Hormone levels and response during de-etiolation in pea

Abstract.

The objective of this study was to increase our understanding of the hormonal regulation of de-etiolation by investigating endogenous hormone levels and response in etiolated pea (Pisum sativum L.) seedlings after exposure to continuous white light. Recent reports suggest that de-etiolation may result from the down-regulation of an enzyme in the brassinosteroid (BR) biosynthesis pathway in pea. A subsequent review highlighted the need for direct measurements of BR levels to support this hypothesis. We have shown that endogenous castasterone and 6-deoxocastasterone levels are not greatly reduced after exposure to light; indeed, 6-deoxocastasterone levels were actually increased. Similarly, the elongation response to exogenous brassinolide was greater in plants grown in continuous light, or in dark-grown plants that had been transferred into the light, than in plants that were grown in continuous darkness. These results provide further evidence to suggest that BRs do not negatively regulate de-etiolation in pea. However, changes in the levels of several other hormones have also been implicated in light-regulated development. We have simultaneously quantified indole-3-acetic acid (IAA), gibberellin (GA), and abscisic acid levels in whole seedlings, which revealed a complex pattern of changes in the levels of these substances after exposure to light. The first and most dramatic of these changes was a significant reduction in GA₁ levels, which reached a minimum 8 h after exposure to light. Whilst GA₁ levels rapidly decreased, IAA levels remained unchanged in the short term after exposure to light, suggesting that GA₁ levels may be the primary factor regulating the reduction in elongation growth during de-etiolation. In the long term after exposure to light, IAA levels underwent a transitory increase, which peaked at 48 h, and had abated by 96 h. However, abscisic acid levels remained unchanged in the first 1 h after exposure to light before undergoing a steady decline over time. The relative importance of these changes in mediating light-induced changes in plant morphology is discussed.