CYTOKININ ACTION IS COUPLED TO ETHYLENE IN ITS EFFECTS ON THE INHIBITION OF ROOT AND HYPOCOTYL ELONGATION IN ARABIDOPSIS-THALIANA SEEDLINGS

Cytokinins have profound effects on seedling development in Arabidopsi s thaliana. Benzyladenine (BA) inhibits root elongation in light- or d ark-grown seedlings, and in dark-grown seedlings BA inhibits hypocotyl elongation and exaggerates the curvature of apical hooks. The latter are characteristic ethylene responses and, therefore, the possible inv olvement of ethylene in BA responses was examined in seedlings. It was found that the inhibitory effects of BA on root and hypocotyl elongat ion were partially blocked by the action of ethylene inhibitors or eth ylene-resistant mutations (ein1-1 and ein2-1). Ethylene production was stimulated by submicromolar concentrations of BA and could account, i n part, for the inhibition of root and hypocotyl elongation. It was de monstrated further that BA did not affect the sensitivity of seedlings to ethylene. Thus, the effect of cytokinin on root and hypocotyl elon gation in Arabidopsis appears to be mediated largely by the production of ethylene. The coupling between cytokinin and ethylene responses is further supported by the discovery that the cytokinin-resistant mutan t ckr1 is resistant to ethylene and is allelic to the ethylene-resista nt mutant ein2.