植物AREB／ABF转录因子及其参与的ABA信号转导

AREB（ABA responsive element binding protein）／ABF（ABRE binding factors）转录因子即ABA（脱落酸）应答元件结合蛋白,参与调控ABA相关基因的表达,提高植物对环境胁迫的适应能力。本文从AREBs的克隆与表达、在抗非生物胁迫中的作用以及参与的ABA信号转导等方面阐述现有的研究进展。     

Structural and functional insights into core ABA signaling

A series of papers in the last year reported major advances in our understanding of abscisic acid (ABA) signaling: the identification of soluble ABA receptors, the elucidation of a core ABA signaling pathway and structural insights into the mechanism of ABA perception and signaling. Here we summarize these advances, which have shown in atomic resolution that the ABA receptors PYR1, PYL1 and PYL2 function as allosteric switches that inhibit type 2C protein phosphatases (PP2Cs) in response to ABA. These receptors function at the apex of a core signaling pathway that regulates ABA responses by controlling SnRK2 kinase activity and the phosphorylation of downstream target proteins such as ABFs, which control nuclear responses, and the ion channel SLAC1, which mediates electrophysiological responses to ABA.     

Interaction between two cis-acting elements,ABRE and DRE,in ABA-dependent expression of Arabidopsis rd29A gene in response to dehydration and high-salinity stresses

Many abiotic stress-inducible genes contain two cis-acting elements, namely a dehydration-responsive element (DRE; TACCGACAT) and an ABA-responsive element (ABRE; ACGTGG/TC), in their promoter regions. We precisely analyzed the 120 bp promoter region (-174 to -55) of the Arabidopsis rd29A gene whose expression is induced by dehydration, high-salinity, low-temperature, and abscisic acid (ABA) treatments and whose 120 bp promoter region contains the DRE, DRE/CRT-core motif (A/GCCGAC), and ABRE sequences. Deletion and base substitution analyses of this region showed that the DRE-core motif functions as DRE and that the DRE/DRE-core motif could be a coupling element of ABRE. Gel mobility shift assays revealed that DRE-binding proteins (DREB1s/CBFs and DREB2s) bind to both DRE and the DRE-core motif and that ABRE-binding proteins (AREBs/ABFs) bind to ABRE in the 120 bp promoter region. In addition, transactivation experiments using Arabidopsis leaf protoplasts showed that DREBs and AREBs cumulatively transactivate the expression of a GUS reporter gene fused to the 120 bp promoter region of rd29A. These results indicate that DRE and ABRE are interdependent in the ABA-responsive expression of the rd29A gene in response to ABA in Arabidopsis.     

ABA调控种子发育的研究进展

种子发育是一个复杂的生物学过程，受各种遗传和外界因素的调节，显著影响农作物特别是禾谷类作物的种子活力和产量与质量。脱落酸(ABA)是调控种子发育和萌发最重要的植物激素之一，其活性水平、信号转导及其LAFL网络在种子发育包括胚胎发生和成熟过程的调控中起关键作用。该文主要综述了近年来ABA调控种子发育的研究取得的重要进展，包括ABA代谢和信号转导对种子发育的调控，ABA与种子成熟转录因子(AFL-B3、FUS3、ABI3、LEC2等)的作用，以及ABA在种子发育中的作用机制，并提出了需要进一步研究的科学问题，为深入理解种子发育的分子机制提供参考，从而提高种子的活力、产量和质量。     

Transport of ABA from the site of biosynthesis to the site of action

There is substantial evidence that abscisic acid (ABA) moves within plants. ABA has been considered as a root-derived signaling molecule that induces stomatal closure in response to dry soil conditions. It has been also reported that ABA synthesized in vegetative tissues is translocated to the seeds. The transport of ABA is an important factor in determining the endogenous concentrations of the hormone at the site of action, and hence, it is an important process in physiological responses. However, the molecular mechanisms that regulate ABA transport are not fully understood. Recent studies using Arabidopsis indicate that ABA is actively synthesized in leaf vascular tissues in response to drought, and that ABA is subsequently transported to the guard cells to close stomata. Identification of the transporters that mediate ABA export from the inside to the outside of the cells at the site of ABA biosynthesis (vascular tissues) and ABA uptake into the cells at the site of action (guard cells), respectively, in this species indicates an active mechanism to regulate ABA transport. Although Arabidopsis represents only one model plant, these findings are useful to discuss common or different regulatory mechanisms among different species and to improve our total understanding of the regulation of ABA transport.     

ABA signal transduction from ABA receptors to ion channels

The plant hormone abscisic acid (ABA) is involved in regulating a number of major processes such as seed dormancy, seedling development, and biotic and abiotic stress responses. The function and effect of ABA on pathogens are still unclear, but the roles of ABA in seed germination and abiotic stress responses have been well characterized. Abiotic stresses elevate ABA levels and activate ABA signaling; thus, inducing a variety of responses, including the expression of stress-related genes and stomatal closure. The past decade has witnessed many significant advances in our understanding of ABA signal transduction due to application of a combination of approaches including genetics, biochemistry, electrophysiology, and chemical genetics. A number of proteins associated with the ABA signal transduction pathway such as PYR/PYL/RCAR family of START proteins, have been identified. These ABA receptors bind to ABA and positively regulate ABA signaling via inactivation of PP2C phosphatase activity, which inhibits SnRK2-type kinases by direct interaction and dephosphorylation. Additionally, SnRK2-type kinases and PP2Cs interact with one another and with other components of ABA signaling and function as positive and negative ABA regulators, respectively. In this review, we focus on ABA function to abiotic stresses and highlight each component in relation to ABA and its interactions.     

ABA transport factors found in Arabidopsis ABC transporters

Abscisic acid (ABA) is a phytohormone that plays an important role in responses to environmental stresses as well as seed maturation and germination. Intracellular signaling by ABA has been rigorously investigated in relation to stomatal guard-cell regulation, seed germination and abiotic stress responses. However, intercellular regulation of ABA, including the molecular basis of ABA transport systems, has hardly been examined in any plant species. Based on genetic and biochemical analyses, we present evidence that one of the ATP-binding cassette (ABC) transporter genes, AtABCG25, encodes a protein that functions as an ABA exporter through the plasma membrane and is involved in the intercellular ABA signaling pathway. The ABC-type transporter is conserved in model species from E. coli to humans and is reported to transport various metabolites or signaling molecules in an ATP-dependent manner. At same time, another ABC transporter in Arabidopsis, AtABCG40, was independently reported to function as an ABA importer in plant cells. These findings strongly suggest the active control of ABA transport between plant cells, and they provide a novel impetus for examining ABA intercellular regulation.Key words: Arabidopsis, ABA, transport, ABC transporter, ABCG, transposontagged lines