Early detection of grapevine graft incompatibility: Insights into translocated and virus-induced incompatibility

In vineyards to control phylloxera (Daktulosphaira vitifolia Ficth) attacks in Vitis vinifera L., heterografted vines are planted using American vines hybrids as rootstocks. However, graft incompatibilities can affect grape yield and plant longevity. Thus, to identify early graft incompatibility factors, we established in vitro micrografting protocols coupled with histology and histochemistry analysis in grapevine graft combinations of known compatibility behavior. The histochemical characterization of the graft union revealed irregular cell arrangement, slower vascular differentiation, persistence of the necrotic layer, accumulation of starch, and lower differentiation of phloem cells in hetero- compared to homografts, indicating the presence of translocated incompatibility symptoms. We highlight the utility of evaluating the graft interface cellular arrangement and starch content via calcofluor and I2KI staining, respectively, as allowed to identify the graft combinations with lower graft success. Wounded and grafted Syrah plantlets pointed out an impaired sucrose distribution in these plants and levels of Grapevine Rupestris Stem Pitting associated Virus (GRSPaV) infections correlated with graft (un)-success in two Syrah clones micrografted onto 110-Ritcher rootstock. Furthermore, silencing GRSPaV before grafting increased graft success rates. We propose that grapevine graft incompatibility is mainly a virus-induced phenomenon that can arise even in certified plants.