title: Rhizobacterial volatile emissions regulate auxin homeostasis and cell expansion in Arabidopsis.
abstract: Certain plant growth-promoting rhizobacteria (PGPR), in the absence of physical contact with a plant stimulate growth via volatile organic compound (VOC) emissions, through largely unknown mechanisms. To probe how PGPR VOCs trigger growth in plants, RNA transcript levels of Arabidopsis seedlings exposed to Bacillus subtilus (strain GB03) were examined using oligonucleotide microarrays. In screening over 26,000 protein-coded transcripts, a group of approximately 600 differentially expressed genes related to cell wall modifications, primary and secondary metabolism, stress responses, hormone regulation and other expressed proteins were identified. Transcriptional and histochemical data indicate that VOCs from the PGPR strain GB03 trigger growth promotion in Arabidopsis by regulating auxin homeostasis. Specifically, gene expression for auxin synthesis was up regulated in aerial regions of GB03-exposed plants; auxin accumulation decreased in leaves and increased in roots with GB03 exposure as revealed in a transgenic DR5::GUS Arabidopsis line, suggesting activation of basipetal auxin transport. Application of the auxin transport inhibitor 1-naphthylphthalamic acid (NPA) restricted auxin accumulation to sites of synthesis thereby preventing GB03-mediated decreases in shoot auxin levels as well as thwarting GB03-mediated growth promotion. In addition, microarray data revealed coordinated regulation of cell wall loosening enzymes that implicated cell expansion with GB03 exposure, which was confirmed by comparative cytological measurements. The discovery that bacterial VOCs, devoid of auxin or other known plant hormones regulate auxin homeostasis and cell expansion provides a new paradigm as to how rhizobacteria promote plant growth.
authors.affiliation: Department of Chemistry/Biochemistry, Texas Tech University, Lubbock, TX 79409, USA.
authors.forename: Huiming
authors.initials: H
authors.lastname: Zhang
authors.forename: Mi-Seong
authors.initials: MS
authors.lastname: Kim
authors.forename: Venkat
authors.initials: V
authors.lastname: Krishnamachari
authors.forename: Paxton
authors.initials: P
authors.lastname: Payton
authors.forename: Yan
authors.initials: Y
authors.lastname: Sun
authors.forename: Mark
authors.initials: M
authors.lastname: Grimson
authors.forename: Mohamed A
authors.initials: MA
authors.lastname: Farag
authors.forename: Choong-Min
authors.initials: CM
authors.lastname: Ryu
authors.forename: Randy
authors.initials: R
authors.lastname: Allen
authors.forename: Itamar S
authors.initials: IS
authors.lastname: Melo
authors.forename: Paul W
authors.initials: PW
authors.lastname: Paré
mesh_terms: D017360:Arabidopsis / Q000254:growth & development / Q000378:metabolism*; D001412:Bacillus subtilis / Q000502:physiology*; D049108:Cell Enlargement; D002473:Cell Wall / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D006706:Homeostasis / Q000502:physiology; D007210:Indoleacetic Acids / Q000378:metabolism*; D018515:Plant Leaves / Q000254:growth & development; D014835:Volatilization
chemical_list: D007210:Indoleacetic Acids