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.

In grapevine, anthocyanins and proanthocyanidins are the main flavonoids in berries, which are associated to organoleptic properties in red wine such as color and astringency. Flavonoid pathway is specifically regulated at transcriptional level and several R2R3-MYB proteins have shown to act as positive regulators. However, some members of this family have shown to repress the flavonoid biosynthesis. In this work, we present the characterization of VvMYB4-like gene, which encodes a putative transcriptional factor highly expressed in the skin of berries at the pre veraison stage in grapevine. Its over-expression in tobacco resulted in the loss of pigmentation in flowers due a decrease in anthocyanin accumulation. Severity in anthocyanin suppression observed in petals could be associated with the expression level of the VvMYB4-like transgene. Expression analysis of flavonoid structural genes revealed the strong down-regulation of the flavonoid-related genes anthocyanidin synthase (ANS) and dihydroflavonol reductase (DFR) genes and also the reduction of the anthocyanin-related gene UDP glucose:flavonoid 3-O-glucosyl transferase (UFGT), which was dependent of the transgene expression. In addition, expression of VvMYB4-like in the model plant Arabidopsis showed similar results, with the higher down-regulation observed in the AtDFR and AtLDOX genes. These results suggest that VvMYB4-like may play an important role in regulation of anthocyanin biosynthesis in grapevine acting as a transcriptional repressor of flavonoid structural genes.

Summary Root‐knot nematodes (RKNs; Meloidogyne spp.) induce new post‐embryogenic organs within the roots (galls) where they stablish and differentiate nematode feeding cells, giant cells (GCs). The developmental programmes and functional genes involved remain poorly defined. Arabidopsis root apical meristem (RAM), lateral root (LR) and callus marker lines, SHORT‐ROOT/SHR, SCARECROW/SCR, SCHIZORIZA/SCZ, WUSCHEL‐RELATED‐HOMEOBOX‐5/WOX5, AUXIN‐RESPONSIVE‐FACTOR‐5/ARF5, ARABIDOPSIS‐HISTIDINE PHOSPHOTRANSFER‐PROTEIN‐6/AHP6, GATA‐TRANSCRIPTION FACTOR‐23/GATA23 and S‐PHASE‐KINASE‐ASSOCIATED‐PROTEIN2B/SKP2B, were analysed for nematode‐dependent expression. Their corresponding loss‐of‐function lines, including those for LR upstream regulators, SOLITARY ROOT/SLR/IAA14, BONDELOS/BDL/IAA12 and INDOLE‐3‐ACETIC‐ACID‐INDUCIBLE‐28/IAA28, were tested for RKN resistance/tolerance. LR genes, for example ARF5 (key factor for root stem‐cell niche regeneration), GATA23 (which specifies pluripotent founder cells) and AHP6 (cytokinin‐signalling‐inhibitor regulating pericycle cell‐divisions orientation), show a crucial function during gall formation. RKNs do not compromise the number of founder cells or LR primordia but locally induce gall formation possibly by tuning the auxin/cytokinin balance in which AHP6 might be necessary. Key RAM marker genes were induced and functional in galls. Therefore, the activation of plant developmental programmes promoting transient‐pluripotency/stemness leads to the generation of quiescent‐centre and meristematic‐like cell identities within the vascular cylinder of galls. Nematodes enlist developmental pathways of new organogenesis and/or root regeneration in the vascular cells of galls. This should determine meristematic cell identities with sufficient transient pluripotency for gall organogenesis.

In plant cells, flavonoids are synthesized in the cytosol and then are transported and accumulated in the vacuole. Glutathione S-transferase-mediated transport has been proposed as a mechanism involved in flavonoid transport, however, whether binding of flavonoids to glutathione S-transferase (GST) or their transport is glutathione-dependent is not well understood. Glutathione S-transferases from Vitis vinífera (VviGSTs) have been associated with the transport of anthocyanins, however, their ability to transport other flavonoids such as proanthocyanidins (PAs) has not been established. Following bioinformatics approaches, we analyzed the capability of VviGST1, VviGST3, VviGST4, and Arabidopsis TT19 to bind different flavonoids. Analyses of protein-ligand interactions indicate that these GSTs can bind glutathione and monomers of anthocyanin, PAs and flavonols. A total or partial overlap of the binding sites for glutathione and flavonoids was found in VviGST1, and a similar condition was observed in VviGST3 using anthocyanin and flavonols as ligands, whereas VviGST4 and TT19 have both sites for GSH and flavonoids separated. To validate the bioinformatics predictions, functional complementation assays using the Arabidopsis tt19 mutant were performed. Overexpression of VviGST3 in tt19-1 specifically rescued the dark seed coat phenotype associated to correct PA transport, which correlated with higher binding affinity for PA precursors. VviGST4, originally characterized as an anthocyanin-related GST, complemented both the anthocyanin and PA deposition, resembling the function of TT19. By contrast, VviGST1 only partially rescued the normal seed color. Furthermore the expression pattern of these VviGSTs showed that each of these genes could be associated with the accumulation of different flavonoids in specific tissues during grapevine fruit development. These results provide new insights into GST-mediated PA transport in grapevine and suggest that VviGSTs present different specificities for flavonoid ligands. In addition, our data provide evidence to suggest that GST-mediate flavonoid transport is glutathione-dependent.