Changes in gibberellin A(1) levels and response during de-etiolation of pea seedlings

The level of gibberellin A(1) (GA(1)) in shoots of pea (Pisum sativum) drop ped rapidly during the first 24 h of de-etiolation. The level then increase d between 1 and 5 d after transfer to white light. Comparison of the metabo lism of [(CH)-C-13-H-3]GA(20) suggested that the initial drop in GA(1) afte r transfer is mediated by a light-induced increase in the 2 beta -hydroxyla tion of GA(1) to GA(8). A comparison of the elongation response to GA(1) at early and late stages of de-etiolation provided strong evidence for a chan ge in GA(1) response during de-etiolation, coinciding with the return of GA (1) levels to the normal, homeostatic levels found in light- and dark-grown plants. The emerging picture of the control of shoot elongation by light i nvolves an initial inhibition of elongation by a light-induced decrease in GA(1) levels, with continued inhibition mediated by a light-induced change in the plant's response to the endogenous level of GA(1). Hence the plant u ses a change in hormone level to respond to a change in the environment, bu t over time, homeostasis returns the level of the hormone to normal once th e ongoing change in environment is accommodated by a change in the response of the plant to the hormone.