SummaryThe abscisic acid-responsive gene rab17 of maize is expressed during late embryogenesis, and is induced by ABA and desiccation in embryo and vegetative tissues. ABRE and DRE cis-elements are involved in regulation of the gene by ABA and drought. Using yeast one-hybrid screening, we isolated two cDNAs encoding two new DRE-binding proteins, designated DBF1 and DBF2, that are members of the AP2/ EREBP transcription factor family. Analysis of mRNA accumulation pro®les showed that DBF1 is induced during maize embryogenesis and after desiccation, NaCl and ABA treatments in plant seedlings, whereas the DBF2 mRNA is not induced. DNA-binding preferences of DBFs were analysed by electrophoretic mobility shift assays, and showed that both DBF1 and DBF2 bound to the wild-type DRE2 element, but not to the DRE2 mutant or to the DRE1 element which differs only in a single nucleotide. Transactivation activity using particle bombardment showed that DBF1 functioned as activator of DRE2-dependent transcription of rab17 promoter by ABA, whereas DBF2 overexpression had a repression action downregulating not only the basal promoter activity, but also the ABA effect. These results show that ABA plays a role in the regulation of DBF activity, and suggests the existence of an ABA-dependent pathway for the regulation of genes through the C-repeat/DRE element.

Crop plants are exposed to many types of abiotic stress during their life cycle. Water deficit derived from drought, low temperature or high salt concentration in the soil, is one of the most common environmental stresses that affects growth and development of plants through alterations in metabolism and gene expression. Adaptation to these conditions may involve passive tolerance or active homeostatic mechanisms for maintaining water balance. Active responses occur at different levels in the plant and may represent a concomitant protection against other types of stress such as pathogen attack. Many morphological and physiological adaptations to water stress are under the control of the plant hormone abscisic acid and involve specific activation of target genes that in one way or another protect cells against water deficit or participate in the regulation of the drought response. Here, we discuss recent advances in our understanding of drought adaptation mediated by specific changes in gene expression and the role of AP2/EREBP nuclear factors in these processes. ß

Tudor-SN (TSN) copurifies with the RNA-induced silencing complex in animal cells where, among other functions, it is thought to act on mRNA stability via the degradation of specific dsRNA templates. In plants, TSN has been identified biochemically as a cytoskeleton-associated RNA binding activity. In eukaryotes, it has recently been identified as a conserved primary target of programmed cell death-associated proteolysis. We have investigated the physiological role of TSN by isolating null mutations for two homologous genes in Arabidopsis thaliana. The double mutant tsn1 tsn2 displays only mild growth phenotypes under nonstress conditions, but germination, growth, and survival are severely affected under high salinity stress. Either TSN1 or TSN2 alone can complement the double mutant, indicating their functional redundancy. TSN accumulates heterogeneously in the cytosol and relocates transiently to a diffuse pattern in response to salt stress. Unexpectedly, stress-regulated mRNAs encoding secreted proteins are significantly enriched among the transcripts that are underrepresented in tsn1 tsn2. Our data also reveal that TSN is important for RNA stability of its targets. These findings show that TSN is essential for stress tolerance in plants and implicate TSN in new, potentially conserved mechanisms acting on mRNAs entering the secretory pathway.

We identified agtA, a gene that encodes the specific dicarboxylic amino acid transporter of Aspergillus nidulans. The deletion of the gene resulted in loss of utilization of aspartate as a nitrogen source and of aspartate uptake, while not completely abolishing glutamate utilization. Kinetic constants showed that AgtA is a high-affinity dicarboxylic amino acid transporter and are in agreement with those determined for a cognate transporter activity identified previously. The gene is extremely sensitive to nitrogen metabolite repression, depends on AreA for its expression, and is seemingly independent from specific induction. We showed that the localization of AgtA in the plasma membrane necessitates the ShrA protein and that an active process elicited by ammonium results in internalization and targeting of AgtA to the vacuole, followed by degradation. Thus, nitrogen metabolite repression and ammonium-promoted vacuolar degradation act in concert to downregulate dicarboxylic amino acid transport activity.Amino acids can be utilized by saprophytic fungi as nitrogen and/or carbon sources. Their uptake is mediated by transmembrane proteins belonging to the specific fungal YAT (TC 2.A.3.10, yeast amino acid transporter) family, a member of the APC (amino acid polyamine choline) superfamily (25), which shows a wide range of substrate specificities and includes representatives in all realms of life (53). In Saccharomyces cerevisiae (51), 18 YAT transporters have been functionally characterized, while only a few have been studied in other organisms (8,25,35,57,67,68,69,72,76; Saccharomyces genome database at http://www.yeastgenome.org/). The YAT family transporters share a common predicted topology, comprising 12 transmembrane domains, and show, even among proteins with completely different substrate specificities, a high degree of sequence identity (2, 25). The transporters of the dicarboxylic amino acids glutamate and aspartate have been characterized in S. cerevisiae (DIP5; 47) and Penicillium chrysogenum (PcDIP5; 68) and are members of this family. In A. nidulans, specific dicarboxylic amino acid transport activity and certain aspects of its regulation were previously reported (24, 26, 42, 51, 52), but the corresponding gene(s) was not characterized.In fungi, the genes that encode transporters and enzyme activities involved in nitrogen source utilization are subject to tight transcriptional and/or posttranscriptional controls. Preferred nitrogen sources (such as ammonium and glutamine) repress the transcription of genes involved in the utilization of metabolically less favorable nitrogen sources such as nitrate, purines, or amino acids. In A. nidulans, nitrogen metabolite repression acts by inactivating the AreA GATA transcriptional activator (30) by a number of concurring mechanisms (36, 66). In S. cerevisiae, a transcriptional repression mechanism similar but not identical to the one described above for A. nidulans is operative (see reference 33 for a review).In addition to being subject to transcriptional regulati...