THE SIGNIFICANCE OF ANTIOXIDANTS IN THE APHID-PLANT INTERACTION - THE REDOX HYPOTHESIS

The hypothesis is advanced that a redox system controls oxidation rates during the responses of plants to attack by sucking insects, that soluble antioxidants, such as ascorbate and glutathione, enhance the effectiveness of the plant's defensive system, and that oxidising enzymes in the saliva of aphids (and other phytophagous sucking insects) serve to counter it. Plants typically respond to wounding, including that caused by sucking insects, by mobilising and oxidising phenolic compounds. The initial phenolic monomers, and especially the monomeric o-quinones to which many give rise on oxidation, are generally deterrent to insects. Their final oxidation products, however, are polymers and phenol-protein conjugates, which are non-toxic, but serve to seal off damaged cells. It is suggested that effective defence by the plant requires oxidation of phenolics at a controlled rate that maintains a deterrent titre of the monomers, while allowing a well ordered deposition of sealants. It is also suggested that the salivary oxidases of the insects hasten oxidation in the affected tissues, thereby decreasing concentrations of monomeric phenols and quinones. It is further suggested that sedentary species may also thereby disorganise the sealing off of affected tissues. A kinetic model is developed to show that inclusion of a reductive cycle in the sequential oxidation of phenolics increases the steady state concentrations of monomeric phenols for a given throughput. Conversely, an increase in oxidation rates diminishes the titre of monomers for the same throughput. In initial biological tests of the hypothesis, infiltration of stems of lucerne cultivars with ascorbate or glutathione reduced the reproductive rate of spotted alfalfa aphid, Therioaphis trifolii maculata (Buckton), and of blue-green aphid, Acyrthosiphon kondo Shinji, feeding thereon. Intrinsically non-deterrent concentrations of ascorbate synergised the deterrence of the plant phenolics chlorogenic acid and catechin to the apple aphid, Aphis pomi (de Geer), and the spotted alfalfa aphid, T. t. maculata, respectively.