Isolation and characterization of novel salt-responsive genes in tomato (Lycopersicon esculentum Mill) roots

In an effort to isolate and characterize novel salt-responsive genes, mRNA differential display-polymerase chain reaction was conducted. Examination of tomato root mRNA profiles revealed that a salt treatment induced, up-regulated or down-regulated the expression of a number of genes. Nine partial cDNAs were isolated and three (JWS19, JWS20 and JWS27) were chosen for further analyses. A full-length cDNA corresponding to JWS27 contained an interrupted open reading frame and two introns, suggesting that JWS27 was potentially derived from either an unprocessed transcript(s) or a pseudogene. Full length cDNAs corresponding to JWS19 and JWS20 encoded polypeptides with similarities to a tomato auxin-regulated protein of unknown function and adioxygenase enzymes, respectively. Alpha4ioxygenases catalyse the oxygenation of fatty acids to produce a newly identified group of oxylipins. In tomato, a-DIOXYGENASE is represented by a small gene family, of which only one member (LEU-DOX1) was salt-responsive. The role of ABA in regulating salt-induced changes of gene expression was explored using genetic and chemical approaches to reduce root ABA levels. In salt-treated roots of the ABA deficient mutant flacca, the expression of genes corresponding to JWS19, JWS20, and JWS27 was similar to that observed in the wild type. A fluridone (FLU) pre-treatment to reduce ABA content had no effect on the level of salt-induced expression of JWS19 and JWS27. However following a salt treatment, the expression of LEU-DOXI was higher in roots of FLU-pre-treated plants than it was in roots that did not receive FLU. An explanation for this relates to the role of ABA in suppressing ethylene accumulation in osmotically stressed roots. Ethephon and the precursor of ethylene biosynthesis markedly elevated LEU-DOXI expression, and this enhanced expression was suppressed by ABA, suggesting that ABA and ethylene can interact to regulate LEU-DOXI expression. LEU-DOXI expression in salt-stressed roots was not markedly affected by an inhibitor of ethylene biosynthesis, 1 -aminoethoxyvinylglycine, AVG, indicating that ABA may be responsible for the enhanced a-DOX expression. However, blocking ethylene signalling with silver ions drastically reduced LEU-DOXl transcript levels, suggesting that a functional ethylene signalling pathway may be required for LEU-DOXI expression in salt-treated roots.