Volatile Indole Produced by Rhizobacterium Proteus vulgaris JBLS202 Stimulates Growth of Arabidopsis thaliana Through Auxin, Cytokinin, and Brassinosteroid Pathways

Volatile compounds produced by bacteria play an important role in plant and bacteria interactions. Volatiles from the rhizobacterium Proteus vulgaris JBLS202 or synthetic indole increased the fresh weight of Arabidopsisthaliana Col-0 by 74.9–80.3 %, and 48.0–56.3 %, respectively. However, exposure to volatiles from JBLS202 or indole was unable to promote growth in the mutant lines of A. thaliana defective in auxin transport (eir1), cytokinin (cre1), and brassinosteroid metabolism (cbb1), whereas growth was significantly stimulated in the ethylene- (etr1) and gibberellin-insensitive (gai1) mutants. In addition, Arabidopsis Col-0 treated with auxin, and brassinosteroid biosynthesis inhibitors was considerably arrested in growth-promoting performance by the volatiles. Moreover, exposure of Col-0 seedlings to JBLS202 or indole for 14 days resulted in overexpression of small auxin up RNA, histidine kinase1, and brassinosteroid biosynthetic cytochrome P450 genes. Overall, the results indicate that the indole emitted by JBLS202 stimulates the growth of A. thaliana through an interplay between the auxin, cytokinin, and brassinosteroid pathways. This is the first report on how bacterial indole influences the plant hormone signaling pathways.