Possible role of sugar metabolism during plant/pathogen interactions

Abstract

Infection of plants by pathogens often results in major metabolic alterations. This is particularly pronounced with biotrophic parasites and viruses where the pathogen obtains nutrients from living cells and/or employs the host´s cellular machinery to replicate. Due to the extra load of metabolic performance enhanced allocation of photoassimilates to infection sites is frequently observed. These assimilates are on the one hand used to support growth of the pathogen and on the other hand to allow the infected tissue to activate a vast array of defence responses. As a consequence new source to sink relations at the whole plant level are established. The outcome of an infection may depend on the competitive abilities of the different sinks. If carbon flow to defence responses outweighs carbon flux to the pathogen, a compatible interaction might be attenuated. Commonly accumulation of soluble sugars is observed at infection sites. These sugars may fulfil dual functions, being fuel for metabolism and signal for the activation of defence responses. Accumulation of sugars may be caused by physical perturbations of the transport path, the inhibition of sugar transport proteins, or by induction of cell wall invertase. Using transgenic tobacco plants expressing a cell wall invertase we could demonstrate that indeed invertase-mediated accumulation of soluble sugars results in the induction of plant defence responses and an increased resistance towards virus infection (Herbers et al., 1996a). The involvement of salicylate in the acquired resistance could be excluded by analysing the offsprings of a crossing between salicylate hydroxylase and a cell wall invertase expressing transgenic plants (Herbers et al., 2000). Infection experiments revealed that high sugar and low salicylate containing offsprings exhibited resistance towards PVY infection. Based on these results we concluded that due to the action of cell wall invertase, plant defence responses are enhanced. Enhancement of defence responses is most likely mediated by increased metabolic flux towards secondary plant compounds (Baumert et al., 2001) and the activation of defence genes via a sugar-mediated signal transduction pathways (Herbers et al., 1996b). To find out whether sugar signals are acting systemically or only locally we investigated transgenic plants expressing a cell wall invertase under control of an ethanol-inducible promoter. Analysis of these plants revealed that sugar signals are not systemically spread throughout the plant. To circumvent sugar-mediated defence mechanisms certain pathogens have evolved strategies to overcome invertase expression/activity or action. In addition to our results on the involvement of invertase in plant defence mechanisms the occurrence of pathogen-derived inhibitors of invertase expression and the possible role of sucrose isomerization by phytopathogenic bacteria will be discussed.