83-Calculum-a-central-regulator-of-plant-growth-and-development

HISTORICALPERSPECTIVEESSAYCalcium:ACentralRegulatorofPlantGrowthandDevelopmentTodaynoonequestionstheassertionthatCa21isacrucialregulatorofgrowthanddevelopmentinplants.Themyriadpro-cessesinwhichthisionparticipatesislargeandgrowingandinvolvesnearlyallaspectsofplantdevelopment(recentreviewsinHarperetal.,2004;HetheringtonandBrownlee,2004;Hirschi,2004;ReddyandReddy,2004;BothwellandNg,2005).Despitethiswealthofresearch,thecon-ceptofCa21asanintracellularregulatorisrelativelyrecentandwithintheprofessionallifespanofmanypeoplewhoarestillactiveandworkingonthistopictoday.TheaimofthisessayistoidentifythoselinesofthoughtandresearchthatledtotheideathatCa21isasecondmessengerinplantcellgrowthanddevelopment.Thisessaythusfocusesprimarilyonworkstartinginthemidsixtiesandextendingtothemideighties.IdonotprovideanexhaustivereviewofthehistoryofCa21research,nordoIattempttotreatmodernaspectsofCa21research.However,IdostrivetoidentifytherootsofmodernCa21researchandtocharttheoriginofthecurrentrevolution.EARLYSTUDIESONPLANTCALCIUMCa21isanessentialelement;however,itsroleiselusive.WhenexaminingtotalCa21inplants,theconcentrationisquitelarge(mM),butitsrequirementisthatofamicro-nutrient(mM).Ca21isnotusuallylimitinginfieldconditions,stillthereareseveraldefectsthatcanbeassociatedwithlowlevelsofthision,includingpoorrootdevelopment,leafnecrosisandcurling,blossomendrot,bitterpit,fruitcracking,poorfruitstorage,andwatersoaking(Simon,1978;WhiteandBroadley,2003).Theunderlyingcausesfortheseeffectsarenotentirelyclear;nevertheless,twoareaswithinthecellhavebeenrecognizedasbeingimportanttargets.Firstisthecellwall,whereCa21playsakeyroleincross-linkingacidicpectinresidues.Thesecondisthecellularmembranesystem,wherelow[Ca21]eincreasesthepermeabilityoftheplasmamembrane.Thesearebrieflydiscussedbelow.Ca21andtheCellWallSincethe19thcentury,ithasbeenappre-ciatedthatCa21playsacrucialroleindeterminingthestructuralrigidityofthecellwall(reviewedinWynJonesandLunt,1967;Burstrom,1968).Duringcellwallformation,theacidicpectinresidues(