Can ABA mediate responses of salinity stressed tomato

Abstract

We tested the hypothesis that responses to root-sourced salt-stress, in the root and shoot, are coordinated by elevated abscisic acid (ABA) concentrations. To do this we examined the responses of wild-type (Ailsa Craig) and ABA-deficient mutant ( notabilis) genotypes of tomato to a range of salinity (0 (control), 30, 60, 90, 120 mM NaCl). Leaf area was reduced and root:shoot ratio increased in response to increasing salinity in the absence of changes in leaf turgor for both genotypes. Stomatal conductance was however reduced suggesting that a non-hydraulic factor may be coordinating responses to the salinity in the rooting medium. ABA concentrations were increased in response to salinity and correlative analysis showed that stomatal conductance exhibited a negative relationship with increasing xylem ABA for both genotypes ( P<0.001). The possibility that ABA controlled leaf area was less apparent as reduced leaf areas were found in both wild-type and notabilis under high salinity (120 mM; P<0.001), but xylem sap and tissue ABA concentrations were much lower in notabilis. More specific evidence for the role of enhanced ABA concentrations on leaf and root growth was found under moderate levels of salinity. At 90 mM NaCl there was a large decrease in leaf area for notabilis compared with the near control levels of leaf area exhibited by the wild-type ( P<0.001). We hypothesised that the poorer growth in the ABA-deficient mutant was due to its inability to produce ABA at wild-type levels. This was confirmed, by feeding synthetic ABA to the rooting medium, and that 1 and 10 μM±ABA appeared necessary for the maintenance of leaf and root growth at wild-type levels, respectively. These data suggested that there was differential sensitivity to ABA in the roots and shoots but that enhanced concentrations at wild-type levels acted primarily to maintain root and shoot growth under moderate salt stress. How ABA was synthesised to affect concentrations within the leaf and root tissue under salinity stress was investigated by the analysis of mRNAs for two enzymes involved in ABA biosynthesis (9- cis-epoxycarotenoid dioxygenase (NCED) and zeaxanthin epoxidase (ZEP). Evidence for increased synthesis in the roots was detected but there was little change in the leaves in response to salinity and alternative mechanisms that may regulate ABA content in the leaf are explored. The possibility that ABA can interact with other hormones to coordinate whole plant responses to high salinity stress is discussed.