THE CONSEQUENCE OF PEROXIDASE OVEREXPRESSION IN TRANSGENIC PLANTS ON ROOT-GROWTH AND DEVELOPMENT

Transgenic tobacco plants that overproduce the tobacco anionic peroxid ase wilt upon reaching maturity, although having functional stomata an d normal vascular anatomy and physiology. These plants were examined f urther to determine the cause for wilting, and thus better understand how the anionic peroxidase functions in plant growth and development. Shoots from young peroxidase overproducing plants were grafted onto wi ld-type tobacco root stock to determine if the roots could absorb and transmit sufficient water to maintain leaf turgidity. These grafted pl ants never wilted when grown in the greenhouse though shoot peroxidase activity remained ten-fold greater than in control plants, thus indic ating that wilting is a consequence of peroxidase expression in the ro ots. Close examination of root systems revealed considerably less root mass in the transformed plant, primarily exhibited through a decrease in branching. At flowering, root growth rate and total root mass in t ransformed plants were less than 50% of control plants although shoot mass and growth rate were unchanged. This is in contrast to root growt h in young seedlings where transformed plants performed equivalently t o controls. Root hydraulic conductivity was measured to evaluate the e ffect of elevated peroxidase expression on water absorption and transp ort; however, no significant change in hydraulic conductivity was foun d in transformed plants. The consequence of anionic peroxidase overexp ression on indoleacetic acid (IAA) metabolism was also examined, No si gnificant difference in IAA levels was observed; however, root elongat ion in plants overexpressing peroxidase was insensitive to exogenous I AA. It can be concluded that the overexpression of the tobacco anionic peroxidase in transformed plants results in diminished root mass from fewer root branches, which contributes to the wilting phenomenon seen in these plants. Further, this developmental change in transformed pl ants may be a consequence of the metabolism of IAA by the anionic pero xidase.