Crop plants resistance genes

The use of plants from extreme environments Wild plants from extreme environments may possess genes and gene combinations which confer stress tolerance. We must realise, however, that many of their characteristics, e.g. leaf pubescence and succulence in drought-resistant plants, are incompatible with the high yield potential required for crop plants. In addition, most of these species contain compounds such as phenolics and mucilages which interfere with conventional molecular biology techniques. 

Several successful examples are found in the literature where defense-related genes have been transformed into other plants, both model systems and crop plants. Some examples of transgenic plants and their conferred resistances are summarized in Table 4. 

The successful transfer of recombinant DNA into plants was vividly illustrated by an experiment in which the luciferase gene from fireflies was introduced into the cells of a tobacco plant (Fig. 9-29)—a favorite plant for transformation experiments because its cells are particularly easy to transform with Agrobacterium. The potential of this technology is not limited to the production of glow-in-the-dark plants, of course. The same approach has been used to produce crop plants that are resistant to herbicides, plant viruses, and insect pests (Fig. 9-30). Potential benefits include increased yields and less need for environmentally harmful agricultural chemicals. 

Because the role of plant enzymes in host-plant resistance to insects is poorly understood at both an ecological and mechanistic level, the enhancement and/or transfer of genes for crop improvement should proceed with caution. The combined input of molecular biologists, agronomists, food scientists, plant pathologists, weed scientists, and entomologists is needed to achieve optimal success in crop improvements. 

However, each new resistance that develops will produce a different genetic situation. The solutions found by weeds to escape herbicide selection pressures may be varied. Due to the high selective value conferred by the resistance genes in herbicide treated areas, mutation events at very low frequencies have high probability to lead to the appearance of resistant plants. In addition, weeds will certainly display after a short delay the bacterial genes transferred to crops for herbicide resistance, and some wild plants could be expected to become new weeds because of resistant genes. 

The very low frequencies of resistant individuals in the field must compete with the crop, with resistant members of other weed species, and (when the herbicide is not present) with susceptible members of the same and other species. If triazine resistance evolved only in populations with plastome mutator genes (see above), there is the strong possibility of multiple mutations in these plastid genomes, including deleterious mutations giving unfit alleles of other genes. These may explain much of the unfitness of resistant plants, as well as the published variabilities of plastid fitness. Deleterious nuclear... 

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