Root transcriptome analysis of Arabidopsis thaliana exposed to beneficial Bacillus subtilis FB17 rhizobacteria revealed genes for bacterial recruitment and plant defense independent of malate efflux

Our previous work has demonstrated that Arabidopsis thaliana can actively recruit beneficial rhizobacteria Bacillus subtilis strain FB17 (hereafter FB17) through an unknown shoot-to-root long-distance signaling pathway post a foliar bacterial pathogen attack. However, it is still not well understood which genetic targets FB17 affects in plants. Microarray analysis of A. thaliana roots treated with FB17 post 24 h of treatment showed 168 and 129 genes that were up- and down-regulated, respectively, compared with the untreated control roots. Those up-regulated include auxin-regulated genes as well as genes involved in metabolism, stress response, and plant defense. In addition, other defense-related genes, as well as cell-wall modification genes were also down-regulated with FB17 colonization. Expression patterns of 20 selected genes were analyzed by semi-quantitative RT-PCR, validating the microarray results. A. thaliana insertion mutants were used against FB17 to further study the functional response of the differentially expressed genes. Five mutants for the up-regulated genes were tested for FB17 colonization, three (at3g28360, at3g20190 and at1g21240) mutants showed decreased FB17 colonization on the roots while increased FB17 titers was seen with three mutants of the down-regulated genes (at3g27980, at4g19690 and at5g56320). Further, these mutants for up-regulated genes and down-regulated genes were foliar infected with Pseudomonas syringae pv. tomato (hereafter PstDC3000) and analyzed for Aluminum activated malate transporter (ALMT1) expression which showed that ALMT1 may be the key regulator for root FB17 colonization. Our microarray showed that under natural condition, FB17 triggers plant responses in a manner similar to known plant growth-promoting rhizobacteria and to some extent also suppresses defense-related genes expression in roots, enabling stable colonization. The possible implication of this study opens up a new dialogin terms of how beneficial microbes regulate plant genetic response for mutualistic associations.