Conclusion
Grafting WT scions onto constitutively ABA-overproducing rootstocks produced local (root) and systemic (scion) responses mediated by root-shoot communication. Evidence that SlNCED1 overexpression in rootstocks caused a change in ABA root-to-shoot signalling included increased ABA concentrations in scion reproductive tissues and increased ABA catabolites in leaves, but lower ABA in leaf phloem. ABA overproduction altered stress-mediated responses by: decreasing root expression of PYL ABA receptors; reduced auxin signalling (lower auxin concentration in leaf phloem and decreased root expression of auxin responsive factors); enhanced root expression of most ethylene signalling gene (ERFs ); and decreased lateral root development. Moreover, rootstock NCED overexpression down-regulated root expression of CK biosynthesis genes and reduced t -Z in root xylem sap and leaf, suggesting reduced CK transport from root to shoot. However, iP increased in the leaf and leaf phloem, potentially as part of feedback loop to restore CK homeostasis. The modified leaf growth and anatomy and associated increase in photosynthesis induced by NCED overexpression in rootstocks could be explained by the known actions of the iP and JA accumulating in the leaf and leaf phloem. Enhanced GA3in truss xylem sap was consistent with the observed increases in truss length, weight and overall yield. Considering whole plant source-sink relationships, the stimulation of leaf photosynthesis and reduction in root assimilate requirements could explain the more productive scion phenotypes (vegetative vigour, truss length, fruit number and yield) when grafted on NCED OE rootstocks. Overall, NCED OE rootstocks may be of great value in generating plants with higher yields under abiotic stresses (Figure 8).