This research was directed at determining how the Muscadinia rotundifolia based rootstock O39-16 induces tolerance to grapevine fanleaf virus (GFLV), and developing genetic markers associated with this trait to facilitate breeding of fanleaf degeneration resistant rootstocks. O39-16 is resistant to GFLV?s dagger nematode vector (Xiphinema index), but the nematode?s test feeding is able to inoculate GFLV which moves freely into the scion. GFLV affects crop yields by disrupting pollination and greatly reducing berry set.
However, if a scion is grafted to O39-16 it maintains normal crops even while infected. Other dagger nematode resistant rootstocks do not possess O39-16?s ability to induce fanleaf tolerance. This effect is likely due to O39-16?s M. rotundifolia parentage and that a flowering-associated phytohormone generated by its root system is compensating for GFLV?s impact on flowering. The most obvious phytohormone is cytokinin, which is produced primarily by the root system and known to be involved in flowering.
In previous studies we determined that rootstocks capable of inducing GFLV tolerance do not impede the virus movement or alter the virus foliar disease symptoms. Various grafted combinations of O39-16, O43-43 (a sibling rootstock with the same fanleaf tolerance), St. George and Cabernet Sauvignon (both highly susceptible to both GFLV and X. index) have been created in standard, reverse, and interstock arrangements. These combinations were bench grafted with both infected and healthy Cabernet Sauvignon. ELISA data indicate that GFLV is able to move freely across the graft union of all genotypes, regardless of positioning of VR rootstock in the grafted vine. Additionally, tissue samples isolated from scions where M. rotundifolia served as an interstock grafted to a GFLV infected rootstock, generated foliar symptoms in the GFLV herbaceous indicator, Chenopodium quinoa. The presence of the portion of the viral genome that encodes for the GFLV coat protein has been verified in scions of vines grown on both St. George and O39-16. Based upon these results, we believe that induced tolerance is best explained by a mechanism which does not directly interact with the virus.
A modified phytohormone separation protocol for HPLC was adopted, this method allows for the separation of auxin, abscisic acid, and cytokinin from a single sample. The initial results of HPLC separation of cytokinin into the nucleoside, riboside and base conformation were verified through competitive ELISA. This method of analysis will be used to track the phytohormonal status of both rootstock and scion of an 80 vine experimental plot planted this year at UC Davis. Samples will be collected throughout the growing season, with particular focus on the ratio of cytokinins to abscisic acid in the developing inflorescence and flowers of GFLV infected vines.
We have begun the process of generating simple sequence repeat (SSR) based linkage map of F1 hybrid of Vitis vinifera x M. rotundifolia. Previous work by H.P. Olmo found this population segregates for resistance to phylloxera, root and dagger nematode, as well as many morphological traits. It is predicted that this population will also segregate forinduced tolerance to fanleaf degeneration.