Changes in gibberellin A1 levels and response during de-etiolation of pea seedlings

The level of gibberellin A1 (GA1) in shoots of pea (Pisum sativum) dropped rapidly during the first 24 h of de-etiolation. The level then increased between 1 and 5 d after transfer to white light. Comparison of the metabolism of [13C3H] GA20 suggested that the initial drop in GA1 after transfer is mediated by a light-induced increase in the 2β-hydroxylation of GA1 to GA8. A comparison of the elongation response to GA1 at early and late stages of de-etiolation provided strong evidence for a change in GA1 response during de-etiolation, coinciding with the return of GA1 levels to the normal, homeostatic levels found in light- and dark-grown plants. The emerging picture of the control of shoot elongation by light involves an initial inhibition of elongation by a light-induced decrease in GA1 levels, with continued inhibition mediated by a light-induced change in the plant's response to the endogenous level of GA1. Hence the plant uses a change in hormone level to respond to a change in the environment, but over time, homeostasis returns the level of the hormone to normal once the ongoing change in environment is accommodated by a change in the response of the plant to the hormone.