Roles of brassinosteroid and cytokinin in photomorphogenesis in the dark

Brassinazole (Brz) is a specific inhibitor of brassinosteroid biosynthesis. The dark-grown Brz-treated Arabidopsis and non-Brz-treated der2, which is a brassinosteroid biosynthesis-deficient Arabidopsis mutant, show characteristics of de-etiolated plants, i.e., short hypocotyls, expanded cotyledons and development of true leaves, when grown in darkness. In this context, brassinosteroids seem to function in the suppression of photomorphogenesis in the dark. In general, the application of cytokinin reduces hypocotyls length and induces development of true leaves in the dark. Here, we examined that brassinosteroid-deficiency and cytokinin-excess exhibit whether common effects or different effects in photomorphogenesis in the dark. In development of true leaves in the dark, Brz induced expanding of leaves mainly and a little promote of proliferation of shoot apex. In contrast, Arabidopsis treated by benzyladenine (BA) exhibited many, slender and wind true leaves. Treatment of Brz plus BA induced many true leaves which is alike to normal leaves in light. Both of brassinosteroid-deficiency and cytokinin-excess may be necessary to shape normal leaves. The Brz--treated and BA-treated plants exhibited fast greening after transferring from dark to light. Then, we examined content of protochlorophylide (Pchlide) and protochlorophyll (Pchl) of Brz-treated and BA-treated Arabidopsis. The results showed that Brz induces accumulation of high contents of Pchl. However, BA induced accumulation of Pchlide rather than Pchl. Thus, brassinosteroid-deficiency and cytokinin-excess exhibited different effects in photomorphogenesis in the dark.