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3 Past Experience with Genetic Modification of Plants and Their Introduction into the Environment
Pages 16-36

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From page 16...
... Each scientific advance has increased our ability to alter the genetic makeup of plants predictably, and several techniques are often used together to improve plants. For example, an existing plant chosen for genetic modification by recombinant DNA techniques knight have been modified by many generations of cIassicat breeding and selection; the recombinant plant derived from the Original could then be reintroduced into a classical breeding program from which its descendants would be released for commercial use.
From page 17...
... 17 CO ~ by Z .
From page 18...
... Classical Techniques of Genetic Modification of Plants Hybridization. Most genetic modification techniques are used by plant breeders whose purpose is to apply the techniques to improve plants with commercial value.
From page 19...
... Cellular Techniques of Genetic Modification of Plants Somaclonal Variation. Somaclonal variation occurs in plants regenerated from cell in tissue culture, presumably as a result of stress imposed on the plant cells.
From page 20...
... TH1: RESIJ[TS OF GEN1:TIC MODIFICATION Plant breeding has sought to make two major kinds of modifications In recipient organisms: those to increase yield and those to increase reliability of performance. Increased Yield and Increased Reliability of Performance Maize breeders have looked for varieties or hybrids that produce larger amounts of grain per unit of land area, potato breeders for increased tuber yields, and cotton breeders for increased yields of lint (fiber)
From page 21...
... Plant varieties have been continually selected for Unproved resistance or tolerance to external factors that inhibit their inherent productivity. They have been selected for resistance to insect pests, to disease organisms, and, in recent years, even to specific herbicides.
From page 22...
... Thus, cultivated Cuphea could easily revert to the self-perpetuating nature of the wild species if other plant traits have not been altered by domestication to hincler survivability. Most of the widely grown grain crops and the horticultural and vegetable crops are at the maize end of the reproductive spectrum; they cannot survive in the wild.
From page 23...
... Domesticated plants and their weeds have thus evolved together, and distinctions between them are sometimes minor. For example, grassy annual sorghums, grown as pasture crops or for cutting as green forage, have often retained their wild ancestors' traits of bearing self-sowing, long-lived seeds with varying periods of dormancy.
From page 24...
... The bulk of the a~ric~,lt~,ra1 nr~rl,,~t.i~n in t.h" United States has depended on the introduction of exotic species such as wheat, soybeans, peaches, cherries, apples, tomatoes, potatoes, and peas. This can be an inconvenience for breeders, because the useful gene pool found in wild relatives may be less readily accessible.
From page 25...
... The need for marine lubricating oils during the Second World War motivated Canadian farmers to initiate commercial growing of rapeseed, but the need disappeared after the war and production declined. Experiments In the 1940s and 1950s demonstrated that erucic acid, one of the major fatty acids in rapeseed oil, Is metabolized poorly by mammals.
From page 26...
... The double-low Brassica napes and B campestris varieties were the first rapeseed to meet specific quality requirements of low erucic acid and low glucosinolates.
From page 27...
... It has been possible to hybridize aIrnost all wild species to the common cultivated potato either directly or indirectly by use of multiple crosses. Through several backcrosses of hybrids to existing cultivars, new, acceptable cultivars were obtained that contain the desired germplasm from the wild species.
From page 28...
... The direct transfer of genes for resistance into highly developed cultivars with gene-transfer methods would be significantly more effective than if done by classical breeding. Potato plants regenerated from protoplasts or other unorganized groups of cells display an outburst of phenotypic variation.
From page 29...
... Two situations are discussed here to exemplify the type of problems that have developed and how they have been readily managed by plant breeders. The first example is breeding for resistance to northern corn leaf blight fungus (XeZminthosporium turcica)
From page 30...
... corn crop was destroyed by the fungal plant pathogen Helminthosporium maydis, which causes southern corn leaf blight (Zadoks and Schein, 1979~. This represented a loss of 20 million metric tons of corn, worth about one billion dollars.
From page 31...
... ~ 1970, with proper weather conditions for disease development, with 85 percent of the corn crop containing Tams' and with an abundant supply of race T ~noculum, a southern corn leaf blight epidemic developed. Fortunately, however, the genetic basis for race-T susceptibility was quickly determined.
From page 32...
... Plant breeders traditionally use confinement procedures to minimize genetic contamination of their field plots by pollen from outside sources such as neighboring fields. In addition, confinement practices are used to keep plant pathogens from spreading into or out of experimental field plots.
From page 33...
... This is done by confinement practices, which limit the plants or their products to a particular site and also protect neighboring fields from contaminating pollen. In this way, any unexpected effects can be observed.
From page 34...
... In tests conducted on a small scale, one uses the smallest numbers of plants that will give the information desired. More elaborate barriers to limit dispersal beyond the site include removing pollinating organs from plants, bagging flowers, and adjusting the time of year the plants are grown to avoid insect pests.
From page 35...
... Quarantine Tillage c ChemicalsBiological control Imgation/flooding Insect vector control Machinery sanitation Runoff water control Solarization Scourer with plastic) Breeding for resistancea Biological controlQuarantine Chemicals Crop rotation Culti~rar rotation Irrigat ion /flooding Heat treatment Soil solarization Induced resistance Meristem/tissue culture Insect Rector control Weed control Erosion control Breeding for resistance Biological control Crop rotation Cultivar rotation Soil amendments Weed control Erosion control Gerrnplasm may be adequately identified for rapid development; otherwise the process normally takes 5 to 10 years.
From page 36...
... Cellular techniques include cell fusion and somaclonal variation produced by tissue culture. Molecular techniques include directly introducing genes by a variety of trance formation procedures.


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