The Arabidopsis bZIP gene AtbZIP63 is a sensitive integrator of transient abscisic acid and glucose signals.

Glucose modulates plant metabolism, growth, and development. In Arabidopsis (Arabidopsis thaliana), Hexokinase1 (HXK1) is a glucose sensor that may trigger abscisic acid (ABA) synthesis and sensitivity to mediate glucose-induced inhibition of seedling development. Here, we show that the intensity of short-term responses to glucose can vary with ABA activity. We report that the transient (2 h/4 h) repression by 2% glucose of AtbZIP63, a gene encoding a basic-leucine zipper (bZIP) transcription factor partially involved in the Snf1-related kinase KIN10-induced responses to energy limitation, is independent of HXK1 and is not mediated by changes in ABA levels. However, high-concentration (6%) glucose-mediated repression appears to be modulated by ABA, since full repression of AtbZIP63 requires a functional ABA biosynthetic pathway. Furthermore, the combination of glucose and ABA was able to trigger a synergistic repression of AtbZIP63 and its homologue AtbZIP3, revealing a shared regulatory feature consisting of the modulation of glucose sensitivity by ABA. The synergistic regulation of AtbZIP63 was not reproduced by an AtbZIP63 promoter-5'-untranslated region::ß-glucuronidase fusion, thus suggesting possible posttranscriptional control. A transcriptional inhibition assay with cordycepin provided further evidence for the regulation of mRNA decay in response to glucose plus ABA. Overall, these results indicate that AtbZIP63 is an important node of the glucose-ABA interaction network. The mechanisms by which AtbZIP63 may participate in the fine-tuning of ABA-mediated abiotic stress responses according to sugar availability (i.e., energy status) are discussed.

Evolutionary pattern of angiosperm bZIP factors homologous to the maize Opaque2 regulatory protein.

Opaque2 (O2) is a bZIP transcriptional regulatory factor involved in the control of seed storage proteins synthesis as well as carbon and nitrogen metabolism during maize seed development. Phylogenetic analysis of a possible complete and nonredundant collection of angiosperm bZIP factors resulted in the identification of 20 angiosperm O2-homologues that defined what we call the O2 gene family. Members of the family share a highly conserved bZIP DNA binding domain and several other motifs which define important functional features. The O2 family was enriched by the identification of 25 new putative angiosperm O2 homologous genes in EST databases and in the rice genome. Based on parsimony analysis, the collection of O2 homologues was organized into one eudicot-monocot and three monocot groups of orthologous genes and two groups of eudicot genes. These results support a model of the evolution of the O2 family that involves two O2 homologous gene duplications before the separation of monocots and eudicots. Further expansion of O2 homologues resulted in at least three and one gene duplications in the monocot and eudicot lineages, respectively. O2 appears to have been the result of a monocot-specific gene duplication event, and the possibility that O2 represents a functional specialization restricted to monocots is suggested.