Not just sweet: phylloxerated roots indicate complex plant response mechanisms

As a worldwide pest grape phylloxera (Daktulosphaira vitifoliae Fitch) threatens viticulture. The compatible phylloxera-grapevine (Vitis spp.) interaction has dramatic impact on the host plant response and in reverse will select for adapted phylloxera biotypes to interact. The understanding of these interactions will enhance our knowledge on the resistance of rootstocks, evolution of phylloxera biotypes, insect-induced plant responses including the carbohydrate allocation of the host plant as well as on plant protection strategies. As a model for a compatible interaction the Rootstock Teleki 5C was studied. Uninfected root tips were compared with phylloxerated root tips at four developmental stages and changes were studied by gene expression analysis, enzymatic assays and GC-MS analysis. Massive import of sucrose from source leaves occurs symplastically towards the feeding site from which phylloxera ingests sucrose directly (instead of directed uptake of glucose by starch degradation by salivary enzymes). This transport is enhanced not due to increased invertase activity, as formerly proposed, but due to elevated sucrose synthase activity and specific insect feeding behavior by effectively tapping the symplast as shown here. We found activity in the raffinose pathway in nodosities which might be a stress-based plant response leading to changes in the rations of hexoses within the gall tissue. The results indicate far more complex and severe effects on the Vitis as a host and especially on its root system, possibly affecting the soil ecosystem. Our work should encourage discussion on new strategies for vineyard floor management in phylloxerated vineyards.