Effects of Thidiazuron on Cytokinin Autonomy and the Metabolism of N6-(Δ2-Isopentenyl)[8-14C]Adenosine in Callus Tissues of Phaseolus lunatus L.

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Abstract
The effects of a highly cytokinin-active urea derivative, N-phenyl-N′-1,2,3-thiadiazol-5-ylurea (Thidiazuron), and zeatin on cytokinin-autonomous growth and the metabolism of N6-(Δ2-isopentenyl)[8-14C]adenosine ([14C]i6 Ado) were examined in callus tissues of two Phaseolus lunatus genotypes, cv Jackson Wonder and P.I. 260415. Tissues of cv Jackson Wonder maintained on any concentration of Thidiazuron became cytokinin autonomous, whereas only tissues exposed to suboptimal concentrations of zeatin displayed cytokinin-autonomous growth. Tissues of P.I. 260415 remained cytokinin dependent under all these conditions. The metabolism of [14C]i6 Ado was similar for the two genotypes, but differed with the medium used. [14C]i6 Ado was rapidly converted to N6-(Δ2-isopentenyl)[8-14C]adenosine 5′-P ([14C]i6 AMP) by tissues grown on zeatin-containing medium, whereas only traces of the nucleotide were formed in tissues grown on medium with Thidiazuron. Incubation with [14C] i6 AMP of tissues grown in the presence of Thidiazuron resulted in rapid conversion to [14C]i6 Ado, while [14C]i6 AMP persisted in tissues maintained on zeatin. Thus, Thidiazuron appears to stimulate enzyme activity converting the ribonucleotide to ribonucleoside. Although the cytokininactive phenylureas and adenine derivatives differ in their effects on cytokinin autonomy as well as nucleotide formation, the two types of effects do not seem to be related.