Production of Stress Tolerant Plants by Overproduction of Enzymatic Oxyradical Scavengers

Most of the objectives that were outlined in the original proposal have been met with two exceptions. Briefly, our goals were to: (1) constract transgenic tobacco plants which overproduce one or more of the enzymatic oxyradical scavengers and associated ancillary enzymes, including superoxide dismutase, ascorbate peroxidase, glutathione peroxidase, glutathione reductase, and monodehydrascorbate reductase; (2) evaluate the tolerance of these transgenic plants to oxidative stress; and (3) extend these studies to an agronomically important crop such as citrus. As can be seen i the following pages, our objectives (1) and (2) have been achieved, although transgenic lines overexpressing phospholipid hydroperoxidase glutathione peroxidase (PHGPX) were not obtained and our evidence to date suggests that constitutive overexpressing of the enzyme is probably lethal. Howeever, transgenic tobacco expressing the antisense construct for PHGPX were obtained. Tobacco plants overexpressing ascorbate peroxidase and those sensesuppressing monodehydroascorbate reductase are more tolerant to oxidative stress, as mediated by the redox-cycling agent paraquant; in contrast, plants expressing the PHGPX-antisense construct are more sensitive to paraquat. Additional research is warranted on each of the six types of transgenic lines which we generated with regard to their tolerance to saline stress. Until recently, attempts to transform citrus were not very successful, and thus additional attention is currently being directed at objective (3). We are optimistic that use of the plant transformation vector, pBIN, will lead to stable transgenic citrus, as preliminary experiments demonstrate stable expression of the GUS reporter gene. Other important contributions resulting from this BARD project include the biochemical characterization of the first plant phospholipid glutathione peroxidase and the biochemical and molecular analysis of another key antioxidant enzyme, monodehydroascorbate reductase. Overall this BARD-supported project was quite successful, and the biological resource of numerous transgenic lines which have altered levels of antioxidant enzymes should be valuable for years to come.