201
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Lee YJ, Szumlanski A, Nielsen E, Yang Z. Rho-GTPase-dependent filamentous actin dynamics coordinate vesicle targeting and exocytosis during tip growth. J Cell Biol 2008; 181:1155-68. [PMID: 18591430 PMCID: PMC2442199 DOI: 10.1083/jcb.200801086] [Citation(s) in RCA: 172] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2008] [Accepted: 06/02/2008] [Indexed: 01/09/2023] Open
Abstract
The dynamic activity of tip-localized filamentous actin (F-actin) in pollen tubes is controlled by counteracting RIC4 and RIC3 pathways downstream of the ROP1 guanosine triphosphatase promoting actin assembly and disassembly, respectively. We show here that ROP1 activation is required for both the polar accumulation and the exocytosis of vesicles at the plasma membrane apex. The apical accumulation of exocytic vesicles oscillated in phase with, but slightly behind, apical actin assembly and was enhanced by overexpression of RIC4. However, RIC4 overexpression inhibited exocytosis, and this inhibition could be suppressed by latrunculin B treatment or RIC3 overexpression. We conclude that RIC4-dependent actin assembly is required for polar vesicle accumulation, whereas RIC3-mediated actin disassembly is required for exocytosis. Thus ROP1-dependent F-actin dynamics control tip growth through spatiotemporal coordination of vesicle targeting and exocytosis.
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Affiliation(s)
- Yong Jik Lee
- Department of Botany and Plant Sciences, Institute for Integrative Genome Biology, University of California, Riverside, Riverside, CA 92521, USA
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202
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Schultheiss H, Preuss J, Pircher T, Eichmann R, Hückelhoven R. Barley RIC171 interacts with RACB in planta and supports entry of the powdery mildew fungus. Cell Microbiol 2008; 10:1815-26. [PMID: 18466344 DOI: 10.1111/j.1462-5822.2008.01167.x] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
RHO-like GTPases of plants (ROPs, also called RACs) are involved in plant development and interaction with the environment. The barley ROP protein RACB is involved in susceptibility to the fungal pathogen Blumeria graminis f.sp. hordei (Bgh). By screening barley sequence databases for potential protein interactors of plant RHO-like proteins, we identified a ROP-interactive CRIB (CDC42/RAC interactive binding) motif containing protein of 171 amino acids (RIC171). The protein interacted with constitutively activated RACB in a targeted yeast two-hybrid assay. By use of split yellow fluorescing protein fusions, we demonstrated that RIC171 interacts with constitutively activated (CA) RACB-G15V but not with dominant negative RACB-T20N in planta. Transient overexpression of RIC171, similar to overexpression of CA RACB-G15V, rendered epidermal cells more susceptible to penetration by Bgh. In contrast, expression of a 46-amino-acid RIC171-CRIB peptide, which was sufficient to interact with CA RACB-G15V, had a dominant negative effect and reduced susceptibility to Bgh. A red fluorescing DsRED-RIC171 fusion protein colocalized with green fluorescing GFP-RACB-G15V at the cell periphery. Coexpression with CA RACB-G15V but not with RACB-T20N increased peripheral localization of DsRED-RIC171. Additionally, DsRED-RIC171 accumulated at sites of fungal attack, suggesting enhanced ROP activity at sites of attempted fungal penetration.
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Affiliation(s)
- Holger Schultheiss
- Lehrstuhl für Phytopathologie, Technische Universität München, Freising, Germany
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203
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Zhang Y, McCormick S. Regulation of pollen tube polarity: Feedback loops rule. PLANT SIGNALING & BEHAVIOR 2008; 3:345-7. [PMID: 19841667 PMCID: PMC2634279 DOI: 10.4161/psb.3.5.5353] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2007] [Accepted: 11/30/2007] [Indexed: 05/25/2023]
Abstract
Targeted delivery of immotile sperm through growing pollen tubes is a crucial step in achieving sexual reproduction in angiosperms. Unlike diffuse-growing cells, the growth of a pollen tube is restricted to the very apical region where targeted exocytosis and regulated endocytosis occur. The plant-specific Rho GTPases, Rops, are central organizers for pollen tube polarity. Through effector binding, Rops regulate the tip-focused Ca(2+) gradient and the actin cytoskeleton during pollen tube growth. Therefore, understanding the spatiotemporal regulation of Rop activity would reveal how establishment and maintenance of tube polarity as well as re-orientation of the growth axis are accomplished. Recent findings indicated that two feedback loops may be fundamental in maintaining a fine-tuned and dynamic Rop activity. The concerted activities of RopGAP and RopGDI prevent lateral diffusion of activated Rop, restricting Rop activity to the apical plasma membrane. Conversely, pollen receptor kinases (PRKs) and RopGEFs positively feedback regulate Rop activity through protein binding and membrane recruitment. Feedback loops would also be essential for pollen tube re-orientation. Shallow extracellular cues amplified by concerted activities of feedback loops would lead to asymmetric activation of Rop and result in tube re-orientation.
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Affiliation(s)
- Yan Zhang
- Plant Gene Expression Center; United States Department of Agriculture/Agricultural Research Service, and Department of Plant and Microbial Biology; University of California at Berkeley; Berkley, California USA
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204
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Cárdenas L, Lovy-Wheeler A, Kunkel JG, Hepler PK. Pollen tube growth oscillations and intracellular calcium levels are reversibly modulated by actin polymerization. PLANT PHYSIOLOGY 2008; 146:1611-21. [PMID: 18263780 PMCID: PMC2287337 DOI: 10.1104/pp.107.113035] [Citation(s) in RCA: 80] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/09/2007] [Accepted: 02/04/2008] [Indexed: 05/17/2023]
Abstract
Prevention of actin polymerization with low concentrations of latrunculin B (Lat-B; 2 nm) exerts a profound inhibitory effect on pollen tube growth. Using flow-through chambers, we show that growth retardation starts after 10 min treatment with 2 nm Lat-B, and by 15 to 20 min reaches a basal rate of 0.1 to 0.2 microm/s, during which the pollen tube exhibits relatively few oscillations. If treated for 30 min, complete stoppage of growth can occur. Studies on the intracellular Ca(2+) concentration indicate that the tip-focused gradient declines in parallel with the inhibition of growth. Tubes exhibiting nonoscillating growth display a similarly reduced and nonoscillating Ca(2+) gradient. Studies on the pH gradient indicate that Lat-B eliminates the acidic domain at the extreme apex, and causes the alkaline band to move more closely to the tip. Removing Lat-B and returning the cells to control medium reverses these effects. Phalloidin staining of F-actin reveals that 2 nm Lat-B degrades the cortical fringe; it also disorganizes the microfilaments in the shank causing the longitudinally oriented elements to be disposed in swirls. Cytoplasmic streaming continues under these conditions, however the clear zone is obliterated with all organelles moving into and through the extreme apex of the tube. We suggest that actin polymerization promotes pollen tube growth through extension of the cortical actin fringe, which serves as a track to target cell wall vesicles to preferred exocytotic sites on the plasma membrane.
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Affiliation(s)
- Luis Cárdenas
- Departamento de Biología Molecular de Plantas Instituto de Biotecnología, Universidad Nacional Autónoma de México, Cuernavaca, Morelos 62271, Mexico.
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205
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Berken A, Wittinghofer A. Structure and function of Rho-type molecular switches in plants. PLANT PHYSIOLOGY AND BIOCHEMISTRY : PPB 2008; 46:380-93. [PMID: 18272378 DOI: 10.1016/j.plaphy.2007.12.008] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2007] [Indexed: 05/20/2023]
Abstract
Molecular switches of the Rho family, in concert with their associated regulators and effectors are well known as important control elements of vital signaling pathways in eucaryotic organisms. Yet, this knowledge has so far been established mainly from animal and fungal studies. However, during the recent years, the Rho switch has gone increasingly green as well, and it turned out that the homologous system in plants holds some distinctive features regarding structures, functions and molecular mechanisms for signal transduction. In this review, we give an overview about the structural characteristics of the Rho proteins of plants, termed ROP, highlighting some exciting differences to their animal and fungal counterparts. We further address the unique regulators and effectors of the ROPs and discuss the structural basis for the function and interaction of those proteins in ROP controlled reaction cascades. We finally intend to stimulate the demand for future three-dimensional structures that advance our understanding of the ROP switch in plants.
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Affiliation(s)
- Antje Berken
- Max Planck Institute of Molecular Physiology, Structural Biology Department, Otto Hahn Strasse 11, 44227 Dortmund, Germany.
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206
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Certal AC, Almeida RB, Carvalho LM, Wong E, Moreno N, Michard E, Carneiro J, Rodriguéz-Léon J, Wu HM, Cheung AY, Feijó JA. Exclusion of a proton ATPase from the apical membrane is associated with cell polarity and tip growth in Nicotiana tabacum pollen tubes. THE PLANT CELL 2008; 20:614-34. [PMID: 18364468 PMCID: PMC2329945 DOI: 10.1105/tpc.106.047423] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/18/2006] [Revised: 02/13/2008] [Accepted: 02/29/2008] [Indexed: 05/18/2023]
Abstract
Polarized growth in pollen tubes results from exocytosis at the tip and is associated with conspicuous polarization of Ca(2+), H(+), K(+), and Cl(-) -fluxes. Here, we show that cell polarity in Nicotiana tabacum pollen is associated with the exclusion of a novel pollen-specific H(+)-ATPase, Nt AHA, from the growing apex. Nt AHA colocalizes with extracellular H(+) effluxes, which revert to influxes where Nt AHA is absent. Fluorescence recovery after photobleaching analysis showed that Nt AHA moves toward the apex of growing pollen tubes, suggesting that the major mechanism of insertion is not through apical exocytosis. Nt AHA mRNA is also excluded from the tip, suggesting a mechanism of polarization acting at the level of translation. Localized applications of the cation ionophore gramicidin A had no effect where Nt AHA was present but acidified the cytosol and induced reorientation of the pollen tube where Nt AHA was absent. Transgenic pollen overexpressing Nt AHA-GFP developed abnormal callose plugs accompanied by abnormal H(+) flux profiles. Furthermore, there is no net flux of H(+) in defined patches of membrane where callose plugs are to be formed. Taken together, our results suggest that proton dynamics may underlie basic mechanisms of polarity and spatial regulation in growing pollen tubes.
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Affiliation(s)
- Ana C Certal
- Instituto Gulbenkian de Ciência, Centro de Biologia do Desenvolvimento, 2780-156 Oeiras, Portugal
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207
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Molendijk AJ, Ruperti B, Singh MK, Dovzhenko A, Ditengou FA, Milia M, Westphal L, Rosahl S, Soellick TR, Uhrig J, Weingarten L, Huber M, Palme K. A cysteine-rich receptor-like kinase NCRK and a pathogen-induced protein kinase RBK1 are Rop GTPase interactors. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2008; 53:909-923. [PMID: 18088316 DOI: 10.1111/j.1365-313x.2007.03384.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
In plants, Rop/Rac GTPases have emerged as central regulators of diverse signalling pathways in plant growth and pathogen defence. When active, they interact with a wide range of downstream effectors. Using yeast two-hybrid screening we have found three previously uncharacterized receptor-like protein kinases to be Rop GTPase-interacting molecules: a cysteine-rich receptor kinase, named NCRK, and two receptor-like cytosolic kinases from the Arabidopsis RLCK-VIb family, named RBK1 and RBK2. Uniquely for Rho-family small GTPases, plant Rop GTPases were found to interact directly with the protein kinase domains. Rop4 bound NCRK preferentially in the GTP-bound conformation as determined by flow cytometric fluorescence resonance energy transfer measurements in insect cells. The kinase RBK1 did not phosphorylate Rop4 in vitro, suggesting that the protein kinases are targets for Rop signalling. Bimolecular fluorescence complementation assays demonstrated that Rop4 interacted in vivo with NCRK and RBK1 at the plant plasma membrane. In Arabidopsis protoplasts, NCRK was hyperphosphorylated and partially co-localized with the small GTPase RabF2a in endosomes. Gene expression analysis indicated that the single-copy NCRK gene was relatively upregulated in vasculature, especially in developing tracheary elements. The seven Arabidopsis RLCK-VIb genes are ubiquitously expressed in plant development, and highly so in pollen, as in case of RBK2. We show that the developmental context of RBK1 gene expression is predominantly associated with vasculature and is also locally upregulated in leaves exposed to Phytophthora infestans and Botrytis cinerea pathogens. Our data indicate the existence of cross-talk between Rop GTPases and specific receptor-like kinases through direct molecular interaction.
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Affiliation(s)
- Arthur J Molendijk
- Institute for Biologie II/Botany, Faculty of Biology, Albert-Ludwigs-University Freiburg, Schänzlestrasse 1, 79104 Freiburg, Germany
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208
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Sakai T, Honing HVD, Nishioka M, Uehara Y, Takahashi M, Fujisawa N, Saji K, Seki M, Shinozaki K, Jones MA, Smirnoff N, Okada K, Wasteneys GO. Armadillo repeat-containing kinesins and a NIMA-related kinase are required for epidermal-cell morphogenesis in Arabidopsis. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2008; 53:157-71. [PMID: 17971038 DOI: 10.1111/j.1365-313x.2007.03327.x] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
The involvement of kinesin motor proteins in both cell-tip growth and cell-shape determination has been well characterized in various organisms. However, the functions of kinesins during cell morphogenesis in higher plants remain largely unknown. In the current study, we demonstrate that an armadillo repeat-containing kinesin-related protein, ARMADILLO REPEAT KINESIN1 (ARK1), is involved in root-hair morphogenesis. Microtubule polymers are more abundant in ark1 null allele root hairs, but analysis shows that these extra microtubules are concentrated in the endoplasm, and not in the cortical array, suggesting that ARK1 regulates tip growth by limiting the assembly and distribution of endoplasmic microtubules. The ARK1 gene has two homologues in the Arabidopsis genome, ARK2 and ARK3, and our results show that ARK2 is involved in root-cell morphogenesis. We further reveal that a NIMA-related protein kinase, NEK6, binds to the ARK family proteins and has pleiotropic effects on epidermal-cell morphogenesis, suggesting that NEK6 is involved in cell morphogenesis in Arabidopsis via microtubule functions associated with these armadillo repeat-containing kinesins. We discuss the function of NIMA-related protein kinases and armadillo repeat-containing kinesins in the cell morphogenesis of eukaryotes.
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Affiliation(s)
- Tatsuya Sakai
- RIKEN Plant Science Center, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan
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209
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Jeon BW, Hwang JU, Hwang Y, Song WY, Fu Y, Gu Y, Bao F, Cho D, Kwak JM, Yang Z, Lee Y. The Arabidopsis small G protein ROP2 is activated by light in guard cells and inhibits light-induced stomatal opening. THE PLANT CELL 2008; 20:75-87. [PMID: 18178769 PMCID: PMC2254924 DOI: 10.1105/tpc.107.054544] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2007] [Revised: 11/20/2007] [Accepted: 12/10/2007] [Indexed: 05/19/2023]
Abstract
ROP small G proteins function as molecular switches in diverse signaling processes. Here, we investigated signals that activate ROP2 in guard cells. In guard cells of Vicia faba expressing Arabidopsis thaliana constitutively active (CA) ROP2 fused to red fluorescent protein (RFP-CA-ROP2), fluorescence localized exclusively at the plasma membrane, whereas a dominant negative version of RFP-ROP2 (DN-ROP2) localized in the cytoplasm. In guard cells expressing green fluorescent protein-ROP2, the relative fluorescence intensity at the plasma membrane increased upon illumination, suggesting that light activates ROP2. Unlike previously reported light-activated factors, light-activated ROP2 inhibits rather than accelerates light-induced stomatal opening; stomata bordered by guard cells transformed with CA-rop2 opened less than controls upon light irradiation. When introduced into guard cells together with CA-ROP2, At RhoGDI1, which encodes a guanine nucleotide dissociation inhibitor, inhibited plasma membrane localization of CA-ROP2 and abolished the inhibitory effect of CA-ROP2 on light-induced stomatal opening, supporting the negative effect of active ROP2 on stomatal opening. Mutant rop2 Arabidopsis guard cells showed phenotypes similar to those of transformed V. faba guard cells; CA-rop2 stomata opened more slowly and to a lesser extent, and DN-rop2 stomata opened faster than wild-type stomata in response to light. Moreover, in rop2 knockout plants, stomata opened faster and to a greater extent than wild-type stomata in response to light. Thus, ROP2 is a light-activated negative factor that attenuates the extent of light-induced changes in stomatal aperture. The inhibition of light-induced stomatal opening by light-activated ROP2 suggests the existence of feedback regulatory mechanisms through which stomatal apertures may be finely controlled.
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Affiliation(s)
- Byeong Wook Jeon
- POSTECH-UZH Global Research Laboratories, Division of Molecular Life Sciences, POSTECH, Pohang 790-784, Korea
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210
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Cheung AY, Wu HM. Structural and signaling networks for the polar cell growth machinery in pollen tubes. ANNUAL REVIEW OF PLANT BIOLOGY 2008; 59:547-72. [PMID: 18444907 DOI: 10.1146/annurev.arplant.59.032607.092921] [Citation(s) in RCA: 188] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
Pollen tubes elongate within the pistil to transport sperms to the female gametophytes for fertilization. Pollen tubes grow at their tips through a rapid and polarized cell growth process. This tip growth process is supported by an elaborate and dynamic actin cytoskeleton and a highly active membrane trafficking system that together provide the driving force and secretory activities needed for growth. A polarized cytoplasm with an abundance of vesicles and tip-focused Ca(2+) and H(+) concentration gradients are important for the polar cell growth process. Apical membrane-located Rho GTPases regulate Ca(2+) concentration and actin dynamics in the cytoplasm and are crucial for maintaining pollen tube polarity. Pollen tube growth is marked by periods of rapid and slow growth phases. Activities that regulate and support this tip growth process also show oscillatory fluctuations. How these activities correlate with the rapid, polar, and oscillatory pollen tube growth process is discussed.
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Affiliation(s)
- Alice Y Cheung
- Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003, USA.
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211
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Bloch D, Hazak O, Lavy M, Yalovsky S. A novel ROP/RAC GTPase effector integrates plant cell form and pattern formation. PLANT SIGNALING & BEHAVIOR 2008; 3:41-3. [PMID: 19704766 PMCID: PMC2633956 DOI: 10.4161/psb.3.1.4838] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/31/2007] [Accepted: 08/06/2007] [Indexed: 05/03/2023]
Abstract
ROPs/RACs are the only known signaling Ras superfamily small GTPases in plants. As such they have been suggested to function as central regulators of diverse signaling cascades. The ROP/RAC signaling networks are largely unknown, however, because only few of their effector proteins have been identified. In a paper that was published in the June 5, 2007 issue of Current Biology we described the identification of a novel ROP/RAC effector designated ICR1 (Interactor of Constitutive active ROPs 1). We demonstrated that ICR1 functions as a scaffold that interacts with diverse but specific group of proteins including SEC3 subunit of the exocyst vesicle tethering complex. ICR1-SEC3 complexes can interact with ROPs in vivo and are thereby recruited to the plasma membrane. ICR1 knockdown or silencing leads to cell deformation and loss of the root stem cells population, and ectopic expression of ICR1 phenocopies activated ROPs/RACs. ICR1 presents a new paradigm in ROP/RAC signaling and integrates mechanisms regulating cell form and pattern formation at the whole plant level.
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Affiliation(s)
- Daria Bloch
- Department of Plant Sciences; Tel Aviv University; Tel Aviv, Israel
| | - Ora Hazak
- Department of Plant Sciences; Tel Aviv University; Tel Aviv, Israel
| | - Meirav Lavy
- Department of Biology; Indiana University; Bloomington, Indiana USA
| | - Shaul Yalovsky
- Department of Plant Sciences; Tel Aviv University; Tel Aviv, Israel
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212
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Wilsen KL, Hepler PK. Sperm Delivery in Flowering Plants: The Control of Pollen Tube Growth. Bioscience 2007. [DOI: 10.1641/b571006] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
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213
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A mutation in MRH2 kinesin enhances the root hair tip growth defect caused by constitutively activated ROP2 small GTPase in Arabidopsis. PLoS One 2007; 2:e1074. [PMID: 17957256 PMCID: PMC2031828 DOI: 10.1371/journal.pone.0001074] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2007] [Accepted: 10/07/2007] [Indexed: 01/02/2023] Open
Abstract
Root hair tip growth provides a unique model system for the study of plant cell polarity. Transgenic plants expressing constitutively active (CA) forms of ROP (Rho-of-plants) GTPases have been shown to cause the disruption of root hair polarity likely as a result of the alteration of actin filaments (AF) and microtubules (MT) organization. Towards understanding the mechanism by which ROP controls the cytoskeletal organization during root hair tip growth, we have screened for CA-rop2 suppressors or enhancers using CA1-1, a transgenic line that expresses CA-rop2 and shows only mild disruption of tip growth. Here, we report the characterization of a CA-rop2 enhancer (cae1-1 CA1-1) that exhibits bulbous root hairs. The cae1-1 mutation on its own caused a waving and branching root hair phenotype. CAE1 encodes the root hair growth-related, ARM domain-containing kinesin-like protein MRH2 (and thus cae1-1 was renamed to mrh2-3). Cortical MT displayed fragmentation and random orientation in mrh2 root hairs. Consistently, the MT-stabilizing drug taxol could partially rescue the wavy root hair phenotype of mrh2-3, and the MT-depolymerizing drug Oryzalin slightly enhanced the root hair tip growth defect in CA1-1. Interestingly, the addition of the actin-depolymerizing drug Latrunculin B further enhanced the Oryzalin effect. This indicates that the cross-talk of MT and AF organization is important for the mrh2-3 CA1-1 phenotype. Although we did not observe an apparent effect of the MRH2 mutation in AF organization, we found that mrh2-3 root hair growth was more sensitive to Latrunculin B. Moreover, an ARM domain-containing MRH2 fragment could bind to the polymerized actin in vitro. Therefore, our genetic analyses, together with cell biological and pharmacological evidence, suggest that the plant-specific kinesin-related protein MRH2 is an important component that controls MT organization and is likely involved in the ROP2 GTPase-controlled coordination of AF and MT during polarized growth of root hairs.
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214
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Yang Z, Fu Y. ROP/RAC GTPase signaling. CURRENT OPINION IN PLANT BIOLOGY 2007; 10:490-4. [PMID: 17709276 PMCID: PMC2956068 DOI: 10.1016/j.pbi.2007.07.005] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/15/2007] [Revised: 07/06/2007] [Accepted: 07/09/2007] [Indexed: 05/16/2023]
Abstract
ROP/RAC GTPases are versatile signaling molecules in plants. Recent studies of ROP/RAC regulators and effectors have generated new insights into the molecular basis of their functional versatility. Significant progress has also been made in our understanding of the mechanism for the localization of ROP/RAC signaling to specific domains of the plasma membrane.
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Affiliation(s)
- Zhenbiao Yang
- Center for Plant Cell Biology, Institute of Integrative Genome Biology, Department of Botany and Plant Sciences, University of California, Riverside, CA 92521, USA.
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215
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Yu GH, Sun MX. Deciphering the Possible Mechanism of GABA in Tobacco Pollen Tube Growth and Guidance. PLANT SIGNALING & BEHAVIOR 2007; 2:393-5. [PMID: 19704611 PMCID: PMC2634224 DOI: 10.4161/psb.2.5.4265] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/12/2007] [Accepted: 04/12/2007] [Indexed: 05/04/2023]
Abstract
gamma-Aminobutyric acid (GABA) is an inhibitory transmitter in animal central and peripheral nervous systems, and also plays an important role in pollen tube growth and guidance. However, the mechanisms underlying these effects in plants are poorly understood, mainly because the GABA receptor in plants has not been elucidated. To address this issue, we recently created quantum dot probes to identify possible GABA receptors on the membrane surfaces of pollen protoplasts. We found that GABA bound to cell membranes and regulated downstream Ca(2+) oscillation in the cells. These results provide important clues to further specifying the nature of the binding sites and deciphering the role of GABA as a signal molecule in pollen tube growth and orientation.
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Affiliation(s)
- Guang-Hui Yu
- College of Life Sciences; South-Central University for Nationalities; Wuhan, China
- Key Laboratory of MOE for Plant Developmental Biology; College of Life Sciences; Wuhan University; Wuhan, China
| | - Meng-Xiang Sun
- Key Laboratory of MOE for Plant Developmental Biology; College of Life Sciences; Wuhan University; Wuhan, China
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216
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Lavy M, Bloch D, Hazak O, Gutman I, Poraty L, Sorek N, Sternberg H, Yalovsky S. A Novel ROP/RAC effector links cell polarity, root-meristem maintenance, and vesicle trafficking. Curr Biol 2007; 17:947-52. [PMID: 17493810 DOI: 10.1016/j.cub.2007.04.038] [Citation(s) in RCA: 163] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2007] [Revised: 04/14/2007] [Accepted: 04/16/2007] [Indexed: 11/15/2022]
Abstract
ROP/RAC GTPases are master regulators of cell polarity in plants, implicated in the regulation of diverse signaling cascades including cytoskeleton organization, vesicle trafficking, and Ca(2+) gradients [1-8]. The involvement of ROPs in differentiation processes is yet unknown. Here we show the identification of a novel ROP/RAC effector, designated interactor of constitutive active ROPs 1 (ICR1), that interacts with GTP-bound ROPs. ICR1 knockdown or silencing leads to cell deformation and loss of root stem-cell population. Ectopic expression of ICR1 phenocopies activated ROPs, inducing cell deformation of leaf-epidermis-pavement and root-hair cells [3, 5, 6, 9]. ICR1 is comprised of coiled-coil domains and forms complexes with itself and the exocyst vesicle-tethering complex subunit SEC3 [10-13]. The ICR1-SEC3 complexes can interact with ROPs in vivo. Plants overexpressing a ROP- and SEC3-noninteracting ICR1 mutant have a wild-type phenotype. Taken together, our results show that ICR1 is a scaffold-mediating formation of protein complexes that are required for cell polarity, linking ROP/RAC GTPases with vesicle trafficking and differentiation.
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Affiliation(s)
- Meirav Lavy
- Department of Plant Sciences, Tel Aviv University, Tel Aviv 69978, Israel
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217
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Lovy-Wheeler A, Cárdenas L, Kunkel JG, Hepler PK. Differential organelle movement on the actin cytoskeleton in lily pollen tubes. ACTA ACUST UNITED AC 2007; 64:217-32. [PMID: 17245769 DOI: 10.1002/cm.20181] [Citation(s) in RCA: 99] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
We have examined the arrangement and movement of three major compartments, the endoplasmic reticulum (ER), mitochondria, and the vacuole during oscillatory, polarized growth in lily pollen tubes. These movements are dependent on the actin cytoskeleton, because they are strongly perturbed by the anti-microfilament drug, latrunculin-B, and unaffected by the anti-microtubule agent, oryzalin. The ER, which has been labeled with mGFP5-HDEL or cytochalasin D tetramethylrhodamine, displays an oscillatory motion in the pollen tube apex. First it moves apically in the cortical region, presumably along the cortical actin fringe, and then periodically folds inward creating a platform that transects the apical domain in a plate-like structure. Finally, the ER reverses its direction and moves basipetally through the central core of the pollen tube. When subjected to cross-correlation analysis, the formation of the platform precedes maximal growth rates by an average of 3 s (35-40 degrees ). Mitochondria, labeled with Mitotracker Green, are enriched in the subapical region, and their movement closely resembles that of the ER. The vacuole, labeled with carboxy-dichlorofluorescein diacetate, consists of thin tubules arranged longitudinally in a reticulate network, which undergoes active motion. In contrast to the mitochondria and ER, the vacuole is located back from the apex, and never extends into the apical clear zone. We have not been able to decipher an oscillatory pattern in vacuole motion. Because this motion is dependent on actin and not tubulin, we think this is due to a different myosin from that which drives the ER and mitochondria.
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Affiliation(s)
- Alenka Lovy-Wheeler
- Department of Biology and Plant Biology Graduate Program, Morrill Science Center III, University of Massachusetts, Amherst, Massachusetts 01003, USA
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218
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Menand B, Calder G, Dolan L. Both chloronemal and caulonemal cells expand by tip growth in the moss Physcomitrella patens. JOURNAL OF EXPERIMENTAL BOTANY 2007; 58:1843-9. [PMID: 17404383 DOI: 10.1093/jxb/erm047] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Tip growth is a mode of cell expansion in which all growth is restricted to a small area that forms a tip in an elongating cell. In green plants, tip growth has been shown to occur in root hairs, pollen tubes, rhizoids, and caulonema. Each of these cell types has a longitudinally elongated shape, longitudinally oriented microtubules and actin microfilaments, and a characteristic cytoplasmic organization at the growing tip which is required for growth. Chloronema are elongated cylindrical shaped cells that form during the development of the moss protonema. Since there are no published reports on the precise mode of chloronema elongation and conflicting interpretations of its cytology, the mechanism of cell growth has remained unclear. To determine if chloronema elongate by tip or diffuse growth, time-lapse light microscopy was employed to follow the movement of fluorescent microspheres attached to the surface of growing cells. It is shown here that chloronemal cells elongate by a form of tip growth. However, the slower growth of chloronema compared with caulonema is probably the result of differences in cytological organization of the growing tip.
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Affiliation(s)
- Benoît Menand
- Department of Cell and Developmental Biology, John Innes Centre, Norwich NR4 7UH, UK
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219
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Yoneda A, Kutsuna N, Higaki T, Oda Y, Sano T, Hasezawa S. Recent progress in living cell imaging of plant cytoskeleton and vacuole using fluorescent-protein transgenic lines and three-dimensional imaging. PROTOPLASMA 2007; 230:129-39. [PMID: 17458628 DOI: 10.1007/s00709-006-0237-4] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2005] [Accepted: 03/17/2006] [Indexed: 05/15/2023]
Abstract
In higher-plant cells, microtubules, actin microfilaments, and vacuoles play important roles in a variety of cellular events, including cell division, morphogenesis, and cell differentiation. These intracellular structures undergo dynamic changes in their shapes and functions during cell division and differentiation, and to analyse these sequential structural changes, the vital labelling technique, using the green-fluorescent protein or other fluorescent proteins, has commonly been used to follow the localisation and translocation of specific proteins. To visualise microtubules, actin filaments, and vacuoles, several strategies are available for selecting the appropriate fluorescent-protein fusion partner: microtubule-binding proteins, tubulin, and plus-end-tracking proteins are most suitable for microtubule labelling; the actin binding domain of mouse talin and plant fimbrin for actin microfilament visualisation; and the tonoplast-intrinsic proteins and syntaxin-related proteins for vacuolar imaging. In addition, three-dimensional reconstruction methods are indispensable for localising the widely distributed organelles within the cell. The maximum intensity projection method is suitable for cytoskeletal structures, while contour-based surface modelling possesses many advantages for vacuolar membranes. In this article, we summarise the recent progress in living cell imaging of the plant cytoskeleton and vacuoles using various fusions with green-fluorescent proteins and three-dimensional imaging techniques.
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Affiliation(s)
- A Yoneda
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba, Japan
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220
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Bushart TJ, Roux SJ. Conserved features of germination and polarized cell growth: a few insights from a pollen-fern spore comparison. ANNALS OF BOTANY 2007; 99:9-17. [PMID: 16867999 PMCID: PMC2802967 DOI: 10.1093/aob/mcl159] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/18/2006] [Accepted: 06/12/2006] [Indexed: 05/11/2023]
Abstract
BACKGROUND The germination of both pollen and fern spores results in the emergence of a cell-pollen tube from pollen, rhizoid from spore-that grows in a polar fashion, primarily at its apical end. In both of these tip-growing cells, the delivery of secretory vesicles to the growing end is guided in part by a calcium gradient, with calcium entering at the tip where it is most highly concentrated. The similarities between the two systems extend beyond tip-focused calcium gradients to encompass signalling pathways and elements including calmodulin, nitric oxide, annexins and Rop-GTPases. SCOPE AND AIMS This review is limited to those pathways and elements that function similarly in fern and pollen systems based on currently available evidence. The aim is to illustrate the common mechanisms by which tip growth occurs, facilitate further investigations into this area, and examine the implications for the evolutionarily conserved control of tip growth. CONCLUSIONS The interplay of calcium, nitric oxide and other effectors in both pollen and fern spores suggests certain signalling pathways became important regulators of germination and growth early in the evolution of land plants. Both large- and small-scale comparative genomic methods have shown to be promising in their ability to find new and relevant comparisons for further research. Cross-species comparisons may serve to speed up this process by highlighting both basic pathways and system-specific deviations.
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Affiliation(s)
| | - Stanley J. Roux
- Molecular Cell and Developmental Biology, University of Texas, Austin, TX 78751, USA
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221
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Chan J, Peter Pauls K. Brassica napus Rop GTPases and their expression in microspore cultures. PLANTA 2007; 225:469-84. [PMID: 16896789 DOI: 10.1007/s00425-006-0362-5] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/30/2006] [Accepted: 07/13/2006] [Indexed: 05/08/2023]
Abstract
Androgenesis in plants involves a shift in development that causes cultured microspore cells to form embryos rather than continue to develop pollen. In Brassica napus microspore culture a mild heat stress is used to switch on embryo development. An early hallmark of embryogenesis in this system is a symmetrical division of the nucleus instead of the asymmetric division that occurs during pollen formation. ROP GTPases act as molecular switches in a variety of developmental processes; therefore, the current study was initiated to examine whether they might be involved in androgenesis. Five distinct Rop genes with nucleic acid similarities ranging from 82 to 93% to Arabidopsis Rop1 were isolated from B. napus cv Topas. A Southern blot hybridization with a BnRop sequence probe suggested that there are 11-15 ROP gene family members in B. napus. RT-PCR reactions with PCR primers specific to BnRop5, BnRop6, BnRop9 and BnRop10 showed that expression of the BnRop5 was restricted to pollen but the others were detected in leaf, root, stem and pollen tissue. Pollen-like cells obtained from 3-day-old cultures by flow cytometric sorting had BnRop5 transcript levels that were 2.8 times higher than in flow sorted embryogenic microspores. Conversely, the BnRop9 transcript levels were 2.5-fold higher in the embryogenic cells than in the pollen-like cells. The potential involvement of specific ROPs in early stage microspore culture responses is discussed.
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Affiliation(s)
- John Chan
- Department of Plant Agriculture, University of Guelph, Guelph, ON, Canada
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222
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Cole RA, Fowler JE. Polarized growth: maintaining focus on the tip. CURRENT OPINION IN PLANT BIOLOGY 2006; 9:579-88. [PMID: 17010659 DOI: 10.1016/j.pbi.2006.09.014] [Citation(s) in RCA: 176] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2006] [Accepted: 09/19/2006] [Indexed: 05/04/2023]
Abstract
Tip growth, a spatially focused cell expansion, has been best characterized in two plant cell types: pollen tubes and root hairs. It has long been established that both cell types require three intracellular components for this process: a tip-high calcium gradient, a polarized actin cytoskeleton, and tip-directed vesicle trafficking. More recently, additional mechanistic parallels have been observed between the two cell types, including roles for ROP and Rab GTPase signaling, phosphoinositides, calcium-dependent protein kinases, and the exocyst. Uncovering pathways that control the three components is beginning to reveal a highly interconnected network, which we call the tip growth LENS (for localization enhancing network, self-sustaining), that coordinates the required cellular activities to allow regulated tip growth, and to maintain itself as the tip advances.
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Affiliation(s)
- Rex A Cole
- Molecular and Cellular Biology Program, Oregon State University, Corvallis, Oregon 97331, USA
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223
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Samaj J, Müller J, Beck M, Böhm N, Menzel D. Vesicular trafficking, cytoskeleton and signalling in root hairs and pollen tubes. TRENDS IN PLANT SCIENCE 2006; 11:594-600. [PMID: 17092761 DOI: 10.1016/j.tplants.2006.10.002] [Citation(s) in RCA: 150] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/26/2006] [Revised: 09/11/2006] [Accepted: 10/23/2006] [Indexed: 05/12/2023]
Abstract
Root hairs and pollen tubes show strictly polar cell expansion called tip growth. Recent studies of tip growth in root hairs and pollen tubes have revealed that small GTPases of the Rab, Arf and Rho/Rac families, along with their regulatory proteins, are essential for spatio-temporal regulation of vesicular trafficking, cytoskeleton organization and signalling. ROP/RAC GTPases are involved in a multiplicity of functions including the regulation of cytoskeleton organization, calcium signalling and endocytosis in pollen tubes and root hairs. One of the most exciting recent discoveries is the preferential localization of vesicles of the trans-Golgi network (TGN), defined by specific RAB GTPases, in the apical "clear zone" and the definition of TGN as a bona fide organelle involved in both polarized secretion and endocytosis. The TGN is thought to serve the function of an early endosome in plants because it is involved in early endocytosis and rapid vesicular recycling of the plasma membrane in root epidermal cells.
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Affiliation(s)
- Jozef Samaj
- Institute of Cellular and Molecular Botany, University of Bonn, Kirschallee 1, D-53115 Bonn, Germany.
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224
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Abu-Abied M, Golomb L, Belausov E, Huang S, Geiger B, Kam Z, Staiger CJ, Sadot E. Identification of plant cytoskeleton-interacting proteins by screening for actin stress fiber association in mammalian fibroblasts. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2006; 48:367-79. [PMID: 17010111 DOI: 10.1111/j.1365-313x.2006.02883.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
Taking advantage of the high conservation of the cytoskeleton building blocks actin and tubulin between plant and animal kingdoms, we developed a functional genomic screen for the isolation of new plant cytoskeleton-binding proteins that uses a mammalian cell expression system. A yellow fluorescent protein (YFP)-fusion cDNA library from Arabidopsis was inserted into rat fibroblasts and screened for fluorescent chimeras localizing to cytoskeletal structures. The high-throughput screen was performed by an automated microscope. An initial set of candidate genes identified in the screen was isolated, sequenced, the full-length cDNAs were synthesized by RT-PCR and tested by biochemical approaches to verify the ability of the genes to bind actin directly. Alternatively, indirect binding via interaction with other actin-binding proteins was studied. The full-length cDNAs were transferred back to plants as YFP chimeras behind the CAMV-35S promoter. We give here two examples of new plant cytoskeletal proteins identified in the pilot screen. ERD10, a member of the dehydrin family of proteins, was localized to actin stress fibers in rat fibroblasts. Its direct binding to actin filaments was confirmed by several biochemical approaches. Touch-induced calmodulin-like protein, TCH2, was also localized to actin stress fibers in fibroblasts, but was unable to bind actin filaments directly in vitro. Nevertheless, it did bind to the IQ domains of Arabidopsis myosin VIII in a calcium-dependent manner. Further evidence for a cytoskeletal function of ERD10 was obtained in planta; GFP-ERD10 was able to protect the actin cytoskeleton from latrunculin-mediated disruption in Nicotiana benthamiana leaves.
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Affiliation(s)
- Mohamad Abu-Abied
- The Institute of Plant Sciences, The Volcani Center, Bet-Dagan 50250 Israel
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225
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Klahre U, Kost B. Tobacco RhoGTPase ACTIVATING PROTEIN1 spatially restricts signaling of RAC/Rop to the apex of pollen tubes. THE PLANT CELL 2006; 18:3033-46. [PMID: 17098809 PMCID: PMC1693941 DOI: 10.1105/tpc.106.045336] [Citation(s) in RCA: 105] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/27/2006] [Revised: 08/09/2006] [Accepted: 10/17/2006] [Indexed: 05/12/2023]
Abstract
Regulation by Rho-type small GTPases, such as RAC5, is important for the maintenance of polarity in tobacco (Nicotiana tabacum) pollen tubes. We previously showed that RhoGDI2 is necessary for RAC5 localization. Here, we describe the GTPase activating protein RhoGAP1 that controls the area of RAC5 activity. RhoGAP1 N-terminal and CRIB (for Cdc42/Rac-interactive binding) domains are both necessary for targeting yellow fluorescent protein-RhoGAP1 fusions to the plasma membrane close to, but not in, pollen tube apices. We propose that this localization restricts apical Rho-type GTPase activity from spreading toward the flanks, which ensures the maintenance of RAC signaling at the apex. The CRIB domain is not required but enhances in vitro RhoGAP1 activity toward the pollen tube-specific-RAC5. A mutation reducing GAP activity of RhoGAP1 leads to ballooning pollen tubes resembling those overexpressing RAC5. To ascertain the specific targeting mechanism of RhoGAP1, we isolated a 14-3-3 protein interacting with RhoGAP1. When overexpressed with RhoGAP1, it counteracts the growth-retarding effect of RhoGAP1 overexpression and attenuates RhoGAP1 membrane localization but, overexpressed alone, induces only small architectural changes. We propose that inactivation of RAC5 by the subapically localized RhoGAP1, together with dynamic relocalization of inactivated RAC5 from flanks to tip by RhoGDI2, leads to spatial restriction of RAC5 to pollen tube apices, thereby sustaining polar growth.
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Affiliation(s)
- Ulrich Klahre
- Heidelberg Institute of Plant Sciences, University of Heidelberg, 69120 Heidelberg, Germany.
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226
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Lovy-Wheeler A, Kunkel JG, Allwood EG, Hussey PJ, Hepler PK. Oscillatory increases in alkalinity anticipate growth and may regulate actin dynamics in pollen tubes of lily. THE PLANT CELL 2006; 18:2182-93. [PMID: 16920777 PMCID: PMC1560910 DOI: 10.1105/tpc.106.044867] [Citation(s) in RCA: 96] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/13/2006] [Revised: 07/14/2006] [Accepted: 07/26/2006] [Indexed: 05/11/2023]
Abstract
Lily (Lilium formosanum or Lilium longiflorum) pollen tubes, microinjected with a low concentration of the pH-sensitive dye bis-carboxyethyl carboxyfluorescein dextran, show oscillating pH changes in their apical domain relative to growth. An increase in pH in the apex precedes the fastest growth velocities, whereas a decline follows growth, suggesting a possible relationship between alkalinity and cell extension. A target for pH may be the actin cytoskeleton, because the apical cortical actin fringe resides in the same region as the alkaline band in lily pollen tubes and elongation requires actin polymerization. A pH-sensitive actin binding protein, actin-depolymerizing factor (ADF), together with actin-interacting protein (AIP) localize to the cortical actin fringe region. Modifying intracellular pH leads to reorganization of the actin cytoskeleton, especially in the apical domain. Acidification causes actin filament destabilization and inhibits growth by 80%. Upon complete growth inhibition, the actin fringe is the first actin cytoskeleton component to disappear. We propose that during normal growth, the pH increase in the alkaline band stimulates the fragmenting activity of ADF/AIP, which in turn generates more sites for actin polymerization. Increased actin polymerization supports faster growth rates and a proton influx, which inactivates ADF/AIP, decreases actin polymerization, and retards growth. As pH stabilizes and increases, the activity of ADF/AIP again increases, repeating the cycle of events.
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Affiliation(s)
- Alenka Lovy-Wheeler
- Department of Biology and Plant Biology Graduate Program, University of Massachusetts, Amherst, 01003, USA
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227
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Brembu T, Winge P, Bones AM, Yang Z. A RHOse by any other name: a comparative analysis of animal and plant Rho GTPases. Cell Res 2006; 16:435-45. [PMID: 16699539 DOI: 10.1038/sj.cr.7310055] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Rho GTPases are molecular switches that act as key regulators of a many cellular processes, including cell movement, morphogenesis, host defense, cell division and gene expression. Rho GTPases are found in all eukaryotic kingdoms. Plants lack clear homologs to conventional Rho GTPases found in yeast and animals; instead, they have over time developed a unique subfamily, ROPs, also known as RAC. The origin of ROP-like proteins appears to precede the appearance of land plants. This review aims to discuss the evolution of ROP/RAC and to compare plant ROP and animal Rho GTPases, focusing on similarities and differences in regulation of the GTPases and their downstream effectors.
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Affiliation(s)
- Tore Brembu
- Department of Botany and Plant Sciences, Center for Plant Cell Biology, Institute of Integrative Genome Biology, University of California, Riverside, California 92521, USA
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228
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Abstract
Polarized cell growth in plants is controlled by Rho-like small GTPases (ROPs), not only through the canonical WAVE/Arp2/3 pathway, but also through newly defined plant-specific pathways involving the regulated release of reactive oxygen species (ROS).
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Affiliation(s)
- Joachim F Uhrig
- University of Köln, Botanical Institute III, Gyrhofstr. 15, D-50931, Köln, Germany.
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229
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Dowd PE, Coursol S, Skirpan AL, Kao TH, Gilroy S. Petunia phospholipase c1 is involved in pollen tube growth. THE PLANT CELL 2006. [PMID: 16648366 DOI: 10.1105/tpc.106.041582.1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
Although pollen tube growth is essential for plant fertilization and reproductive success, the regulators of the actin-related growth machinery and the cytosolic Ca2+ gradient thought to determine how these cells elongate remain poorly defined. Phospholipases, their substrates, and their phospholipid turnover products have been proposed as such regulators; however, the relevant phospholipase(s) have not been characterized. Therefore, we cloned cDNA for a pollen-expressed phosphatidylinositol 4,5-bisphosphate (PtdInsP2)-cleaving phospholipase C (PLC) from Petunia inflata, named Pet PLC1. Expressing a catalytically inactive form of Pet PLC1 in pollen tubes caused expansion of the apical Ca2+ gradient, disruption of the organization of the actin cytoskeleton, and delocalization of growth at the tube tip. These phenotypes were suppressed by depolymerizing actin with low concentrations of latrunculin B, suggesting that a critical site of action of Pet PLC1 is in regulating actin structure at the growing tip. A green fluorescent protein (GFP) fusion to Pet PLC1 caused enrichment in regions of the apical plasma membrane not undergoing rapid expansion, whereas a GFP fusion to the PtdInsP2 binding domain of mammalian PLC delta1 caused enrichment in apical regions depleted in PLC. Thus, Pet PLC1 appears to be involved in the machinery that restricts growth to the very apex of the elongating pollen tube, likely through its regulatory action on PtdInsP2 distribution within the cell.
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Affiliation(s)
- Peter E Dowd
- Department of Biochemistry and Molecular Biology, Pensylvania State University, University Park, Pensylvania 16802, USA
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230
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Nibau C, Wu HM, Cheung AY. RAC/ROP GTPases: 'hubs' for signal integration and diversification in plants. TRENDS IN PLANT SCIENCE 2006; 11:309-15. [PMID: 16737841 DOI: 10.1016/j.tplants.2006.04.003] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/21/2006] [Revised: 04/04/2006] [Accepted: 04/26/2006] [Indexed: 05/04/2023]
Abstract
RAC/ROP GTPases are a family of plant-specific signaling molecules solely representing the Ras and Rho family of Ras-related G proteins in plants. RAC/ROPs potentially interact with cell surface-associated signal perception apparatus for a broad range of extracellular stimuli, including hormones, pathogen elicitors and abiotic stress, and mediate diverse cellular pathways in response to these signals. They are also known to interact with multiple effectors, affecting cellular and biochemical systems that regulate actin dynamics, reactive oxygen species production, proteolysis, and gene expression. RAC/ROPs are, thus, ideally suited as integrators for multiple signals and as coordinators of diverse cellular pathways to control growth, differentiation, development and defense responses. Recent findings that suggest how RAC/ROP signaling activity is regulated and how functional specificity can be achieved are discussed here.
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Affiliation(s)
- Candida Nibau
- Plant Biology Graduate Program, University of Massachusetts, Lederle Graduate Research Tower, Department of Biochemistry and Molecular Biology, Amherst, MA 10003, USA
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231
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Dowd PE, Coursol S, Skirpan AL, Kao TH, Gilroy S. Petunia phospholipase c1 is involved in pollen tube growth. THE PLANT CELL 2006; 18:1438-53. [PMID: 16648366 PMCID: PMC1475500 DOI: 10.1105/tpc.106.041582] [Citation(s) in RCA: 163] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/01/2006] [Revised: 03/17/2006] [Accepted: 04/05/2006] [Indexed: 05/08/2023]
Abstract
Although pollen tube growth is essential for plant fertilization and reproductive success, the regulators of the actin-related growth machinery and the cytosolic Ca2+ gradient thought to determine how these cells elongate remain poorly defined. Phospholipases, their substrates, and their phospholipid turnover products have been proposed as such regulators; however, the relevant phospholipase(s) have not been characterized. Therefore, we cloned cDNA for a pollen-expressed phosphatidylinositol 4,5-bisphosphate (PtdInsP2)-cleaving phospholipase C (PLC) from Petunia inflata, named Pet PLC1. Expressing a catalytically inactive form of Pet PLC1 in pollen tubes caused expansion of the apical Ca2+ gradient, disruption of the organization of the actin cytoskeleton, and delocalization of growth at the tube tip. These phenotypes were suppressed by depolymerizing actin with low concentrations of latrunculin B, suggesting that a critical site of action of Pet PLC1 is in regulating actin structure at the growing tip. A green fluorescent protein (GFP) fusion to Pet PLC1 caused enrichment in regions of the apical plasma membrane not undergoing rapid expansion, whereas a GFP fusion to the PtdInsP2 binding domain of mammalian PLC delta1 caused enrichment in apical regions depleted in PLC. Thus, Pet PLC1 appears to be involved in the machinery that restricts growth to the very apex of the elongating pollen tube, likely through its regulatory action on PtdInsP2 distribution within the cell.
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Affiliation(s)
- Peter E Dowd
- Department of Biochemistry and Molecular Biology, Pensylvania State University, University Park, Pensylvania 16802, USA
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232
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Schütz I, Gus-Mayer S, Schmelzer E. Profilin and Rop GTPases are localized at infection sites of plant cells. PROTOPLASMA 2006; 227:229-35. [PMID: 16736261 DOI: 10.1007/s00709-005-0151-1] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/25/2005] [Accepted: 09/08/2005] [Indexed: 05/09/2023]
Abstract
We have found 5 profilin cDNAs in cultured parsley cells, representing a small gene family of about 5 members in parsley. Specific antibodies were produced using heterologously expressed parsley profilin as antigen. Western blot analysis revealed the occurrence of similar amounts of profilin in roots and green parts of parsley plants. Immunocytochemical staining of parsley cells infected with the oomycetous plant pathogen Phytophthora infestans clearly revealed that profilin accumulates at the site on the plasma membrane subtending the oomycetous appressorium, where the actin cables focus. We also observed the accumulation of Rop GTPases around this site, which might point to a potential function in signaling to the cytoskeleton.
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Affiliation(s)
- I Schütz
- Central Microscopy, Max Planck Institute for Plant Breeding Research, Cologne
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233
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Palanivelu R, Preuss D. Distinct short-range ovule signals attract or repel Arabidopsis thaliana pollen tubes in vitro. BMC PLANT BIOLOGY 2006; 6:7. [PMID: 16595022 PMCID: PMC1489931 DOI: 10.1186/1471-2229-6-7] [Citation(s) in RCA: 181] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/09/2006] [Accepted: 04/05/2006] [Indexed: 05/08/2023]
Abstract
BACKGROUND Pollen tubes deliver sperm after navigating through flower tissues in response to attractive and repulsive cues. Genetic analyses in maize and Arabidopsis thaliana and cell ablation studies in Torenia fournieri have shown that the female gametophyte (the 7-celled haploid embryo sac within an ovule) and surrounding diploid tissues are essential for guiding pollen tubes to ovules. The variety and inaccessibility of these cells and tissues has made it challenging to characterize the sources of guidance signals and the dynamic responses they elicit in the pollen tubes. RESULTS Here we developed an in vitro assay to study pollen tube guidance to excised A. thaliana ovules. Using this assay we discerned the temporal and spatial regulation and species-specificity of late stage guidance signals and characterized the dynamics of pollen tube responses. We established that unfertilized A. thaliana ovules emit diffusible, developmentally regulated, species-specific attractants, and demonstrated that ovules penetrated by pollen tubes rapidly release diffusible pollen tube repellents. CONCLUSION These results demonstrate that in vitro pollen tube guidance to excised A. thaliana ovules efficiently recapitulates much of in vivo pollen tube behaviour during the final stages of pollen tube growth. This assay will aid in confirming the roles of candidate guidance molecules, exploring the phenotypes of A. thaliana pollen tube guidance mutants and characterizing interspecies pollination interactions.
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Affiliation(s)
- Ravishankar Palanivelu
- Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, IL 60637, USA
| | - Daphne Preuss
- Howard Hughes Medical Institute, The University of Chicago, Chicago, IL 60637, USA
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234
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Yoon GM, Dowd PE, Gilroy S, McCubbin AG. Calcium-dependent protein kinase isoforms in Petunia have distinct functions in pollen tube growth, including regulating polarity. THE PLANT CELL 2006; 18:867-78. [PMID: 16531501 PMCID: PMC1425858 DOI: 10.1105/tpc.105.037135] [Citation(s) in RCA: 106] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2005] [Revised: 02/07/2006] [Accepted: 02/15/2006] [Indexed: 05/07/2023]
Abstract
Calcium is a key regulator of pollen tube growth, but little is known concerning the downstream components of the signaling pathways involved. We identified two pollen-expressed calmodulin-like domain protein kinases from Petunia inflata, CALMODULIN-LIKE DOMAIN PROTEIN KINASE1 (Pi CDPK1) and Pi CDPK2. Transient overexpression or expression of catalytically modified Pi CDPK1 disrupted pollen tube growth polarity, whereas expression of Pi CDPK2 constructs inhibited tube growth but not polarity. Pi CDPK1 exhibited plasma membrane localization most likely mediated by acylation, and we present evidence that suggests this localization is critical to the biological function of this kinase. Pi CDPK2 substantially localized to as yet unidentified internal membrane compartments, and this localization was again, at least partially, mediated by acylation. In contrast with Pi CDPK1, altering the localization of Pi CDPK2 did not noticeably alter the effect of overexpressing this isoform on pollen tube growth. Ca(2+) requirements for Pi CDPK1 activation correlated closely with Ca(2+) concentrations measured in the growth zone at the pollen tube apex. Interestingly, loss of polarity associated with overexpression of Pi CDPK1 was associated with elevated cytosolic Ca(2+) throughout the bulging tube tip, suggesting that Pi CDPK1 may participate in maintaining Ca(2+) homeostasis. These results are discussed in relation to previous models for Ca(2+) regulation of pollen tube growth.
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Affiliation(s)
- Gyeong Mee Yoon
- School of Biological Sciences and Center for Reproductive Biology, Washington State University, Pullman, Washington 99164, USA
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235
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Gu Y, Li S, Lord EM, Yang Z. Members of a novel class of Arabidopsis Rho guanine nucleotide exchange factors control Rho GTPase-dependent polar growth. THE PLANT CELL 2006; 18:366-81. [PMID: 16415208 PMCID: PMC1356545 DOI: 10.1105/tpc.105.036434] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/21/2005] [Revised: 11/28/2005] [Accepted: 12/13/2005] [Indexed: 05/06/2023]
Abstract
Rho family small GTPases are signaling switches controlling many eukaryotic cellular processes. Conversion from the GDP- to GTP-bound form is catalyzed by guanine nucleotide exchange factors (GEFs). Rho GEFs in animals fall into two structurally distinct classes containing DH and DOCKER catalytic domains. Using a plant Rho GTPase (ROP1) as bait in yeast two-hybrid screens, we identified a family of Rho GEFs, named RopGEFs. The Arabidopsis thaliana RopGEF family of 14 members contains a conserved central domain, the domain of unknown function 315 (DUF315), and variable N- and C-terminal regions. In vitro GEF assays show that DUF315 but not the full-length version of RopGEF1 has high GEF activity toward ROP1. Our data suggest that the variable regions of RopGEF1 are involved in regulation of RopGEF through an autoinhibitory mechanism. RopGEF1 overexpression in pollen tubes produced growth depolarization, as does a constitutively active ROP1 mutant. The RopGEF1 overexpression phenotype was suppressed by expression of a dominant-negative mutant of ROP1, probably by trapping RopGEF1. Deletion mutant analysis suggested a requirement of RopGEF activity for the function of RopGEF1 in polar growth. Green fluorescent protein-tagged RopGEF1 was localized to the tip of pollen tubes where ROP1 is activated. These results provide strong evidence that RopGEF1 activates ROP1 in control of polar growth in pollen tubes.
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Affiliation(s)
- Ying Gu
- Department of Botany and Plant Sciences, Center for Plant Cell Biology, Institute of Integrative Genome Biology, University of California, Riverside, California 92521, USA
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236
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Abstract
The cytoskeleton plays important roles in plant cell shape determination by influencing the patterns in which cell wall materials are deposited. Cortical microtubules are thought to orient the direction of cell expansion primarily via their influence on the deposition of cellulose into the wall, although the precise nature of the microtubule-cellulose relationship remains unclear. In both tip-growing and diffusely growing cell types, F-actin promotes growth and also contributes to the spatial regulation of growth. F-actin has been proposed to play a variety of roles in the regulation of secretion in expanding cells, but its functions in cell growth control are not well understood. Recent work highlighted in this review on the morphogenesis of selected cell types has yielded substantial new insights into mechanisms governing the dynamics and organization of cytoskeletal filaments in expanding plant cells and how microtubules and F-actin interact to direct patterns of cell growth. Nevertheless, many important questions remain to be answered.
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Affiliation(s)
- Laurie G Smith
- Section of Cell and Developmental Biology, University of California, San Diego, La Jolla, California 92093-0116, USA.
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237
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Hussey PJ, Ketelaar T, Deeks MJ. Control of the actin cytoskeleton in plant cell growth. ANNUAL REVIEW OF PLANT BIOLOGY 2006; 57:109-25. [PMID: 16669757 DOI: 10.1146/annurev.arplant.57.032905.105206] [Citation(s) in RCA: 201] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Plant cells grow through increases in volume and cell wall surface area. The mature morphology of a plant cell is a product of the differential rates of expansion between neighboring zones of the cell wall during this process. Filamentous actin arrays are associated with plant cell growth, and the activity of actin-binding proteins is proving to be essential for proper cell morphogenesis. Actin-nucleating proteins participate in cell expansion and cell plate formation whereas the recycling of actin monomers is required to maintain actin dynamics and controlled growth. Coordination of actin-binding protein activity and other aspects of cytoskeletal behavior during cell development maintains cohesive cell expansion. Emerging plant signaling networks are proving to be powerful regulators of morphology-shaping cytoskeletal activity, and in this review we highlight current research in actin network regulation.
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Affiliation(s)
- Patrick J Hussey
- 1The Integrative Cell Biology Laboratory, School of Biological and Biomedical Sciences, University of Durham, Science Laboratories, Durham DH1 3LE, United Kingdom.
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238
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Blancaflor EB, Wang YS, Motes CM. Organization and function of the actin cytoskeleton in developing root cells. INTERNATIONAL REVIEW OF CYTOLOGY 2006; 252:219-64. [PMID: 16984819 DOI: 10.1016/s0074-7696(06)52004-2] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
The actin cytoskeleton is a highly dynamic structure, which mediates various cellular functions in large part through accessory proteins that tilt the balance between monomeric G-actin and filamentous actin (F-actin) or by facilitating interactions between actin and the plasma membrane, microtubules, and other organelles. Roots have become an attractive model to study actin in plant development because of their simple anatomy and accessibility of some root cell types such as root hairs for microscopic analyses. Roots also exhibit a remarkable developmental plasticity and possess a delicate sensory system that is easily manipulated, so that one can design experiments addressing a range of important biological questions. Many facets of root development can be regulated by the diverse actin network found in the various root developmental regions. Various molecules impinge on this actin scaffold to define how a particular root cell type grows or responds to a specific environmental signal. Although advances in genomics are leading the way toward elucidating actin function in roots, more significant strides will be realized when such tools are combined with improved methodologies for accurately depicting how actin is organized in plant cells.
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Affiliation(s)
- Elison B Blancaflor
- Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73401, USA
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239
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Xu J, Scheres B. Cell polarity: ROPing the ends together. CURRENT OPINION IN PLANT BIOLOGY 2005; 8:613-8. [PMID: 16182602 DOI: 10.1016/j.pbi.2005.09.003] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/01/2005] [Accepted: 09/12/2005] [Indexed: 05/04/2023]
Abstract
Cell polarity plays an important role in plant development, but the mechanisms that first establish polarity cues remain obscure. By contrast, a flurry of information has recently emerged on the elaboration of cell shape from such unknown initial cell-polarity cues. Recent studies suggest that Rho-related GTPases in plants (ROPs), and their effector targets among the ROP-interactive CRIB motif-containing proteins (RICs), mediate two antagonistic pathways that have opposing action on cell polarization. ROP proteins appear to interact directly with upstream regulators of the ARP2/3 complex, which are conserved modulators of the actin cytoskeleton. ROP function is dependent on the class 1 ADP-ribosylation factors (ARFs), which are core components of the vesicle transport machinery that are also involved in the polar localization of PIN-FORMED (PIN) family auxin efflux facilitators.
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Affiliation(s)
- Jian Xu
- Department of Molecular Genetics, Utrecht University, Padualaan 8, 3584CH Utrecht, The Netherlands
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240
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Hwang JU, Gu Y, Lee YJ, Yang Z. Oscillatory ROP GTPase activation leads the oscillatory polarized growth of pollen tubes. Mol Biol Cell 2005; 16:5385-99. [PMID: 16148045 PMCID: PMC1266434 DOI: 10.1091/mbc.e05-05-0409] [Citation(s) in RCA: 149] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2005] [Revised: 07/12/2005] [Accepted: 08/30/2005] [Indexed: 11/11/2022] Open
Abstract
Oscillation regulates a wide variety of processes ranging from chemotaxis in Dictyostelium through segmentation in vertebrate development to circadian rhythms. Most studies on the molecular mechanisms underlying oscillation have focused on processes requiring a rhythmic change in gene expression, which usually exhibit a periodicity of >10 min. Mechanisms that control oscillation with shorter periods (<10 min), presumably independent of gene expression changes, are poorly understood. Oscillatory pollen tube tip growth provides an excellent model to investigate such mechanisms. It is well established that ROP1, a Rho-like GTPase from plants, plays an essential role in polarized tip growth in pollen tubes. In this article, we demonstrate that tip-localized ROP1 GTPase activity oscillates in the same frequency with growth oscillation, and leads growth both spatially and temporally. Tip growth requires the coordinate action of two ROP1 downstream pathways that promote the accumulation of tip-localized Ca2+ and actin microfilaments (F-actin), respectively. We show that the ROP1 activity oscillates in a similar phase with the apical F-actin but apparently ahead of tip-localized Ca2+. Furthermore, our observations support the hypothesis that the oscillation of tip-localized ROP activity and ROP-dependent tip growth in pollen tubes is modulated by the two temporally coordinated downstream pathways, an early F-actin assembly pathway and a delayed Ca2+ gradient-forming pathway. To our knowledge, our report is the first to demonstrate the oscillation of Rho GTPase signaling, which may be a common mechanism underlying the oscillation of actin-dependent processes such as polar growth, cell movement, and chemotaxis.
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Affiliation(s)
- Jae-Ung Hwang
- College of Life Sciences, China Agricultural University, Beijing 100094, China
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241
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Schultheiss H, Hensel G, Imani J, Broeders S, Sonnewald U, Kogel KH, Kumlehn J, Hückelhoven R. Ectopic expression of constitutively activated RACB in barley enhances susceptibility to powdery mildew and abiotic stress. PLANT PHYSIOLOGY 2005; 139:353-62. [PMID: 16126850 PMCID: PMC1203384 DOI: 10.1104/pp.105.066613] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
Small RAC/ROP-family G proteins regulate development and stress responses in plants. Transient overexpression and RNA interference experiments suggested that the barley (Hordeum vulgare) RAC/ROP protein RACB is involved in susceptibility to the powdery mildew fungus Blumeria graminis f. sp. hordei. We created transgenic barley plants expressing the constitutively activated RACB mutant racb-G15V under control of the maize (Zea mays) ubiquitin 1 promoter. Individuals of the T1 generation expressing racb-G15V were significantly more susceptible to B. graminis when compared to segregating individuals that did not express racb-G15V. Additionally, racb-G15V-expressing plants showed delayed shoot development from the third leaf stage on, downward rolled leaves, and stunted roots. Expression of racb-G15V decreased photosynthetic CO(2)-assimilation rates and transpiration of nonstressed leaves. In contrast, racb-G15V-expressing barley leaves, when detached from water supply, showed increased water loss and enhanced transpiration. Water loss was associated with reduced responsiveness to abscisic acid in regard to transpiration when compared to segregants not expressing racb-G15V. Hence, RACB might be a common signaling element in response to both biotic and abiotic stress.
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Affiliation(s)
- Holger Schultheiss
- Institute of Phytopathology and Applied Zoology, University of Giessen, Germany
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