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Department of Entomology and Plant Pathology, Auburn University, Auburn, AL 36849, USA (Received on January 17, 2005; Accepted on February 11, 2005)Plant growth-promoting rhizobacteria (PGPR) are awide range of root-colonizing bacteria with the capacityto enhance plant growth and control plant pathogens.Here we review recent progress that indicate somePGPR strains release a blend of volatile organiccompounds (VOCs) that promote growth in Arabidopsisseedlings and induce resistance against Erwiniacarotovora subsp. carotovora. In particular, the volatilecomponents 2,3-butanediol and acetoin released ex-clusively from the PGPR strains triggered the greatestlevel of growth promotion and induced systemicresistance. Pharmacological applications of 2,3-butan-ediol promoted the plant growth and induced resistance,while bacterial mutants blocked in 2,3-butanediol andacetoin synthesis was devoid of growth-promotion andinduced resistance capacities. The results suggested thatthe bacterial VOCs play a critical role in the plantgrowth promotion and induced resistance by PGPR.Using transgenic and mutant lines of Arabidopsis, weprovide evidences that the signal pathway activated byvolatiles from one PGPR strain is dependent on cyto-kinin activation for growth promotion and dependenton an ethylene-signaling pathway for induced pathogenresistance. This discovery provides new insight into therole of bacterial VOCs as initiators of both plant growthpromotion and defense responses in plants.Keywords : bacterial volatiles, induced systemic resistance,PGPR, volatile organic compoundsFree-living root colonizing bacteria (rhizobacteria) havebeen studied for the past century as possible inoculants forincreasing plant productivity and controlling microbialpathogens (Kloepper, 1992). Soil or seed applications withplant growth-promoting rhizobacteria (PGPR) have beenused to enhance the growth of several crops (Glick, 1995)as well as to suppress the growth of plant pathogens(Kloepper et al., 2004). PGPR that colonize root systemsthrough seed applications and protect plants from foliardiseases include Pseudomonas spp. Bacillus spp. Paebacillusspp., and Serratia sp. (Kloepper et al., 1999, 2004; Pieterseet al., 2002). The mechanisms for plant growth promotionand induced systemic resistance (ISR) by PGPR have beenextensively studied in the past decade. There are severaldeterminants for mechanisms of growth promotion thatinclude bacterial synthesis of the plant hormones (indole-3-acetic acid (IAA), cytokinin, and gibberellin), breakdownof plant-produced ethylene by bacterial production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase, andincreased mineral and N availability in the soil (Glick,1999; Kloepper, 1991; Timmusk et al., 1999). Recently, thephenomenon that PGPR elicit plant defense has also beenfound to lead to a state of ISR in the treated plant (Kloepperet al., 1999; van Loon et al., 1998). ISR occurs when theplant’s defense mechanisms are stimulated and primed toresist infection by pathogens (Conrath et al., 2002). ISR isdifferent from systemic acquired resistance (SAR) thattriggers systemically plant defense response followinghypersensitive response after inoculation of plant pathogens(Durrant and Dong, 2004; Mysore and Ryu, 2004; Ryals etal., 1996; van Loon et al., 1998). Previous worksdemonstrated that several bacterial determinants such assiderophores, salicylic acid (SA), and lipopolysaccharides(LPS) contributed to ISR (van Loon et al., 1998). In thefurther studies to define defense signaling components ofplant, many researchers found novel plant defense signaling

Invisible Signals from the Underground: Bacterial Volatiles Elicit Plant Growth Promotion and Induce Systemic Resistance