Our recent study demonstrated that foliar infection by Pseudomonas syringae pv. tomato (hereafter PstDC3000) induced malic acid (MA) transporter (ALMT1) expression leading to increased beneficial rhizobacteria Bacillus subtilis FB17 (hereafter FB17) colonization in plants against PstDC3000. Having shown that a live pathogen could induce an intra-plant signal from shoot-to-root to recruit FB17 belowground, we hypothesized that pathogen derived microbe-associated molecular patterns (MAMPs) may relay a similar response specific to FB17 recruitment. Plants subjected to known MAMPs such as a flagellar peptide, flagellin (flg22), and a pathogen-derived phytotoxin, coronatine (COR) induced a shoot-to-root signal regulating ALMT1 for recruitment of FB17. The data suggests that MAMPs-induced signaling to regulate ALMT1 is salicylic acid (SA) and JAR1/JIN1/MYC2 independent. Interestingly, a cell culture filtrate of FB17 suppressed flg22-induced MAMPs-activated root defense responses, which are similar to suppression of COR-mediated MAMPs-activated root defense, revealing a diffusible bacterial component that may regulate plant immune responses. Further analysis showed that the biofilm formation in B. subtilis, negate suppression of MAMPs-activated defense responses in roots. Moreover, B. subtilis suppression of MAMPs-activated root defense does require JAR1/JIN1/MYC2. The ability of FB17 to block the MAMPs-elicited signaling pathways related to antibiosis reflects a strategy adapted by FB17 for efficient root colonization. These experiments demonstrate a remarkable strategy adapted by beneficial rhizobacteria to suppress a host defense response which may facilitate rhizobacterial colonization and host-mutualistic association.
Venkatchalam Lakshmannan, Sherry Kitto, Jeff Caplan, Yi-Huang Hsueh, Dan Kearns, Yu-Sung Wu, and Harsh Bais
Via Nicolas Denancé, Guogen Yang