Characterization of salt-induced changes in gene expression in tomato ( Lycopersicon esculentum) roots and the role played by abscisic acid

Examination of tomato ( Lycopersicon esculentum Mill) root mRNA profiles by differential display-polymerase chain reaction (DD-PCR) revealed that a salt treatment induced, promoted or repressed the expression of a number of genes. The majority of the observed changes were indicative of a rapid and transient salt-induced alteration in gene expression. Twenty partial cDNAs corresponding primarily to salt-induced or up-regulated mRNAs were subsequently cloned and sequenced. The role of abscisic acid (ABA) in regulating salt-responsive gene expression in roots was explored. The DD-PCR data indicate that the majority of the salt-induced changes in the root mRNA profile occurred in an ABA-independent manner. The expression of genes corresponding to six cDNAs was shown unequivocally to be responsive to a salt treatment by RNA blot hybridization. Just two of these were responsive to exogenous ABA and, in salt-treated roots of the ABA-deficient mutant flacca, all were expressed to a level comparable to that in the wild-type. The identity of two of the salt-responsive partial cDNAs is known. The deduced amino acid sequence of one was similar to that of laccases that polymerize a variety of substrates to form resilient structures within the cell wall. One other shared amino acid sequence similarity with the C-terminus of a tobacco pathogen-induced oxygenase (PIOX). It is possible that the PIOX is involved in generating signaling molecules that mediate a general stress response.