The significance of cell wall
invertase (cwINV) for plant defense was investigated by comparing wild type (wt) tobacco Nicotiana tabacum L. Samsun NN (SNN) with plants with RNA interference-mediated repression of cwINV (SNN::cwINV) during the interaction with the oomycetic phytopathogen Phytophthora nicotianae. We have previously shown that the transgenic plants developed normally under standard growth conditions, but exhibited weaker defense reactions in infected source leaves and were less tolerant to the pathogen. Here, we show that repression of cwINV was not accompanied by any compensatory activities of intracellular
sucrose-cleaving
enzymes such as vacuolar and alkaline/neutral invertases or
sucrose synthase (SUSY), neither in uninfected controls nor during
infection. In wt source leaves vacuolar
invertase did not respond to
infection, and the activity of alkaline/neutral invertases increased only slightly. SUSY however, was distinctly stimulated, in parallel to enhanced cwINV. In SNN::cwINV SUSY-activation was largely repressed upon
infection. SUSY may serve to allocate
sucrose into
callose deposition and other
carbohydrate-consuming defense reactions. Its activity, however, seems to be directly affected by cwINV and the related reflux of
carbohydrates from the apoplast into the mesophyll cells.