* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Signaling tip growth in plants Zhenbiao Yang
Survey
Document related concepts
Cell encapsulation wikipedia , lookup
Cell membrane wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Cellular differentiation wikipedia , lookup
Cell culture wikipedia , lookup
Extracellular matrix wikipedia , lookup
Endomembrane system wikipedia , lookup
Cell growth wikipedia , lookup
Programmed cell death wikipedia , lookup
Rho family of GTPases wikipedia , lookup
Signal transduction wikipedia , lookup
Cytoplasmic streaming wikipedia , lookup
Cytokinesis wikipedia , lookup
Transcript
525 Signaling tip growth in plants Zhenbiao Yang Tip growth is an extreme form of polar growth modulated by both intrinsic and extrinsic spatial cues. Pollen tubes and root hairs have been used as model systems to investigate tip growth signaling in higher plants. Recent studies have focused on tip-localized Ca2+ gradients and Rho GTPases in pollen tubes and a series of mutants affecting root hair tip growth. These molecular and genetic markers will serve as stepping stones towards uncovering tip growth pathways in plants. Addresses Plant Biotechnology Center, The Ohio State University, 1060 Carmack Road, Columbus, OH 43210, USA; e-mail: [email protected] Current Opinion in Plant Biology 1998, 1:525–530 http://biomednet.com/elecref/1369526600100525 © Current Biology Ltd ISSN 1369-5266 Introduction Cell polarity, characterized by asymmetric distribution of subcellular structures and molecules, is a fundamental attribute to the development of all eukaryotic organisms. The unique significance of cell polarity in plant development is underscored by the fact that, in plants, cell morphogenesis is largely defined by spatially organized cell walls. Moreover, the position of sessile cells rather than their lineage has a predominant role in dictating their developmental fates. Because it is difficult to study the control of cell polarity in tissues or organs composed of multiple interacting cell types, most studies on plant cell polarity have been focused on readily accessible tip-growing cells such as fucoid zygotes, root hairs, and pollen tubes. These cells exhibit many crucial aspects of cell polarity development. maintaining this cytoplasmic polarity [7]. Obviously the regulation of actin and microtubule dynamics needs to be coupled to the tip growth signaling events in order to modulate growth direction and rate. Major questions about tip growth are: first, what are the signaling pathways that control the selection and establishment of the cortical site for tip growth? Second, what is the machinery involved in polar secretion and how is it linked to the signaling pathways? Finally, how are the pathways coupled to the cytoskeleton to regulate vesicle transport and cytoplasmic polarity? Similar questions regarding polarized growth such as the formation of buds and mating tubes (‘shmoos’) in Saccharomyces cerevisiae have been investigated extensively [1,8]. In yeast, a signaling network involving several small GTPases controls polarity establishment, cytoskeletal organization, polar secretion, and cell wall assembly. The heart of this network is CDC42, which is a Rho family small GTPase that regulates actin polarization upon activation by various spatial cues [1]. In addition, a CDC42-independent Sec3p pathway is required for polarized growth in yeast [9]. Sec3p shows polar localization to the site of growth and recruits to exocysts (containing complexes of proteins critical for polar secretion) to the same site. Are there similar pathways involved in the regulation of tip growth in plants or are there unique tip growth pathways in plants? This review focuses on recent advances in addressing these questions in the pollen tube and root hair systems. For tip growth in Fucus zygotes, the reader is referred to recent reviews [2••,10]. Recognition and interpretation of spatial cues Intrinsic and extrinsic tip growth cues Tip growth is dependent upon polarized exocytosis of Golgi vesicles to a single defined site of the cell. On the basis of what is known about tip growth in plant cells and cell polarity development in yeast and mammals [1,2••,3•], a general model for tip growth pathways is proposed (Figures 1 and 2). Tip growth begins with the selection and establishment of a cortical site of the cell at which vesicles will dock and fuse [2••]. These processes require a signaling network to recognize and interpret intrinsic and extrinsic spatial cues that determine the site and the direction of tip growth. The signaling network must orchestrate the secretory machinery in such a way that Golgi vesicles dock and fuse only at the specified site. To maintain tip growth, vesicles have to be actively targeted to the tip via the actin cytoskeleton [4]. Consequently, tip-growing cells are organized such that they are composed of the vesicle-rich apex, the subapical region containing various organelles and the nucleus, and the vacuole-occupied basal region [5,6]. Microtubules appear to be involved in Both internal and external cues are known to determine the site of tip growth initiation and the direction of tip growth. In the absence of external cues, Arabidopsis pollen tends to germinate near one end of the pollen grain (R Heath and Z Yang, unpublished data), and root hairs emerge from the apical end of epidermal cells and invariably grow away from the root axis [11,12•]. The nature of these internal cues and their mode of action are unknown. Pollen presents the best example of externally controlled tip growth. When pollen grains land on the stigma, a pollen tube will emerge from the aperture closest to papillar cells, suggesting the presence of a stigma-derived signal [13•]. To deliver sperm to the egg, pollen tubes must penetrate the stigma, travel through the transmitting track, and enter the ovule. Cues for guidance within the stigma and the transmitting track are most likely to involve extracellular matrix components such as glycoproteins and lipids [14,15], and then targeting of tubes to 526 Cell biology Figure 1 Spatial Cues Internal cues External cues Signal recognition Recruitment and activation of signaling apparatus (e.g.,Rop1 GTPase, Ca2+) Establishment of tip growth sites Actin filaments Secretory machinery Polarity maintenance Feedback regulation Microtubules Vesicle accumulation in the apex Localized secretion Cytoplasmic polarization Plasma membrane growth and cell wall assembly Tip growth Current Opinion in Plant Biology A working model for tip growth pathways. The model describes the generalization of current understanding of tip growth in pollen tube, root hair and Fucus zygote systems. It should be noted that detailed mechanisms for polarized growth are most likely to differ among various systems. Substantial evidence is available for the participation of Rop1 and Ca2+ in the establishment of tip growth sites and for the involvement of the actin and microtubule cytoskeleton in tip growth. Many events or components in the pathways remain to be identified, however, and the actual signaling pathways are likely to be complex. ovules requires chemoattractants derived from the female gametophyte [16]. Repulsive cues are also expected to be involved in guidance as only one tube grows towards each ovule [13•]. Establishment of tip growth sites For in vitro germinated pollen, the direction of tube growth is manipulated by electrical currents, sugar and Ca2+ gradients, but roles of these external cues in vivo guidance remain unclear [17]. Nonetheless, pollen tube guidance seems to share striking similarities with animal axon guidance that requires adhesive, attractive, repulsive and electric cues [18,13•]. How the guidance cues are recognized by pollen is unknown, but, in Arabidopsis, POP2 and POP3 genes are good candidates for this role [18]; pollen tubes from the pop2 and pop3 double mutant grow normally within the transmitting tract but are unable to find their way to the ovule, suggesting their specific role in the signaling of ovule-derived factors [18]. Pollen-specific receptor-like kinases (PRKs) from tomato have been localized to the plasma membrane of pollen tubes and might play a role in pollen recognition [19]. In yeast, various spatial cues recruit and activate a unifying signaling apparatus which controls the establishment of a polar site [1,8]. Evidence is emerging that a unifying signaling mechanism also controls the establishment of tip growth sites. Ca2+ is one of the best studied molecules involved in this mechanism. Localized accumulation of Ca 2+ at the cortical site for tip growth has been demonstrated in various systems including pollen tubes, root hairs, Fucus zygotes and fungal hyphae [20–23]. For instance, pollen tubes contain a tip-focused intracellular Ca 2+ gradient ranging from 10 µM at the tip to < 0.2 µM at the base [22,24 •]. Several recent studies suggest a critical role for the gradient in the control of tip growth [22,25–27,24•]. Experiments involving localized release of caged Ca 2+ or Ca2+ chelators in the apex of pollen tubes and root hairs indicate that asymmetric distribution of intracellular Ca2+ alone is sufficient to establish the site of tip growth [26,12 •]. Signaling tip growth in plants Yang 527 Figure 2 (a) MT GV . .. . . . . . . .. . V . .. . . . . . . .. . ER M MF Rop1 Ca 2+ channel M: mitochondria ER: endoplasmic reticulum MT: microtubules GV: Golgi vesicles Ca 2+ MF: actin microfilaments V: vacuoles (b) Current Opinion in Plant Biology Molecular and structural polarity in tip growing cells. (a) Major structures and molecules known to be critical for tip growth in pollen tubes or root hairs. (b) Immunolocalization of Rop1Ps in pea pollen tubes (reproduced with permission from [30]). Tip-localized plasma membrane Ca2+ channel activities are at least partially responsible for the generation of the tip-focused Ca2+ gradient [25–27,12•]. Although Ca2+ channels appear to be localized throughout the apical dome of pollen tubes, only those localized to the extreme apex are the most active [26,27]. Putative plasma membrane Ca2+ channels have also been polarly localized to the tip of Fucus zygotes [28]. Intracellular release, however, may also be involved in the generation of the Ca2+ gradient [29••]. Molecular identification of Ca2+ channels in plants will be a critical step in elucidating the mechanism behind the formation of the Ca2+ gradients. Uncovering other signaling events involved in the establishment of tip growth sites should also provide clues to this mechanism. An exciting recent discovery in tip growth signaling came from studies on tip-localized Rop GTPases [30,31••]. Rop is a plant-specific GTPase of the Rho family that includes CDC42, Rac, and Rho from fungi and animals [32,33•]. The Rho-family GTPases are key regulators of actin cytoskeletal organization and many other cellular processes in fungi and animals [34]. Rop evolved prior to the divergence of Rac and CDC42 and might have functions similar to those of CDC42 and Rac [33•]. A portion of Rop1, a pollen-specific isoform, appears to be localized to the apical plasma membrane with a tip high gradient, implying a recruitment of Rop1 to the growth site [30]. This localization is particularly interesting, because it is analogous to that of yeast CDC42 at the bud emergence and growth sites [35]. When expressed as a GFP fusion protein in fission yeast, Rop1 is also localized to the site of growth [33 •]. Overexpression of Rop1 in fission yeast caused non-polarized growth [33•], which is also induced by constitutively active cdc42 mutants. Microinjected anti-Rop antibodies specifically inhibit pollen tube growth [31••]. Interestingly, this effect is more similar to loss-of-function cdc42 mutations in fission yeast (growth arrest) than in budding yeast (enlarged cells unable to bud) [8]. Preliminary results show that overexpression of Rop1 transformed normal cylindrical 528 Cell biology tubes into bulbous tubes (H Li and Z Yang, unpublished data). These results indicate that Rop controls not only the establishment of cell polarity but also tip growth, as does CDC42 in tip-growing fission yeast [8]. The bulbous phenotype is analogous to tip swelling accompanied by non-localized intracellular Ca2+ accumulation when arrested pollen tubes resume growth [22,26]. Further, anti-Rop antibody-induced growth inhibition is potentiated by a lower threshold of extracellular Ca2+ concentration and a subinhibitory concentration of caffeine [31••]. Although the mechanism behind the Rop–Ca2+ interaction is unknown, an attractive explanation for these data is that Rop regulates localized accumulation of intracellular Ca2+, which is thought to be involved in the regulation of exocytosis through annexins [27,36]. The Rho-family GTPases also regulate intracellular Ca2+ release and plasma membrane Ca2+ channels in animal cells [37,38]. Thus, Rop could modulate the Ca2+ gradient by integrating various tip growth spatial cues. In this regard, Rop may be analogous to the yeast CDC42, which integrates bud site selection, pheromone, and pseudohyphal growth signaling pathways to control polarity establishment through actin polarization [1]. Whether Rop regulates actin organization in pollen tubes remains to be determined. Nonetheless, this role would be difficult to compromise with recent findings that suggest pollen tubes do not contain apical actin networks [39]. Tipassociated actin filaments have been demonstrated in fucoid zygotes and play a crucial role in the establishment of polarity [2••,40]. A Rac-like GTPase has been identified in Fucus and has also been localized to the tip, providing a good candidate for regulating polarity establishment through organizing the tip-associated actin or other tip plasma membrane-associated asymmetries (J Fowler and R Quatrano, personal communication). In summary, current evidence suggests that a Rop GTPase controls the establishment of tip growth sites, analogous to CDC42 in the control of polarity establishment in yeasts. The mechanism of Rop-dependent polarity control, however, seems to be unique in that it might regulate the tip-focused Ca2+ gradient, which has yet to be shown in yeasts. Cytoskeletal regulation of tip growth Recent identification of several cytoskeleton-associated proteins [4] may provide a starting point for understanding how cytoskeletal dynamics is regulated by the signaling network that controls tip growth. Multiple isoforms of the actin-binding protein profilin are found in pollen [41]. Profilins have been implicated in the regulation of actin dynamics in Tradescantia stamen hair cells and thus may also have a similar role in pollen tubes [41]. In addition, pollen profilins may also participate in signaling, as suggested by their ability to alter the phosphorylation of specific pollen proteins [42]. Unfortunately, the uniform cytoplasmic localization of profilins in pollen tubes revealed by immunolocalization and in live cells was not informative regarding their roles in pollen [43]. Maize pollen profilin isoforms show differential capacities to interact with proline-rich motifs, which are correlated with their activities in altering actin dynamics in plant cells [44••]. In animals and yeast, profilin interacts with prolinerich formin-homology domain proteins, apparent downstream targets of Rho GTPases in the regulation of actin dynamics [45]. Interestingly, a Rho-like GTPase may also regulate actin organization in pollen tubes [31••]. Another potential regulator of the actin cytoskeleton is calcium-dependent protein kinases that associate with microfilaments and are implicated in pollen germination [46,47]. More attention needs to be given to the involvement of these actin-associated proteins in the regulation of the actin cytoskeleton in tip-growing cells. Tip growth mutants Although biochemical and molecular studies described above have shed some light on tip growth mechanisms, these studies must be complemented with genetic approaches to fully elucidate tip growth pathways, as in the study of yeast cell polarity control. Indeed, some new insights into tip growth in plants have been obtained by mutant analyses. Root hairs serve as a model system for genetic analyses of tip growth because they are dispensable and easy for mutant screen [48,49]. Three loci affecting maize root hair elongation and morphology were identified [49]. Mutations in RTH1 are defective in hair elongation. RTH1 encodes a protein with similarity to Sec3p (P Schnable, personal communication), raising the possibility that the Sec3p pathway and exocyst might also be involved in tip growth in plants. Exocyst is also critical for polarized secretion in mammalian epithelial cells [50]. Seven complementation groups of tip growth mutants from Arabidopsis have been reported and are divided into three classes according to their phenotypes and genetic interactions [11,48,51,52] (see Figure 3). The hair initiation mutants (rhd6) show reduced hair numbers and altered initiation sites and are rescued by auxin or ethylene, suggesting a role for RHD6 in controlling the site of root hair initiation [11]. Growth site establishment mutants exhibit abnormal growth site morphology (rhd1) or fail to undergo tip growth (rhd2). Tip growth maintenance mutants (rhd3, cow1, tip1 and rhd4) display various morphological changes. Consistent with pleiotropic effects of some of these mutations in cell types other than root hairs [52,53], this class of genes is most likely to be involved in polar secretion, cytoskeletal organization, cell wall assembly, and cell expansion. RHD3 affects vacuole enlargement and Golgi vesicle accumulation at the tip [53]. Recently, RHD3 has been shown to encode a novel GTP-binding protein, suggesting its potential role in Signaling tip growth in plants Yang tubes, and root hairs. The review focuses on the development of cortical sites that control polar growth and asymmetric cell division in Fucus zygotes. Evidence is presented that the cortical site involves localized cytosolic Ca2+, plasma membrane Ca2+ channels, and the actin cytoskeleton, which are all required for the establishment of cell polarity and for polar secretion. Figure 3 Selection Establishment Maintenance/growth COW1 RHD6 RHD2 RHD1 RHD4 RHD3 TIP1 529 Tip growth Current Opinion in Plant Biology A genetic pathway for root hair tip growth in Arabidopsis. The pathway was proposed on the basis of phenotypic analyses of single and double mutants (see [11,48,51]). The mutants described are classified into three groups according to the proposed tip growth pathways described in Figure 1. The initiation mutants show changes in the number of root hairs and the position of hair initiation. Affected genes are predicted to participate in early signaling of root hair initiation cues. The establishment mutants show loss of polarity in the root hair proper. These genes are expected to be required for the interpretation of tip growth cues. The maintenance/growth mutants affect root hair morphology and elongation. This class of genes are most likely involved in cytoskeletal organization, secretion, and cell wall asembly. signaling these processes [54••]. Genetic analyses of tip growth in root hairs, therefore, will not only reveal tip growth mechanisms but also shed light on other fundamental cellular processes. Conclusions Significant progress has been made towards understanding signaling pathways leading to tip growth in plants. The tip-focused Ca2+ gradient is a unifying mechanism for the establishment of tip growth sites in many tip growth systems, and a Rop GTPase is emerging as another key player in regulating this process in pollen tubes, probably by interacting with the Ca2+ signaling. Future work should focus on determining whether Rop has a unifying role in polarized plant cell growth, uncovering the mechanistic relationship between Rop and Ca2+, and understanding how these signaling events are mediated by specific spatial cues and how they control polarized secretion and cytoskeletal organization. Clearly, a genetic approach will continue to provide a new inroad into tip growth signaling pathways in plants. Acknowledgements I am grateful to John Fowler and Desh Pal S Verma for their valuable comments on the manuscript and to members of my laboratory for helpful discussions. This work is supported by National Science Foundation and United States Department of Agriculture. References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: 3. Taylor LP, Hepler PK: Pollen germination and tube growth. Annu • Rev Plant Physiol Plant Mol Biol 1997, 48:461-491. A comprehensive review on mechanisms of pollen germination and tube growth. Topics discussed include calcium influxes, cytoskeletal dynamics, extracellular matrix, and pollen gene expression. 4. Cai G, Moscatelli A, Cresti M: Cytoskeletal organization and pollen tube growth. Trends Plant Sci 1997, 2:86-91. 5. Lancelle SA, Hepler PK: Ultrastructure of freeze-substituted pollen tubes of Lilium longiflorum. Protoplasma 1992, 167:215-230. 6. Miller DD, de Ruijter NCA, Emons AMC: From signal to form: aspects of the cytoskeleton–plasma membrane–cell wall continuum in root hair tips. J Exp Bot 1997, 48:1881-1896. 7. Joos U, van Aken J, Kristen U: Microtubules are involved in maintaining the cellular polarity in pollen tubes of Nicotiana sylvestris. Protoplasma 1994, 179:5-15. 8. Mata J, Nurse P: Discovering the poles in yeast. Trends Cell Biol 1998, 8:163-167. 9. Finger FP, Hughes TE, Novick P: Sec3p is a spatial landmark for polarized secretion in budding yeast. Cell 1998, 92:559-571. 10. Kropf DL: Induction of polarity in fucoid zygotes. Plant Cell 1997, 9:1011-1020. 11. Masucci JD, Schiefelbein JW: The rhd6 mutation of Arabidopsis thaliana alters root-hair initiation through an auxin- and ethyleneassociated process. Plant Physiol 1994, 106:1335-1346. 12. Bibikova TN, Zhigilei A, Gilroy S: Root hair growth in Arabidopsis • thaliana is directed by calcium and an endogenous polarity. Planta 1997, 203:495-505. The paper shows that an artificially generated localized Ca2+ influx can reorient root hair tip growth. The reoriented root hair tip, however, will return to the original direction of growth. These results suggest that the direction of root hair tip growth is controlled by tip-localized Ca2+ influxes which in turn are regulated by an endogenous signal(s). 13. Wilhelmi LK, Preuss D: Blazing new trails: pollen tube guidance in • flowering plants. Plant Physiol 1997, 113:307-312. A well-written review focusing on pollen–stigma interactions and various external cues that control pollen tube guidance. 14. Wu H-M, Wang H, Cheung AY: A pollen tube growth stimulatory glycoprotein is deglycosylated by pollen tubes and displays a glycosylation gradient in the flower. Cell 1995, 82:394-403. 15. Wolters-Arts M, Lush WM, Mariani C: Lipids are required for directional pollen tube growth. Nature 1998, 392:818-821. 16. Ray S, Park S-S, Ray A: Pollen tube guidance by the female gametophyte. Development 1997, 124:2489-2498. 17. Mascarenhas JP: Molecular mechanisms of pollen tube growth and differentiation. Plant Cell 1993, 5:1303-1314. 18. Wilhelmi LK, Preuss D: Self-sterility in Arabidopsis due to defective pollen tube guidance. Science 1996, 274:1535-1537. 19. Muschietti J, Eyal Y, McCormick S: Pollen tube localization implies a role in pollen–pistil interactions for the tomato receptor-like protein kinases LePRK1 and LePRK2. Plant Cell 1998, 10:319-330. 20. Berger F, Brownlee C: Ratio confocal imaging of free cytoplasmic calcium gradients in polarizing and polarized Fucus zygote. Zygote 1993, 1:9-15. Drubin DG, Nelson WJ: Origins of cell polarity. Cell 1996, 84:335-344. 21. Garrill A, Jackson SL, Lew RR, Heath IB: Ion channel activity and tip growth: tip localized stretch-activated channels generate an essential Ca2+ gradient in the oomycete, Saprolegnia ferax. Eur J Cell Biol 1993, 60:358-365. Fowler JE, Quatrano RS: Plant cell morphogenesis: plasma membrane interactions with the cytoskeleton and cell wall. Annu Rev Cell Dev Biol 1997, 13:697-743. An excellent review on the principles of morphogenesis in plants derived from recent studies in several model systems including fucoid zygotes, pollen 22. Pierson ES, Miller DD, Callaham DA, Shipley AM, Rivers BA, Cresti M, Hepler PK: Pollen tube growth is coupled to the extracellular calcium ion flux and the intracellular calcium gradient: effect of BAPTA-type buffers and hypertonic media. Plant Cell 1994, 6:1815-1828. • of special interest •• of outstanding interest 1. 2. •• 530 Cell biology 23. Wymer CL, Bibikova TN, Gilroy S: Cytoplasmic free calcium distributions during the development of root hairs of Arabidopsis thaliana. Plant J 1997, 203:427-439. 39. Miller DD, Lancelle SA, Hepler PK: Actin microfilaments do not form a dense meshwork in Lilium longiflorum pollen tube tips. Protoplasma 1996, 195:123-132. 24. Messerli M, Robinson KR: Tip localized Ca2+ pulses are coincident • with peak pulsatile growth rates in pollen tubes of Lilium longiflorum. J Cell Sci 1997, 110:1269-1278. This paper together with [29••] shows that the tip-localized Ca2+ oscillates in phase with the growth of pollen tubes. 40. Kropf DL, Bisgrove SR, Hable WE: Cytoskeletal control of polar growth in plant cells. Curr Opin Cell Biol 1998, 10:117-122. 25. Malhó R, Read ND, Trewavas AJ, Salomé Pais M: Calcium channel activity during pollen tube growth and reorientation. Plant Cell 1995, 7:1173-1184. 26. Malhó R, Trewavas AJ: Localized apical increases of cytosolic free calcium control pollen tube orientation. Plant Cell 1996, 8:1935-1949. 27. Pierson ES, Miller DD, Gallaham DA, van Aken J, Hackett G, Hepler PK: Tip-localized calcium entry fluctuates during pollen tube growth. Dev Biol 1996, 174:160-173. 28. Shaw SL, Quatrano RS: Polar localization of a dihydropyridine receptor on living Fucus zygotes. J Cell Sci 1996, 109:335-342. 29. Holdaway-Clarke TL, Feió JA, Hackett GR, Kunkel JG, Hepler PK: •• Pollen tube growth and the intracellular cytosolic calcium gradient oscillate in phase while extracellular calcium influx is delayed. Plant Cell 1997, 9:1999-2010. The paper describes some elegant experiments to demonstrate that although the tip-focused Ca2+ gradient oscillates in phase with growth, Ca2+ influxes lag by 11 seconds. The phase delay in extracellular Ca2+ influx is interpreted as a result of changes in some types of Ca2+ stores. In other words, the measured Ca2+ influx represents either the refilling of intracellular Ca2+ stores or the binding of Ca2+ within the cell wall domain. 30. Lin Y, Wang Y, Zhu J, Yang Z: Localization of a rho GTPase implies a role in tip growth and movement of the generative cell in pollen tubes. Plant Cell 1996, 8:293-303. 31. Lin Y, Yang Z: Inhibition of pollen tube elongation by microinjected •• anti-Rop1Ps antibodies suggests a crucial role for Rho-type GTPases in the control of tip growth. Plant Cell 1997, 9:1647-1659. The demonstration that microinjected anti-Rop1Ps antibodies specifically cause rapid growth arrest. This effect is potentiated by low extracellular Ca2+ and by treatments with subinhibitory concentrations of caffeine. Further, the antibody-induced growth inhibition is not associated with changes in cytoplasmic streaming, whereas growth arrest induced by the bacterial C3 toxin, an ADP-ribosyltransferase that specifically inactivates the Rho subfamily of Rho GTPases, is accompanied by the cessation of cytoplasmic streaming. It is proposed that Rop1 works in junction with Ca2+ signaling to control polarized secretion, while a C3-sensitive Rho GTPase is involved in cytoplasmic streaming that delivers vesicles to the tip. 32. Winge P, Brembu T, Bones AM: Cloning and characterization of raclike cDNAs from Arabidopsis thaliana. Plant Mol Biol 1997, 35:483-495. 33. Li H, Wu G, Ware D, Davis KR, Yang Z: Arabidopsis Rop GTPases: • differential gene expression in pollen and polar localization in fission yeast. Plant Physiol 1998, in press. Rop1At, a member of the Arabidopsis Rop subfamily of Rho GTPases, is specifically expressed in mature pollen. Overexpression of Rop1At or Rop1At fused to the jellyfish green fluorescence protein (GFP) induced isotropic growth in fission yeast, analogous to the phenotype caused by constitutively active cdc42 mutations. The GFP:Rop1At fusion appears to be localized to the site of exocytosis in fission yeast, that is, growing ends and septa of the cell. The results suggest a functional similarity between Rop1At and yeast CDC42. 34. Hall A: Rho GTPases and the actin cytoskeleton. Science 1998, 279:509-514. 35. Ziman M, Preuss D, Mulholland J, O’Brien JM, Botstein D, Johnson DI: Subcellular localization of Cdc42p, a Saccharomyces cerevisiae GTP-binding protein involved in the control of cell polarity. Mol Biol Cell 1993, 4:1307-1316. 36. Battey NH, Blackbourn H: The control of exocytosis in plant cells. New Phytologist 1993, 125:307-338. 37. Grönroos E, Andersson T, Schippert Å, Zheng L, Sjölander: Leukotriene D4-induced mobilization of intracellular Ca2+ in epithelial cells is critically dependent on activation of the small GTP-binding protein rho. Biochem J 1996, 316:239-245. 38. Peppelenbosch MP, Tertoolen LGJ, de Vries-Smits AMM, Qui R-G, M’Rabet L, Symons MH, de Laat SW, Bos JL: Rac-dependent and independent pathways mediate growth factor-induced Ca2+ influx. J Biol Chem 1996, 271:7883-7886. 41. Staiger CJ, Gibbon BC, Kovar DR, Zonia LE: Profilin and actin depolymerizing factor: modulators of actin organization in plants. Trends Plant Sci 1997, 2:275-281. 42. Clarke SR, Staiger CJ, Gibbon BC, Franklin-Tong VE: A potential signaling role for profilin in pollen of Papaver rhoeas. Plant Cell 1998, 10:967-979. 43. Vidali L, Hepler P: Characterization and localization of profilin in pollen grains and tubes of Lilium longiflorum. Cell Motil Cytoskel 1997, 36:323-338. 44. Gibbon BC, Zonia LE, Kovar DR, Hussey PJ, Staiger CJ: Pollen •• profilin function depends on interaction with profilin rich motifs. Plant Cell 1998, 10:981-994. This paper describes an important finding that the pollen profilin isoform ZmPRO4 is functionally distinct from other maize pollen profilins in that it has greater capacities to interact with poly-L-proline and to alter actin dynamics. Further, a gain-of-function mutant (ZmPRO1-Y6F) has ZmPRO4-like properties in its interaction with proline-rich motifs and actin regulation. These results may have significant implications in understanding the signaling mechanism modulating actin dynamics, given recent findings in yeast and animals indicating that proteins with proline-rich formin-homology (FH) domains connect the Rho-family GTPases with profilins in the regulation of actin dynamics [45]. It should be noted that FH-containing sequences are present in the plant DNA database. 45. Watanabe N, Madaule P, Reid T, Ishizaki T, Watanabe G, Kakizuka A, Saito Y, Nakao K, Jockusch BM, Narumiya S: p14mDia, a mammalian homolog of Drosophila diaphanous, is a target for protein Rho small GTPase and is a ligand for profilin. EMBO J 1997, 16:3044-3056. 46. Putnam-Evans CL, Harmon AC, Palevitz BA, Fechheimer M, Cormier MJ: Calcium-dependent protein kinase is localized with F-actin in plant cells. Cell Motil Cytoskel 1989, 12:12-22. 47. Estruch J, Kadwell S, Merlin E, Crossland L: Cloning and characterization of a maize pollen-specific calcium-dependent calmodulin-independent protein kinase. Proc Natl Acad Sci USA 1994, 91:8837-8841. 48. Schiefelbein JD: Genetic control of root hair development in Arabidopsis thaliana. Plant Cell 1990, 2:235-243. 49. Wen T-J, Schnable PS: Analyses of mutants of three genes that influence root hair development in Zea mays (Gramineae) suggest that root hairs are dispensable. Am J Bot 1994, 81:833-842. 50. Grindstaff KK, Yeaman C, Anandasabapathy N, Hsu S-C, Rodriguez-Boulan E, Scheller RH, Nelson WJ: Sec6/8 complex is recruited to cell-cell contacts and specifies transport vesicle delivery to the basal-lateral membrane in epithelial cells. Cell 1998, 93:731-740. 51. Grierson CS, Roberts KR, Feldman KA, Dolan L: The COW1 locus of Arabidopsis acts after RHD2, and in parallel with RHD3 and TIP1, to determine the shape, rate of elongation, and number of root hairs produced from each site of hair formation. Plant Physiol 1997, 115:981-990. 52. Ryan E, Grierson CS, Carell A, Steer M, Dolan L: TIP1 is required for both tip growth and non-tip growth in Arabidopsis. New Phytol 1998, 138:49-58. 53. Galway ME, Heckman JW, Schiefelbein JW: Growth and ultrastructure of Arabidopsis root hairs: the rhd3 mutation alters vacuole enlargement and tip growth. Planta 1997, 201:209-218. 54. Wang H, Lockwood SK, Hoeltzel MF, Schiefelbein JW: The ROOT •• HAIR DEFECTIVE3 gene encodes an evolutionarily conserved protein with GTP-binding motifs and is required for regulated cell enlargement in Arabidopsis. Genes Dev 1997, 11:799-811. This is the first report on the mutant-based cloning of a gene involved in tip growth in plants. The RDH3 gene, which may be involved in vacuole enlargement and Golgi vesicle targeting to the tip, encodes a novel 87-kD polypeptide with GTP-binding motifs. RDH3-like genes are found in other eukaryotes, suggesting that they may control fundamental cellular processes. This is consistent with the pleiotropic effects of rdh3 mutations on cell expansion in Arabidopsis.