Genetic Engineering of Plants: Progress and Prospects

Abstract

Gene-vector systems have been improved and diversified and used to study the structure and function of regulatory DNA sequences specifying quantitative and qualitative gene expression. Improvements of gene-vector systems, based on the Agrobacterium Ti and Ri plasmids, are based on a better understanding of the T-DNA transfer mechanism. Thus special purpose vectors for gene-expression studies, shotgun cloning insertional mutagenesis in active genes and the convenient regeneration of transformed cells into plants have been developed. Recent research is expanding the host range of such gene vectors to a number of crop plants, e.g. tomato, potato, alfalfa, soya- Brassica, etc. Promoter DNA sequences derived from T-DNA genes or from plant viruses such as the cauliflower mosaic virus were successfully used to express enzymes such as neomycine phosphotransferase, hygromycine phosphotransferase, chloramphenicol transacetylase, methotrexate resistant dehydrofolate reductase, enolpyruvateskikimate synthase, etc. Cells, calluses and whole plants expressing chimeric genes coding for these enzymes were shown to be resistant to toxic agents such as kanamycin, G-418, hygromycin, chloramphenicol, methotrexate, glyphosphate, etc. The dominant selectable marker genes thus developed were subsequently used to produce various methods for direct DNA uptake by plant protoplasts. These methods open realistic prospects for the genetic engineering of cereals. Transgenic plants were also used to test expression vectors. Regulatory sequences located in 5′-upstream regions of regulated genes have been shown to be sufficient to direct the regulated expression of chimeric genes in transgenic plants. The following examples of regulated expressions were achieved: low or high level, organ specific, light and chloroplast dependent and heatshock dependent. It was also shown that nuclear DNA sequences coding for transit peptides can be used to direct the synthesis in plants of chimeric precursor proteins which are transported into chloroplasts and specifically processed. Promoter sequences can be put together with regulatory elements derived from different genes. Such chimeric promoter sequences can be used to predetermine the expression of foreign genes in trans-genic plants.