Brassinosteroid-induced de-etiolation of Arabidopsis thaliana seedlings resembles the long-term effects of cytokinins

Abstract

Cytokinins can cause de-etiolation of Arabidopsis thaliana (L.) Heynh. seedlings growing in the dark. Brassinosteroids (BRs) have been considered to regulate negatively the de-etiolation in dark-grown Arabidopsis seedlings. We show here that epi-brassinolide (epi-BL) can partially produce the phenotype of de-etiolation as caused by treatment with cytokinins in the dark, including the development of leaves and epicotyls in the wild-type and the BR-deficient mutant det2. But BRs cannot inhibit hypocotyl elongation, nor restore all the inhibition caused by cytokinins and light. We have found that there are distinct short term and long term phases of induction of de-etiolation by cytokinins. The short-term effect is probably coupled to ethylene in the inhibition of the hypocotyl elongation; the long-term effect causes morphogenesis of leaves and epicotyls. BRs can only regulate de-etiolation in the long term. We propose that the inhibition of hypocotyl elongation of det2 in darkness is caused by the absence of BR-dependent elongation rather than the inhibition caused by the expression of genes for photomorphogenesis. We propose that BRs resemble cytokinins in regulating de-etiolation as positive regulators, and that the inhibition of hypocotyl elongation and the development of leaves and epicotyls in de-etiolation are independent processes.