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 vira1 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 photosynthstic gene transcripts (chlorophyll a/b binding protein) were inversely correlated and required the same threshold leve1 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.
We have transformed potato with Nt-inhh cDNA, encoding a putative vacuolar homolog of a tobacco cell wall invertase inhibitor, under the control of the CaMV 35S promoter. In transgenic tubers, cold-induced hexose accumulation was reduced by up to 75%, without any effect on potato tuber yield. Processing quality of tubers was greatly improved without changing starch quantity or quality, an important prerequisite for the biotechnological use of Nt-inhh for potato transformation.
To test whether carbohydrates may play a signalling function during plant pathogenesis, we investigated the interaction between tobacco and potato virus Y (PVY(N)). Four days after PVY(N) infection, leaves started to accumulate soluble sugars and leaf photosynthesis decreased. The accumulation of soluble sugars was accompanied by an induction of cell wall invertase and a gradual decrease in the sucrose-to-hexose ratio. In parallel to changes in carbohydrate metabolism and photosynthesis, transcripts encoding PR-proteins accumulated. Based on this coincidence, it was hypothesized that elevated hexose levels may enhance the expression of defence-related functions and might possibly explain the phenomenon of high sugar resistance in plants. This notion has been supported by the fact that cell wall invertase-expressing transgenic tobacco plants were found to be resistant against PVY(N) (Herbers et al., 1996b). To exclude the possibility that salicylate, which accumulates in plants expressing invertase, may be responsible for the observed resistance, these transgenic plants were crossed with salicylate hydroxylase-expressing plants (nahG). The progeny were selected for high levels of sugar and low levels of salicylate. Necrotic lesions also developed, typically formed on the leaves of plants expressing invertase, and transcripts encoding PR-Q accumulated in the absence of salicylate. On the other hand, accumulation of PR-1b transcripts decreased, indicating that sugars are not sufficient for PR-1b induction. Infection experiments using these plants as hosts revealed resistance towards PVY(N). Thus, the mechanism of apoplastic invertase induced virus resistance is salicylate independent and most likely sugar mediated.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.