抄録
Brassinosteroid-deficient plants show characteristics of de-etiolated plants, i.e., short hypocotyls, expanded cotyledons and development of true leaves, when grown in darkness. In contrast, it is known that the application of cytokinin induces de-etiolated phenotype in the dark. In this context, brassinosteroid and cytokinin seem to exhibit opposite roles in photomorphogenesis. Here, we observed cell division and plastid development of dark-grown plants of brassinosteroid-deficiency and cytokinin-excess. Four-days-grown cyclin : : GUS transgenic Arabidopsis showed no GUS-staining in the dark. In contrast, Brz (an specific brassinosteroid biosynthesis inhibitor)-treated or benzyladenine (BA)-treated cyclin : : GUS plants showed intense GUS expression in shoot apex. In Brz plus BA-treated plants, additive GUS expression was observed. These results suggest that both brassinosteroid-deficiency and cytokinin-excess function cell division in development of true leaves in the dark. Brz-treated or BA-treated plants exhibited fast greening after transferring from dark to light. In Brz plus BA-treated plants, however, the fast greening effect was not additive. Transmission electron microscopical observation revealed that plastids of Brz-treated plants were etioplasts containing large prolammellar bodies, while those of BA-treated plants were young chloroplasts containing monolayer thylakoid membranes. Thus, brassinosteroid-deficiency and cytokinin-excess exhibited different effects in plastid development in the dark.
