Cell-wall invertases, key enzymes in the modulation of plant metabolism during defence responses

Most plant-pathogen interactions do not result in pathogenesis because of pre-formed defensive plant barriers or pathogen-triggered activation of effective plant immune responses. The mounting of defence reactions is accompanied by a profound modulation of plant metabolism. Common metabolic changes are the repression of photosynthesis, the increase in heterotrophic metabolism and the synthesis of secondary metabolites. This enhanced metabolic activity is accompanied by the reduced export of sucrose or enhanced import of hexoses at the site of infection, which is mediated by an induced activity of cell-wall invertase (Cw-Inv). Cw-Inv cleaves sucrose, the major transport sugar in plants, irreversibly yielding glucose and fructose, which can be taken up by plant cells via hexose transporters. These hexose sugars not only function in metabolism, but also act as signalling molecules. The picture of Cw-Inv regulation in plant-pathogen interactions has recently been broadened and is discussed in this review. An interesting emerging feature is the link between Cw-Inv and the circadian clock and new modes of Cw-Inv regulation at the post-translational level.