EFFECT OF AUXIN ON EXPRESSION OF THE ISOPENTENYL TRANSFERASE GENE (IPT) IN TRANSFORMED BEAN (PHASEOLUS-VULGARIS L.) SINGLE-CELL CLONES INDUCED BY AGROBACTERIUM-TUMEFACIENS C58

The effect of auxin on the endogenous cytokinin content and on the exp ression of isopentenyl transferase gene (ipt) was investigated in bean (Phaseolus vulgaris L. cv. Palgong) tumor single-cell clones induced by Agrobacterium tumefaciens C58. The major endogenous cytokinins of t umor single-cell clones were zeatin and zeatin riboside. Endogenous ze atin and zeatin riboside levels in tumor single-cell clones cultured o n an auxin-supplemented medium were reduced by six-fold and eight-fold , respectively, while tumor single-cell clones cultured on the 5.0 mu M kinetin-supplemented medium did not exhibit any reduction in the lev els of these cytokinins. The mRNAs isolated from normal single-cell cl ones cultured on 5.0 mu M kinetin and 2.5 mu M picloram-supplemented m edium, from transformed single-cell clones cultured on hormone-free me dium, and from transformed single-cell clones cultured on 2.5 mu M pic loram-supplemented medium, were subjected to Northern blot hybridizati on. The ipt transcript was not detected in tumor single-cell clones cu ltured on picloram-supplemented medium, but the ipt mRNA was detected in tumor single-cell clones cultured on hormone-free medium. The amoun t of ipt mRNA in tumor single-cell clones was found to decrease with t ime in cultures grown on picloram-supplemented medium. The nopaline sy nthase gene (nos) transcript was detected in the tumor single-cell clo nes ft om both culture conditions. It is concluded that picloram regul ates the ipt mRNA steady state level, either at the transcriptional le vel or by affecting ipt mRNA stability.