The past year has seen considerable advances in our understanding of signaling in pollen tubes. Evidence
suggesting that lipids are involved in pollen tube guidance has opened up new avenues. Major advances have been made in understanding the roles of Rho-like GTPases and protein kinases in regulating pollen tube growth. Light is being shed on how signals may be integrated. It is becoming clear that the role of Ca2+in pollen tube growth is perhaps more complex than originally anticipated.
Addresses
Wolfson Laboratory for Plant Molecular Biology, School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK; e-mail: [email protected]
Current Opinion in Plant Biology1999, 2:490–495
1369-5266/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
Abbreviations
ADF actin depolymerizing factor AGP arabinogalactan protein
CaM calmodulin
CDPK Ca2+-dependent protein kinase Ins(1,4,5)P3 inositol triphosphate
PKC protein kinase C
PtdIns(4,5)P2 phosphatidylinositol 4,5-bisphosphate
Introduction
When pollen lands on the stigma of a flower a complex series of events, described loosely as pollination, are set in motion. After hydration and germination the pollen tube faces a long journey to the ovary where it may eventually effect fertilization and produce seed. A cartoon of a gener-alized pistil, outlining events and the route of the pollen tube, is shown in Figure 1. Pollination in flowering plants begins with a recognition process that is poorly understood at the molecular level. Inter-specific pollination is con-trolled very rigidly so that pollen from species other than that of the pistil is rejected. If the pollen is accepted there are often further barriers to prevent intra-specific self-fer-tilization, such as self-incompatibility. Self-incompatibility is controlled by the Sgenes and pollen is inhibited if it
car-ries Salleles that are genetically identical to those carried
by the pistil. If the pollen germinates and a pollen tube is successfully formed it must grow all the way to the ovary where the sperm cells are delivered to the ovule. Pollen tube growth and its control are of considerable importance and interest for both fundamental studies of the control of fertility and reproduction, and also as an attractive model system for the investigation of polarized tip growth, cell–cell interactions and signal transduction.
Pollen tubes provide a classic example of tip growth. They establish polarity and extend by vesicle fusion in a highly
defined apical region. Pollen germination and tube growth are dependent on a functional actin cytoskeleton. This allows vigorous cytoplasmic streaming which moves organelles around the pollen tube and brings vesicles to the tip region. There they fuse to produce new cell mem-brane and cell wall. Evidence from other eukaryotic cells increasingly suggests that the actin cytoskeleton of pollen acts not only as a structural element, but also as the target of signaling pathways. It has always been assumed that pollen tubes respond to a myriad of chemical and physical signals and cues which regulate and guide their growth.
The pollen tube has become a relatively well-characterized model system in which to study cell–cell recognition, intra-and intercellular signaling intra-and responses to signals in higher plants (see [1] for a recent review). There is now compelling evidence that signal transduction occurs in pollen tubes, implicating a function in pollination. A central role for Ca2+
in regulating pollen tube growth has been demonstrated [1–3]. Alterations in the concentration of cytostolic free cal-cium ([Ca2+]
i) result in changes in the rate, direction and
inhibition of pollen growth in response to defined proteins known to interact with the pollen [1,3]. With the importance of Ca2+-mediated signaling established, the next challenge
is to identify other components of the signaling pathway. My review highlights some of the most recent steps forward that have significantly changed our perception and understand-ing of the signalunderstand-ing components and pathways involved in modulating pollen tube growth.
Lipids play a role in guidance
The debate on how pollen tube guidance operates has been ongoing for decades, with many suggestions as to what might stimulate the reorientation of pollen tubes. Although pollen tubes can be grown in vitroin the absence
of a pistil, they are known to interact with the extracellular matrix components of the transmitting tract through which they grow [4,5]. Until recently, consensus opinion has been that several pistil components play an important role in pollen adhesion, the stimulation of pollen tube growth, directional guidance and signaling. One particular class of molecule that is found in the style, and which is implicat-ed in the regulation of pollen tube growth, is the arabinogalactan protein (AGP) family. Not only do some AGPs, such as tobacco transmitting tissue glycoprotein stimulate pollen tube growth, but there is also evidence that pollen tubes grow towards these AGPs [5].
New evidence has demonstrated that triacylglycerides play a role in penetration of tissues by the pollen tube [6••,7••].
It has been proposed that lipids can control directional growth by forming a water gradient that regulates the flow of water to pollen. The particular physico-chemical prop-erties of these lipids, rather than their potential signaling
capacity, are thought to be important. Moreover, Ca2+and
sugars were shown not to be required for directional growth as elimination of these compartments did not affect the directional growth of the pollen tubes [7••]. This
con-tentious and interesting proposition overturns many of the accepted hypotheses on how pollen tubes direct their ori-entation. It remains to be seen if this system operates together with other signals thought to be responsible for directional growth during pollination.
What is the role of Ca
2+?
Apical [Ca2+]igradients and localized Ca2+influx at the tip
are general characteristics of growing pollen tubes. Until recently it was thought that Ca2+influx might regulate pollen
tube growth directly. There is now convincing evidence of a positive correlation between changes in [Ca2+]
iand both the
rate and direction of pollen tube growth. Changes in the direction of pollen tube growth are preceded by increases in Ca2+detected in the side of the pollen tube towards which
the pollen tube subsequently turns. Furthermore, [Ca2+] ihas
been shown to act as a second messenger mediating the inhi-bition of pollen tube growth in the self-incompatibility response [1,3]. The role of Ca2+influx, however, needs
re-evaluation. The apical [Ca2+]
i in growing pollen tubes
oscillates with a very similar periodicity as does pollen tube growth. The suggestion is, therefore, that there is a causal link between these two pulses. A detailed discussion of pollen tube oscillations may be found in [8].
It is becoming apparent, however, that the role of Ca2+in
pollen tube growth is more complex than originally antici-pated. Recent studies have demonstrated that although increases in [Ca2+]
iat the tip virtually coincide with pulses
in growth, Ca2+ influx does not [9,10]; there is new
evi-dence that [Ca2+]
iincreases lag slightly behind the growth
pulses (MA Messerli, KR Robinson, personal communica-tion). Further studies are urgently needed to clarify this matter. The unexpected finding that apical intracellular and extracellular Ca2+fluxes appear not to be directly
con-nected has important implications for models of pollen tube growth regulation. Several puzzling questions remain unanswered. One is, ‘Where do the apical increases in [Ca2+]
icome from, if it is not directly from Ca2+influx at
the tip?’ A second is, ‘Why do pollen tubes have an oscil-latory mode of growth?’ Despite earlier failures to detect pH gradients, several reports have identified H+fluxes at
the pollen tube tip [11,12]. Although it has not been shown that they are necessary for tip growth, H+fluxes oscillate
with a similar periodicity as growth [12]. Clearly much more investigation needs to be undertaken before we can have a clear idea of what controls ion fluxes and growth at the pollen tube tip.
It is becoming evident that [Ca2+]
i fluxes also occur in
regions of the pollen tube other than the apical region, espe-cially in response to external signals (see [1]). Extracellular Ca2+is not the only source of Ca2+in pollen tubes though
the signals involved in releasing Ca2+ from other sources
remain largely unknown. There is emerging evidence, how-ever, that a functional phosphoinositide pathway operates in growing pollen tubes (see [1]). This pathway is thought to be involved in co-ordinating and regulating pollen tube growth and in transmitting signals through the pollen tube, perhaps via Ca2+waves. Recent data [13] provide further
evidence for intracellular pools of Ca2+in the sub-apical and
nuclear regions of the pollen tube which are sensitive to inositol triphosphate (Ins[1,4,5]P3). This implies that Ca2+
stores are present compartments which have Ins(1,4,5)P3 receptors within distinct and localized regions, mainly with-in the ‘shank’ of the pollen tube. The next challenge is to identify and define the nature of the Ca2+pools and how
they are regulated.
Ca2+has many roles within the cell, including that of a
sec-ond messenger for numerous signaling pathways (see [3,14] for recent reviews). An important Ca2+-interacting
Figure 1
A generalized diagram of a pistil and the pollination route that pollen takes on its journey to accomplish fertilization. Pollen hydrates, germinates and grows through the transmitting tract to the ovules, where the pollen tube has to find the correct entry point before it effects fertilization.
Pollen tube growth
Transmitting
Pollen tube tract
Ovules Pollen
hydration Pollen germination
Stigma
Style
Ovary
Fertilization
protein is calmodulin (CaM). A recent study of pollen tube myosin has demonstrated an association between CaM and myosin and that, at least in vitro, both this association and
the myosin-based motility of F-actin are inhibited by high Ca2+concentrations [15•]. It also provides a possible
expla-nation as to why no cytoplasmic streaming is detected in the ‘clear zone’ at the pollen tube tip where the [Ca2+]
iis
high. Interestingly, imaging of CaM in growing pollen tubes suggests that it is distributed evenly throughout the cytosol [16]; however, this imaging does not distinguish between free CaM and CaM which is bound to a ligand.
The association of CaM and myosin, together with its reg-ulation by Ca2+, implicate a potential role for Ca2+–CaM in
the regulation of actomyosin activity, and therefore in the regulation of not only intracellular movement in pollen tubes, but also of pollen tube growth. Direct evidence that the actin cytoskeleton is a target for Ca2+-/CaM-dependent
protein kinase signaling cascades has been obtained in liv-ing plant cells usliv-ing biophysical measurements of actin tension [17]. Increases in [Ca2+]
iand consequent
stimula-tion of Ca2+-/CaM-dependent protein kinases or regulatory
proteins are implicated in changing the tension and organi-zation of the actin cytoskeleton. Whether such a mechanism operates in pollen tubes is still open to ques-tion, but on the current evidence it seems plausible.
Evidence for a modulatory protein
kinase activity
Relatively little is known about the mechanisms by which protein kinases and their phosphorylation are involved in pollen tube growth, although several protein kinases have been shown to be active in pollen extracts. The evidence indicates that protein kinase activity is involved in the modulation of pollen tube growth, including the self-incompatibility response [1,18,19]. Until recently there has been little direct evidence for a major role for protein kinase activity during pollination, but the cloning and char-acterization of several classes of protein kinase has triggered renewed interest in their potential role(s) in pollen tube growth [1]. Furthermore, the recent identifica-tion of receptor kinases in pollen [20,21] has provided clear evidence that signal transduction occurs during pollination.
The novel approach of imaging Ca2+-dependent protein
kinase activity in the apical region has recently provided evi-dence of a functional role for protein kinases in pollen tube growth [22••]. Interestingly, this localization appears to be
dif-ferent from the apical Ca2+ gradient in pollen tubes. This
difference may be as a result of the extremely high [Ca2+] iin
the apical region which is reported to reach concentrations as high as 10µM; such concentrations might be expected to inhibit protein kinase activity in vivo. An exciting finding
from the imaging approach is that a spatial re-distribution of kinase activity in the apical dome could result in the reorien-tation of pollen tube growth [22••], suggesting that protein
kinases may be intimately involved in pollen tube reorienta-tion. Although the imaging probe used — which was
designed to detect protein kinase C (PKC)-type activities — was shown to report a Ca2+-dependent PKC-like activity, the
protein kinase(s) to which it binds has not yet been identified. It is therefore important to bear in mind that the probe might also be reporting other protein kinase activities. Nevertheless, this study represents a real step forward in our understanding of pollen tube signaling by demonstrating the involvement of protein kinase activity in pollen tube reorientation.
Futher evidence supporting the involvement of protein kinases and phosphatases in the modulation of pollen tube growth comes from studies of the actin cytoskeleton. After receiving an external stimulus, many eukaryotic cells rapidly rearrange their actin cytoskeleton. Emerging data suggest that the cytoskeleton is a target for signaling path-ways. As a functional actomyosin-based cytoskeleton is thought to play a major role in the modulation of pollen tube growth, the perception and transduction of signals which mediate changes in the cytoarchitecture are of con-siderable interest. In addition to the regulation of myosin activity by Ca2+which was mentioned previously, several
studies implicate the actin cytoskeleton, and those pro-teins that interact with it, as a target for protein kinases.
Ca2+ modulates the activity of actin-binding proteins,
phosphoinositidases and protein kinases that are implicat-ed as key regulators of tip growth. This Ca2+modulation
provides a potential link between second messenger sig-naling and remodeling of the cytoarchitecture at the apex of the pollen tube. Interestingly, pulsatile changes in actin organization at the tip of the pollen tube, which would be expected from the fluxes in Ca2+and presence of
actin-binding proteins, have not been demonstrated, possibly because of the limited temporal resolution. One actin-binding protein which has been identified in pollen is actin depolymerizing factor (ADF) [23]. The phosphorylation of animal ADF inhibits G-actin-binding activity. Evidence that this is also the case for a plant ADF, and that this process is Ca2+-stimulated, has recently been reported in vitro[24], suggesting an important functional role for this
protein. Interestingly, in animal cells a LIM-kinase has recently been shown to be responsible for the phosphory-laton of ADF, thereby inactivating it and allowing the formation of F-actin [25,26].
One of the most abundant actin-binding proteins in pollen, profilin, is known to regulate microfilament formation and dynamics. Profilin also interacts with phosphoinositides, such as phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P2),
and proline-rich proteins that are thought to target it to sub-cellular regions where it might be associated with membrane-linked signaling complexes [27,28]. Although pollen profilin is known to interact with actin and PtdIns(4,5)P2, proline-rich proteins that interact with
profil-in have not yet been identified profil-in plants. It has recently been demonstrated, however, that (in vitroat least) profilin
soluble pollen phosphoproteins [29•]. This evidence goes
some way towards indicating the involvement of profilin in modulating the activity of signaling component(s) which affect protein phosphorylation, most likely by regulating protein kinase or phosphatase activity. Recently, evidence that profilin itself can be phosphorylated has been obtained ([30], CJ Staiger, VE Franklin-Tong, unpublished data). As pollen profilin has been shown to play a role in regulating the actin cytoskeleton in vivo[31] it is not inconceivable that
profilin may act in a signaling capacity, regulating pollen tube growth through its modulation of protein kinase activ-ity. Although there is compelling evidence that profilin plays a role in signaling pathways in pollen tubes, how this is achieved is, as yet, unestablished.
Although Ca2+-dependent protein kinases (CDPKs) were
shown to co-localize with microfilaments in plant cells a decade ago [32] more direct evidence for the involvement of protein kinases in the regulation of the actin cytoskele-ton has been found only recently. A pollen protein that cross-reacts with the actin-binding protein caldesmon, which is implicated in regulating the cytoarchitecture in animal cells, has recently been identified [33].
Dephosphorylation is, unsurprisingly, as important as phos-phorylation in controlling signaling. It has recently been shown that pollen tube growth and polarity are disturbed by protein phosphatase inhibitors [34]. It appears that pollina-tion events are likely to involve a complex interplay between signaling pathways involving protein kinase and phosphatase cascades which may regulate pollen tube growth by acting on the actin-based cytoskeleton organization and assembly.
A role for Rho-GTPases
Recent progress in understanding the role of Rho-GTPases in pollen tube growth is particularly exciting. A highly conserved
Rho family of GTPases that act as molecular switches regu-late actin organization and actin-associated cellular responses. The identification of this class of GTPase in pollen has prompted studies into the possible roles played by Rho- and Rac-related GTPases in pollen tube growth [35]. This new work has provided further evidence in support of the idea that signaling at the cytoskeleton interface controls apical growth in pollen tubes.
The localization of Rop, a pollen-specific Rho-type GTPase, to the apical region of the pollen tube plasma membrane [36] first drew attention to the its potential role of this protein in pollen tip growth. More recently, Rop has been demonstrated to play a pivotal role in the signaling controlling the polarized apical growth typical of pollen tubes [4,37,38•,39••,40••]. Furthermore, the localization of
Rop is critical for its function in signaling in tip growth [39••,40••]. Recent data strongly indicate that Rop and Rac,
another Rho-like GTPase, are functionally involved in actin cytoskeleton signaling and the control of polar growth in both yeast [38•] and pollen tubes [39••]. Over-expression of
Rop or Rac induces dramatic morphological alterations, resulting in shorter, rounded cells, and the formation of ‘bal-loons’ with abnormal F-actin arrangements in pollen tube tips [38•,39••]. A novel approach has been used to visualize
the actin cytoskeleton in living pollen tubes: this involved the fusion of green fluorescent protein to a portion of verte-brate talin (which binds filamentous actin) [41]. Use of this technique to visualize changes in actin in pollen tubes that had altered Rop expression has clearly demonstrated the pivotal role of Rop in specifying apical growth in pollen tubes [40••]. It is suggested that polarity is controlled by the
activation of Rop by localized spatial cues [40••]. Rop also
appears to have a role in determining pollen tube growth rate. The Rop-signaling pathway may therefore control and integrate both the direction and rate of pollen tube growth. Figure 2
Ca2+
PIP kinase PIP2 CaM/
CDPKs CDPK
ADF P Profilin
PIP2
Target Target
proteins? Protein kinases and phosphatases
IP3
LIM kinase?
? P
Ca2+
Rac/Rop
Ca2+
Ca2+
Myosin Actin cytoskeleton
CDPK
Current Opinion in Plant Biology
Some of the signaling components thought to be involved in regulating pollen tube growth are indicated in this cartoon. In this review I describe the recent identification of some of the key players that provide important links in what is emerging as a complex interplay between signaling cascades [Ca2+]
i, Rho-like GTPases (Rac and Rop), phosphoinositides,
CaM, calcium-dependent protein kinases and other protein kinases, and
How might this be achieved? Rac has been shown to asso-ciate with a phosphatidylinositol monophosphate kinase (PIP kinase) [39••], which is implicated as a potential
effec-tor of Rac/Rho. As PIP kinase produces PtdIns(4,5)P2, it is
possible that Rac/Rho might act in the phosphoinositide signaling pathway. There is, however, no direct evidence as yet to show that Rac/Rop are able to regulate phospho-inositide turnover. In animal cells, Rac has recently been shown to interact with a LIM-kinase [25,26] that regulates the phosphorylation of ADF. This discovery suggests that both protein kinase activity and Rac/Rho-mediated signal-ing pathways could be involved in inducsignal-ing actin reorganization. Importantly, Rop has also been shown to regulate Ca2+influx at the pollen tube tip [40••]. These
studies represent a significant step forward in our under-standing of how Rho-type GTPases function in the control of polarized growth of pollen tubes. Whether or not Rac/Rop connects several signaling pathways in pollen tubes is an important question which is as yet unanswered.
Conclusions
Exciting progress is being made in piecing together some of the signaling components and pathways involved in the regulation of pollen tube growth. There is growing evi-dence that changes in cytosolic calcium levels, rate of phosphoinositide turnover, protein kinases and the Rho family of GTPases are key regulators of the tip growth of pollen tubes. This evidence provides important pieces in solving what is emerging as a complex jigsaw puzzle. An attempt to indicate how these and o ther events known to be involved in pollen tube signaling may modulate pollen tube growth is shown in Figure 2. The essential role of the cytoskeleton in pollen tube growth together with its possi-ble role as a link between external stimuli, perhaps mediated through transmembrane receptors and remodel-ing of the cytoarchitecture at the apex of the pollen tube, is beginning to define its role as a target for signaling path-ways. The cytoskeleton may also provide a mechanism whereby signals are integrated. We are at an exciting stage in our understanding of pollen tube growth and can antici-pate further progress in fully understanding its modulation.
Acknowledgements
I would like to thank Chris Staiger, Zhenbiao Yang and Ken Robinson for sharing ideas and communicating unpublished results. I also thank Chris Franklin and Jason Rudd for stimulating discussions and useful comments on the manuscript. Work in the author’s lab is funded by the Biotechnology and Biological Sciences Research Council.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest ••of outstanding interest
1. Franklin-Tong VE: Signalling and the modulation of pollen tube growth.Plant Cell 1999, 11:727-738.
2. Taylor LP, Hepler PK:Pollen germination and tube growth.Annu Rev Plant Physiol Plant Mol Biol1997, 48:461-491.
3. Rudd JJ, Franklin-Tong VE: Calcium signalling in plants.Cell Mol Life Sci1999, 55:214-232.
4. Sanders LC, Lord EM: A dynamic role for the stylar matrix in pollen tube extension.Int Rev Cytol1992,140:297-318.
5. Wu H, Wang H, Cheung AY: A floral transmitting tissue specific glycoprotein attracts pollen tubes and stimulates their growth.
Cell 1995, 82:383-393.
6. Wolters-Arts M, Lush WM, Mariani C: Lipids are required for •• directional pollen tube growth.Nature 1998, 392:818-821. The authors demonstrate that lipids are essential for pollen tube penetration of the pistil and show that, in the presence of lipids, pollen tubes can pene-trate leaf tissue. They propose that the flow of water is a crucial factor in directing pollen tube growth.
7. Lush WM, Grieser F, Wolters-Arts M:Directional guidance of •• Nicotiana alatapollen tubes in vitroand on the stigma.Plant
Physiol1999,118:733-741.
A more detailed account of the importance of water supply in determining pollen tube guidance. The paper provides contentious evidence that chemi-cal gradients are unlikely to be important in determining the directional growth of pollen tubes.
8. Feijó JA: The pollen tube oscillator: towards a molecular mechanism of growth?In Fertilization in Higher Plants. Molecular and Cytological Aspects. Edited by Cresti M, Cai G, Moscatelli A. Berlin; Heidelberg: Springer Verlag; 1999:317-336.
9. Holdaway-Clarke TL, Feijó 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.
10. Messerli M, Danuser G, Robinson KR: Pulsatile influxes of H+, K+ and Ca2+lag growth pulses of Lilium longiflorumpollen tubes. J Cell Sci1999, 112:1497-1509.
11. Messerli M, Robinson KR: Cytoplasmic acidification and current influx follow growth pulses Lilium longiflorumpollen tubes. Plant J 1998,16:87-91.
12. Feijó JA, Sainhas J, Hackett GR, Kunkel JG, Hepler PK: Growing pollen tubes possess a constitutive alkaline band in the clear zone and a growth-dependent acidic tip.J Cell Biol1999, 144:483-496.
13. Malhó R: Role of 1,4,5-inositol triphosphate-induced Ca2+release in pollen tube orientation.Sex Plant Reprod1998, 11:231-235.
14. Trewavas AJ, Malhó R: Ca2+signalling in plant cells: the big network!Curr Opin Plant Biol1998, 1:428-434.
15. Yokota E, Muto S, Shimmen T: Inhibitory regulation of higher plant • myosin by Ca2+ions.Plant Physiol1999, 119:231-239.
The first demonstration in angiosperms that high Ca2+inhibits the motile
activity of myosin and that CaM interacts with myosin in a Ca2+-dependent
manner. Taken together with evidence that this myosin acts as a motor pro-tein that is involved with cytoplasmic streaming, the findings in this paper suggest that Ca2+and CaM regulate cytoplasmic streaming.
16. Moutinho A, Love J, Trewavas AJ, Malhó R: Distribution of calmodulin protein and mRNA in growing pollen tubes. Sex Plant Reprod1998, 11:131-139.
17. Grabski S, Arnoys E, Busch B, Schindler M: Regulation of actin tension in plant cells by kinases and phosphatases. Plant Physiol 1998, 116:279-290.
18. Estruch JJ, 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.
19. Rudd JJ, Lord JM, Franklin FCH, Franklin-Tong VE: Increased phosphorylation of a 26 kD pollen protein is induced by the self-incompatibility response in Papaver rhoeas.Plant Cell 1996,
8:713-724.
20. 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.
21. Takahashi T, Mu J-H, Gasch A, Chua N-H: Identification by PCR of receptor-like protein kinases from Arabidopsisflowers.
Plant Mol Biol 1998, 37:587-596.
22. Moutinho A, Trewavas AJ, Malhó R:Relocation of a Ca2+-dependent •• protein kinase activity during pollen tube reorientation. Plant Cell
1998, 10:1499-1509.
23. Lopez I, Anthony RG, Maciver SK, Jiang C-J, Khan S, Weeds AG, Hussey PJ: Pollen specific expression of maize genes encoding actin depolymerising factor-like proteins.Proc Natl Acad Sci USA 1996, 93:7415-7420.
24. Smertenko AP, Jiang C-J, Simmons NJ, Weeds AG, Davies DR, Hussey PJ: Ser6 in the maize actin-depolymerizing factor, ZmADF3, is phosphorylated by a calcium-stimulated protein kinase and is essential for the control of functional activity.
Plant J1998, 14:187-194.
25. Arber S, Barbayannis FA, Hanser H, Schneider C, Stanyon CA, Bernard O, Caroni P:Regulation of actin dynamics through phosphorylation of cofilin by LIM-kinase.Nature 1998, 393:805-809.
26. Yang N, Higuchi O, Ohashi K, Nagata K, Wada A, Kangawa K, Nishida E, Mizuno K: Cofilin phosphorylation by LIM-kinase 1 and its role in Rac-mediated actin reorganization.Nature 1998, 392:809-812.
27. Wasserman S: FH proteins as cytoskeletal organizers.Trends Cell Biol1998, 8:111-115.
28. Carlier M-F:Control of actin dynamics.Curr Opin Cell Biol1998,
10:45-51.
29. 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.
The finding that addition of profilin to cytosolic extracts alters several protein kinase activities provides the first evidence that plant profilin may modulate protein phosphorylation. This implicates a functional signaling role for profil-in profil-in plant cells.
30. Guillén G, Valdés-López V, Noguez R, Olivares J, Rodríguez-Zapata LC, Pérez H, Vidali L, Villanueva MA, Sánchez F:Profilin in Phaseolus vulgarisis encoded by two genes (only one expressed in root nodules) but multiple isoforms are generated in vivoby phosphorylation on tyrosine residues.Plant J1999, 19:497-508.
31. Staiger CJ, Yuan M, Valenta R, Shaw PJ, Warn RM, Lloyd CW:
Microinjected profilin affects cytoplasmic streaming in plant-cells by rapidly depolymerizing actin microfilaments.Curr Biol1997,
4:215-219.
32. Putnam-Evans C, Harmon AC, Palevitz BA, Fechheimer M, Cormier MJ:
Calcium-dependent protein kinase is localized with F-actin in plant cells.Cell Motil Cytoskeleton 1989, 12:12-22.
33. Krauze K, Makuch R, Steptka M, Dabrowska R: The first caldesmon-like protein in higher plants.Biochem Biophys Res Commun 1998,
247:576-579.
34. Obermeyer G, Klaushofer H, Nagl M, Hoftberger M, Bentrup FW: In vitrogermination and growth of lily pollen tubes is affected by protein phosphatase inhibitors.Planta 1998, 207:303-312.
35. Yang Z: Signaling tip growth in plants.Curr Opin Plant Biol1998,
1:525-530.
36. Lin YK, Wang Y, Zhu J, Yang ZB: 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.
37. Lin YK, Yang ZB: 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.
38. Li H, Wu G, Ware D, Davis KR, Yang Z: ArabidopsisRho-related • GTPases: differential gene expression in pollen and polar
localization in fission yeast.Plant Physiol1998, 118:407-417. Rop, a pollen-specific Rho-like GTPase, when overexpressed in yeast, caus-es morphological abnormaliticaus-es. This providcaus-es evidence that Rop is impor-tant in the regulation of polar growth.
39. Kost B, Lemichez E, Spielhofer P, Hong Y, Tolias K, Carpenter C,
•• Chua N-H: Rac homologues and compartmentalized
phosphatidylinositol 4,5-bisphosphate act in a common pathway to regulate polar pollen tube growth.J Cell Biol1999, 145:317-330. Several lines of evidence suggest that Rac homologues play a role in con-trolling the polarity of pollen tube growth. Rac is associated with compo-nents of the phosphoinositide signaling pathway and may act in the same pathway to control polarized pollen tube growth.
40. Li H, Lin Y, Heath R, Zhu M, Yang Z: Control of pollen tube tip •• growth by a Rop GTPase-dependent pathway that leads to the
tip-localized calcium influx.Plant Cell1999, 11:1731-1742. The authors report important information on a signaling pathway controlling pollen tube tip growth with evidence that Rop couples both spatial and tem-poral control of tip growth in pollen tubes. Furthermore, Rop is implicated in the control of Ca2+influx at the pollen tube tip.
