Abstract
To determine whether root-supplied ABA alleviates saline stress, tomato
(Solanum lycopersicum L. cv. Sugar Drop) was grafted onto two
independent lines overexpressing the SlNCED1(9-cis -epoxycarotenoid dioxygenase) gene (NCED OE) and wild type
rootstocks. After 200 days of salinity irrigation (EC = 3.5 dS
m-1), plants with NCED OE rootstocks had 30% higher
fruit yield, but root biomass and lateral root development was reduced.
Although NCED OE rootstocks upregulated ABA-signalling (AREB,ATHB12), ethylene-related (ACCs, ERFs ), aquaporin (PIP s)
and stress-related (TAS14, KIN, LEA ) genes, downregulation ofPYL ABA receptors and signalling components (WRKYs ),
ethylene synthesis (ACO s) and auxin responsive factors occurred.
Elevated SlNCED1 expression enhanced ABA levels in reproductive
tissue while ABA catabolites accumulated in leaf and xylem sap
suggesting homeostatic mechanisms. NCED OE also reduced xylem cytokinin
transport to the shoot and stimulated foliar 2-isopentenyl adenine (iP)
accumulation and phloem transport. Moreover, increased xylem gibberellin
GA3 levels in growing fruit trusses was associated with
enhanced reproductive growth. Improved photosynthesis without changes in
stomatal conductance was consistent with hormone-mediated alteration of
leaf growth and mesophyll structure, which combined with lower
assimilate requirement in the roots and systemic changes in hormone
balances could explain enhanced vigour, reproductive growth and yield
under saline stress.