucleus and addition of a chloroplasttargeting sequence to the protein is a major step toward producing plants with an "engineered" rubisco gene, which will yield a more efficient enzyme.
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From page 377... ...
The biochemistry and molecular genetics of nitrogen fixation have been greatly advanced by studies on a model organism, Klebsiella pneumonias, whose relationship to the common colon bacterium Escherichia cold allows the application of many sophisticated techniques that have been developed for use with its extensively studied relative. Other bacteria have also been important as experimental material for investigations on how nitrogen fixation functions in various ecological niches and takes place in connection with a number of different biochemical strategies.
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From page 378... ...
Unicellular cyanobacteria that show temporal separation between oxygen-producing photosynthesis and oxygen-sensitive nitrogen fixation are ideal models for the possible compatibility of nitrogen fixation and photosynthesis in plants in the absence of symbiosis. The most extensively studied association between plants and nitrogen-fixing microorganisms is that of Rhizobium and its legume hosts, which include such important crop plants as alfalfa, soybean, peanut, vetch, cowpea, beans, peas, and clover, as well as a number of important tropical timber trees, the winged bean, and the "miracle tree," Leucaena, now being used to vegetate large areas in the Asian and Pacific tropics and as a ready source of fuel.
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From page 379... ...
Soybean leghemoglobin genes have been cloned, and their synthesis is controlled at the transcriptional level by an unknown signal from the bacterium. The primary amino acid sequence of plant leghemoglobin strikingly resembles that of animal myoglobin.
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From page 380... ...
~ ~ ~ ~ ~ MESSENGER ~MOLE:C:ULE~S IN ~BA~lTERIAL-PlLANT~ INTE~RAG:1TIONS~: :: 380 OPPORTUNITIES INBIOLOGY I: ~ :~ ~ ~ ~ ~ ~ ~:~ Soil bacteria ~irte~t~:~with~ :~plants~in a Variety of ~ways. ~ Semis ::~eslatblish~:: : ~ ~ ~bendf~cial symbol relationships with~s~cific ~hosts, whereas others invade : ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~.~ ~ ~ ~ ~ ~ ~ ~ :~ ~ ~ ~ ~ ~ ~ ~ ~ ~ .
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From page 381... ...
PLANT GROWTH AND DEVELOPMENT Plants Have an Open System of Growth in Touch the Role of a Very Few Kinds of Plant Hormones Is of Critical Importance Many developmental processes in plants are regulated by a relatively small number of substances called plant hormones. In addition, environmental cues, such as the duration of the daily light and dark period or the ambient temperature, help synchronize the life cycle of plants with the changing seasons.
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From page 382... ...
The cytokinin gene encodes an enzyme that is similar to an enzyme that has been isolated from plants. The bacterial genes for indoleacetic acid synthesis encode two enzymes, a ayptophan monooxygenase and indoleacetamide hydrolase.
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From page 383... ...
The targeted use of plant hormones and synthetic plant growth regulators requires detailed knowledge of their mode of action. In most instances, the response is well characterized at the physiological level.
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From page 384... ...
The timing of such events as seed germination, flowering, the onset of dormancy, and the breaking of dormancy has to be coordinated with the seasons of the year. Plants achieve this coordination by measuring the duration of day and night length and the time over which they are exposed to low temperatures.
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From page 385... ...
These questions are among the most difficult ones in plant biology because basic concepts, on which testable hypotheses can be built, are largely lacking. Precisely because of this gap in our knowledge, work in this area may be particularly rewarding.
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From page 386... ...
is the basis for the production of hybrids with increased vigor in such important crop plants as corn and sorghum. More than 140 plant species have genes for CMS.
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From page 387... ...
PLANT BIOLOGY AND AGRICULTURE 387 ~l:he~ ~ S :~locus~:: is Being ~ analyzed ~ by Bring: thi~nucleic~amd~ ~SeqlJ~Qncss ::: Derived from: different IS genolypes.~ ~:~:this~man:ner ~ re~laitive:ly~::conserv.ed ~ :~: - ions ~ The ~ele-specffie;~:g~n3terns, ~ Ala ~ well As bark; ~ 0:ar:able~ ::: region ::~::wh~may~determine~a:~¢y ~ :have~been~Wentifidd~ ~:~ Se~incompAtibilRy: h~as::alr~adibjen u~we9~in ~e~ ~bf;new~hybrid~strains~of~kohl~::o~ilseed~pe Andover: - mm~y~ar:~:~ :~ :~:~:species~Brdssica :: The~m~anipulation~of~g~netic ~s"HA:ncompati~'lity~is::~:both~ ~:~ :~ag~ ~cu~lt~urally:~:i:mportant~::and~::of:~:funda~me~htal~::biologiCal ~im:porthnce;~blearly~ :~::~these Systems play" a:: sign~ificar~t~role~in~the~:33voh~n of :the~flowa:ring:~ :~: :: ~:~plants ~the~:~dominant~photosynthetic~organisms~on~land.
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From page 388... ...
However, some of these proteins lack essential amino acids, and people whose diet is based largely on such seeds may experience net amino acid deficiencies. Biochemical studies carried out during the 1960s and 1970s showed that seed-storage proteins are specific to certain stages of embryonic development or to particular embryonic organs and that they are contained within protein storage vacuoles.
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From page 389... ...
Genetic engineering methods could be used to introduce barriers to fertilization in cases in which the production of hybrid progeny may increase yields, and they could also be used to help to remove such barriers when self-pollination would prove advantageous. Much remains to be learned about CMS.
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From page 390... ...
Unlike their untransformed, normal counterparts, tumorous plant cells grown on a synthetic culture medium require no exogenous sources of the growth substances cytokinin and indoleacetic acid. Crown-gall tumor cells have therefore acquired the capacity to produce these growth regulators as a result of their transformation.
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From page 391... ...
Both cytokinins and auxins are natal plant constituents, and their overproduction in the transformed plant cells leads to undifferentiated rapid proliferation characteristic of tumorous growth. T-DNA also contains a gene coding for the synthesis of a novel opine amino acid, such as octopine or nopaline, substances that can serve as nitrogen sources for Be bacterium.
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From page 392... ...
No transformation of the plant genome occurs. The nucleotide sequences in the coding regions of the genes for cytokinin and indoleacetic acid synthesis show a high degree of homology with those of the corresponding genes from T-DNA.
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From page 393... ...
Because toxins produced by plant pathogens kill or injure plant cells, they may be viewed as natural herbicides. The activity of such toxins in selective instances provides a conceptual basis for the use of chemicals for weed control.
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From page 395... ...
lion molecules also mediate the interactions between pathogens and their plant hosts. An example for this is the specific messenger function of small polysaccharide fragments of fungal cell walls, called elicitors, which induce plants to produce chemicals called phytoalexins, which in turn might confer disease resistance to plants because they are toxic to the microorganisms that induce phytoalexin production.
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From page 396... ...
In solanaceous plants, such as tomato and potato, small cell-wall fragments called oligogalacturonides are released when plant tissue is injured by chewing insects or mechanical rupture. Such oligogalacturonides, or perhaps some other signal molecules, are transported throughout the plant and systemically induce the production of a powerful proteinase inhibitor that interferes with the digestion of proteins.
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From page 397... ...
In addition, plants such as Arabidopszs thaliana are being used as models for the study of plant molecular genetics, which will provide basic insights into plant biology. Tissue Culture Plant Improvement Through Tissue Culture Is Feasible, but Remains Technically Difficult Plant cell and tissue culture is an important tool for improving plant characteristics.
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From page 398... ...
.. , ~ ~ ~ ~ ~ ~:~:su~on~u~rea~ne~roicides::~'nh~bR::acetola~ate~ synt:hase,~ an~enzyme i:n~:the~: bb-:~:~ :: sYnth:et:ic::~Dathwav of :bra:nch~:~amino acid:s.:~:~ ThQ~m~:~tated~a:Q:nQ~:fnr :~f=t~l:~:: :: : ~:: ~lale symnas~e ~was Clonec:: trom Ar~ aops/s :and :::was~::used: to: transform : tobacco ce:lls.::::: : The: :~:::regenera ed: t:obacco: plants ::::showed stable: h~erbicide ~ ::resistance.
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From page 399... ...
Altematively, plant cell walls can be digested and calli regenerated from single protoplasts. The fusion of protoplasts permits the recom bination of genetic material, even from unrelated kinds of organisms.
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From page 400... ...
Plant Cell Transformation Plant Cell Transformation in Agrobacterium Has Become an Important Research Tool in Plant Molecular Biology Such fundamental questions as the way gene expression is regulated in plants have been investigated through the use of Agrobacterium-induced transformation. Nuclear genes under phytochrome control for example, the gene encoding the small subunit of the chloroplast enzyme rubisco-have been transferred from one species into another in such a way that the regulation of gene expression by light can be studied against a background of precisely defined gene constructions.
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From page 401... ...
The Techniques of Genetic Engineering Hold Enormous Promise for Agriculture Potential improvements in crop plants through genetic engineering include increased yield, lowered production costs, improved nutritional qualities, adaptations to unfavorable growing conditions, and new biosynthetic capacities.
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From page 402... ...
"Custom" crop plants capable of synthesizing specific proteins, valuable oils, or secondary metabolites for medical use could be the bases of new agricultural industries. To attain these goals, research in the plant sciences must proceed at all levels.
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