The Roles of CATALASE2 in Abscisic Acid Signaling in Arabidopsis Guard Cells

We investigated the roles of catalase (CAT) in abscisic acid (ABA)-induced stomatal closure using a cat2 mutant and an inhibitor of CAT, 3-aminotriazole (AT). Constitutive reactive oxygen species (ROS) accumulation due to the CAT2 mutation and AT treatment did not affect stomatal aperture in the absence of ABA, whereas ABA-induced stomatal closure, ROS production, and [Ca²⁺]_cyt oscillation were enhanced.     

植物ABA受体及其介导的信号转导通路

ABA是调控植物体生长发育和响应外界应激的重要植物激素之一。近年来, ABA受体的筛选和鉴定取得了突破性进展, 为植物中ABA信号转导通路的阐明奠定了重要基础。该文主要综述了ABA-binding protein/H subunit of Mgchelatase (ABAR/CHLH)、G protein-coupled receptor 2 (GCR2)、GPCR-type G protein 1/2 (GTG1/2)和pyrabactin resistant/PYR-like/regulatory component of ABA (PYR/PYL/RCAR)被报道为ABA受体的研究历程, 重点介绍了以ABAR/CHLH PYR/PYL/RCAR为受体的ABA信号转导通路模型的构建, 旨在为ABA受体及其信号转导通路的相关研究提供参考。     

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 Represses Growth of the Arabidopsis Embryonic Axis after Germination by Enhancing Auxin Signaling

Under unfavorable environmental conditions, the stress phytohormone ABA inhibits the developmental transition from an embryo in a dry seed into a young seedling. We developed a genetic screen to isolate Arabidopsis thaliana mutants whose early seedling development is resistant to ABA. Here, we report the identification of a recessive mutation in AUXIN RESISTANT1 (AUX1), encoding a cellular auxin influx carrier. Although auxin is a major morphogenesis hormone in plants, little is known about ABA–auxin interactions during early seedling growth. We show that aux1 and pin2 mutants are insensitive to ABA-dependent repression of embryonic axis (hypocotyl and radicle) elongation. Genetic and physiological experiments show that this involves auxin transport to the embryonic axis elongation zone, where ABA enhances the activity of an auxin-responsive promoter. We propose that ABA represses embryonic axis elongation by potentiating auxin signaling in its elongation zone. This involves repression of the AUXIN INDUCIBLE (Aux/IAA) gene AXR2/IAA7, encoding a key component of ABA- and auxin-dependent responses during postgerminative growth.     

The Barley Magnesium Chelatase 150-kD Subunit Is Not an Abscisic Acid Receptor

Magnesium chelatase is the first unique enzyme of the chlorophyll biosynthetic pathway. It is composed of three gene products of which the largest is 150 kD. This protein was recently identified as an abscisic acid receptor in Arabidopsis (Arabidopsis thaliana). We have evaluated whether the barley (Hordeum vulgare) magnesium chelatase large subunit, XanF, could be a receptor for the phytohormone. The study involved analysis of recombinant magnesium chelatase protein as well as several induced chlorophyll-deficient magnesium chelatase mutants with defects identified at the gene and protein levels. Abscisic acid had no effect on magnesium chelatase activity and binding to the barley 150-kD protein could not be shown. Magnesium chelatase mutants showed a wild-type response in respect to postgermination growth and stomatal aperture. Our results question the function of the large magnesium chelatase subunit as an abscisic acid receptor.     

The Arabidopsis Kelch Repeat F-box E3 Ligase ARKP1 Plays a Positive Role for the Regulation of Abscisic Acid Signaling

Ubiquitination is one of the most common posttranslational modifications. A series of E3 ligases are implicated in plant abiotic stress signaling, regulating the degradation of multiple specific target proteins. Here, we showed that a novel gene ABA-RESPONSE KELCH PROTEIN 1 (AtARKP1), which encodes an F-box subunit of Skp-cullin-F-box (SCF) ubiquitin ligase complex, was localized in the nucleus and could be induced by phytohormone abscisic acid (ABA) in Arabidopsis. ARKP1 interacted with ASK1 and ASK2, which tethered the rest of the complex to an F-box protein, suggesting that they might form an SCF ubiquitin ligase complex. Further analysis revealed that ARKP1 was exclusively expressed in the seed, rosette leaf, and root. arkp1 T-DNA insertion mutant plants were insensitive to ABA, displaying reduced ABA-mediated inhibition of seed germination, root elongation, and water loss rate of detached leaves. In contrast, transgenic plants showed enhanced sensitivity to ABA and tolerance to water deficit. Accordingly, the expressions of ABA and drought responsive marker genes were markedly upregulated in ARKP1 overexpressing plants than the wild-type and arkp1 mutant plants. Taken together, our findings suggest that AtARKP1 plays a positive role in ABA signaling network.