View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Springer - Publisher Connector Plant Mol Biol DOI 10.1007/s11103-012-9965-3 Roles of the different components of magnesium chelatase in abscisic acid signal transduction Shu-Yuan Du • Xiao-Feng Zhang • Zekuan Lu • Qi Xin • Zhen Wu • Tao Jiang • Yan Lu • Xiao-Fang Wang • Da-Peng Zhang Received: 18 May 2012 / Accepted: 26 August 2012 Ó The Author(s) 2012. This article is published with open access at Springerlink.com Abstract The H subunit of Mg-chelatase (CHLH) was not to the other Mg-chelatase components/subunits CHLI, shown to regulate abscisic acid (ABA) signaling and the I CHLD (D subunit) and GUN4. A new rtl1 mutant allele of subunit (CHLI) was also reported to modulate ABA sig- the CHLH gene in Arabidopsis thaliana showed ABA- naling in guard cells. However, it remains essentially insensitive phenotypes in both stomatal movement and unknown whether and how the Mg-chelatase-catalyzed seed germination. Upregulation of CHLI1 resulted in ABA Mg-protoporphyrin IX-production differs from ABA sig- hypersensitivity in seed germination, while downregulation naling. Using a newly-developed surface plasmon reso- of CHLI conferred ABA insensitivity in stomatal response nance system, we showed that ABA binds to CHLH, but in Arabidopsis. We showed that CHLH and CHLI, but not CHLD, regulate stomatal sensitivity to ABA in tobacco (Nicotiana benthamiana). The overexpression lines of the Accession numbers Sequence data from this article can be found CHLD gene showed wild-type ABA sensitivity in Ara- in the Arabidopsis Genome Initiative database under the following bidopsis. Both the GUN4-RNA interference and overex- accession numbers: At5g13630 (CHLH/ABAR), At4g18480 (CHLI1), At5g45930 (CHLI2), At1g08520 (CHLD), and At3g59400 (GUN4). pression lines of Arabidopsis showed wild-type phenotypes Germplasm identification numbers: abi4-1, CS8104; abi5-1, CS8105; in the major ABA responses. These findings provide clear cch, CS6499; ch1-3, CS3121. evidence that the Mg-chelatase-catalyzed Mg-ProtoIX Shu-Yuan Du, Xiao-Feng Zhang and Zekuan Lu contributed equally production is distinct from ABA signaling, giving infor- to this work. mation to understand the mechanism by which the two cellular processes differs at the molecular level. Electronic supplementary material The online version of this article (doi:10.1007/s11103-012-9965-3) contains supplementary material, which is available to authorized users. Keywords H subunit of Mg-chelatase Á I subunit of Mg-chelatase Á D subunit of Mg-chelatase Á GUN4 Á S.-Y. Du Á X.-F. Zhang Á Z. Wu Á X.-F. Wang (&) Á ABA binding Á ABA signal transduction D.-P. Zhang (&) MOE Systems Biology and Bioinformatics Laboratory, School of Life Sciences, Tsinghua University, Beijing 100084, China e-mail: [email protected] Introduction D.-P. Zhang e-mail: [email protected]; Abscisic acid (ABA) is an essential hormone to regulate [email protected] plant growth and development and to control plant adapta- tion to environmental challenges (reviewed in Finkelstein Z. Lu State Key Laboratory of Integrated Management of Pest Insects and Rock 2002; Adie et al. 2007). As one of the highly and Rodents, Institute of Zoology, Chinese Academy of complex plant cell signaling systems, ABA signaling begins Sciences, Beijing 100101, China with signal perception, which triggers downstream signaling cascades to induce the final physiological responses. It Q. Xin Á T. Jiang Á Y. Lu College of Biological Sciences, China Agricultural University, has been believed that the ABA signal is sensed by cells Beijing 100094, China with multiple receptors including plasma membrane and 123 Plant Mol Biol intracellular receptors (Assmann 1994; Finkelstein et al. ABA signaling components with the PYR/PYL/RCAR ABA 2002; Verslues and Zhu 2007). In the past decades, ABA receptors remain to be explored. signal transduction has been extensively studied, and We previously reported that the magnesium-protopor- numerous signaling components, including ABA receptors, phyrin IX (Mg-ProtoIX) chelatase large subunit (Mg- have been identified. These ABA signaling regulators chelatase H subunit CHLH/putative ABA receptor ABAR), involve diverse proteins, which localize to different cellular a chloroplast/plastid protein, binds ABA and functions in compartments including plasma membrane, cytosolic space ABA signaling, thus meeting the essential criteria of a can- and nucleus. Functioning on cell surface, two candidate didate receptor for ABA in Arabidopsis thaliana (Shen et al. plasma membrane ABA receptors—an unconventional 2006; Wu et al. 2009). We further identified a CHLH-med- G-protein-coupled receptor (GPCR) GCR2 and a novel class iated ABA signaling pathway in which CHLH antagonizes a of GPCR-type G proteins GTG1 and GTG2—have been WRKY-domain transcription repressor to relieve ABA- reported (Liu et al. 2007a, b; Johnston et al. 2007; Pandey responsive genes of inhibition (Shang et al. 2010). Although et al. 2009), though it is controversial whether GCR2 regu- the identity of CHLH as an ABA receptor is controversial lates ABA-mediated inhibition of seed germination and post- (Mu¨ller and Hansson 2009; Tsuzuki et al. 2011), we provide germination growth (Gao et al. 2007; Guo et al. 2008). GTGs multiple lines of evidence to show that CHLH binds ABA are positive regulators of ABA signaling and interacts with (Shen et al. 2006; Wu et al. 2009; Wang et al. 2011) on the the sole Arabidopsis G protein a subunit GPA1 (Pandey and one hand, and on the other, consistent with our observations Assmann 2004), which may negatively regulate ABA sig- (Shen et al. 2006; Wu et al. 2009; Shang et al. 2010), evi- naling by inhibiting the activity of GTG-ABA binding dence from independent groups reveals that CHLH mediates (Pandey et al. 2009). Many other membrane-associated ABA signaling in guard cells of both Arabidopsis (Legnaioli proteins, such as phospholipases C/D (Fan et al. 1997; San- et al. 2009; Tsuzuki et al. 2011) and peach (Prunus persica) chez and Chua 2001; Zhang et al. 2004, 2009), other GPCR leaves (Jia et al. 2011a). Also, it has been demonstrated that members (such as GCR1) and G proteins (Wang et al. 2001; CHLH is a key component connecting the circadian clock Pandey and Assmann 2004; Pandey et al. 2006), and recep- with ABA-mediated plant drought responses in Arabidopsis tor-like kinases (Osakabe et al. 2005), have been reported to (Legnaioli et al. 2009) and mediates ABA signaling in fruit be involved in ABA signaling. However, whether these ripening of both peach (Jia et al. 2011a) and strawberry plasma membrane-localized proteins cooperates with (Fragaria ananassa, Jia et al. 2011b). These data consis- plasma membrane ABA receptors to regulate early events of tently demonstrate that CHLH is an essential ABA signaling ABA signaling processes on the cell surface remains an regulator in plant cells. interesting and open question. CHLH has multiple functions in plant cells. One of its Intracellular ABA signaling regulators involve numerous functions is to chelate magnesium to protoporphyrin IX, proteins of diverse identities such as various protein kinases, which provides Mg-ProtoIX in the chlorophyll biosynthesis type-2C/A protein phosphatases (PP2C/A), ubiquitin E3 pathway (Gibson et al. 1996; Willows et al. 1996; Walker ligases involved in degradation of ABA signaling proteins, and Willows 1997; Guo et al. 1998; Papenbrock et al. and various classes of transcription factors (for reviews, see 2000). The second role of CHLH is to mediate plastid-to- Shinozaki et al. 2003; Fan et al. 2004; Seki et al. 2007; Cutler nucleus retrograde signaling, known as Genomes Uncou- et al. 2010). Most recently, PYR/PYL/RCAR proteins, the pled 5 (GUN5), and this function in the retrograde sig- members of a START domain superfamily, were reported to naling may be connected with its role in catalyzing function as cytosolic ABA receptors by inhibiting directly production of Mg-ProtoIX (Mochizuki et al. 2001; Nott type 2C protein phosphatases (Ma et al. 2009; Park et al. et al. 2006). It has been well established that Mg-chelatase 2009; Santiago et al. 2009). A PYL/PYR/RCAR-mediated functions in catalyzing Mg-ProtoIX production as a hetero- ABA signaling pathway from ABA perception to down- tetramer, which is composed of Mg-chelatase subunits H, I stream gene expression has been reconstituted in vitro (Fujii (CHLI), D (CHLD) (Gibson et al. 1996; Willows et al. et al. 2009; Cutler et al. 2010). In this PYR/PYL/RCAR- 1996; Walker and Willows 1997; Guo et al. 1998; Papen- mediated ABA signaling pathway, PP2Cs relay ABA signal brock et al. 2000) and a supplementary and essential directly from the PYR/PYL/RCAR ABA receptors to their component GUN4 (Genomes Uncoupled 4) that binds downstream regulators SNF1-related protein kinase 2s CHLH and activates Mg-chelatase (Larkin et al. 2003; (SnRK2s), which activate an ABF/AREB/ABI5 clade of Peter and Grimm 2009; Adhikari et al. 2011). A recent bZIP-domain transcription factors via a protein phosphory- report showed that, besides CHLH, CHLI also mediates lation process, and finally induce physiological ABA guard cell signaling in response to ABA (Tsuzuki et al. responses (Fujii et al. 2009; Cutler et al. 2010). However, it is 2011). However, it remains essentially unknown whether widely believed that the networks of ABA signaling path- Mg-chelatase heterotetramer complex or only two subunits ways are highly complex, and connections of other numerous CHLH and CHLI function in ABA signaling, and why the 123 Plant Mol Biol Mg-ProtoIX production process may differ from the both CHLD and CHLI1 in the yeast two-hybrid system CHLH-mediated ABA signaling. To explore this mecha- (Fig. 1a, b, d). The different negative controls showed no nism is of importance to understanding complex ABA interaction signal (Fig.