Evaluating the Impacts of Grapevine Red Blotch Virus on Grape Metabolism to Develop Viticultural and Enological Mitigation Strategies
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Evaluating the Impacts of Grapevine Red Blotch Virus on Grape Metabolism to Develop Viticultural and Enological Mitigation Strategies

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Grapevines are one of the most economically important crops around the world. Like all crops, the productivity and vitality of grapevines are threatened by pathogens such as bacteria, fungi, and viruses. Numerous viral pathogens, both RNA and DNA, impact the metabolism of the grapevine and the grape berry, resulting in downstream effects for winemakers. Grapevine fanleaf virus and grapevine leafroll virus are two of the most prominent and detrimental viruses affecting grapevines in the world. However, in 2011, a new disease termed grapevine red blotch disease (GRBD) was discovered, and shortly thereafter, the etiological agent was determined to be a new geminivirus, grapevine red blotch virus (GRBV). Since then, researchers have focused on understanding the functioning of the virus, identifying potential insect vectors, and evaluating the viral impacts on grapevine performance, grape metabolism, and resulting wine composition. The current body of knowledge regarding GRBV is described in length in Chapter 1. The aim of this work was to investigate the extent of the pathogenicity of GRBV under genotypic and environmental factors. We evaluated grape chemical composition through ripening and at harvest of Vitis vinifera L. cv. Cabernet Sauvignon was grafted on two different rootstocks (110R and 420A) in 2016 and 2017. Our research brought to light the variable influence different rootstocks and seasons could have on disease outcome in grapevines. A more drought-resistant, vigorous rootstock (110R) experienced worsened grape composition due to GRBV infection than 420A, whereas grapevines in a warmer season with heat exceeding 35C (2017) outperformed those grown in a cooler season under GRBV infection. This work is explained in detail in Chapter 2. In Chapter 3, a more extensive study examined the impact of GRBV on grape metabolism on the same set of grapevines described in Chapter 2. By analyzing the transcriptome and metabolome of the grapes, we were able to identify specific pathways and compounds that were differentially affected by GRBV infection, such as the phenylpropanoid pathway and amino acid composition. In addition, we were able to uncover conserved responses to GRBV infection across both rootstocks and seasons. Our work determined a conserved upregulation of photosynthetic processes at harvest with a simultaneous increase in malate concentrations indicating an irregularity in energy metabolism. More importantly, differential co-expression analysis revealed the enrichment of a Dicer-like (DCL) protein, specifically DCL2, which is responsible for viral-induced gene silencing. This plant immune response can decrease viral load and symptomology in plants infected with a geminivirus. DCL2 was only induced at veraison across genotypes and environments, suggesting for the first time a phenological association with this antiviral plant immune response. Additionally, in 2017 the upregulation of DCL2 was higher than 2016 with concurrent decreases in viral gene expression suggesting a warmer season led to increased viral immunity and improved grape metabolism. Overall, the lower total soluble sugar levels and higher titratable acidity at harvest in grapes infected with GRBV suggested a delay in ripening events, which corroborated previous studies. However, no study to date has investigated the impact of GRBV on grape cell wall composition even though the cell wall plays a large role in viral transport and phenolic extractability during winemaking. Therefore, in 2019 V. vinifera L. cv. Merlot grapes were collected through ripening to investigate the impact of GRBV on grape cell wall metabolism (Chapter 4). GRBV caused the induction of several transcripts encoding for cell wall modifying enzymes at harvest; however, this was not translated into the overall composition of the cell wall. This may indicate a post-transcriptional regulation of cell wall modification processes. Interestingly, GRBV upregulated genes associated with pathogenesis-related (PR) proteins and pectin methylesterase inhibitors which correlated to higher levels of soluble proteins and pectin in the grape cell wall. Both PR proteins and pectin are known to retain important phenolic compounds during winemaking ultimately affecting the chemical and sensorial characteristics of a final wine. This work is examined in Chapter 4 which expresses the need for further investigation into the impact of GRBV on the grape cell wall. Finally, in Chapter 5, we utilized our understanding from the previous findings to explore potential viticulture and enological mitigation strategies to alleviate the impact of GRBV on grape and wine composition. Since GRBV causes a delay in ripening events in grapes, an extended ripening was employed in two seasons to improve primary and secondary metabolite levels in the grapes and the resulting wines. In addition, since ethanol levels are positively correlated with phenolic extractability, chaptalization of GRBV grape musts was performed in one season to increase ethanol concentration during fermentation. Interestingly, a delayed harvest of GRBV fruit did increase phenolic extractability during winemaking, yet chaptalization did not. This suggests that grape maturity plays a larger role in the phenolic extraction of GRBV fruit than ethanol concentration during winemaking. Corresponding to results obtained in Chapter 4, the maturity of the grape cell wall potentially could cause the effects observed in Chapter 5. Extended ripening did improve metabolites levels in wines made with GRBV fruit; however, this was variable depending on the rootstock and season. Although the final wine composition of GRBV chaptalized wines was similar to the wines made from healthy fruit, sensorially they were differentiated, suggesting that this technique may not sufficiently alleviate the impact of GRBV on wine composition.

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