SUBSTRATE EFFECTS ON CYTOKININ METABOLISM IN SOYBEAN CALLUS-TISSUE

The metabolism of three different concentrations of [8-3H] zeatin riboside (ZR; 0.6 .mu.M alone, or supplemented with 5.4 or 59.4 .mu.M unlabelled ZR) supplied to cytokinin-dependent soybean callus tissue, previously maintaind on medium containing unlabelled ZR, was studied in relation to cytokinin oxidase-like degradative metabolism in vivo. The tissue was extracted and analysed after 20 h incubation. With increasing substrate concentration the proportion of degradative metabolites of ZR decreased, with a concomitant increase in residual ZR and ZR metabolites with an intact cytokinin moiety. The metabolism of 0.6 .mu.M [8-3H] ZR was also studied when supplied together with unlabelled benzyladenine (BA, which is not a substrate for cytokinin oxidase), and in callus which had been maintained previously on BA instead of ZR. These BA treatments resulted in a slightly greater uptake of [8-3H] ZR. Degradative metabolism of [8-3H] ZR was reduced in callus that had been maintained previously on BA, and in ZR-maintained callus supplied with 5.4 .mu.M unlabelled BA, instead of unlabelled ZR, at the time of incubation. However, the pattern of ZR metabolism was very similar in treatments where the incubation medium was supplemented with either 59.4 .mu.M unlabelled ZR or the same concentration of unlabelled BA. These results suggest that at lower cytokinin concentrations a substrate-inducible cytokinin oxidase-like system dominates ZR metabolism in this tissues, but as cytokinin concentration increases enzymes other than cytokinin oxidase play a major role in determining the overall pattern of cytokinin metabolism.