GIBBERELLINS AND PHYTOCHROME REGULATION OF STEM ELONGATION IN PEA

In garden pea (Pisum sativum L.) neither etiolation nor the phytochrom e B (phyB)-response mutation lv substantially alters the level of the major active endogenous gibberellin, GA(1) in the apical portion of yo ung seedlings. The phyB-controlled responses to continuous red light a nd end-of-day far-red light are retained even in a GA-overproducing mu tant (sln). Comparison of the effects of the lv mutation and GA(1) app lication on seedling development shows important differences in rate o f node development, cell extension and division, and leaf development. These results suggest that in pea the control of stem elongation by l ight in general and phyB in particular is not mediated by changes in G A(1) content. Instead, the increased elongation of dark-grown and lv p lants appears to result from increased responsiveness of the plant to its endogenous levels of GA(1). Three GA(1)-deficient mutants, na, ls and le have been used to investigate these changes in responsiveness, and study of these and the double mutants na lv, ls lv and le lv has d emonstrated that the relative magnitude of the change in responsivenes s is dependent on GA(1) level. The difference in pleiotropic effects o f GA(1) application and the Iv mutation suggest that light and GA(1) i nteract late in their respective transduction pathways. A model for th e relationship between light, GA(1) level and elongation in pea is pre sented and discussed.