Phosphoinositides and plant growth substance action.
Synergism between 0.5 mM inositol and cytokinin in the stimulation of plant tissue culture growth suggests a role for inositol in the mediation of cytokinin action. Investigation of the effects of cytokinin on the pattern of labelling of lipids from radioactive precursors by cytokinin-responsive cells of a cytokinin-dependent soybean cell suspension culture did not detect any reproducible link between cytokinin action and lipid labelling after 10 min. Evidence for links between auxin action and phosphoinositide metabolism in other systems would benefit from confirmation of long term repeatability and more rigorous chemical characterization of the compounds involved. For plant growth substance action to be mediated by release of inositol trisphosphate from phosphatidylinositol bisphosphate, activation of phospholipase C, possibly requiring potentiation by a GTP-binding protein, would be expected. Reports of G-protein effects on plant phospholipase C activity are conflicting. There is evidence for G-protein stimulation of activity from the results of assays using endogenous substrates, although the products released have not been fully characterized. Other results, from assays using exogenous substrates, have shown no effect of GTP analogues on the enzymic breakdown of phosphatidylinositol bisphosphate. Using endogenously labelled membranes from soybean cells, we were unable to detect effects attributable to G-protein potentiation. None of a range of growth substances at physiologically active concentrations proved able to alter detectably the lack of response of polyphosphoinositidase activity to GTP-analogues. The activity was, however, strongly stimulated by Ca2+ at micromolar levels, a characteristic widely reported. In consequence, the possibility that transient increases in the labelling of inositol phosphate fractions may be a result of increases in cytosolic Ca2+ levels needs to be addressed. If there is a role for inositol in soybean cell activation by cytokinin, we have no evidence that it involves polyphosphoinositide cleavage. That there is a special role for inositol in the mitotic cycle of soybean cells and in addition to the maintenance of viability is shown by the results of experiments in which endogenous inositol synthesis was inhibited. Further research aims to identify the inositol-requiring steps and their relationship, if any, with the auxin-requiring and cytokinin-requiring steps in the mitotic cycle of cultured soybean cells.