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 apop lastic yeast-derived invertase (vaclnv and cwlnv, respectively) develo p spontaneous necrotic lesions similar to hypersensitive responses cau sed by avirulent pathogens. Therefore, SAR and metabolic alterations l eading to the activation of defense-related responses were studied in these plants. Defense-related gene transcripts, callose content, perox idase activities, and levels of salicylic acid were found to be elevat ed. The defense reactions were accompanied by increased resistance tow ard potato virus Y and were measured as decreased viral spreading and reduced multiplication in systemic leaves of the transgenic plants, In terestingly, the accumulation of pathogenesis-related (PR) protein tra nscripts (PR-Q) and repression of photosynthetic gene transcripts (chl orophyll alb binding protein) were inversely correlated and required t he same threshold level of hexoses for induction and repression. Expre ssion of a cytosolic yeast-derived invertase in transgenic tobacco pla nts with equally increased levels of sugars neither displayed SAR resp onses nor showed decreased levels of photosynthetic genes. It is sugge sted that hexose sensing in the secretory pathway is essential for med iating the activation of defense-related genes as well as repression o f photosynthetic genes in vaclnv and cwlnv plants.