Compatible Phylloxera - Grape interaction

In the second part of the 18th century a small aphid, the grapevine louse (Daktulosphaira vitifoliae FITCH) Grape Phylloxera, was introduced to Europe with devastating effects on indogenous European Grapevine cultivars and enormous economic losses. The root system of susceptible cultivars is severely damaged by the formation of gall tissues, called nodosities and tuberosities. Scientists and agronomists have circumvented the problem by grafting V. vinifera cultivars on tolerant rootstocks gained from North American Vitis species. This strategy has been working well for over 100 years, but recently reports show that this tolerance is endangered by appearance of aggressive Phylloxera biotypes. Therefore the need to understand the Phylloxera - plant interaction has dramatically increased. In this proposal we will focus on the processes taking place in grape roots upon Phylloxera feeding with a special interest on nodosity development and on the nutrient supply of the aphid. We will analyse the sugar and starch metabolism of nodosities from two angles: the genes involved in sugar and starch metabolism and their transport mechanisms in developing nodosities. The transcriptional changes in root galls, especially concerning genes involved in sugar and starch metabolism, will be analysed with GeneChip and qPCR analyses. Transport mechanisms of sugars to feeding and storage cells of nodosities will be determined with microscopical, histochemical and molecular biological means. The results will give new insights into Phylloxera - grape interaction, a still not well understood plant - pathogen system, and will provide essential knowledge for the identification of crucial physiological processes for the development of new strategies to fight aggressive Phylloxera biotypes.

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 improve our knowledge on the resistance of rootstocks, evolution of phylloxera biotypes, insect induced plant responses including the carbohydrate allocation of the host plant and enhances the knowledge base on future plant protection strategies against phylloxera. The aim of the project was to analyze the transcriptome of the phylloxera induced root gall (nodosity) to elucidate significant plant-based responses mainly affecting the carbohydrate metabolism and sink activity of the gall.The experimental approaches covered gene expression analyses combined with functional tests. As a model, the partially resistant Vitis genotype T5C was tested with a defined phylloxera single founder lineage. Uninfected root tips were compared with nodosities at different development stages and changes between these stages were studied by different methods: gene expression (gene chip, qPCR analysis), starch content (enzymatic assay), sugar contents, secondary metabolites (GC-MS analysis) and sugar-transport (histological analysis). Our results indicate that the actual physiological and functional role of the root gall needs revision. The biosynthesis of starch for storage and to buffer sugar metabolites is not the main and not the only effect altering the gall-bound carbohydrate metabolism. Massive import of sucrose from source leaves is symplastically transported towards the feeding site, transcending endodermis and casparian strip where phylloxera ingests sucrose directly (instead of directed uptake of glucose by starch degradation by salivary enzymes). The sink acitivity is enhanced but not due to increased invertase activity, as formerly proposed. Instead, our work showed that the activity is enhanced through sucrose synthase activity, activity of expansin genes and the insect feeding activity by effectively tapping the symplast. We found activity in the raffinose pathway in nodosities which maybe a stress-based plant response leading to changes in the rations of hexoses within the gall tissue. Results beyond the leading questions showed for the first time that root-produced volatiles are present in the nodosity aphid induced galls with potential effect to manipulate the interactions with other plants and insects in the rhizosphere. The results will impact the scientific field by providing a reference transcriptome of nodosities allowing comparable studies on the host-pest interaction in Vitis roots and also provides a modified gene chip to do so. The results also indicate a far more complex and severe effect on the Vitis host and especially in the Vitis root system, possibly affecting the soil ecosystem and should induce discussion on new strategies for vineyard floor management in phylloxerated vineyards.