Responses of a compatible lucerne variety to attack by spotted alfalfa aphid: changes in the redox balance in affected tissues

Abstract

AbstractThe spotted alfalfa aphid, Therioaphis trifolii maculata (Buckton), caused local browning of cells surrounding feeding sites on lucerne plants (cv. Hunter River). Aqueous extracts of infested leaves underwent a marked browning process that did not occur in extracts of healthy leaves. The process was accelerated by addition of tyrosinase and peroxidase and reversed by reducing agents such as ascorbate and glutathione. In the presence of added reducing agents, the extracts produced brown precipitates, probably conjugates of phenolics with leaf proteins similar to those involved in the sealing of damaged tissues in vivo.Partially autoxidised catechin (PAC) solutions showed an absorbance peak at 438 nm that was increased by polyphenol oxidase and decreased by ascorbate and glutathione. When extraction of tissues into PAC was used to assess redox activities, healthy tissues showed a rapid, short lived oxidising activity combined with a much more persistent reducing activity, whereas infested leaves had even greater oxidising activity and no detectable reducing activity.Soluble phenolics increased in infested leaves and stems. Total protein decreased, but the specific activity of peroxidase, catechol oxidase and superoxide dismutase relative to protein content increased. The ability of extracts to reduce cytochrome c increased, indicating an overall increase in superoxide radicals in attacked tissues.These results are consistent with a general disturbance of redox balance induced in tissues by aphid feeding, including accumulation of oxidases and phenolic substrates and loss of reducing activity and protein.