Systemic Acquired Resistance Mediated by the Ectopic Expression of Invertase: Possible Hexose Sensing in the Secretory Pathway.

Systemic acquired resistance (SAR) has been reported to be associated with lesion-mimic mutants. Tobacco plants expressing vacuolar and apoplastic yeast-derived invertase (vaclnv and cwlnv, respectively) develop spontaneous necrotic lesions similar to hypersensitive responses caused by avirulent pathogens. Therefore, SAR and metabolic alterations leading to the activation of defense-related responses were studied in these plants. Defense-related gene transcripts, callose content, peroxidase activities, and levels of salicylic acid were found to be elevated. The defense reactions were accompanied by increased resistance toward potato virus Y and were measured as decreased viral spreading and reduced multiplication in systemic leaves of the transgenic plants. Interestingly, the accumulation of pathogenesis-related (PR) protein transcripts (PR-Q) and repression of photosynthetic gene transcripts (chlorophyll a/b binding protein) were inversely correlated and required the same threshold level of hexoses for induction and repression. Expression of a cytosolic yeast-derived invertase in transgenic tobacco plants with equally increased levels of sugars neither displayed SAR responses nor showed decreased levels of photosynthetic genes. It is suggested that hexose sensing in the secretory pathway is essential for mediating the activation of defense-related genes as well as repression of photosynthetic genes in vaclnv and cwlnv plants.