Resistant plant varieties

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... 

Intercropping (seep.259) a disease-susceptible variety with a resistant one in the vegetable plot is a technique that is also looking promising, on a field scale at least. Research trials have shown that lettuces susceptible to downy mildew can be protected if, in the row, each plant alternates with one of a resistant lettuce variety. 

Incorporate into plant varieties, perhaps through genetic engineering, specific antifungal genes expressed in the specific plant tissues, e. g. seed tissues contaminated by aflatoxigenic strains (a host-plant resistance approach), and... 

By late 1996, Monsanto had planned to divide itself into two entities Monsanto Life Sciences and a chemical entity (later given the name Solutia) (Reisch). This occurred within a year, during which Monsanto purchased a plant biotechnology company (Calgene) that held patents for improved fresh produce, cotton seeds, specialty industrial and edible oils (derived from seeds) and plant varieties (Anonymous, 1997). Based on analysis of 1995 sales, the life sciences products accounted for annual sales of S5.3 billion based on the major products of Round-up herbicide, Roundup-resistant soybeans, Bollguard insect-protected cotton, Nutrasweet sweetner, and prescription drugs for arthritis and insomnia. 

Once the desired plant variety has been selected, the marker gene serves no useful purpose in the new plant, but it does continue to produce the gene product, APH(.V)I1 in the case of kanamycin resistance. This enzy me is present at very low concentrations in food. 

Breeders have selected celery with relatively high amounts of psoralens because farmers prefer to grow insect resistant plants and consumers prefer to buy undamaged produce. Unfortunately, workers who harvest such celery sometimes develop a severe skin rash (NAS 2004), an unintended consequence of this conventional breeding. It is possible that if the gene encoding the toxic protein had been cloned and studied before being introduced into the new varieties, farm workers would have learned of the harmful effects before exposure. 

One must distinguish between resistant species and resistance within species. The use of the former as substitutes for susceptible species in parks, industrial areas, and other urban settings is more properly discussed in ecological engineering terms. We are primarily interested here in the identification and use of resistant variants within species and the use of several plant breeding techniques to obtain new and improved resistant varieties. These techniques include selection of resistant plants within varieties, selection of resistant varieties within species, and crosses between varieties with subsequent selection of improved progenies. 

Often, however, it is necessary to resort to a more complex procedure. If none of the plants in the population of interest is resistant, the breeder must look outside that population for a source of resistance, either in another variety or in a wild relative. The resistant plants in the second population may be susceptible to a disease, may yield less than desired, have poor color or quality, or be sensitive to heat or cold. 

Plant varieties that are resistant or have low susceptibility to mildew. 

Growth Rates. Because pathogen growth rates are so important in the evolution of fungicide resistance, plant pathologists should continue to recommend complete, effective systems to curtail growth rates of both resistant and sensitive forms (Table III). These complete systems are likely to include cultural controls, sanitation, resistant varieties, as well as protectant and systemic fungicides. 

The development of crop plant varieties with resistance to pests including Insects and diseases has several advantages compared with other approaches to pest control The application of pesticides may be decreased or eliminated, resulting in decreased costs of production and environmental pollution The use of resistant varieties also does not require the adoption of complicated technologies by the user. 

Resistance of vTKM6f plants to SSB is probably due to the production of certain allomones which inhibit oviposition and disrupt the insect s growth and development. Some of these factors, such as compound A, have recently been identified and synthesized in collaboration with the Tropical Development Research Institute (formerly Tropical Products Institute), London (20). The allomones in the resistant TKM6 variety thus account for inhibition of SSB oviposition and for adverse effects on egg (embryonic), larval and pupal (post-embryonic) stages. On the other hand, nine wild rices (four . minuta J.S. Presl., four 0 officinalis Wall ex. Watt, and one (). punctata Kotschy ex. Steud.) had no compound A in their plant volatiles, but a few were still toxic to SSB (21). This confirms that allomones other than compound A may also be involved in SSB resistance. The role of nutritional factors in growth of SSB larvae remains yet to be studied. 

---