Molecular memory of Flavescence dorée phytoplasma in recovering grapevines.

Flavescence dorée (FD) is a destructive phytoplasma disease of European grapevines. Spontaneous and cultivar-dependent recovery (REC) may occur in the field in FD-infected vines starting the year following the first symptoms. However, the biological underpinnings of this process are still largely unexplored. In this study, transcriptome sequencing (RNAseq), whole-genome bisulphite sequencing (WGBS) and metabolite analysis were combined to dissect molecular and metabolic changes associated to FD and REC in leaf veins collected in the field from healthy (H), FD and REC plants of the highly susceptible Vitis vinifera 'Barbera'. Genes involved in flavonoid biosynthesis, carbohydrate metabolism and stress responses were overexpressed in FD conditions, whereas transcripts linked to hormone and stilbene metabolisms were upregulated in REC vines. Accumulation patterns of abscisic acid and stilbenoid compounds analysed in the same samples confirmed the RNAseq data. In recovery conditions, we also observed the persistence of some FD-induced expression changes concerning inhibition of photosynthetic processes and stress responses. Several differentially expressed genes tied to those pathways also underwent post-transcriptional regulation by microRNAs, as outlined by merging our transcriptomic data set with a previously conducted smallRNAseq analysis. Investigations by WGBS analysis also revealed different DNA methylation marks between REC and H leaves, occurring within the promoters of genes tied to photosynthesis and secondary metabolism. The results allowed us to advance the existence of a "molecular memory" of FDp infection, involving alterations in the DNA methylation status of REC plants potentially related to transcriptional reprogramming events, in turn triggering changes in hormonal and secondary metabolite profiles.