Leaf hydraulic resistance is not affected by the root-specific aquaporin VvPIP2;4N overexpression.

After overexpressing the grape aquaporin gene VvPIP2;4N in Vitis vinifera L. ‘Brachetto’ we analysed i) the expression of VvPIP2;4N and five other aquaporins, ii) whole-plant, root, and leaf ecophysiological parameters, and iii) leaf abscisic acid content. The availability of six different transgenic grapevine lines with different VvPIP2;4N transcript levels allowed us to study the correlation between expression of this aquaporin and water transport processes at the whole-plant level. Expression of transgenic VvPIP2;4N inhibited neither the expression of the endogenous gene nor that of other PIP aquaporins in both root and leaf. The expression level of VvPIP2;4N (endogenous + transgene) was inversely correlated to root hydraulic resistance. Partitioning of plant hydraulic resistance at the organ level showed that leaf resistance was not affected, despite the fact that the transgene was expressed in both roots and shoots. Upon water stress, the overexpression of VvPIP2;4N induced a surge in leaf abscisic acid content, and a decrease in stomatal conductance and leaf gas exchange. Our results show that aquaporin-mediated modifications of root, but not leaf, hydraulics play a substantial role in the regulation of water flow in irrigated grapevine plants, while they have a minor role upon drought, probably because other signals, such as ABA, take over control of water flow.