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).