CYTOKININ INHIBITION OF ARABIDOPSIS ROOT-GROWTH - AN EXAMINATION OF GENOTYPE, CYTOKININ ACTIVITY, AND N-6-BENZYLADENINE METABOLISM

The effects of cytokinins on the in vitro growth of the roots of Arabi dopsis thaliana seedlings were examined. Root growth was inhibited in a manner dependent upon the type of cytokinin compound, the cytokinin concentration, the Ai Arabidopsis genotype, and the duration of exposu re to cytokinin. For the cytokinins N-6-benzyladenine (BA), isopenteny l 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 thr ee genotypes, A brief 1-day exposure of seeds to a root-inhibiting con centration of BA increased root growth compared with seedlings grown w ithout BA; exposure to BA for 3-6 days inhibited root growth. BA metab olism was evaluated after 6 h and 1, 3, and 6 days in Columbia seedlin gs. BA, N-6-benzyladenosine (BAR), and N-6-benzyladenosine-5'-monophos phate (BAMP) decreased with time, whereas N-6-benzyladenine-7-beta-D-g lucopyranoside (BA-7-G) and N-6-benzyladenine-9-beta-D-glucopyranoside (BA-9-G) accumulated in the growing seedlings. Seven aromatic cytokin ins were compared at 5 mu M for their effect on Col-3 root growth. BA, BAR, N-6-(m-hydroxybenzylamino)adenine, and N-6-(o- hydroxybenzylamin o)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.