| Plant Growth-Promoting Rhizobacteria (PGPR) were evaluated for plant growth promotion and induced systemic resistance (ISR) in Arabidopsis thaliana. Nine different PGPR strains including Bacillus subtilis GB03, B. amyloliquefaciens IN937a, B. pumilus SE-34, B. pumilus T4, B. pasteurii C9, Paenibacillus polymyxa E681, Pseudomonas fluorescens 8913-61, Serratia marcescens 90-166 and Enterobacter cloacae JM-22 were evaluated for plant growth promotion and ISR capacity against Pseudomonas syringae pv. tomato DC3000 and P. s. pv. maculicola.Strain GB03 inhibited the growth of Arabidopsis plants in vitro when the bacteria were inoculated 2 cm from the plants, but significantly increased plant growth when the bacteria were inoculated 6 cm from the plants. In vivo four of the nine PGPR strains significantly promoted shoot fresh weight under greenhouse conditions 7 weeks after germination. In ISR tests, six PGPR strains significantly reduced development of symptoms caused by P. syringae pv. tomato DC3000 and P. syringae pv. maculicola.PGPR strains 90-166 and SE34 significantly protected Arabidopsis thaliana ecotype Columbia (Col-0) against Cucumber mosaic virus (CMV). Soil drench with strain 90-166 and strain SE34 also reduced symptom development in NahG plants indicating that the protection did not require salicylic acid. ISR-negative mutant (90-1662882) of strain 90-166 induced resistance of Arabidopsis at a level similar to that of the parental strain. Using NahG, npr1-1, and fad3-2 fad7-2 fad8 plants, protection of Arabidopsis by strain 90-166 follows a novel-signaling pathway that is independent of salicylic acid and NPR1 but dependent on jasmonic acid.Identification of bacterial chemical messengers that trigger growth promotion has been limited in part by the understanding of how plants respond to external stimuli. Chemical and plant-growth assays show that some PGPR release a blend of volatile components that promote growth of Arabidopsis thaliana . Moreover, several volatile components are released from two bacterial strains that trigger the greatest level of growth promotion and induced resistance against Erwinia carotovora in Arabidopsis. The demonstration that PGPR strains release different volatile blends and that plant growth is stimulated by differences in these volatile blends adds a new dimension to our understanding of the role of chemical cues in mediating plant microbe interactions. |
