In vitro cytokinin binding to a particulate cell fraction from protonemata of Funaria hygrometrica

Cytokinin-induced bud formation in moss protonemata is specific for cytokinin bases, their ribosides being relatively inactive. Binding of [(3)H]benzyladenine (BA) to a 13,000-80,000 x g subcellular fraction from extracts of Funaria hygrometrica (L.) Sibth. was measured by a centrifugation assay. Increasing concentrations of non-radioactive BA decreased the binding proportionally to the logarithm of the BA concentration between 3×10(-8) and 10(-4)M. [(3)H]Zeatin also bound to these fractions, although the extent of binding was not as great as with [(3)H]BA. Biologically active cytokinins, including BA, zeatin, 6-(3-methyl-2-enylamino)purine (IPA) and kinetin, competed for the binding of [(3)H]BA, whereas the ribosides of BA, zeatin and IPA competed poorly. Other biologically inactive compounds, such as adenine and 9-methyl-BA, were also ineffective as competitors. The ability to bind BA by the 13,000-80,000 x g fraction was greatly reduced by treatment with 1% Triton X-100, and heat treatment eliminated more than one-half of the binding activity. Competitive binding appeared to be pH-dependent, with maximal activity between pH 6.0 and 6.5. After fractionation by differential centrifugation, the ability to bind cytokinins was not correlated with the RNA content of the fraction and thus probably did not represent binding to ribosomes which has been reported in other plant tissues. Cytokinins also exhibited competitive binding to non-biological materials, e.g., talc. The detailed characteristics of the binding of BA to talc were different from those to the biological fractions. However, the problem remains, in all studies of cytokinin binding, to distinguish between binding that is biologically meaningful, and biological (biologically) non-meaningful physical adsorption.