Resistant varieties, crop plant selection

There are a number of possibilities for using plant secondary chemistry to control herbivory in crop plants. One possibility is to select for insect resistant lines and though it has been done in only a few cases, select for specific allomones. There are, however, some potential problems with this approach. There is a cost for the production of the secondary compounds which may be useful for defense ( ). Insect resistant soybean cultivars produce lower yields of seeds and accumulate nitrogen at a slower rate than insect susceptible varieties in the absence of herbivores 3 ). Conversely, varieties of crop plants selected for high yield are often more susceptible to insects, pathogens, and weeds (35). 

Classically, plant breeders selecting for insect resistance in crop plants have utilized crosses between varieties within the same species or between plants of closely related species. Higher levels of chemical resistance factors can indeed be built up in this way. However, insects can potentially become adapted to these substances as a consequence of previous exposure at lower concentrations, thus being preadapted to detoxify or otherwise tolerate these chemicals. We desire to introduce completely exotic chemicals, i. e, substances to which specific pests have never been exposed. This could be done by crossing plants that are only distantly related to the crop plant and would include species from other genera, possibly from other families. 

Air pollutants cause plant damage which varies both in type and degree. A few crop, ornamental, and forest species have shown a variability in the degree of damage so that some varieties within these species may be classed as resistant and others as susceptible. This variability probably exists in many species in which it has not yet been demonstrated. Further, in a few species this variability is at least in part genetically based. Therefore, the opportunity exists to select those plants resistant to the toxicants and to breed new varieties which can be successfully grown in a polluted atmosphere. Obviously, this solves only part of the air pollution problem. Resistant varieties will alleviate some effect of air pollution on plants but will not eliminate the pollutants themselves. 

Cloned herbicide resistant ALS genes have been used to transform both homologous and heterologous plant species. ALS genes can be modified in vitro in order to achieve selective resistance toward broad or narrow classes of inhibitors. The modified genes can be introduced into a variety of commercial crops. 

Herbicide-resistant plant varieties have proven to be valuable experimental tools In determining the molecular mode of action of herbicides (2,8). In addition, such varieties are likely to be an Important source of selectable markers for use In plant molecular genetics and In the engineering of resistant crop species ( ) The earliest herbicide-resistant blotypes described arose spontaneously from weed populations which had been repeatedly exposed to a herbicide (10,11). More recently, mutagenesis and selection on defined media have been used to Isolate herbicide-resistant or herbicide-tolerant mutants of higher plants from populations of cells In tissue culture (4,5.7,12). While the use of plant tissue culture has proven useful for Isolation of some types of mutants, the... 

The harvest index of wheat has been substantially increased by the breeding of semi-dwarf varieties, and a closely related trait, improved resistance to lodging, has resulted in improvements both to yield and crop quality. Improved yields have been an important trait in all of the other major crops. In some cases improvements have been obtained by plant breeders selecting for winter hardy types which can be autumn sown. Oilseed rape is an important example, others, less successful, have included linseed, peas and lupins. 

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