Roberto A. Gaxiola Ariza, Ph.D.

Research

Up-regulation of the vacuolar H+-PPase had a very unexpected effect in plant growth and development. Arabidopsis AVP1OX plants AVP1-1 and AVP1-2 had more rosette leaves (3 and 16, respectively) and significantly greater leaf area (60 and 40% respectively, p<0.05) than="" wild="" type="" plants.="" furthermore,="" avp1-1="" and="" avp1-2="" exhibited="" enhanced="" root="" growth="" and="" dry="" weight="" (2.6="" and="" 9.4x,="" p="">< 0.01)="" compared="" to="" wild="" type.="" thus,="" avp1="" appears="" to="" function="" in="" both="" shoot="" and="" root="" development.="" our="" initial="" hypothesis,="" that="" was="" still="" vacuole-centric,="" predicted="" that="" the="" cells="" of="" the="" avp1ox="" plants="" would="" be="" larger="" than="" controls="" due="" to="" enhanced="" vacuolar="" functions.="" this="" hypothesis="" was="" soon="" challenged="" when="" we="" learned="" that="" the="" enhanced="" biomass="" of="" these="" plants="" was="" due="" to="" increased="" cell="" numbers.="" so="" how="" could="" we="" explain="" that="" the="" overexpression="" of="" a="" vacuolar="" h+-ppase="" enhances="" cell="" division="" and="" biomass?="" in="" order="" to="" solve="" this="" puzzle="" we="" had="" to="" question="" the="" established="" notion="" that="" the="" h+-ppase="" functions="" exclusively="" at="" the="" tonoplast="" and="" basically="" as="" a="" backup="" device="" to="" the="" ubiquitous="" v-atpase="" (sze="" et="" al.,="" 1999;="" sze="" et="" al.,="" 2002).="" by="" integrating="" several="" disciplines="" in="" plant="" biology,="" including="" hormone="" metabolism,="" development,="" transport,="" and="" cell="" biology,="" and="" through="" the="" establishment="" of="" collaborations="" with="" us="" and="" international="" experts="" in="" the="" different="" fields,="" we="" demonstrated="" that="" increases="" and="" decreases="" in="" the="" expression="" of="" avp1="" affect="" the="" distribution="" and="" abundance="" of="" the="" plasma="" membrane="" h+-atpase="" with="" implications="" in="" apoplastic="" (cell="" wall="" space)="" and="" rhizosphere="" acidification="" (acidification="" of="" the="" environment="" surrounding="" the="" root).="" furthermore,="" our="" data="" showed="" that="" avp1="" plays="" a="" key="" role="" in="" development="" through="" facilitating="" the="" fluxes="" of="" auxin="" (plant="" growth="" hormone)="" that="" regulate="" organogenesis="" in="" arabidopsis="" (li="" et="" al.,="" 2005).="" is="" there="" a="" common="" denominator?="" however,="" auxin="" fluxes="" do="" not="" explain="" the="" increase="" in="" shoot="" and="" root="" biomass="" triggered="" by="" the="" h+-ppase="" up-regulation.="" how="" can="" we="" explain="" these="" phenotypes?="" is="" there="" a="" common="" denominator?="" our="" current="" working="" hypothesis="" is="" that="" the="" up-regulation="" of="" the="" h+-ppase="" avp1="" enhances="" sucrose="" fluxes="" from="" source="" to="" sink="" tissues="" via="" improving="" phloem="" sucrose="" loading="" capacity.="" sucrose="" produced="" by="" photosynthesis,="" is="" the="" cornerstone="" of="" higher="" plant="" metabolism.="" it="" is="" the="" main="" substrate="" for="" respiration="" and="" biosynthesis.="" thus,="" an="" enhanced="" availability="" of="" sucrose="" could="" explain="" both="" larger="" and="" more="" energized="" root="" systems="" with="" an="" enhanced="" apoplast="" and="" rhizosphere="" acidification="" capacity.="" apoplast="" acidification="" is="" known="" to="" enhance="" auxin="" transport="" and="" rhizosphere="" acidification="" is="" a="" central="" mechanism="" for="" plant="" mineral="" nutrition.="" in="" a="" sense="" a="" domino="" effect="" for="" integral="" plant="" growth="" and="" development.="" we="" are="" using="" a="" combined="" genetic,="" molecular="" biology,="" biochemistry,="" cytology="" and="" ultra-structural="" approach="" to="" address="" our="" main="">