The G-box is a cis-acting DNA sequence present in several plant promoters that are regulated by diverse signals such as UV irradiation, anaerobiosis, abscissic acid and light. Several basic/leucine zipper (bZIP) proteins from different plant species have been identified as high affinity G-box binding proteins. Although their capability to enhance transcription has been demonstrated, their precise function in transcriptional activation is still unknown. We have isolated three cDNAs from young tomato fruit that encode bZIP G-box binding proteins (GBF4, GBF9 and GBF12). They bind to the G-box sequence in the tomato rbcS1, rbcS2 and rbcS3A promoters. GBF9 binding resulted in a DNase I footprint identical to that obtained with tomato nuclear extract and different from the DNase I protection obtained with GBF4 and GBF12. The mRNAs of all three GBFs were most abundant in tomato fruit and seeds, moderately abundant in root and least abundant in leaves. Protein sequences outside of the bZIP domains were compared with the known GBFs from other plants and seven conserved motifs of seven to 35 amino acids length have been identified. Based on the presence of these motifs, three classes of GBFs can be defined that are conserved among plant species. GBF9, the predominantly expressed tomato GBF, is the first member of its class isolated from dicot plants. Three conserved motifs from two of the classes are highly hydrophilic and are predicted to be exposed on the surface of the proteins. These motifs likely define novel interactive domains in the different classes of GBFs that could provide a new tool to determine how distinct regulatory signals are transmitted through GBFs to activate transcription.

To unravel the molecular mechanisms of drought responses in tomato, gene expression profiles of two drought-tolerant lines identified from a population of Solanum pennellii introgression lines, and the recurrent parent S. lycopersicum cv. M82, a drought-sensitive cultivar, were investigated under drought stress using tomato microarrays. Around 400 genes identified were responsive to drought stress only in the drought-tolerant lines. These changes in genes expression are most likely caused by the two inserted chromosome segments of S. pennellii, which possibly contain drought-tolerance quantitative trait loci (QTLs). Among these genes are a number of transcription factors and signalling proteins which could be global regulators involved in the tomato responses to drought stress. Genes involved in organism growth and development processes were also specifically regulated by drought stress, including those controlling cell wall structure, wax biosynthesis, and plant height. Moreover, key enzymes in the pathways of gluconeogenesis (fructose-bisphosphate aldolase), purine and pyrimidine nucleotide biosynthesis (adenylate kinase), tryptophan degradation (aldehyde oxidase), starch degradation (beta-amylase), methionine biosynthesis (cystathionine beta-lyase), and the removal of superoxide radicals (catalase) were also specifically affected by drought stress. These results indicated that tomato plants could adapt to water-deficit conditions through decreasing energy dissipation, increasing ATP energy provision, and reducing oxidative damage. The drought-responsive genes identified in this study could provide further information for understanding the mechanisms of drought tolerance in tomato.