Light regulation of gibberellin A1 content and expression of genes coding for GA 20-oxidase and GA 3β-hydroxylase in etiolated pea seedlings

White light (WL) inhibited the stem elongation of etiolated pea (Pisum sativum L.) seedlings and the inhibition was partially reversed by the application of gibberellin A(1) (GA(1)), the active GA in shoot growth, The amount of GA(1) in the apical shoot was reduced to about 25% after 2 h of WL, and to a trace level after 4 h, The effect of light on GA(1) content was reversed when the plants were transferred again to the dark after 6 h of WL. The effect of light on the expression of GA 20-oxidase and GA 3 beta-hydroxylase genes, coding for the last steps of GA(1) biosynthesis, was also investigated. Contrary to expectations, the amounts of GA 20-oxidase and GA 3 beta-hydroxylase transcripts increased in the entire apical shoot of WL-irradiated seedlings, and this increase was negated in seedlings treated with GA(1) before WL irradiation, probably as a result of negative feed-back regulation. The effect of WL on GA 20-oxidase transcripts was mainly localized in the apex (hook) whereas the effect on GA 3 beta-hydroxylase transcripts was mainly localized in the subapical tissues, Red and far-red light also enhanced the GA 20-oxidase transcript level, but not that of GA 3 beta-hydroxylase, suggesting that different photoreceptors are involved in the regulation of these genes by WL, The results presented indicate that the inhibition of stem elongation by light is due, at least partially, to a decrease of GA(1) content by a still unknown mechanism. The increase of GA(8) upon WL irradiation raises the possibility that an inactivation activity may be involved in the control of the content of GA(1) by light.