Grapevine rootstocks differently affect physiological and molecular responses of the scion under water deficit condition

Grapevine rootstocks play a pivotal role in plant responses to water deficiency (WD); therefore, the selection of new genotypes is a promising strategy for future agricultural managements aimed to cope with climate changes. Recent studies reinforced the central role of the root system in modulating WD responses, as it not only controls water uptake and transport to the leaves, but it also participates in stress perception and stress signalling to the shoot. The present work evaluated the performance of the 101.14 and M4 rootstocks in graft combination with the cultivar Cabernet Sauvignon (Cab) by assessing some of the canonical molecular, biochemical and physiological responses induced by WD. The autograft Cab/Cab was also included in the experimental design as a control. Under WD, Cab/M4 showed a greater capacity to sustain CO2 assimilation rate (A(n)) and stomatal conductance (g(s)), while limiting the decrease of leaf potential (Psi(leaf)) compared with the other graft combinations. The enhanced adaptability of Cab/M4 to WD was also supported by the higher uptake of water from the soil, estimated by measuring the daily water lost of plants, and by the reduced effect of the drought treatment on the total root biomass. Quantification of ABA in both root and leaf organs revealed a reduced accumulation in Cab/M4 plants, thus confirming the lower sensitivity of the Cab/M4 combination to water deficit. At the molecular level, the expression of selected stress-responsive ABA-related genes was investigated, including genes involved in ABA biosynthesis (VviNCED3), ABA signalling (VviPP2C9, VviPP2C4, VviSnRk2.6), regulation of gene expression (VviABF2) and stomatal opening (VviSIRK, VviMYB60). Results indicated a tight correlation between the level of gene expression and of ABA accumulation in roots and leaves, suggesting that ABA synthesis and signalling were attenuated in Cab/M4 as compared with Cab/101.14 and Cab/Cab. As a whole, our data demonstrated the capacity of M4 to satisfy the water demand of the scion under limited water availability, as revealed by delayed stomatal closure and higher photosynthetic activity. Importantly, these physiological adaptive traits related to attenuated ABA-mediated responses in roots and leaves.