Characterization and Utilization of Rootstock that Promote Normal Scion Fruit Set, despite presence pf Grapevine Fanleaf Virus (GFLV), in Muscadinia

This proposal is directed at determining how the Muscadinia rotundifolia based rootstock O39-16 induces tolerance to grapevine fanleaf virus (GFLV), and applying that ability to rootstock breeding. O39-16 is resistant to GFLV’s dagger nematode vector (Xiphinema index), but the nematode’s test feeding is able to inoculate GFLV and it 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. We are hypothesizing that this effect is due to O39-16’s unique M. rotundifolia parentage and that a flowering associated phytohormone generated by its root system is compensating for GFLV’s effect on flowering. The most obvious phytohormone is cytokinin, which is produced primarily by the root system and known to be involved in flowering. It is also possible that O39-16 only allows non-infectious
empty virus particles to be transmitted to the scion. ELISA testing does not distinguish
between empty and complete virus particles.

We have been developing assays to evaluate these possibilities. 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 created with both infected and healthy Cabernet Sauvignon. Preliminary 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. Primers have been designed in order to synthesize a cDNA library
from the viral RNA species in the rootstocks and scions of this trial. 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. We want to verify that the
infectious portions of the GFLV RNA are present in tissue across graft unions. A new
method of high-through put RNA extraction is being developed in order to extract greater
amounts of viral RNA in less time.

A pilot research study was undertaken to develop a method of correlating cytokinin levels
with induced tolerance to GFLV. Apical, foliar and reproductive tissues were collected
throughout the course of a single growing season from two separate fanleaf rootstock
trials at key reproductive developmental stages. It quickly became clear that it was too
time consuming and expensive to collect enough samples from these field plots, and
current cytokinin assay methods are too expensive and not sensitive enough to be
meaningful. We are optimizing a technique to measure cytokinin production in root
tissue to compare rootstocks and species that induce fanleaf tolerance and those that do
not while grown under controlled conditions.