Membrane Receptors for Fusicoccin: A Molecular Study

Abstract

The phytotoxic metabolite of the fungus Fusicoccum amygdali Del. (Ballio, Chain, De Leo, Erlanger, Mauri, Tonolo, 1964) interacts with higher plant cells promoting cell extension growth and membrane transport processes (Marre, 1979). The interest of plant physiologists in this toxin has been increased by the finding that these effects are similar to those regulated by some plant hormones. It is now well established that all the hormone-like responses stimulated by fusicoccin (FC) depend on the activation of a protontranslocating plasmalemma ATPase, which is present in all higher plants and is necessary for nutrient transport. The stimulation of this enzyme represents the more direct FC action, but so far its mechanism is unclear. Many studies directed to elucidate the nature of the signal triggering the H+-ATPase showed that the specificity of FC interaction with plant cells depends on the presence of FC binding sites at the plasmalemma. Results obtained so far show that they can be considered true receptors (Aducci, Marra, Ballio, 1990). Their presence in higher plants which are not hosts of the fungus F. amygdali and therefore do not meet the toxin in natural conditions is explained by the occurrence, in several plants, of a physiological ligand mimicked by FC (Aducci, Crosetti, Federico, Ballio, 1980; Ballio and Aducci, 1987; Marra, Ballio, Aducci, 1988). It is very likely that the interaction of this ligand with FC receptors is the first step in a chain of reactions leading to the biological responses known to be promoted by FC. Therefore, the system recognised by FC can, under physiological conditions, play a crucial role in cell metabolism.