Arpa Boghozian, Cristina Medina-Plaza, Reddy Kishorekumar, Mysore Sudarshana, and Anita Oberholster*
*University of California, Davis, 595 Hilgard Lane, Davis, CA, 95616 (aoberholster@ucdavis.edu)

Grapevine red blotch virus (GRBV), a member of the Geminiviridae family and vectored by the three-corned alfalfa treehopper Spissistilus festinus, causes delayed grape ripening and decreased wine phenolic content. However, wine phenolic content does not correlate linearly with concentrations in grape skin cell walls (GSCW). Phenolics are stored in GSCW vacuoles with extraction into macerating juice affected by the hemicellulose, pectin, and structural protein content of cell walls. GRBV’s effect on GSCW was studied using virus-infected, GRBV(+), and noninfected, GRBV(-) vines from a commercial vineyard in Paso Robles, CA in 2021 and 2022. Biochemical analysis involved GSCW isolations as alcohol-insoluble residue, total protein content (TPC) using the Coomassie blue protein assay, and pectin concentration measured as uronic acid. TPC was lower in GRBV(+) than in GRBV(-) in 2021 and 2022 grapes at harvest and in postveraison GRBV(+) grapes in 2021. In 2022, there was less TPC in GRBV(+) and GRBV(-) grapes than in 2021. TPC decline in 2022 can be attributed to increased heat degradation or suspension of total protein production due to environmental factors such as more growing degree days and lower annual precipitation. Further investigation will uncover the relationship of TPC with pathogenesis-related protein production during infection. GRBV(+) GSCW exhibited greater pectin in both seasons, possibly due to a ripening delay with downregulated pectin enzyme activity or upregulated pectin synthesis, since concentrations in GRBV(+) grapes at preveraison, postveraison, and harvest in both vintages were greater. Pectin concentration has been previously proven to be inversely correlated with phenolic extractability due to firmness of the GSCW. Future steps involve using gas chromatography coupled with mass spectrometry to assess the effect of GRBV on GSCW monosaccharide linkages.

Understanding the viral effect on GSCW and its correlation with phenolic extractability will help determine mitigation strategies for the winemaking industry.

Funding Support: CDFA-PD/GWSS Board, Jastro-Shields Research Award, American Society of Enology and Viticulture Traditional Scholarship, NIFA Specialty Crops Research Initiative