Impact of grapevine leafroll virus infections on vine physiology and the berry transcriptome

Context and purpose of the study - Grapevine leafroll associated virus (GLRaV) infections deteriorate vine physiological performance and cause high losses of yield and fruit quality and are thus causing serious economic losses in the wine industry. Physiological and molecular studies on the impact of leafroll infections on plant and fruit metabolism are relatively scare and the molecular mechanisms associated with the plant response to the virus during berry ripening are not well understood so far. Commonly observed phenotypic alterations consist in a ripening delay, a reduction in anthocyanin and sugar accumulation. The few molecular studies associated in particular the lack of anthocyanin of berries of infected vines to a repression of key genes of anthocyanin biosynthesis. However such studies did never account for berry heterogeneity and potential phenological shifts induced by virus infection, which could have introduced biases in gene expression studies. Material and methods - In the present study a long-term experiment was established in the year 2000, with the aim to investigate the effects of infections with different GLRaVs (GLRaV 1 & GLRaV 1+3) on vine and grape physiology. Physiological data (yield, vigor, photosynthesis, berry quality) has been collected from 2015 to 2018. In 2018 a transcriptomic (RNA-seq) analysis of 2 reconstituted berry ripening stages was performed. Therefore 245 berries were individually sampled and individually analyzed for sugar and organic acids in order to re-constitute 2 homogenous ripening stages to circumvent intercluster berry heterogeneity and thus to compensate for phenological shifts induced by virus infections. RNA of reconstituted samples was extracted and sequenced by single end sequencing and subsequently analyzed for differentially expressed genes (DEGs). Results - Physiological measurement showed a significant decrease in photosynthesis, yield and sugar content, which were highly significant in the co-infected vines (GLRaV 1+3). RNA-sequencing of berries revealed a total of 2136 DEGs between control and virus infections. Several transcriptions factors related to abiotic and biotic stress could be identified and showed interesting variation in dependence to ripening stage and infection severity. Strikingly previously reported repression of the anthocyanin biosynthesis and sugar metabolism could not be confirmed by gene expression. This illustrates that the main damaging effect on GLRaV infection is rather related to a phenological shift than to a direct impact on metabolism. The here reported results give new insight in the mechanism of leafroll infection and emphasize the importance of the sampling protocol of molecular studies investigating berry metabolism