Induced Systemic Drought and Salt Tolerance by Pseudomonas chlororaphis O6 Root Colonization is Mediated by ABA-independent Stomatal Closure

Abstract

Institute of Environmentally-Friendly Agriculture, Department of Plant Biotechnology, Chonnam National University, Gwangju500-757, Korea (Received on November 13, 2011; Revised on January 5, 2012; Accepted on January 11, 2012)Root colonization by the rhizobacterium Pseudomonaschlororaphis O6 in Arabidopsis thaliana Col-0 plantsresulted in induced tolerance to drought and salinitycaused by halide salt-generated ionic stress but not byosmotic stress caused by sorbitol. Stomatal aperturesdecreased following root colonization by P. chlororaphisO6 in both wild-type and ABA-insensitive Arabidopsismutant plants. These results suggest that an ABA-independent stomatal closure mechanism in the guardcells of P. chlororaphis O6-colonized plants could be akey phenotype for induced systemic tolerance todrought and salt stress. Keywords : abiotic stress, induced tolerance to environ-mental stress, priming, stomatal closurePlants possess various survival systems against abioticstresses, such as cold, drought, and salinity (Zhu, 2001).Under environmental stresses, the level of the planthormone, abscisic acid (ABA) increases triggering adaptiveresponses essential for survival (Zhu, 2001). Duringdrought and salt stresses, ABA induces stomatal closure tominimize water loss through transpiration (Leung andGiraudat, 1998). Consequently, ABA-biosynthesis mutants(aba mutants) and some of the ABA-response plantmutants (i.e. the ABA-insensitive abi mutants) aresusceptible to drought stress, due to problem of stomatalaperture regulation (Leung and Giraudat, 1998; Schroederet al., 2001). However, another plant growth regulator,jasmonate (Evans, 2003; Suhita et al., 2004), also stimu-lates stomatal closure under drought conditions (Creelmanand Mullet, 1997).Root colonization of certain plant-associated microbeselicits physiological and biochemical change to enhancesystemic resistance against various biotic and abioticstresses in plants (Kim et al., 2011; Yang et al., 2010),termed to as “induced systemic resistance (ISR)” or“induced systemic tolerance (IST)”. IST against drought orsalt stresses in plants can be induced with systemicapplication of certain rhizobacteria including Gram-positive Bacillus strains (Ryu et al., 2004; Timmusk andWagner 1999; Zhang et al., 2010), an endophytic fungalisolate, Trichoderma harziarum (Bae et al., 2010), andcertain Gram-negative bacterial isolates, such as ACC-deaminase producing bacteria (Mayak et al., 2004). Rootcolonization by Pseudomonas chlororaphis O6 inducedsystemic resistance against a broad spectrum of plantdiseases caused by viral, bacterial, and fungal pathogens invarious plants by jasmonic acid-ethylene related pathways(Cho et al., 2008; Kim et al., 2004; Ryu et al., 2007;Spencer et al., 2003). Additionally, root colonization by P.chlororaphis O6 induced systemic tol erance against drought,a process accompanied by stomatal closure. Applying2R,3R-butanediol, a volatile produced by P. chlororaphisO6, resulted in induced tolerance through a salicylic acid(SA), jasmonic acid (JA) and ethylene-dependent mech-anism (Cho et al., 2008). However, mechanisms involvedin microbe-mediated IST against drought have not beencharacterized or elucidated.In this study, we tested the hypothesis that P. chloro-raphis O6 induces tolerance to other abiotic stresses, suchas salinity, osmotic pressure, cold and heat. We used addi-tional Arabidopsis mutants altered in ABA signalingpathways to identify the role of ABA in induced abioticstress tolerance. Parental A. thaliana ecotypes Columbia(Col-0) or Landsberg erecta (Ler), and mutant and trans-genic lines were obtained from the Ohio State University