Mechanism of the Growth-promoting Action of Fusicoccin: Interaction with Auxin, and Effects of Inhibitors of Respiration and Protein Synthesis

Analogies and differences between the growth-promoting action of fusicoccin (FC), the toxic agent produced by Fusicoccum amygdali, and of indole-3-acetic acid (IAA) in etiolated pea internode segments were further investigated. It was confirmed that the effect of FC, at optimal concentration, on growth by cell enlargement is markedly higher (ca. 70%) than that of IAA. The lack of an inhibitory effect of FC at high concentrations corresponds to a lack of capacity of the toxin to induce ethylene synthesis. When FC and IAA are present together at suboptimal concentrations, the effects of the two substances are clearly additive. Growth stimulation by a mixture of FC and IAA at optimal concentrations is equal to that by FC alone. NaF, 2,4-DNP, actinomycin D, and cycloheximide strongly depress FC-induced stimulation of cell enlargement. These data are in agreement with the hypothesis that FC promotes growth through some effect on cell-wall extensibility probably identical to the one mediating auxin-induced growth, while the primary site of action of FC is different from that of the natural and synthetic auxins.