Signaling with Ions: The Keystone for Apical Cell Growth and Morphogenesis in Pollen Tubes1 ...

21 Pages·2017·2.89 MB·English

Update on Ion Transport in Pollen Tubes
Signaling with Ions: The Keystone for Apical Cell
1[OPEN]
Growth and Morphogenesis in Pollen Tubes
Erwan Michard, Alexander A. Simon, Bárbara Tavares, Michael M. Wudick, and José A Feijó*
Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742–
5815 (E.M., A.A.S., M.M.W., J.A.F.); and Instituto Gulbenkian de Ciência, Oeiras 2780–901, Portugal (B.T.)
ORCID IDs:0000-0002-4048-8915(B.T.);0000-0002-4332-369X(M.M.W.);0000-0002-1100-5478 (J.A.F.).
Pollen tubes (PTs) are one of the best characterized transcriptomicsandproteomicsonpracticallyallofits
plant cell types in many respects. The identification of biologicalcontexts(HonysandTwell,2003;Pinaetal.,
keyplayersinvolvedintubegrowthofferstheperspec- 2005;Borgesetal.,2008;Qinetal.,2009;Boavidaetal.,
tive of an integrative understanding of cell morpho- 2011;Mayanketal.,2012;Pertl-Obermeyeretal.,2014;
genesisprocesses.OneoutstandingfeatureofPTsistheir Lang et al., 2015). All these features define a unique
prominentdependenceoniondynamicstopromoteand cell type so evolutionarily streamlined to fast growth
regulate growth. Many reports have identified and andspermdelivery(Williams,2008)thatitremained
characterized membrane transport proteins, such as basically conserved as the only gametophyte de-
channels,transporters,andpumps,aswellastheirreg- velopmentalendproduct formalefunctionsincethe
ulatorymechanisms,someofwhichthemselvesarede-
pendent on ions such as Ca2+ and H+. The signaling
networkthatgovernsgrowthisbasedonastrictspatial
distribution of signaling molecules, including apical
gradients of Ca2+, H+, and reactive oxygen species. A
central role for ion homeostasis, and more generally
membrane transport systems, is proposed to underlie
the spatiotemporal establishment of the signaling net-
work that controls the PT self-organization and mor-
phogenesis. Here, we review the latest progress on
understanding tube growth from the perspective of
membrane transporters and ion homeostasis. The on-
going molecular characterization of the Ca2+-signaling
pathways, as well as the recent identification of female
externalcuesandcorrespondingreceptorsonthepollen
that control growth orientation, offer a firm biological
contexttoboostthefieldevenfurther.
POLLENTUBESASATAILORED MODELFOR
STUDYING IONDYNAMICSATTHECELL
BIOLOGYLEVEL
Pollen tubes (PTs) have long been considered out-
standingmodelsforcellbiologyforavarietyofreasons.
Ontheonehand,theydisplaydramaticfeaturesatthe
level of cell polarity, cytoskeleton dynamics, growth
rates, membrane recycling, cell-cell interaction mecha-
nisms,etc.(CheungandWu,2007;Michardetal.,2009;
QinandYang,2011;Hepler,2016).Ontheotherhand,
their study is backed up by extensive databases on
1This work was supported by the National Science Foundation
(grantno.MCB1616437/2016)andtheUniversityofMaryland.
*[email protected].
[OPEN]Articlescanbeviewedwithoutasubscription.
www.plantphysiol.org/cgi/doi/10.1104/pp.16.01561
Plant Physiology(cid:1), January2017,Vol. 173, pp.91–111, www.plantphysiol.org (cid:3)2017American Society of Plant Biologists.AllRights Reserved. 91
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Michard et al.
Cretaceous (Rudall and Bateman, 2007). PTs in an- lagtimeduringagrowthperiodinPT(forreview,see
giosperms can grow up to 4 mm$s21 and are charac- Holdaway-ClarkeandHepler,2003;Hepleretal.,2013)
terizedbytheperiodicformationofcalloseplugsthat and root hair (Monshausen et al., 2007, 2008). In such
isolate older parts of the tube to die so that growth studies, the minimum pH or maximum Ca2+ oscil-
can be maintained continuously restricted to the ap- lations and growth peak display a time lag of a few
icalend(forreview,seeBoavidaetal.,2005a,2005b). secondsinbothPTandroothairs,suggestiveofsimilar
In plants, PTs share the same type of apical growth regulation mechanisms of growth. Of note, the flux of
mechanism with root hairs, a fact that is reflected at Cl2wasfoundtobeinphasewithgrowth(Zoniaetal.,
the molecular level bytheexistenceofanapicalsigna- 2002). Nevertheless, different estimates of advances
tureintranscriptomicsprofiles(Beckeretal.,2014).Root anddelayshavebeencollectedinavarietyofbiological
hairs, however, grow slower than PTs. In addition, root systemslikelily(Liliumlongiflorum),tobacco(Nicotiana
hair length is controlled through a signaling network tabacum),petunia (Petunia hybrida), less in Arabidopsis
involving ROOTHAIR DEFECTIVE SIX-LIKE (RSL) (Arabidopsis thaliana), using imaging techniques (dif-
transcriptionfactors(HonkanenandDolan,2016).In ferential interference contrast, wide-field or confocal
spite of those main differences, the apical growth of fluorescence), and electrophysiology methods in such
roothairsandPTsischaracterizedbyapicalexocytosis ways that comparisons of the published delays and
ofnewcellwallmaterial,similariongradients,fluxes proposed sequences of events are subject to potential
at the tip, and a mechanism depending on actin cyto- distortions (Portes et al., 2015; Daminelli et al., 2017).
skeleton supporting cell elongation (Gu and Nielsen, Lastbutnotleast,correlationdoesnotimplycausation,
2013; Ketelaar, 2013; Mendrinna and Persson, 2015; and not much can be deduced from those studies,
Manganoetal.,2016). particularlybecausewedonotknowthekineticprop-
Withsomanypeculiaritiesandextremeadaptations erties of key reactions within the networks, such as
tofunction,theapplicabilityofPT-specificmechanisms molecular diffusion, protein phosphorylation, exocy-
to other plant somatic cells, namely diffuse growing tosis,etc.(Daminellietal.,2017)
ones, is not always straightforward. This assumption Figure 1 highlights the peculiar correlation between
isclearlyreflectedinthewell-differentiatedtranscriptomic ion dynamics and cell structure. Spatial correlations
profiles between PTs and those of other organs and betweenfeaturesofthecytosolicgradients(Fig.1,B,C,
tissues(forsnail-viewrepresentations,seeBeckeretal., and E) and other cellular structures are conspicuous
2003; Pina et al., 2005). The one feature in PTs that and easily observed at the level of zonation of organ-
stands out the most is their strict dependence on ion elles along the clear zone (Fig. 1A) or the actin cyto-
dynamicstogrowandsustaintheirfunctions(Michard skeleton (Fig. 1D). Characterizing the transport
et al., 2009). Different ions, namely calcium (Ca2+), molecules that generate these gradients may be a first
protons (H+ or pH), and chloride (Cl2), form stable/ stepintheirmanipulationandeventuallymaytestthe
standing gradients of cytosolic concentration in the hypothesis that spatial correlations are not a mere
clear zone (Fig. 1). Of relevance, these spatial patterns phenomenological coincidence but may actually be
and their temporal and spatial variations or choreog- causal and part of a network of regulatory feedback
raphies,correlateremarkablywellwiththeintracellular loops. One first step in that direction is the establish-
structure of PTs, be it the grading of organelle sizes ment of a functional correlation between the transport
defining the so-called clear zone, the cytoplasmic re- moleculesandthepredictedoutcomeoftheiractivityin
verse fountain (Cheung and Wu, 2007; Hepler and termsofiondynamics,whetheratthelevelofcytosolic
Winship, 2015), and, to some extent, the definition of concentration orofmembranetransport.One such ex-
the membrane-recycling domains in the tip (Parton ampleisalsoofferedinFigure1,wherethelocalization
et al., 2001; Bove et al., 2008; Kost, 2008; Bloch et al., of the H+-ATPase NICOTIANA TABACUM AUTO-
2016;Wangetal.,2016;Fig.1).Theexistenceoftheseion INHIBITEDH+-ATPASE(NtAHA1)(Fig.1F)correlates
gradients as a putative central coordinating mecha- perfectlywiththeexistenceofintracellularpHdomains
nismforcellulargrowthandmorphogenesisinPTshas (Fig. 1E) and extracellular H+ fluxes (Fig. 1G; Certal
beenconceptualizedelsewhere(Feijóetal.,1995,2001; et al., 2008; Michard et al., 2008). The fact that this
Michard et al., 2009; Daminelli et al., 2017). Largely crucialpumpissegregatedfromthetipPMtriggersa
believedtobegeneratedfromplasmamembrane(PM) numberoftestablemodelsandbyitselfalreadydefines
activity,theionchoreographyofPTsiseasilytraceable anexperimentalparadigmoffereduniquelybyPTs.
by noninvasive methodssuch as ion-specific vibrating InArabidopsis,morethan800transportertranscripts
probe electrophysiology and ion-specific probe imag- have been identified in pollen using the ATH1 mRNA
ing to show nearly perfect correlation with growth microarray(Pinaetal.,2005;Bocketal.,2006),andthis
variationswhilealsoallowingonetoscoreforverysubtle overrepresentationisconfirmedbyRNAsequencingin
phenotypeshardlydetectableinothernongrowingcells Arabidopsis and lily (Loraine et al., 2013; Lang et al.,
(Michardetal.,2011). 2015).ThisisperhapsoneofthereasonswhyPTshave
Of relevance, growth rate, ion fluxes, and concen- beenwidelyexploredinrecentyearsforphenotypingan
trationsmayoscillateinPTs,aswellasduringroothair increasing repertoire of channels, transporters, and
growth.Somestudiespresentthechoreographyofion pumps,renderingthevegetativecellofthePTlikelyone
fluxesandintracellularionconcentrationsbyarelative of the best studied cells in plants in terms of ion
92 Plant Physiol. Vol. 173, 2017
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Ion Transport in Pollen Tubes
Figure 1. The ion dynamicsregulatory paradigm of the PT structure is supported by strong spatial correlations. A, Lily PTs
(differential interference contrast imaging)display the quintessence of the dramatic polarization of these cells. Actin-driven
reverse-fountaincytoplasmicmovementdrivesorganellestothetipbytheperipheryandbackthroughthetubecore.Yet,this
back-turningmovementoccurssome20to50mmbackfromthetip,resultinginapatternbestdescribedasifthereisasize-sorting
orsievingmechanism,firstdispatchingbackbigstarchplastids(blackarrows),thenmitochondria(whitearrow)andotherlarger
endomembraneformations,andonlyallowingsmallexocyticvesiclestotheverytip(asterisk).Thispatterngeneratestwoclearly
observablecytologicdomains:theclearzone,definedbytheabsenceoflargeplastidsandmanytimesseenatlowmagnification
(doubleasteriskintheinset),andaninvertedcone(Lancelleetal.,1987)ofsmallvesiclesthat,contrarytotherestofthetube
shank,moveinanonorganizedBrownian-likemotion(dashedlinesonthetip).BtoG,IntobaccoPTs,thesedomainsarere-
markablycorrelatedwithcytosolicionconcentrationsofCl2(B),Ca2+(C),andH+(E)whenliveimagedwithspecificfluorescent
2
geneticprobes.Theacidictip(E)verycloselydefinestheinvertedconeandisalsocorrelatedwithalowerconcentrationofCl .In
tobacco,theinvertedconealsoseemstobelinedupbyaconcentrationofsmall,highlydynamicactinfilaments(D)thatareoften
organizedasabasketorfringe(Roundsetal.,2014).TheclearzoneiscorrelatedwithboththefadingoftheCa2+andtheacidic
gradientsfocusedonthetip.Thebaseoftheclearzonealsodefinestheappearanceofthelongactinfilamentsthatsupport
organellestreaming.Acausalcorrelationbetweenaspecifictransporterdistributionandcytosolicconcentrationofanionisbest
seenforH+,wherethelocalizationofthemosthighlyexpressedP-typeATPaseintobacco,NtAHA1(F;Certaletal.,2008),closely
correlateswiththeextracellularH+fluxes(G;effluxesinredandinfluxesinblue)andcytosolicpH(E).H+effluxes,ontheother
hand,correlatewiththepresenceofthechimericNtAHA1::GFPprotein,andinfluxescorrelatewiththeirabsenceonthetip.Last
butnotleast,theacidtip(redinE)correlateswiththelargestinfluxesandthesubmembranaralkalinelinings(purpleinE)correlate
witheffluxes.Bars=10mm.(TheimageinAisbyN.Moreno,adaptedfromTaizetal.[2015]andSmithetal.[2009];theimagein
BisadaptedfromGutermuthetal.[2013];theimagesinCandEareadaptedfromMichardetal.[2008]).
dynamics. Figure 2 and Table I summarize this accu- downstream targets of specific ions? And (3) how do
mulated knowledge. They incorporate not only genes cellularprocessesfeedbacktoregulateiontransport?
thathavealreadybeencharacterizedbutalsogenesthat
canbepredictedfromtranscriptomics,proteomics,and
OPPOSINGFORCES: TURGORANDCELL
comparative physiology to play roles in PT growth. In
WALLDEPOSITION
thisreview,wefocusonorganizingorsystematizingthis
growing repertoire, focusing on three different angles. When growingPTs orroothairsstop inresponse to
(1) Is this knowledge sufficient to define regulatory an osmotic shock, the exocytosis of vesicles ensuring
mechanisms around a specific ion? (2) What are the cellwalldepositioncontinuesatthetip(Schroeterand
Plant Physiol. Vol. 173, 2017 93
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Michard et al.
Figure2. MolecularbasisforthetransmembranetransportinthePT.AschematicoverviewofionandsolutetransportinthePTis
shown.Gradientsofdifferentionconcentrations(e.g.H+,Ca2+,andCl2)differentiatetheclearzonefromtheshankregionandare
essentialforthepolarizedgrowthofthetube.Wepositthat,likecytosolicpH(Fig.1),ionhomeostasisalongthesegradientsis
reachedbytheintegratedtransportactivityofanetworkofdifferenttransportmoleculesdistributedbetweenthetip,shank,and
internaldomainsofthetube.Depictedontheimagesarethechannels,transporters,andpumpsthatwereeithergenetically
describedorputativebutdeemedtobeessentialfromtranscriptomicanalysis(questionmarks)toformtheionchoreographyofa
pollentune(fordetails,seeTableI).At,Arabidopsisthaliana;Cs,Cucumissativus;Le,Solanumlycopersicum;Ll,Liliumlong-
iflorum;Np,Nicotianaplumbaginifolia;Nt,Nicotianatabacum;Os,Oryzasativa.(Thisimageisnotdrawntoscale.)
Sievers, 1971; Li et al., 1996; Zerzour et al., 2009). Ap- andfiniteelementanalysismethods(Fayantetal.,2010;
parently,themaincontroloftheapicalgrowthprocess Vogler et al., 2012). A discussion of the opportunities
does not depend on turgor as much as on other plant andcaveatsofthesemodelsisbeyondthescopeofthis
cells (Cosgrove, 2014; Ali and Traas, 2016). Quantifi- review,andherewefocusonthefactsthat(1)turgorisa
cationoftheopposinggrowthforcesinlilyPTsledtoa direct consequence of water transport driven by small
difference of 2 orders of magnitude between the inter- solutes,notablyions,and(2)ionssuchasCa2+andH+
nal turgor pressure (approximately 0.3 MPa) and the areinvolvedinthemechanicalmaturationofcellwalls.
cellwallelasticity(approximately20–90MPa;Vogler PTs can appropriately adjust turgor pressure by
et al., 2012), clearly bringing other growth control adapting to changes in external osmolarity (Benkert
mechanisms than turgor to the board. Supporting this et al., 1997), but no osmosensor has yet been charac-
concept, growth can be arrested by nonrelated turgor terized. Mechanosensitive ion channels like the cation
means, such as caffeine treatment (Li et al., 1996). Yet, channel REDUCEDHYPEROSMOLALITY-INDUCED
despite the fact that there is no correlation between [Ca2+] INCREASE1 (AtOSCA1) (Yuan et al., 2014) or
i
turgor and growth rate, a minimal turgor pressure of the anion channel MECHANOSENSITIVE CHANNEL
approximately 0.3 MPa is necessary to sustain PT OFSMALLCONDUCTANCE-LIKE(AtMSL8)(Hamilton
growth in lily (Benkert et al., 1997). The general con- etal.,2015)offeraconceptualbasisforasensor,butsofar
sensusisthatturgordrivesthePTgrowthbyproviding thereportedioncurrentsandphenotypesofthesechan-
a minimal mechanical force necessary for cell wall nelsdonotwarrantthattheymaybeactinginPTgrowth.
elongation at the tip but that it plays no or a minor Several arguments can be raised regarding the role
regulatory role. Various theoretical approaches have of aquaporins in facilitating water transport in PTs
tried to bridge these opposing forces at workby mod- (Obermeyer, 2017). Pollen aquaporins of the SMALL
elinganisotropic-viscoplasticproperties(Dumaisetal., BASICINTRINSICPROTEINS(SIP)andTONOPLAST
2006),theincorporationofnewcellwallmaterial,par- INTRINSICPROTEIN(TIP)cladehavebeenlocatedat
ticularly pectine esters, asa keyfactor in softeningthe endomembranes (Ishikawa et al., 2005; Wudick et al.,
wallbyaffectingpolymercross-links(Rojasetal.,2011), 2014), while NOD26-LIKE INTRINSIC PROTEINS
94 Plant Physiol. Vol. 173, 2017
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Ion Transport in Pollen Tubes
TableI.Summaryofmoleculardataavailableonthephysiologicalroleofionorturgor-relatedmembranetransportproteinsinPTgrowth,gatheredbyfunction(pumps,channels,cotransporters,andaquaporins)andselectivity(sugars,anions,potassium,andcations) Foreachtransporterandcorrespondinggenomicidentifier,themolecularfunctionandthebiologicalsysteminwhichithasbeenestablishedareindicatedaswellastheintracellularlocalizationandthelocalizationmethod(s)used.Physiologicalrelevanceisdefinedbyphenotypesofknockdownoroverexpressinglines,whenavailable,orotherphysiologicaltraits.Putativegenesareindicatedwhen(1)transcriptomicsindicatedpollenselectiveandhighexpressionand(2)functionhasbeendefinedforthegenefamilyinothertissues.n/a,Notavailable. abcdNameLocusIdentifierFunction(System)Localization(Method)PhysiologicalRelevanceReference PumpsNtAHA1AY383599P-typeATPase;protonPT,PM,shank(FP)OX:slowedgrowthrate,Certaletal.(2008)pump(P)calloseplugdeformationNpPMA5AY772462AY772468P-typeATPase;protonShankPM(I)?Lefebvreetal.(2005)pump(Y)+LlHA1AY029190P-typeATPase;protonPG,PM(PC)HcurrentsunderpatchclampGehwolfetal.(2002)pump(P)AtAHA3At5g57350P3A-typeATPase;protonPM?ExpressedduringpollenRobertsonetal.(2004)pump(P)development(GUS);KO:lethalAtAHA6At2g07560P3A-typeATPase;protonPM?Putative,basedonexpressionPinaetal.(2005);Bockpump?levels(AHA8),homologywithetal.(2006);Langetal.AtAHA7At3g60330NtandLl,andpollenselective(2015)AtAHA8At3g42640(AHA6,AHA7,andAHA9)AtAHA9At1g80660AtACA9At3g21180P2B-typeATPase;calciumPM(FP)KO:partialmalesterilitySchiottetal.(2004)pump(Y)+AtVHA-E1At4g11150At3g08560VacuolarH-ATPase(P)E1,vacuolesandendosomesE1,KOisembryolethal;E2,KOStrompenetal.(2005);At1g64200ofspermcell;E2,hasnophenotype;E3,partiallyDettmeretal.(2010)AtVHA-E2vegetativecell,pollenredundanttoE1,possiblestressAtVHA-E3specific;E3,vegetativecellresponseimplicationsandspermcellvacuole(FP)SuctransportersAtSUC1At1g71880Succarrier(P)PM,aroundcalloseplugsandKO:reducedpollengerminationStadleretal.(1999);SivitzcytoplasmnearPTtip(FP)rateetal.(2008)OsSUT1Os03g07480Suctransporter(P)?KO:impairedgerminationrateHiroseetal.(2010)LeSUT2Solyc11g017010Suctransporter(P)PT,PM(I)AS:decreasedamountsofsolubleHackeletal.(2006)sugars,inhibitedPTgrowthCsHT1GenBankHQ202746Hexosetransporter,highPM(FP)OX:higherpollengerminationChengetal.(2015)affinityforGlc(Y)rate,increasedtubegrowth;AS:inhibitedgerminationandelongation,fewerseedsAtSWEET8/AtRPG1At5g40260Suctransporter(P,H,Y)PM(FP)KO:malefertilityphenotype,Guanetal.(2008);microsporogenesis,exineChenetal.(2010);formationandcellintegritySunetal.(2013)AnionchannelsandtransportersAtSLAH3At5g24030Anionchannel(P,O)PM(FP)Voltageclamp:regulatedbyGutermuthetal.(2013)AtCPK2andAtCPK20Hamiltonetal.(2015)AtMSL8At2g17010Anionchannel(O)PM,endomembranes(FP)WT:mechanosensitive;KO:improvedgermination;OX:inhibitedgermination(negativelyregulates)(Tablecontinuesonfollowingpage.)
Plant Physiol. Vol. 173, 2017 95
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Michard et al.
e.)
g
Reference Colmenero-Floresetal.(2007);Kongetal.(2011);Hendersonetal.(2015) Moulineetal.(2002) Safiarianetal.(2015) Beckeretal.(2004) Pinaetal.(2005) Tunc-Ozdemiretal.(2013a) Tunc-Ozdemiretal.(2013b) Frietschetal.(2007);Gaoetal.(2016) Pinaetal.(2005);Morrisetal.(2008)Michardetal.(2011) Michardetal.(2011) Teardoetal.(2015) Houetal.(2014);Yuanetal.(2014)Staeletal.(2012);Wagneretal.(2015) econtinuesonfollowingpa
bl
a
dPhysiologicalRelevance ollenexpression;KO:abortedsiliques,seed-setreduction WT:voltagedependent,inwardcurrents,CsClinhibition;KO:disruptedpollengermination,slowertubegrowth,fertilityaffected WT:regulatedinpH,calcium-dependentmannerutative,pollenselective WT:pollenfertility,initiationofPTgrowth;doubleKO:malesterile WT:PTgerminationandgrowthduringstress;KO:reducedcompetitivefitness,fewerseeds,lowpollentransmission2+WT:Cainflux;KO:sterile utative,pollenselective 2+WT:Cainflux,PTgrowth,andmorphogenesis;KO:AS,partialmalesterilityO:decreasedgrowthrate,partialmalesterilityO:disruptstheformationof2+mitochondriaandCauptake 2+O:higherfreeCainmitochondria,fasterandhigher2+accumulationinresponseCatoauxinandextracellularATP (T
P ? P P K K ? K
bcLocalization(Method) PM,Golgi(FP) PM(PC) Cytoplasm,punctate;PMintobaccoepidermis(FP)PM(PC) ? AtCNGC7:tipPMduringtubeemergence,PTshankduringelongation(FP)? PTapicalPM(FP) Endomembranes 2+influxatPTtipPM(VP)Ca ? Mitochondria,chloroplast(FP)PM? Mitochondria(I,FP)
aFunction(System)2Cl/cationcotransporter(O) +Kchannel(C) +Kchannel(P,Y) +Kchannel(P,O,Y) +Outward-rectifyingKchannel Cationchannel? Cationchannel? 2+Capermeation(E),nonselectivecationchannel(H)2+Cation/Caexchanger 2+Cationchannel,Capermeable(P) Cationchannel? Cationchannel? 2+Ca-permeable?osmolaritygated?2+Ca-bindingprotein(P)
er n
ntifi otei
de pr
I 0 0 20 0 00 0 0 000 0 0 0 0
ouspage.) Locus At1g3045 At2g2560 Genen/a;A3RG9At1g0251 At3g0285 ortersAt1g1599At1g1978 At3g4801 At5g1487 At5g0149At3g1407At5g4840 At2g3240 At2g3239 At4g0290 At4g3206
evi nsp
bleI.(Continuedfrompr Name AtCCC1 PotassiumchannelsAtSPIK(AtAKT6) LilKT1 AtTPK4 AtSKOR CationchannelsandtraAtCNGC7AtCNGC8 AtCNGC16 AtCNGC18 AtCAX4AtCAX9AtGLR1.2 AtGLR3.7 AtGLR3.5 AtOSCA1.7 AtMICU +H/cationcotransporters
a
T
96 Plant Physiol. Vol. 173, 2017
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Ion Transport in Pollen Tubes
P,
V
Reference Padmanabanetal.(2016) Szeetal.(2004);Zhaoetal.(2008) Szeetal.(2004);Zhaoetal.(2015) Szeetal.(2004);Evansetal.(2011);Luetal.(2011)Bassiletal.(2011) DiGiorgioetal.(2016) Botsetal.(2005) Sotoetal.(2008);Wudicketal.(2014) Sotoetal.(2008);Wudicketal.(2014) detection;PC,patchclamp;
o
n
dPhysiologicalRelevance KO:chx17/chx18/chx19mutantpollennormal,reciprocalcrossexperimentsindicatealargelymaledefectRootsandseedlings:KO:sensitive+toKdeficiency;OX:reduced+deficiency;nosensitivitytoKpollenphenotypeKOanddoubleKOchx13/14:root+growthsensitivetohighK;OX:+;rootgrowthincreaseinhighKnopollenphenotypeKO:PTnavigation DoubleKO:pollenunaffected;+regulatespHandKhomeostasis KO:fewerseeds,reducedpollengerminationandPTlength Pollendehydrationanddehiscence DoubleKOwithAtTIP5;1revealedpoorseeddevelopmentandsiliquegrowthDoubleKOwithAtTIP1;3revealedpoorseeddevelopmentandsiliquegrowth FP,Fluorescentproteinfusion;I,immuline.
T, c pe
bcLocalization(Method) PM(FP) PM(FP) PM(FP) Endomembranes(FP) Vacuole(FP) AtNIP4;1,PMandintracellularvesiclesinPpollengrains;AtNIP4;2,PMandintracellularvesiclesofPTonly(FP)PM? TPofvegetativecell(FP) TPofspermcells(FP) byeast.TP,Tonoplast.expressionline;WT,wild-ty
Y,er
aFunction(System) Cationchannel? +Kacquisition,high-affinity+Kuptake(Y) +Low-affinityKefflux(P,Y) +AtCHX23:KuptakeinapH-dependentway(E) ++Na/Hantiporter(P) Waterandnonionicsolutechannels(O) Waterchannels(O) Regulationofwaterfluxes?,waterandsolutetransport(O)Regulationofwaterfluxes?,waterandsolutetransport(O) puslaevisoocytes;P,plant;KO,knockoutline;OX,ov
80 Xenoline;
dentifier At1g055 cells;O,Antisense
ouspage.) LocusI At3g17630 At2g30240 At1g06970 At2g31910 At5g27150At3g05030 At5g37810At5g37820 AF440271AF440272At4g01470 At3g47440 coli;H,HEKdbe.AS,
evi hiapro
ableI.(Continuedfrompr Name AtCHX19 AtCHX13 AtCHX14 AtCHX21AtCHX23 AtNHX1AtNHX2AquaporinsAtNIP4;1AtNIP4;2 NtPIP1;1NtPIP2;1AtTIP1;3 AtTIP5;1 aC,COScells;E,Eschericalcium-selectivevibrating
T c
Plant Physiol. Vol. 173, 2017 97
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Michard et al.
(NIP)aquaporinswerelocalizedinthepollenPM(Lang 2010). The rupture point always being the tip suggests
etal.,2015).HeterologousoverexpressionofPLASMA anisotropy in the cell wall mechanical properties, char-
MEMBRANE INTRINSIC PROTEIN (AtPIP) aqua- acterized by a stronger shank. In hyphae, the turgor
porins yielded an increase of the water permeability pressure plays a minor role in polarization; rather, the
of lily pollen but no evident functional phenotype apical localization of lytic enzymes that loosen the cell
(Sommer et al., 2008). Aquaporins from the SIP, TIP, wall determines the growth rate and polarity (Money
andNIPfamilieswereshowntotransportwaterand/ andHill,1997).Similarly,PTgrowthissustainedbythe
or solutes and appear to be involved in PT growth depositionofprimarycellwallmaterialattheapex;once
(Ishikawaetal.,2005;Sotoetal.,2008;DiGiorgioetal., deposited at the tip, the wall is subject to a maturation
2016) and fertilization (Wudick et al., 2014). Interest- processthatstiffensit, creatinga gradientofviscosity/
ingly,althoughnotexpressedinpollen,itwasreported elasticitybetweenthegrowingtipandthenongrowing
recentlythatAtPIP2;1alsoshowsanonselectivecation tube (Hepler et al., 2013; Cosgrove, 2016). Despite dis-
channelactivity(Byrtetal.,2016),afeaturethatmight crepancies over the quantification of the mechanical
befoundforothermembersoftheaquaporinfamily. properties of the cell wall (Fayant et al., 2010; Vogler
Ion-drivenosmoticchangesinduceelectricpotential et al., 2012), many biochemical data demonstrate its
shiftsatthePMinadditiontoexternalpHalongthePT, anisotropiccompositionandsuggestaviscositygradient
and osmoregulation depends on an active transport along the tube (Steer and Steer, 1989; Geitmann, 2010;
system driven by the proton pump, through 14-3-3 Cheblietal.,2012;Hepleretal.,2013).Theprimarycell
proteinregulation(Pertletal.,2010).SeveralH+-ATPase wallofthePTdepositedattheapexisessentially com-
pumps are expressed in pollen (Pina et al., 2005; Bock posed of pectin plus 2% to 3% cellulose (Aouar et al.,
et al., 2006), with AtAHA8 being the most highly 2010; Derksen et al., 2011). Pectins are exported as
expressedandAtAHA6,AtAHA7,andAtAHA9being methylestersand,inparallel,somepectinmethylester-
pollenspecific(TableI).Intobacco,aclosehomologof aseenzymes(PME)aresecretedbythePTandcatalyze
AtAHA6 andAtAHA9, NtNHA1,was found to belo- pectin deesterification. This pectin chemomechanical
calized on the PM but segregated from the tip and in- structure largely determines the growth of the tube, as
volved in tube growth and callose plug formation revealedbypectinasetreatmentthataffectsgrowthprop-
(Certaletal.,2008).Thesepumpsarelikelytoenergize erties and induces tube swelling (Parre and Geitmann,
thetransportofothermoleculesthatunderlieturgorin 2005)andbythePMEmutantvanguard1PTs,whichare
PTs, such as sucrose (Stadler et al., 1999; Goetz et al., slowerandburstprecociously(Jiangetal.,2005).BothCa
21
2001).WhilenotnecessarilyaffectingonlyPTgrowth, and H either cross-link pectin polymers to induce the
1
the Arabidopsis AtSUC1 and rice (Oryza sativa) formationofagelorregulatetheactivityofPMEs,coor-
OsSUT1 sucrose transporters have defective male ga- dinating the stiffening of the cell wall (Bosch et al., 2005;
metophytephenotypes(Sivitzetal.,2008;Hiroseetal., Bosch andHepler, 2005; Parre andGeitmann, 2005; Tian
2010). In cucumber (Cucumis sativus), the hexose trans- etal.,2006;VieiraandFeijo’,2016).pHregulationofroot
porter CsHT1 is necessary for PT growth (Cheng et al., cellelongationhasbeendemonstrateddirectly(Fendrych
2015). Despite probably being related to microsporo- et al., 2016). Thus, while never directly demonstrated,
genesis and exine pattern formation, mutants of the the regulation of the excretion of these ions to the
PM-localized sucrose transporterAtRPG1/AtSWEET8 apoplastcouldhavearegulatoryroleintheanisotropy
display fertility defects (Guan et al., 2008; Chen et al., ofcellwallmechanicsofPTs.
2010;Sunetal.,2013).
However,andimportantly,ionfluxessuchasanions
(Zoniaetal.,2002)orK+mayparticipateinturgorgen-
ION FLUXESAND GRADIENTSATTHETIP:AN
eration.K+inwardconductivitieshavebeenrecordedby
ELECTRIFYING AFFAIR?
patchclampandvoltageclampinlilyandArabidopsis
(Mouline et al., 2002; Griessner and Obermeyer, 2003; The particular constitution and regulation of the tip
Beckeretal.,2004).TheinwardrectifierAtSPIKchannel domainofthePTPMissuchthatlargeextracellularion
is involved in PT growth (Mouline et al., 2002), and fluxes and cytosolic gradients are formed (Fig. 1). Ion
AtTPK4 mediates nonrectifying currents and also may fluxes,andinparticularthehugeanioneffluxatthetip
participateinosmoticregulationofthePT(Beckeretal., (Zoniaetal.,2002;Gutermuthetal.,2013),areexpected
2004). For anions, major solutes associated with water togenerateanosmoticgradient,anextracellularelectric
movement and turgor in animals and plants, only field,and,asdiscussedbelow,eventuallyamembrane
AtSLAH3 has been characterized in PTs (Gutermuth voltage gradient along the length of the PM. In Arabi-
etal.,2013),butitonlyaccountsforasmallpercentageof dopsis, the accumulation of vesicles at the apex is
the total anion flux. Yetthe demonstrationofa role for enoughtosustaina30-sgrowthperiod(Ketelaaretal.,
anion channels in stomatal turgor regulation offers an 2008). It has been proposed that vesicles in the clear
analogy that could eventually serve as a conceptual zone are governed mainly by Brownian dynamics be-
templatetoscreenfortheiridentityinPTs(seeTextBox1; cause of the apparent disorganization of the actin cy-
Fig.3).Highturgorpressuretypicallyinducestheburst- toskeleton in this region (Kroeger et al., 2009). But the
inginbothhyphea,anothertip-growingcell(Moneyand existenceofsuchdramaticiongradientsalsomayplaya
Hill, 1997), and PTs (Benkert et al., 1997; Amien et al., rolein the movementof vesicles by either electrostatic
98 Plant Physiol. Vol. 173, 2017
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Ion Transport in Pollen Tubes
orosmoticinfluence.PMvoltagehasbeendetermined potentialcanbeinferredtobemorepositivecompared
tobearound2130mVwhenmeasuredontheshankof withtheshank,sincelargepassiveionfluxesatthetip
PTs (Mouline et al., 2002; Griessner and Obermeyer, have a depolarizing effect. Furthermore, H+-ATPases
2003; Becker et al., 2004), a resting potential slightly are excluded from the tip (Certal et al., 2008; Michard
morenegativethantheK+equilibrium(Moulineetal., et al., 2009), where there is high NADPH oxidase ac-
2002). This hyperpolarization is likely driven by tivity (Potocky et al., 2007). This kind of oxidase was
H+-ATPase activity(Langetal.,2014).Unfortunately, proposedrecentlytogenerateelectrochemicalfluxesof
the apical voltage remains unknown, since it is not ions on their own (Segal, 2016). Voltage at the tip PM
possible to impale an electrode at the tip without pro- and at the shank PM tend to equilibrate to the same
voking tubeburst. Theoretically,apicalmembrane valuebychargefluxesalongthemembranethroughthe
Plant Physiol. Vol. 173, 2017 99
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.
Michard et al.
equivalent of an electrical circuit, implying electrical the tube is reversibly stopped by caffeine or low tem-
resistance of the cytosol and capacitance of the mem- perature, the Ca2+ gradient dissipates and the Ca2+ in-
brane (Hille, 2001; Michard et al., 2009). Thus, a com- flux is lowered to a basallevel; regrowth reconstitutes
bination of high enough resistance of the cytosol and theCa2+gradientandinflux(Piersonetal.,1996).Ca2+
strongchargefluxdensityacrossthePMatthetipcould channelactivitywasearlydeducedbyquenchingwith
induce a membrane voltage gradient between the tip Mn2+ (Malho et al., 1995). Several types of channels
and the shank, supporting the notion of electrostatic permeable to Ca2+ have been characterized by patch
movementofcharges.Exocyticvesicleshavelongbeen clamp either as inward rectifiers in Arabidopsis and
foundtobearanegativesurfacecharge(Heslop-Harrison pear (Pyrus communis; Wang et al., 2004; Shang et al.,
and Heslop-Harrison, 1982), and the existence of 2005;Wuetal.,2010,2014)orwithnoclearrectification
electrostaticfieldsinplantcellswasrecentlyproposed in tobacco (Michard et al., 2011). Among the 20 cy-
to underlie cell identity and signaling, namely by reg- clic nucleotide-gated channels (CNGCs) in plants, at
ulatingthetransferofproteinsfromendomembraneto least CNGC7, CNGC8, CNGC16, and CNGC18 are
PMonthebasisofchargealone(Simonetal.,2016).In expressed in pollen (Pina etal.,2005; Bock et al.,2006;
animals, membrane depolarization promotes the vesi- Kaplan et al., 2007). Some CNGC channels have been
cle exocytosis of pancreas b-cells (Yang et al., 2014; presentedasinward-rectifyingchannelswhenexpressed
CardenasandMarengo,2016).Ifdifferentialconditions inHEKcellsoroocytes(Lengetal.,1999,2002;Maetal.,
of cytosolic resistance exist, then theoretically such 2007). The disruption of AtCNGC18, localized in the
mechanismsalsocouldplayaroleinvesiclemigration subapical membrane, induces male sterility due to
andfusioninPTs. defectivePTgrowth(Frietschetal.,2007).AtCNGC18
But other cues for polarity could come from anion was recently shown to drive inward cationic currents
fluxes. Large anion conductivity has been recorded in activated by cAMP/cGMP in an animal heterologous
both Arabidopsis and tobacco (Tavares et al., 2011a, system(Gaoetal.,2016).AtCNGC7,whichlocalizesat
2011b) with effluxes up to 60 nmol$cm22$s21 (Zonia the flank of the growing tip, and AtCNGC8 have
etal.,2002),resultinginananiongradient(Gutermuth weaker phenotypes, but the double knockout is male
et al., 2013; Fig. 1B). The anion channel AtSLAH3 is sterile(Tunc-Ozdemiretal.,2013a).Lastly,AtCNGC16
partly responsible for this conductance (Gutermuth onlyplaysaroleunderstressconditions(Tunc-Ozdemir
et al.,2013), but other anion channels expressed in the et al., 2013b). In animals, CNGCs have calmodulin
PT may be involved (Tavares et al., 2011a), like mem- (CaM)-binding domains with regulatory functions,
bersfromtheAtALMTfamily(Meyeretal.,2010).The a mechanism recently confirmed for AtCNGC12 in
existence of a gradient and fluxes of anions of such plants (DeFalco et al., 2016). In tobacco, the Gluta-
magnitude has been proposed to be physically suffi- mate receptor agonist D-Ser induces a Ca2+ current
cienttocreateconditionsforanosmoticgradientstrong while the antagonist CNQX inhibits the Ca2+ con-
enoughtogeneratethrustforvesiclestomovetowarda ductivityofPTprotoplasts(Michardetal.,2011).This
minimum osmotic potential at the apex through the finding led to the description of PT growth pheno-
process of osmophoresis (Lipchinsky, 2015). While types for the Arabidopsis Glutamate receptor-like
proposed on theoretical grounds, the existence of (GLR) mutants glr1.2 and glr3.7. Additional GLRs
biophysicalmechanismsforthevectorialmovementof alsoareexpressedinPTs(Pinaetal.,2005;Bocketal.,
vesicles in the clear zone is an exciting new prospect 2006) and may be involved in Ca2+ homeostasis. In
callingforexperimentalvalidation. accordance,otherplantGLRshavebeenlocalizedon
Althoughthedirectionofpotassiumfluxesinthetip thePM,andAtGLR3.4hasbeenshowntoinduceCa2+
is still debated, anion fluxes must be compensated by accumulation in HEK cells (Meyerhoff et al., 2005;
cationefflux(Michardetal.,2009).K+outwardcurrents TapkenandHollmann,2008;Teardoetal.,2010;Vincill
have been recorded by patch clamp (Griessner and et al., 2013). Interestingly, GLRs heterodimerize, as
Obermeyer,2003)butremainunaccountedforinterms demonstrated by the sensitivity profile to amino acids
ofthechannelgeneratingthem.TheSKORK+channel in knockout plants (Stephens et al., 2008), yeast two-
isknowntobeexpressedinpollen(Pinaetal.,2005)and hybrid analysis (Price et al., 2013; Vincill et al., 2013),
constitutes a good candidate for that function. Other orFRET(Vincilletal.,2013).Utilizingachimerastrat-
channels, namely nonselective cationic channels, as egy by introducing the pore of plant AtGLR1.1 and
well as transporters from the cation-proton antiporter AtGLR1.4 into the animal GluR1 channel demonstrated
familyalsocouldplayaroleinanionfluxcompensation low Ca2+ permeability and low rectification with nonse-
(seebelow). lectivecationpores(TapkenandHollmann,2008).Other
Ca2+-permeable channels are putatively active in pollen
TheCa2+Affair on the basis of transcriptomics/proteomics, namely the
annexins (Lee et al., 2004; Zhu et al., 2014) and the
A Ca2+ gradient in which intensity correlates with mechanosensitivechannelAtOSCA1(Yuanetal.,2014).
growthratehaslongbeendescribedinPTs(Reissand One common point of the Ca2+-permeable channels
Herth,1985;ObermeyerandWeisenseel,1991;Rathore identified so far is their weak selectivity: they appear
etal.,1991;Piersonetal.,1994,1996;Malhoetal.,1995; to be nonselective cation channels rather than Ca2+-
Michardetal.,2008;Iwanoetal.,2009;Fig.1C).When selective channels. Their gating also is still poorly
100 Plant Physiol. Vol. 173, 2017
Downloaded from on April 4, 2019 - Published by www.plantphysiol.org
Copyright © 2017 American Society of Plant Biologists. All rights reserved.