Abscisic Acid Content of Algae under Stress*

The distribution of the phytohormone, abscisic acid (ABA), within the phylum of Phycophyta was investigated by an enzyme-linked immunoassay (ELISA). Of 64 algal species tested (originating from 9 divisions, 20 classes and 36 orders, including procaryotes) all species contained ABA, whereas no ABA could be detected in the bacteria Escherichia coli, Rhodospirillum rubrum, and Halobacterium halobium. It is concluded that ABA is universally distributed within the algal kingdom and is not restricted to cormophytes. The ability to synthesize ABA must have been developed even within the procaryotes. The physiological role of ABA in some selected algae was studied by investigating 1. the distribution of ABA between the cells and the culture medium, 2. the responses of endogenous ABA to stress, 3. the synthesis of ¹⁴C-ABA from externally applied ¹⁴C-mevalonic acid, 4. the metabolism of ABA, 5. the effect of externally applied ABA on various physiological reactions of the algae, and the effect of norflurazon on ABA content. ¹⁴C-mevalonic acid served as precursor of ¹⁴C-ABA synthesis in Dunaliella cells and ABA was metabolised to the same products which have been observed in higher plants. In D. parva the internal ABA level increased upon hyperosmotic salt shocks, and in D. acidophila upon alkalization of the medium. Norflurazon caused an increase of ABA content in Dunaliella. Externally applied ABA did not affect photosynthesis, respiration and K⁺ content of the cells. The permeability of the plasma membrane of D. acidophila to water was slightly decreased by ABA. The possible physiological function of ABA in algae is discussed.     

Abscisic Acid Transport in Human Erythrocytes

Abscisic acid (ABA) is a plant hormone involved in the response to environmental stress. Recently, ABA has been shown to be present and active also in mammals, where it stimulates the functional activity of innate immune cells, of mesenchymal and hemopoietic stem cells, and insulin-releasing pancreatic β-cells. LANCL2, the ABA receptor in mammalian cells, is a peripheral membrane protein that localizes at the intracellular side of the plasma membrane. Here we investigated the mechanism enabling ABA transport across the plasmamembrane of human red blood cells (RBC). Both influx and efflux of [³H]ABA occur across intact RBC, as detected by radiometric and chromatographic methods. ABA binds specifically to Band 3 (the RBC anion transporter), as determined by labeling of RBC membranes with biotinylated ABA. Proteoliposomes reconstituted with human purified Band 3 transport [³H]ABA and [³⁵S]sulfate, and ABA transport is sensitive to the specific Band 3 inhibitor 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid. Once inside RBC, ABA stimulates ATP release through the LANCL2-mediated activation of adenylate cyclase. As ATP released from RBC is known to exert a vasodilator response, these results suggest a role for plasma ABA in the regulation of vascular tone.     

Abscisic Acid and Desiccation Tolerance in Mosses

In higher plants the phytohormone ABA is involved in processes that are connected to water deficit, like stomatal closure or desiccation tolerance. In bryophytes, also containing ABA in their tissues, physiological functions remained uncertain for a long time. Quite recently, several papers have shown different effects of exogenously applied ABA: stomatal closure in Anthoceros, drought hardening in Funaria and production of the landform in Riccia. In all these cases the relevant conditions (water deficit) enhance the endogenous ABA level significantly. For induced desiccation tolerance, ABA serves as a mediator to induce specific proteins (dehydrins) strongly connected with this tolerance. Therefore, it can be concluded that in bryophytes ABA has the same function as in higher plants. It acts as a mediator in stress conditions.     

Abscisic acid levels in tomato ovaries are regulated by <Emphasis Type="Italic">LeNCED1</Emphasis> and <Emphasis Type="Italic">SlCYP707A1</Emphasis>

Although the hormones, gibberellin and auxin, are known to play a role in the initiation of fruits, no such function has yet been demonstrated for abscisic acid (ABA). However, ABA signaling and ABA responses are high in tomato (Solanum lycopersicum L.) ovaries before pollination and decrease thereafter (Vriezen et al. in New Phytol 177:60–76, 2008). As a first step to understanding the role of ABA in ovary development and fruit set in tomato, we analyzed ABA content and the expression of genes involved in its metabolism in relation to pollination. We show that ABA levels are relatively high in mature ovaries and decrease directly after pollination, while an increase in the ABA metabolite dihydrophaseic acid was measured. An important regulator of ABA biosynthesis in tomato is 9-cis-epoxy-carotenoid dioxygenase (LeNCED1), whose mRNA level in ovaries is reduced after pollination. The increased catabolism is likely caused by strong induction of one of four newly identified putative (+)ABA 8′-hydroxylase genes. This gene was named SlCYP707A1 and is expressed specifically in ovules and placenta. Transgenic plants, overexpressing SlCYP707A1, have reduced ABA levels and exhibit ABA-deficient phenotypes suggesting that this gene encodes a functional ABA 8′-hydroxylase. Gibberellin and auxin application have different effects on the LeNCED1 and SlCYP707A1 gene expression. The crosstalk between auxins, gibberellins and ABA during fruit set is discussed.     

The Lesion-Mimic Mutant cpr22 Shows Alterations in Abscisic Acid Signaling and Abscisic Acid Insensitivity in a Salicylic Acid-Dependent Manner

A number of Arabidopsis (Arabidopsis thaliana) lesion-mimic mutants exhibit alterations in both abiotic stress responses and pathogen resistance. One of these mutants, constitutive expresser of PR genes22 (cpr22), which has a mutation in two cyclic nucleotide-gated ion channels, is a typical lesion-mimic mutant exhibiting elevated levels of salicylic acid (SA), spontaneous cell death, constitutive expression of defense-related genes, and enhanced resistance to various pathogens; the majority of its phenotypes are SA dependent. These defense responses in cpr22 are suppressed under high-humidity conditions and enhanced by low humidity. After shifting plants from high to low humidity, the cpr22 mutant, but not the wild type, showed a rapid increase in SA levels followed by an increase in abscisic acid (ABA) levels. Concomitantly, genes for ABA metabolism were up-regulated in the mutant. The expression of a subset of ABA-inducible genes, such as RD29A and KIN1/2, was down-regulated, but that of other genes, like ABI1 and HAB1, was up-regulated in cpr22 after the humidity shift. cpr22 showed reduced responsiveness to ABA not only in abiotic stress responses but also in germination and stomatal closure. Double mutant analysis with nahG plants that degrade SA indicated that these alterations in ABA signaling were attributable to elevated SA levels. Furthermore, cpr22 displayed suppressed drought responses by long-term drought stress. Taken together, these results suggest an effect of SA on ABA signaling/abiotic stress responses during the activation of defense responses in cpr22.Plants have evolved a large number of defense systems to protect themselves against pathogen invasion. Whether these defenses are successful depends on the speed and intensity of their activation. The first line of defense is the basal immune system that is activated by molecules that are conserved among many pathogens (microbe-associated molecular patterns). Pathogens in turn have evolved a number of effector molecules that can block the basal resistance response (Jones and Dangl, 2006; Bent and Mackey, 2007). A second, stronger response to pathogen infection is mediated by resistance (R) genes that can interact with particular effectors (previously termed avirulence factors) from the pathogen or that can recognize effector-induced modifications of plant proteins (Flor, 1971; Bent and Mackey, 2007). One defense mechanism activated by R gene-mediated pathogen recognition is the hypersensitive response (HR), which is characterized by apoptosis-like cell death at and around the site of pathogen entry (Hammond-Kosack and Jones, 1996; Heath, 2000). HR development is usually accompanied by an increase in salicylic acid (SA) and the accumulation of defense-related proteins such as the pathogenesis-related (PR) proteins (Vlot et al., 2008). At later times after infection, elevated SA levels and PR gene expression are also detected in the uninoculated leaves, concurrent with the development of systemic acquired resistance (SAR), a long-lasting, broad-based resistance to subsequent infection (Durrant and Dong, 2004; Grant and Lamb, 2006; Vlot et al., 2008).Many studies have demonstrated that SA is an important signaling molecule in the pathways conferring local and systemic resistance (Dempsey et al., 1999; Vlot et al., 2008). To identify other components in the pathogen resistance signal transduction pathway, many Arabidopsis (Arabidopsis thaliana) mutants with altered resistance to pathogens have been isolated. One class exhibits constitutively increased SA levels and PR gene expression as well as heightened resistance to pathogen infection. This group includes dnd1, dnd2/hlm1, copine1 (cpn1), constitutive expresser of PR genes22 (cpr22), and ssi4 (Yu et al., 1998; Jambunathan et al., 2001; Yoshioka et al., 2001; Shirano et al., 2002; Balague et al., 2003; Jurkowski et al., 2004). The majority of these mutants share similar phenotypes such as spontaneous HR-like lesions and thus are categorized as lesion-mimic mutants (Moeder and Yoshioka, 2008). Interestingly, it has been reported that some lesion-mimic mutants are environmentally sensitive (i.e. their resistance phenotypes are conditional; Moeder and Yoshioka, 2009). For instance, under high-humidity conditions such as on agar plates or when grown at high temperature, both the spontaneous HR and the enhanced pathogen resistance are suppressed (Jambunathan et al., 2001; Yoshioka et al., 2001; Jambunathan and McNellis, 2003; Xiao et al., 2003; Zhou et al., 2004; Noutoshi et al., 2005). On the other hand, relatively low humidity or cold temperature enhances their SA-related phenotypes, including HR-like cell death (Jambunathan et al., 2001; Zhou et al., 2004).Some of these lesion-mimic phenotypes are caused by mutations in R genes, such as SSI4 and SLH1 (Shirano et al., 2002; Noutoshi et al., 2005), or by the overexpression of an R gene, such as RPW8 (Xiao et al., 2003), indicating the involvement of environmental factors on R gene-mediated signaling pathway(s). Indeed, similar environmental effects were also reported for the response of wild-type R genes. It is well known that the HR induced by the recognition of Tobacco mosaic virus by the N protein can be completely suppressed when plants are kept above 28°C. When plants are shifted back to 22°C, the HR starts to develop, indicating that there is a temperature-sensitive step in the signaling pathway (Samuel, 1931). Both basal and R gene-mediated resistance against the bacterial pathogen, Pseudomonas syringae, is attenuated by a moderate increase in temperature (Wang et al., 2009). It has also been reported that high humidity (greater than 95% relative humidity [RH]) delayed or reduced the HR and other resistance responses induced by the interaction of the Cladosporium fulvum avirulence factors Avr2, Avr4, and Avr9 and their cognate tomato R proteins Cf-2, Cf-4, and Cf-9, respectively (Hammond-Kosack et al., 1996; May et al., 1996; Wang et al., 2005). These findings suggest that there is a universal factor(s) in defense signaling that is environmentally sensitive.Abscisic acid (ABA) controls various environmental (abiotic) stress responses, including drought, salinity, and temperature stress, and many components involved in these responses have been identified (Shinozaki et al., 2003). Additionally, it is becoming clear that ABA is also involved in biotic stress responses in a complex manner. For instance, treatment with exogenous ABA prior to pathogen infection induces enhanced susceptibility in various plant species (Mauch-Mani and Mauch, 2005). Mohr and Cahill (2003, 2006) suggested that the mechanisms behind this phenomenon are likely related to the antagonistic effect of ABA on SA signaling. Similarly, several groups have reported that virulent P. syringae DC3000 enhances the production of ABA during pathogenesis (Schmelz et al., 2003; de Torres-Zabala et al., 2007). Furthermore, Yasuda et al. (2008) suggested the antagonism between SA and ABA signaling in SAR. These studies suggest that ABA plays a negative role in pathogen resistance. In contrast, Melotto and colleagues (2006) reported that ABA-dependent stomata closure is part of plant innate immunity against bacterial invasion and that SA is required for this response. They also reported that aba3-1, an ABA-deficient mutant, was more susceptible to P. syringae DC3000, suggesting a positive role of ABA in innate immunity (Melotto et al., 2006).Here, we attempt to characterize the effects of humidity on pathogen resistance responses using the lesion-mimic mutant cpr22. Previously, we reported that most phenotypes of cpr22, such as spontaneous lesion formation, SA accumulation, and constitutive PR gene expression, were suppressed under high RH (Yoshioka et al., 2001). cpr22 contains a deletion that fuses two cyclic nucleotide-gated ion channel (CNGC)-encoding genes, AtCNGC11 and AtCNGC12, generating the novel chimeric AtCNGC11/12 (Yoshioka et al., 2006). We proposed that the expression of AtCNGC11/12 activates pathogen resistance responses through the same signal transduction pathway used by R genes and that cell death induced by the expression of AtCNGC11/12 is HR-like programmed cell death (Yoshioka et al., 2006; Urquhart et al., 2007). Here, we report intriguing alterations in ABA-related phenotypes in cpr22. Our data demonstrate that elevated SA accumulation is the cause of these alterations, suggesting complex SA-ABA cross talk during lesion formation.     

Abscisic Acid and Its Catabolites in Leaves and Berries of Chardonnay are Affected by Water Status

The biosynthetic pathway of abscisic acid (ABA) is well known. The aim of this study was to investigate the relationship among various ABA catabolites in leaves and berries of Chardonnay grapevines grown under various irrigation regimes. An irrigation trial was set up in one vineyard, located in Niagara-on-the-Lake, ON, Canada, consisting of seven treatments: control (non-irrigated), plus three water levels (100, 50, and 25 % of estimated crop evapotranspiration) combined with two irrigation imposition times (fruit set, veraison). No irrigation occurred prior to treatment imposition. ABA, phaseic acid (PA), dihydrophaseic acid (DPA), 7′-hydroxy-ABA, 8′-hydroxy-ABA, neophaseic acid, and ABA glucose ester (ABA-GE) were quantified in leaves and berries by HPLC–MS. ABA was likely catabolized by conjugation to form ABA-GE in treatments under high levels of water deficit, while in treatments with high water status, the oxidation pathway leading to DPA or PA predominated. Concentrations of ABA and its catabolites therefore reflected vine water status, whereby the specific ABA catabolic pathways in leaves and berries were determined by water status level. Hormonal profiles suggested a direct relationship between ABA and vine water status. The concentration of ABA in Chardonnay may explain why and how white cultivars adapt to drought stress versus red cultivars.

     

Functional Mechanism of the Abscisic Acid Agonist Pyrabactin

Pyrabactin is a synthetic abscisic acid (ABA) agonist that selectively inhibits seed germination. The use of pyrabactin was pivotal in the identification of the PYR1/PYL/RCAR family (PYL) of proteins as the ABA receptor. Although they both act through PYL proteins, pyrabactin and ABA share no apparent chemical or structural similarity. It remains unclear how pyrabactin functions as an ABA agonist. Here, we report the crystal structure of pyrabactin in complex with PYL1 at 2.4 Å resolution. Structural and biochemical analyses revealed that recognition of pyrabactin by the pocket residues precedes the closure of switch loop CL2. Structural comparison between pyrabactin- and ABA-bound PYL1 reveals a general principle in the arrangements of function groups of the two distinct ligands. The study provides a framework for the development of novel ABA agonists that may have applicable potentials in agriculture.     

ROS-Mediated ABA Signaling

In plants, reactive oxygen species (ROS) are short-lived molecules produced through various cellular mechanisms in response to biotic and abiotic stimuli. ROS function as second messengers for hormone signaling, development, oxygen deprivation, programmed cell death, and plant–pathogen interactions. Recent research on ROS-mediated responses has produced stimulating findings such as the specific sources of ROS production, molecular elements that work in ROS-mediated signaling and homeostasis, and a ROS-regulated gene network (Neill et al., Curr Opin Plant Biol 5:388–395, 2002a; Apel and Hirt, Annu Rev Plant Biol 55:373–399, 2004; Mittler et al., Trends Plant Sci 9:490–498, 2004; Mori and Schroeder, Plant Physiol 135:702–708, 2004; Kwak et al., Plant Physiol 141:323–329, 2006; Torres et al., Plant Physiol 141:373–378, 2006; Miller et al., Physiol Plant 133:481–489, 2008). In this review, we highlight new discoveries in ROS-mediated abscisic acid (ABA) signaling. Drs. Daeshik Cho and June M. Kwak are the corresponding authors for this paper.     

Changes in Salicylic and Abscisic Acid Contents during Heat Treatment and Their Effect on Thermotolerance of Grape Plants

Heat treatment (38°C) of young grape plants (Vitis vinifera L., cv. Jingxiu) or leaf spraying with 100 µM salicylic acid (SA) increased leaf thermotolerance. Both treatments led to an increase in ABA content and a decrease in lipoxygenase (LOX) activity. ABA content showed a drastic rise by one hour after treatments and then sharply declined while LOX activity continuously decreased. In the course of heat treatment of grape plants, endogenous SA level and phenylalanine ammonia-lyase activity rapidly increased during the first hour, then declined. These results showed that endogenous SA and ABA can be involved in grape plant response to heat treatment resulted in improved thermotolerance.From Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 578–583.Original English Text Copyright © 2005 by Wang, Huang, Liu, Zhan.The text was submitted by the authors in English.     

脱落酸在植物花发育过程中的作用

植物激素脱落酸(ABA)对植物的生长发育具有多方面的调节作用,比如种子休眠、萌发,营养生长,环境胁迫反应等。大量研究显示,ABA也参与了植物的成花调控。影响植物成花调控的环境因子,包括光周期变化、春化作用、干旱等均会导致植物体内ABA代谢的变化。本文从调控植物开花的4条主要途径与植物体内ABA代谢变化之间的相互关系,花芽分化时期ABA在植物叶芽和花芽中的动态分布以及离体培养条件下ABA对花芽分化的影响等方面总结了ABA与植物花发育这一领域的最新研究进展。对ABA在植物成花诱导和花发育中的作用进行了综合分析。     

保卫细胞的ABA信号转导

植物激素脱落酸(ABA)调节植物体多种生理过程,尤其在一些逆境条件下,植物体中ABA大量合成,诱导气孔关闭,从而有效地调控植物体内的水分平衡.尽管人们对ABA诱导气孔关闭作用已得到共识,但有关信号转导的细节还很不清楚.该文简要介绍了研究气孔保卫细胞信号转导途径的相关技术以及与ABA信号转导直接相关的ABA受体、第二信使、蛋白质磷酸化和离子通道调节等方面的最新妍究进展.并在前人研究工作的基础上,勾画出气孔保卫细胞ABA、H2O2的信号转导模式图.     

Effect of divalent cations on the structure of the antibiotic daptomycin

Daptomycin, a cyclic anionic lipopeptide antibiotic, whose three-dimensional structure was recently solved using solution state NMR (Ball et al. 2004; Jung et al. 2004; Rotondi and Gierasch 2005), requires calcium for function. To date, the exact nature of the interaction between divalent cations, such as Ca²⁺ or Mg²⁺, has not been fully characterized. It has, however, been suggested that addition of Ca²⁺ to daptomycin in a 1:1 molar ratio induces aggregation. Moreover, it has been suggested that certain residues, e.g. Asp3 and Asp7, which are essential for activity (Grunewald et al. 2004; Kopp et al. 2006), may also be important for Ca²⁺ binding (Jung et al. 2004). In this work, we have tried: (1) to further pinpoint how Ca²⁺ affects daptomycin structure/oligomerization using analytical ultracentrifugation; and (2) to determine whether a specific calcium binding site exists, based on one-dimensional ¹³C NMR spectra and molecular dynamics (MD) simulations. The centrifugation results indicated that daptomycin formed micelles of between 14 and 16 monomers in the presence of a 1:1 molar ratio of Ca²⁺ and daptomycin. The ¹³C NMR data indicated that addition of calcium had a significant effect on the Trp1 and Kyn13 residues, indicating that either calcium binds in this region or that these residues may be important for oligomerization. Finally, the molecular dynamics simulation results indicated that the conformational change of daptomycin upon calcium binding might not be as significant as originally proposed. Similar studies on the divalent cation Mg²⁺ are also presented. The implication of these results for the biological function of daptomycin is discussed. Electronic supplementary material The online version of this article (doi:) contains Supplemental Material, which is available to authorized users. Steven W. Ho and David Jung have contributed equally to this work.     

Changes in Abscisic Acid Biosynthesis and Catabolism during Dormancy Breaking in Fagus sylvatica Embryo

At harvest, embryos of Fagus sylvatica are dormant. A cold pretreatment without medium at 30% moisture content allowed them to germinate. A comparison of the abscisic acid (ABA) content before and after the pretreatment has no significant relevance since dormancy is expressed during the culture at 23°C. During this culture, both de novo biosynthesis and conjugate hydrolysis contributed to maintain a high level of ABA in the dormant axis. The level of conjugates and the rate of hydrolysis were not modified substantially by the cold pretreatment. In contrast, the dormancy release was associated with a strong decrease in the capacity for ABA synthesis. Moreover, feeding (+)-[³H]ABA to untreated and pretreated embryos proved that the cold treatment also induced a hastening of ABA catabolism. Received August 15, 1996; accepted December 6, 1996