Is poor detection of grapevine leafroll associated virus 3 in South African Rootstocks due to host resistance?
ARC Plant Protection Research Institute
Objectives and Rationale
In this project we wished to determine whether difficulties in detection of grapevine leafroll associated virus 3 (GLRaV-3) are due to tolerance or immunity of rootstock clones to this virus. By “tolerance” we mean that the virus is able to replicate in the vine but it does not affect the vines phenotype in any way (eg. no symptoms). By “immunity” we mean the virus is incapable of replicating in the vine.
Rationale: To successfully control GLRaV-3 in rootstocks within the Wine Grape Certification Scheme, it is extremely important to ensure that rootstock material supplied to industry is free of this virus. It is therefore imperative 1) to determine whether the observed poor detection of the virus in rootstocks is due to low levels of virus (tolerant host defence response), 2) if so, which rootstock cultivars have this property and 3) whether, in some instances, the virus is absent due to immunity by the host. Proving immunity to GLRaV-3 would allow these rootstocks to be used in industry without any need to test that they are GLRaV-3 free.
To identify rootstocks in which GLRaV-3 is poorly detected, lignified rootstock sprouts of various cultivars from grafted leafroll symptomatic red cultivar scions are collected, and the respective scion and rootstocks tested for GLRaV-3. Rootstock samples testing negative in spite of the presence of virus in the scion were grafted onto Cabernet franc in order to determine if the virus is present at sub-detectable levels for the techniques used, or whether the rootstocks are immune to GLRaV-3 infection. Tests were also conducted to determine whether GLRaV-3 is able to be transmitted through such rootstocks.
It has been confirmed that GLRaV-3 is poorly detected in most individuals of R99, Ruggeri, 101-14, R110, Paulsen, and also US 8/7 relative to the corresponding scion of such plants. We have also shown that transmission of the virus to the commonly utilized rootstocks Richter99, Richter110, 1010-14 and Ramsay can be achieved, but that not all plants became infected. We cannot differentiate whether this is due to inefficient graft transmission of GLRaV-3 (which is known) or differences in susceptibility of specific rootstock individuals. We have also demonstrated that GLRAV-3 can move through a rootstock intergraft both in an acropetal (upward) and basipetal (downward) manner, we are unable to determine whether this is passive via the vascular tissue or whether the virus actively replicated in the interstock.
Key Conclusion of Discussion
We hypothesize that the continued inconsistent results obtained in the industry as well as in experiments performed during this project and the previous one, may be due to genetic differences with regards GLRAV-3 susceptibility in rootstock material and that any given rootstock cultivar is potentially not genetically homogeneous. Our answer to the question “pose at the start of this project “Is poor detection of grapevine leafroll associated Virus 3 in South African rootstocks due to host resistance?” can be answered in the affirmative, but that it differs in individuals of any rootstock.
Take Home message for Industry
Under high infection pressure GLRaV-3 is only detected in some individuals of rootstock cultivars tested thus far. This represents a resistance mechanism to GLRaV-3 which does not appear to be uniform within individuals of the rootstock cultivars. Future studies should identify those individuals with immunity to utilise those as future clonal sources of propagation for each rootstock cultivar. The gene(s) involved in resistance should also be identified and used in breeding or biotechnological programs to introduce these into important wine cultivars for resistance to GLRaV-3 .