Members of the abscisic acid-responsive element binding protein (AREB)/abscisic acid-responsive element binding factor (ABF) subfamily of basic leucine zipper (bZIP) transcription factors have been implicated in abscisic acid (ABA) and abiotic stress responses in plants. Here we describe two members identified in cultivated tomato (Solanum lycopersicum), named SlAREB1 and SlAREB2. Expression of SlAREB1 and SlAREB2 is induced by drought and salinity in both leaves and root tissues, although that of SlAREB1 was more affected. In stress assays, SlAREB1-overexpressing transgenic tomato plants showed increased tolerance to salt and water stress compared to wild-type and SlAREB1-down-regulating transgenic plants, as assessed by physiological parameters such as relative water content (RWC), chlorophyll fluorescence and damage by lipoperoxidation. In order to identify SlAREB1 target genes responsible for the enhanced tolerance, microarray and cDNA-amplified fragment length polymorphism (AFLP) analyses were performed. Genes encoding oxidative stress-related proteins, lipid transfer proteins (LTPs), transcription regulators and late embryogenesis abundant proteins were found among the up-regulated genes in SlAREB1-overexpressing lines, especially in aerial tissue. Notably, several genes encoding defence proteins associated with responses to biotic stress (e.g. pathogenesis-related proteins, protease inhibitors, and catabolic enzymes) were also up-regulated by SlAREB1 overexpression, suggesting that this bZIP transcription factor is involved in ABA signals that participate in abiotic stress and possibly in response to pathogens.

Wild relatives of cultivated tomato (Solanum lycopersicum) are resistant to a wide range of abiotic and biotic stress conditions. In an effort to understand the molecular mechanisms of salt stress resistance in the wild and cultivated Solanum species, a basic leucine zipper (bZIP) transcription factor was identified in S. chilense, S. peruvianum and S. lycopersicum and named ScAREB1, SpAREB1 and SlAREB1, respectively. Deduced amino acid sequences of the three proteins are 97% identical among them and present high homology with the ABF/AREB subfamily of transcription factors described in different plant species, including Arabidopsis (ABF2, 54% identical) and tobacco (PHI-2, 50% identical). Expression of these orthologous genes is upregulated similarly in the three species by salt stress. The expression of SlAREB1 was further investigated in S. lycopersicum and found to be induced by drought, cold and abscisic acid. To investigate the possible role of this transcription factor in response to abiotic stress, a simple transient expression assay was used for rapid analysis of genes regulated by SlAREB1 in tomato and tobacco by means of Agrobacterium-mediated transformation. Tobacco leaves expressing SlAREB1 showed upregulation of stress-responsive genes such as RD29B, the LEA genes ERD10B and TAS14, the transcription factor PHI-2 and a trehalose-6-phosphate phosphatase gene. These results suggest that this class of bZIP plays a role in abiotic stress response in the Solanum genus.