Down-regulation of VvPLL3 enhances grapevine resistance to Erysiphe necator via defense activation and cell wall remodeling.

Grapevine powdery mildew (PM) is a major cause of yield loss worldwide, highlighting the need for durable resistance strategies. Recessive resistance against PM has been previously reported by silencing a pectate lyase-like (PLL) susceptibility gene in the model plant Arabidopsis thaliana (AtPMR6), but this susceptibility gene function is unknown in crop plants. Grapevine has multiple copies of PLL genes but their precise role in PM resistance is not fully understood. Here we report using an artificial microRNA-based gene silencing approach in PM-susceptible Vitis vinifera cv. Chardonnay to stably silence the 'Chardonnay' ortholog of AtPMR6, VvPLL3. Downregulation of VvPLL3 improved foliar resistance against the PM fungus. A significant reduction in tertiary hyphal branching was observed at 48 hours post-inoculation (hpi) that negatively affected colony success rate, but not penetration success. RNA-seq expression analyses of the VvPLL3 silenced lines (vs. empty-vector control lines) identified 41 genes whose expressions were significantly affected. Pathway enrichment analysis showed enrichment of genes mainly associated with pathways involved in the production of defense related secondary metabolites (phenylpropanoid and flavonoid biosynthesis), stress related transcription factors (WRKY, NAC), amino acids (phenylalanine and cyanoamino acids), and carbohydrate metabolism (glucosinolate biosynthesis). The VvPLL3 silenced lines showed altered composition of neutral cell wall carbohydrates relative to the empty vector control. The data presented here indicate compensating pathways involved in VvPLL3-mediated PM resistance in grapevine.