Cytokinin inhibition of Arabidopsis root growth: An examination of genotype, cytokinin activity, and N6-benzyladenine metabolism

The effects of cytokinins on the in vitro growth of the roots of Arabidopsis thaliana seedlings were examined. Root growth was inhibited in a manner dependent upon the type of cytokinin compound, the cytokinin concentration, the Arabidopsis genotype, and the duration of exposure to cytokinin. For the cytokinins N 6-benzyladenine (BA), isopentenyl adenine (iP), or dihydrozeatin (DHZ), the concentration required for 50% root growth inhibition differed for each cytokinin and in each of three Arabidopsis genotypes tested. iP was the most active cytokinin in inhibiting the root growth of the Ler-0 genotype, whereas iP and BA had equal activity when tested with the Col-2 and Columbia genotypes. DHZ had the lowest activity of the three cytokinins tested in all three genotypes. A brief 1-day exposure of seeds to a root-inhibiting concentration of BA increased root growth compared with seedlings grown without BA; exposure to BA for 3–6 days inhibited root growth. BA metabolism was evaluated after 6 h and 1, 3, and 6 days in Columbia seedlings. BA, N 6-benzyladenosine (BAR), and N 6-benzyladenosine-5′-monophosphate (BAMP) decreased with time, whereas N 6-benzyladenine-7-β-d-glucopyranoside (BA-7-G) and N 6-benzyladenine-9-β-d-glucopyranoside (BA-9-G) accumulated in the growing seedlings. Seven aromatic cytokinins were compared at 5 μm for their effect on Col-3 root growth. BA, BAR, N 6-(m-hydroxybenzylamino)adenine, and N 6-(o-hydroxybenzylamino)adenine were highly effective in inhibiting root growth, whereas N 6-(p-hydroxybenzylamino)adenine produced only a slight decrease in root growth. BA-7-G and BA-9-G did not affect root growth.