The phytohormone abscisic acid (ABA) plays a vital role in plant development and response to environmental challenges, but the complex networks of ABA signaling pathways are poorly understood. We previously reported that a chloroplast protein, the magnesium-protoporphyrin IX chelatase H subunit (CHLH/ABAR), functions as a receptor for ABA in Arabidopsis thaliana. Here, we report that ABAR spans the chloroplast envelope and that the cytosolic C terminus of ABAR interacts with a group of WRKY transcription factors (WRKY40, WRKY18, and WRKY60) that function as negative regulators of ABA signaling in seed germination and postgermination growth. WRKY40, a central negative regulator, inhibits expression of ABA-responsive genes, such as ABI5. In response to a high level of ABA signal that recruits WRKY40 from the nucleus to the cytosol and promotes ABAR-WRKY40 interaction, ABAR relieves the ABI5 gene of inhibition by repressing WRKY40 expression. These findings describe a unique ABA signaling pathway from the early signaling events to downstream gene expression.

Transitory starch is stored during the day inside chloroplasts and broken down at night for export. Maltose is the primary form of carbon export from chloroplasts at night. We investigated the influence of daylength and circadian rhythms on starch degradation and maltose metabolism. Starch breakdown was faster in plants of Arabidopsis (Arabidopsis thaliana) ecotype Wassilewskija growing in long days. Transcript levels of genes encoding enzymes involved in starch degradation and maltose metabolism showed a strong diurnal rhythm. Under altered photoperiods, the transcript levels and the rate of starch degradation changed within one day/night cycle. However, the amount of proteins involved in starch degradation was maintained relatively constant throughout the day/night cycle. To investigate whether the diurnal cycling of the transcript levels is only a response to light or is also regulated by a circadian clock, we measured the amount of messenger RNAs in Arabidopsis leaves under continuous light and continuous darkness. The expression of genes encoding starch degradationrelated enzymes was under very strong circadian control in continuous light. Under continuous light, the amount of maltose also showed a strong endogenous rhythm close to 24 h, indicating that maltose metabolism is under circadian control. Light is necessary for the cycling of transcript levels and maltose levels. Under continuous darkness, these genes were barely expressed, and no cycling of maltose levels was observed.Starch is the most abundant carbohydrate reserve in plants. There are two types of starch: storage starch and transitory starch. Transitory starch is stored during the day inside chloroplasts and broken down at night for export. At night, starch is converted to maltodextrin by several enzymes, such as debranching enzyme, and appears to be influenced by a glucan, water dikinase (GWD) and phosphoglucan, water dikinase (PWD) (Ritte et al., 2000(Ritte et al., , 2002Trethewey and Smith, 2000;Yu et al., 2001;Smith et al., 2003;Zeeman et al., 2004;Kotting et al., 2005). a-Amylase (AtAMY3) was thought to be involved in the conversion of starch to maltodextrin (Trethewey and Smith, 2000), but recent data indicate that AtAMY is not required for transitory starch breakdown in Arabidopsis (Arabidopsis thaliana) leaves (Yu et al., 2005). Maltodextrin is then converted to maltose and Glc by b-amylase (CT-BMY) and disproportionating enzyme (DPE1) in the chloroplast (Lao et al., 1999;Critchley et al., 2001;Scheidig et al., 2002;Schneider et al., 2002). Recent evidence indicates that maltose and Glc are the two major forms of carbon exported from chloroplasts at night (Weber et al., 2000;Servaites and Geiger, 2002;Weise et al., 2004). Maltose is exported by the maltose transporter MEX1 (Niittylä et al., 2004) and is metabolized in the cytosol by several enzymes, including disproportionating enzyme (DPE2) and possibly glucan phosphorylase (AtPHS2) Lu and Sharkey, 2004;Schupp and Ziegler, 2004). Localization of DPE2 in the literature is conflicting. ...

ERF transcription factors play important roles in regulating gene expression under abiotic and biotic stresses. The first member of the ERF gene family in wheat (Triticum aestivum L.) was isolated by screening a drought-induced cDNA library and designated as T. aestivum ethylene-responsive factor 1 (TaERF1), which encoded a putative protein of 355 amino acids with a conserved DNA-binding domain and a conserved N-terminal motif (MCGGAIL). The TaERF1 gene was located on chromosome 7A. Protein interaction assays indicated that TaERF1, with a putative phosphorylation site (TPDITS) in the C-terminal region, was a potential phosphorylation substrate for TaMAPK1 protein kinase. Deletion of the N-terminal motif enhanced the interaction of TaERF1 with TaMAPK1. The predicted TaERF1 protein contained three putative nuclear localization signals (NLSs), and three NLSs modulated synergistically the activity of subcellular localization. As a trans-acting factor, TaERF1 was capable of binding to the GCC-box and CRT/DRE elements in vitro, and of trans-activating reporter gene expression in tobacco (Nicotiana tabacum L.) leaves. Transcription of the TaERF1 gene was induced not only by drought, salinity and low-temperature stresses and exogenous ABA, ethylene and salicylic acid, but also by infection with Blumeria graminis f. sp. tritici. Furthermore, overexpression of TaERF1 activated stress-related genes, including PR and COR/RD genes, under normal growth conditions, and improved pathogen and abiotic stress tolerance in transgenic plants. These results suggested that the TaERF1 gene encodes a GCC-box and CRT/DRE element binding factor that might be involved in multiple stress signal transduction pathways.

Transitory starch is stored during the day inside chloroplasts and then broken down at night for export. Recent data indicate that maltose is the major form of carbon exported from the chloroplast at night but its fate in the cytosol is unknown. An amylomaltase gene ( malQ) cloned from Escherichia coli is necessary for maltose metabolism in E. coli. We investigated whether there is an amylomaltase in the cytosol of plant leaves and the role of this enzyme in plants. Two mutants of Arabidopsis thaliana (L) Heynh. were identified in which the gene encoding a putative amylomaltase enzyme [ disproportionating enzyme 2, DPE2 (DPE1 refers to the plastid version of this enzyme)] was disrupted by a T-DNA insertion. Both dpe2-1 and dpe2-2 plants exhibited a dwarf phenotype and accumulated a large amount of maltose. In addition, dpe2 mutants accumulated starch and a water-soluble, ethanol/KCl-insoluble maltodextrin in their chloroplasts. At night, the amount of sucrose in dpe2 plants was lower than that in wild-type plants. These results show that Arabidopsis has an amylomaltase that is involved in the conversion of maltose to sucrose in the cytosol. We hypothesize that knocking out amylomaltase blocks the conversion from maltose to sucrose, and that the higher amount of maltose feeds back to limit starch degradation reactions in chloroplasts. As a result, dpe2 plants have higher maltose, higher starch, and higher maltodextrin but lower nighttime sucrose than wild-type plants. Finally, we propose that maltose metabolism in the cytosol of Arabidopsis leaves is similar to that in the cytoplasm of E. coli.