Salt-induced protein synthesis in tomato roots: the role of ABA

The role played by abscisic acid (ABA) in regulating salt-induced protein synthesis was investigated in roots of tomato (Lycopersicon esculentum Mill. cv, Ailsa Craig). Roots of 9-d-old Ailsa Craig (AC) seedlings and the near-isogenic ABA-deficient mutant, flacca (flc), were exposed to salt which elicited the appearance of novel polypeptides and both repressed and enhanced the synthesis of others. The polypeptide profiles of salt-treated AC and flc roots were similar suggesting that the synthesis of most novel polypeptides in salt-treated roots is not dependent on an elevated level of endogenous ABA. Exogenous ABA and a combined ABA/salt treatment were applied to the roots of AC and flc. Exogenous ABA, in the absence of salt, induced the accumulation of several polypeptides that were unique to this treatment as well as a subset of those synthesized in salt-treated roots. Interestingly, in roots exposed to the combined ABA/salt treatment, only those polypeptides that accumulated in both ABA or salt-treated roots were synthesized. Endogenous ABA levels increased 2-fold in salt-treated AC roots and 14-fold in salt-treated flc roots. Although the absolute level of ABA was lower in salt-treated flc than in AC, this demonstrates that flc has the capacity to accumulate some ABA in its roots following a salt treatment. Since it is possible that this level of ABA was sufficient to induce the changes in polypeptide synthesis observed in salt-treated roots of flc, the salt-induced accumulation of endogenous ABA was prevented by treating AC roots with fluridone. In these roots, the set of salt-induced polypeptides was similar to that observed in salt-treated roots indicating that an elevated level of endogenous ABA may not play a major role in regulating the accumulation of most salt-induced proteins in tomato roots.