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Sze H, Chanroj S. Plant Endomembrane Dynamics: Studies of K +/H + Antiporters Provide Insights on the Effects of pH and Ion Homeostasis. Plant Physiol 2018; 177:875-895. [PMID: 29691301 PMCID: PMC6053008 DOI: 10.1104/pp.18.00142] [Citation(s) in RCA: 101] [Impact Index Per Article: 16.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/09/2018] [Accepted: 04/04/2018] [Indexed: 05/17/2023]
Abstract
Plants remodel their cells through the dynamic endomembrane system. Intracellular pH is important for membrane trafficking, but the determinants of pH homeostasis are poorly defined in plants. Electrogenic proton (H+) pumps depend on counter-ion fluxes to establish transmembrane pH gradients at the plasma membrane and endomembranes. Vacuolar-type H+-ATPase-mediated acidification of the trans-Golgi network is crucial for secretion and membrane recycling. Pump and counter-ion fluxes are unlikely to fine-tune pH; rather, alkali cation/H+ antiporters, which can alter pH and/or cation homeostasis locally and transiently, are prime candidates. Plants have a large family of predicted cation/H+ exchangers (CHX) of obscure function, in addition to the well-studied K+(Na+)/H+ exchangers (NHX). Here, we review the regulation of cytosolic and vacuolar pH, highlighting the similarities and distinctions of NHX and CHX members. In planta, alkalinization of the trans-Golgi network or vacuole by NHXs promotes membrane trafficking, endocytosis, cell expansion, and growth. CHXs localize to endomembranes and/or the plasma membrane and contribute to male fertility, pollen tube guidance, pollen wall construction, stomatal opening, and, in soybean (Glycine max), tolerance to salt stress. Three-dimensional structural models and mutagenesis of Arabidopsis (Arabidopsis thaliana) genes have allowed us to infer that AtCHX17 and AtNHX1 share a global architecture and a translocation core like bacterial Na+/H+ antiporters. Yet, the presence of distinct residues suggests that some CHXs differ from NHXs in pH sensing and electrogenicity. How H+ pumps, counter-ion fluxes, and cation/H+ antiporters are linked with signaling and membrane trafficking to remodel membranes and cell walls awaits further investigation.
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Affiliation(s)
- Heven Sze
- Department of Cell Biology and Molecular Genetics and Department of Plant Science and Landscape Architecture, University of Maryland, College Park, Maryland 20742
- College of Life Sciences, Zhejiang University, Hangzhou 310058, China
| | - Salil Chanroj
- Department of Biotechnology, Burapha University, Chon-Buri 20131, Thailand
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Padmanaban S, Czerny DD, Levin KA, Leydon AR, Su RT, Maugel TK, Zou Y, Chanroj S, Cheung AY, Johnson MA, Sze H. Transporters involved in pH and K+ homeostasis affect pollen wall formation, male fertility, and embryo development. J Exp Bot 2017; 68:3165-3178. [PMID: 28338823 PMCID: PMC5853877 DOI: 10.1093/jxb/erw483] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/05/2016] [Accepted: 12/01/2016] [Indexed: 05/20/2023]
Abstract
Flowering plant genomes encode multiple cation/H+ exchangers (CHXs) whose functions are largely unknown. AtCHX17, AtCHX18, and AtCHX19 are membrane transporters that modulate K+ and pH homeostasis and are localized in the dynamic endomembrane system. Loss of function reduced seed set, but the particular phase(s) of reproduction affected was not determined. Pollen tube growth and ovule targeting of chx17chx18chx19 mutant pollen appeared normal, but reciprocal cross experiments indicate a largely male defect. Although triple mutant pollen tubes reach ovules of a wild-type pistil and a synergid cell degenerated, half of those ovules were unfertilized or showed fertilization of the egg or central cell, but not both female gametes. Fertility could be partially compromised by impaired pollen tube and/or sperm function as CHX19 and CHX18 are expressed in the pollen tube and sperm cell, respectively. When fertilization was successful in self-pollinated mutants, early embryo formation was retarded compared with embryos from wild-type ovules receiving mutant pollen. Thus CHX17 and CHX18 proteins may promote embryo development possibly through the endosperm where these genes are expressed. The reticulate pattern of the pollen wall was disorganized in triple mutants, indicating perturbation of wall formation during male gametophyte development. As pH and cation homeostasis mediated by AtCHX17 affect membrane trafficking and cargo delivery, these results suggest that male fertility, sperm function, and embryo development are dependent on proper cargo sorting and secretion that remodel cell walls, plasma membranes, and extracellular factors.
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Affiliation(s)
- Senthilkumar Padmanaban
- Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, MD, USA
| | - Daniel D Czerny
- Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, MD, USA
| | - Kara A Levin
- Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, MD, USA
| | - Alexander R Leydon
- Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, RI, USA
| | - Robert T Su
- Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, MD, USA
| | - Timothy K Maugel
- Laboratory for Biological Ultrastructure, University of Maryland, College Park, MD, USA
| | - Yanjiao Zou
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA, USA
| | - Salil Chanroj
- Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, MD, USA
- Department of Biotechnology, Burapha University, Chon-Buri, Thailand
| | - Alice Y Cheung
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA, USA
| | - Mark A Johnson
- Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, RI, USA
| | - Heven Sze
- Department of Cell Biology & Molecular Genetics, University of Maryland, College Park, MD, USA
- Department of Plant Science and Landscape Architecture, University of Maryland, College Park, MD, USA
- Maryland Agricultural Experiment Station, University of Maryland, College Park, MD, USA
- Correspondence:
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Chanroj S, Padmanaban S, Czerny DD, Jauh GY, Sze H. K+ transporter AtCHX17 with its hydrophilic C tail localizes to membranes of the secretory/endocytic system: role in reproduction and seed set. Mol Plant 2013; 6:1226-46. [PMID: 23430044 DOI: 10.1093/mp/sst032] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The importance of sorting proteins and wall materials to their destination is critical for plant growth and development, though the machinery orchestrating membrane trafficking is poorly understood. Transporters that alter the environment across endomembrane compartments are thought to be important players. Using Escherichia coli and yeast, we previously showed that several Arabidopsis Cation/H(+) eXchanger (AtCHX) members were K(+) transporters with a role in pH homeostasis, though their subcellular location and biological roles in plants are unclear. Co-expression of markers with CHX16, CHX17, CHX18, or CHX19 tagged with a fluorescent protein indicated these transporters associated with plasma membrane (PM) and post-Golgi compartments. Under its native promoter, AtCHX17(1-820)-GFP localized to prevacuolar compartment (PVC) and to PM in roots. Brefeldin A diminished AtCHX17-GFP fluorescence at PM, whereas wortmannin caused formation of GFP-labeled ring-like structures, suggesting AtCHX17 trafficked among PVC, vacuole and PM. AtCHX17(1-472) lacking its carboxylic tail did not associate with PVC or PM in plant cells. Single chx17 mutant or higher-order mutants showed normal root growth and vegetative development. However, quadruple (chx16chx17chx18chx19) mutants were reduced in frequency and produced 50%-70% fewer seeds, indicating overlapping roles of several AtCHX17-related transporters in reproduction and/or seed development. Together, our results suggest that successful reproduction and seed development depend on the ability to regulate cation and pH homeostasis by AtCHX17-like transporters on membranes that traffic in the endocytic and/or secretory pathways.
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Affiliation(s)
- Salil Chanroj
- Department of Cell Biology and Molecular Genetics, and Maryland Agricultural Experiment Station, University of Maryland, College Park, MD, USA
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Aranda-Sicilia MN, Cagnac O, Chanroj S, Sze H, Rodríguez-Rosales MP, Venema K. Arabidopsis KEA2, a homolog of bacterial KefC, encodes a K(+)/H(+) antiporter with a chloroplast transit peptide. Biochim Biophys Acta 2012; 1818:2362-71. [PMID: 22551943 DOI: 10.1016/j.bbamem.2012.04.011] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2012] [Revised: 04/10/2012] [Accepted: 04/16/2012] [Indexed: 01/10/2023]
Abstract
KEA genes encode putative K(+) efflux antiporters that are predominantly found in algae and plants but are rare in metazoa; however, nothing is known about their functions in eukaryotic cells. Plant KEA proteins show homology to bacterial K(+) efflux (Kef) transporters, though two members in the Arabidopsis thaliana family, AtKEA1 and AtKEA2, have acquired an extra hydrophilic domain of over 500 residues at the amino terminus. We show that AtKEA2 is highly expressed in leaves, stems and flowers, but not in roots, and that an N-terminal peptide of the protein is targeted to chloroplasts in Arabidopsis cotyledons. The full-length AtKEA2 protein was inactive when expressed in yeast; however, a truncated AtKEA2 protein (AtsKEA2) lacking the N-terminal domain complemented disruption of the Na(+)(K(+))/H(+) antiporter Nhx1p to confer hygromycin resistance and tolerance to Na(+) or K(+) stress. To test transport activity, purified truncated AtKEA2 was reconstituted in proteoliposomes containing the fluorescent probe pyranine. Monovalent cations reduced an imposed pH gradient (acid inside) indicating AtsKEA2 mediated cation/H(+) exchange with preference for K(+)=Cs(+)>Li(+)>Na(+). When a conserved Asp(721) in transmembrane helix 6 that aligns to the cation binding Asp(164) of Escherichia coli NhaA was replaced with Ala, AtsKEA2 was completely inactivated. Mutation of a Glu(835) between transmembrane helix 8 and 9 in AtsKEA2 also resulted in loss of activity suggesting this region has a regulatory role. Thus, AtKEA2 represents the founding member of a novel group of eukaryote K(+)/H(+) antiporters that modulate monovalent cation and pH homeostasis in plant chloroplasts or plastids.
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Chanroj S, Wang G, Venema K, Zhang MW, Delwiche CF, Sze H. Conserved and diversified gene families of monovalent cation/h(+) antiporters from algae to flowering plants. Front Plant Sci 2012; 3:25. [PMID: 22639643 PMCID: PMC3355601 DOI: 10.3389/fpls.2012.00025] [Citation(s) in RCA: 141] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/20/2011] [Accepted: 01/21/2012] [Indexed: 05/18/2023]
Abstract
All organisms have evolved strategies to regulate ion and pH homeostasis in response to developmental and environmental cues. One strategy is mediated by monovalent cation-proton antiporters (CPA) that are classified in two superfamilies. Many CPA1 genes from bacteria, fungi, metazoa, and plants have been functionally characterized; though roles of plant CPA2 genes encoding K(+)-efflux antiporter (KEA) and cation/H(+) exchanger (CHX) families are largely unknown. Phylogenetic analysis showed that three clades of the CPA1 Na(+)-H(+) exchanger (NHX) family have been conserved from single-celled algae to Arabidopsis. These are (i) plasma membrane-bound SOS1/AtNHX7 that share ancestry with prokaryote NhaP, (ii) endosomal AtNHX5/6 that is part of the eukaryote Intracellular-NHE clade, and (iii) a vacuolar NHX clade (AtNHX1-4) specific to plants. Early diversification of KEA genes possibly from an ancestral cyanobacterium gene is suggested by three types seen in all plants. Intriguingly, CHX genes diversified from three to four members in one subclade of early land plants to 28 genes in eight subclades of Arabidopsis. Homologs from Spirogyra or Physcomitrella share high similarity with AtCHX20, suggesting that guard cell-specific AtCHX20 and its closest relatives are founders of the family, and pollen-expressed CHX genes appeared later in monocots and early eudicots. AtCHX proteins mediate K(+) transport and pH homeostasis, and have been localized to intracellular and plasma membrane. Thus KEA genes are conserved from green algae to angiosperms, and their presence in red algae and secondary endosymbionts suggest a role in plastids. In contrast, AtNHX1-4 subtype evolved in plant cells to handle ion homeostasis of vacuoles. The great diversity of CHX genes in land plants compared to metazoa, fungi, or algae would imply a significant role of ion and pH homeostasis at dynamic endomembranes in the vegetative and reproductive success of flowering plants.
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Affiliation(s)
- Salil Chanroj
- Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of MarylandCollege Park, MD, USA
| | - Guoying Wang
- Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of MarylandCollege Park, MD, USA
| | - Kees Venema
- Departmento de Bioquímica, Biología Celular y Molecular de Plantas, Estación Experimental del Zaidín, Consejo Superior de Investigaciones CientíficasGranada, Spain
| | - Muren Warren Zhang
- Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of MarylandCollege Park, MD, USA
| | - Charles F. Delwiche
- Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of MarylandCollege Park, MD, USA
| | - Heven Sze
- Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of MarylandCollege Park, MD, USA
- *Correspondence: Heven Sze, Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of Maryland, Bioscience Research Building # 413, College Park, MD 20742, USA. e-mail:
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Chanroj S, Lu Y, Padmanaban S, Nanatani K, Uozumi N, Rao R, Sze H. Plant-specific cation/H+ exchanger 17 and its homologs are endomembrane K+ transporters with roles in protein sorting. J Biol Chem 2011; 286:33931-41. [PMID: 21795714 PMCID: PMC3190763 DOI: 10.1074/jbc.m111.252650] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2011] [Revised: 07/08/2011] [Indexed: 11/30/2022] Open
Abstract
The complexity of intracellular compartments in eukaryotic cells evolved to provide distinct environments to regulate processes necessary for cell proliferation and survival. A large family of predicted cation/proton exchangers (CHX), represented by 28 genes in Arabidopsis thaliana, are associated with diverse endomembrane compartments and tissues in plants, although their roles are poorly understood. We expressed a phylogenetically related cluster of CHX genes, encoded by CHX15-CHX20, in yeast and bacterial cells engineered to lack multiple cation-handling mechanisms. Of these, CHX16-CHX20 were implicated in pH homeostasis because their expression rescued the alkaline pH-sensitive growth phenotype of the host yeast strain. A smaller subset, CHX17-CHX19, also conferred tolerance to hygromycin B. Further differences were observed in K(+)- and low pH-dependent growth phenotypes. Although CHX17 did not alter cytoplasmic or vacuolar pH in yeast, CHX20 elicited acidification and alkalization of the cytosol and vacuole, respectively. Using heterologous expression in Escherichia coli strains lacking K(+) uptake systems, we provide evidence for K(+) ((86)Rb) transport mediated by CHX17 and CHX20. Finally, we show that CHX17 and CHX20 affected protein sorting as measured by carboxypeptidase Y secretion in yeast mutants grown at alkaline pH. In plant cells, CHX20-RFP co-localized with an endoplasmic reticulum marker, whereas RFP-tagged CHX17-CHX19 co-localized with prevacuolar compartment and endosome markers. Together, these results suggest that in response to environmental cues, multiple CHX transporters differentially modulate K(+) and pH homeostasis of distinct intracellular compartments, which alter membrane trafficking events likely to be critical for adaptation and survival.
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Affiliation(s)
- Salil Chanroj
- From the Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
| | - Yongxian Lu
- From the Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
| | - Senthilkumar Padmanaban
- From the Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
| | - Kei Nanatani
- the Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
| | - Nobuyuki Uozumi
- the Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
| | - Rajini Rao
- the Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, and
| | - Heven Sze
- From the Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
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Lu Y, Chanroj S, Zulkifli L, Johnson MA, Uozumi N, Cheung A, Sze H. Pollen tubes lacking a pair of K+ transporters fail to target ovules in Arabidopsis. Plant Cell 2011; 23:81-93. [PMID: 21239645 PMCID: PMC3051242 DOI: 10.1105/tpc.110.080499] [Citation(s) in RCA: 120] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/17/2023]
Abstract
Flowering plant reproduction requires precise delivery of the sperm cells to the ovule by a pollen tube. Guidance signals from female cells are being identified; however, how pollen responds to those cues is largely unknown. Here, we show that two predicted cation/proton exchangers (CHX) in Arabidopsis thaliana, CHX21 and CHX23, are essential for pollen tube guidance. Male fertility was unchanged in single chx21 or chx23 mutants. However, fertility was impaired in chx21 chx23 double mutant pollen. Wild-type pistils pollinated with a limited number of single and double mutant pollen producing 62% fewer seeds than those pollinated with chx23 single mutant pollen, indicating that chx21 chx23 pollen is severely compromised. Double mutant pollen grains germinated and grew tubes down the transmitting tract, but the tubes failed to turn toward ovules. Furthermore, chx21 chx23 pollen tubes failed to enter the micropyle of excised ovules. Green fluorescent protein-tagged CHX23 driven by its native promoter was localized to the endoplasmic reticulum of pollen tubes. CHX23 mediated K(+) transport, as CHX23 expression in Escherichia coli increased K(+) uptake and growth in a pH-dependent manner. We propose that by modifying localized cation balance and pH, these transporters could affect steps in signal reception and/or transduction that are critical to shifting the axis of polarity and directing pollen growth toward the ovule.
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Affiliation(s)
- Yongxian Lu
- Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
| | - Salil Chanroj
- Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
| | - Lalu Zulkifli
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
| | - Mark A. Johnson
- Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, Rhode Island 02912
| | - Nobuyuki Uozumi
- Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
| | - Alice Cheung
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003
| | - Heven Sze
- Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742
- Address correspondence to
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Li X, Chanroj S, Wu Z, Romanowsky SM, Harper JF, Sze H. A distinct endosomal Ca2+/Mn2+ pump affects root growth through the secretory process. Plant Physiol 2008; 147:1675-89. [PMID: 18567829 PMCID: PMC2492598 DOI: 10.1104/pp.108.119909] [Citation(s) in RCA: 84] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/10/2008] [Accepted: 06/14/2008] [Indexed: 05/17/2023]
Abstract
Ca(2+) is required for protein processing, sorting, and secretion in eukaryotic cells, although the particular roles of the transporters involved in the secretory system of plants are obscure. One endomembrane-type Ca-ATPase from Arabidopsis (Arabidopsis thaliana), AtECA3, diverges from AtECA1, AtECA2, and AtECA4 in protein sequence; yet, AtECA3 appears similar in transport activity to the endoplasmic reticulum (ER)-bound AtECA1. Expression of AtECA3 in a yeast (Saccharomyces cerevisiae) mutant defective in its endogenous Ca(2+) pumps conferred the ability to grow on Ca(2+)-depleted medium and tolerance to toxic levels of Mn(2+). A green fluorescent protein-tagged AtECA3 was functionally competent and localized to intracellular membranes of yeast, suggesting that Ca(2+) and Mn(2+) loading into internal compartment(s) enhanced yeast proliferation. In mesophyll protoplasts, AtECA3-green fluorescent protein associated with a subpopulation of endosome/prevacuolar compartments based on partial colocalization with the Ara7 marker. Interestingly, three independent eca3 T-DNA disruption mutants showed severe reduction in root growth normally stimulated by 3 mm Ca(2+), indicating that AtECA3 function cannot be replaced by an ER-associated AtECA1. Furthermore, root growth of mutants is sensitive to 50 microm Mn(2+), indicating that AtECA3 is also important for the detoxification of excess Mn(2+). Curiously, Ateca3 mutant roots produced 65% more apoplastic protein than wild-type roots, as monitored by peroxidase activity, suggesting that the secretory process was altered. Together, these results demonstrate that the role of AtECA3 is distinct from that of the more abundant ER AtECA1. AtECA3 supports Ca(2+)-stimulated root growth and the detoxification of high Mn(2+), possibly through activities mediated by post-Golgi compartments that coordinate membrane traffic and sorting of materials to the vacuole and the cell wall.
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Affiliation(s)
- Xiyan Li
- Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742-5815, USA
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Padmanaban S, Chanroj S, Kwak JM, Li X, Ward JM, Sze H. Participation of endomembrane cation/H+ exchanger AtCHX20 in osmoregulation of guard cells. Plant Physiol 2007; 144:82-93. [PMID: 17337534 PMCID: PMC1913804 DOI: 10.1104/pp.106.092155] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/14/2023]
Abstract
Guard cell movement is induced by environmental and hormonal signals that cause changes in turgor through changes in uptake or release of solutes and water. Several transporters mediating these fluxes at the plasma membrane have been characterized; however, less is known about transport at endomembranes. CHX20, a member of a poorly understood cation/H+ exchanger gene family in Arabidopsis (Arabidopsis thaliana), is preferentially and highly expressed in guard cells as shown by promoterbeta-glucuronidase activity and by whole-genome microarray. Interestingly, three independent homozygous mutants carrying T-DNA insertions in CHX20 showed 35% reduction in light-induced stomatal opening compared to wild-type plants. To test the biochemical function of CHX20, cDNA was expressed in a yeast (Saccharomyces cerevisiae) mutant that lacks Na+(K+)/H+ antiporters (Deltanhx1 Deltanha1 Deltakha1) and plasma membrane Na+ pumps (Deltaena1-4). Curiously, CHX20 did not enhance tolerance of mutants to moderate Na+ or high K+ stress. Instead, it restored growth of the mutant on medium with low K+ at slightly alkaline pH, but had no effect on growth at acidic pH. Green fluorescent protein-tagged CHX20 expressed in mesophyll protoplasts was localized mainly to membranes of the endosomal system. Furthermore, light-induced stomatal opening of the Arabidopsis mutants was insensitive to external pH and was impaired at high KCl. The results are consistent with the idea that, in exchanging K+ for H+, CHX20 maintains K+ homeostasis and influences pH under certain conditions. Together, these results provide genetic and biochemical evidence that one CHX protein plays a critical role in osmoregulation through K+ fluxes and possibly pH modulation of an active endomembrane system in guard cells.
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Affiliation(s)
- Senthilkumar Padmanaban
- Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD 20742-5815, USA
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