Microbe-Associated Molecular Patterns-Triggered Root Responses Mediate Beneficial Rhizobacterial Recruitment in Arabidopsis      

Abstract

This study demonstrated that foliar infection by Pseudomonas syringae pv tomato DC3000 induced malic acid (MA) transporter (ALUMINUM-ACTIVATED MALATE TRANSPORTER1 [ALMT1]) expression leading to increased MA titers in the rhizosphere of Arabidopsis (Arabidopsis thaliana). MA secretion in the rhizosphere increased beneficial rhizobacteria Bacillus subtilis FB17 (hereafter FB17) titers causing an induced systemic resistance response in plants against P. syringae pv tomato DC3000. Having shown that a live pathogen could induce an intraplant 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. The involvement of MAMPs in triggering plant innate immune response is well studied in the plant's response against foliar pathogens. In contrast, MAMPs-elicited plant responses on the roots and the belowground microbial community are not well understood. It is known that pathogen-derived MAMPs suppress the root immune responses, which may facilitate pathogenicity. Plants subjected to known MAMPs such as a flagellar peptide, flagellin22 (flg22), and a pathogen-derived phytotoxin, coronatine (COR), induced a shoot-to-root signal regulating ALMT1 for recruitment of FB17. Micrografts using either a COR-insensitive mutant (coi1) or a flagellin-insensitive mutant (fls2) as the scion and ALMT1 pro :b-glucuronidase as the rootstock revealed that both COR and flg22 are required for a graft transmissible signal to recruit FB17 belowground. The data suggest that MAMPs-induced signaling to regulate ALMT1 is salicylic acid and JASMONIC ACID RESISTANT1 (JAR1)/JASMONATE INSENSITIVE1 (JIN1)/MYC2 independent. Interestingly, a cell culture filtrate of FB17 suppressed flg22-induced MAMPsactivated 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 negates 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 MAMPselicited 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.

A Formate Dehydrogenase Confers Tolerance to Aluminum and Low pH

“…7). While it remains to be investigated whether our transgenic tobacco plants are also resistant to the attack of bacterial pathogens, the expression of Arabidopsis for Aluminum-activated Malate Transporter1 has been documented to be involved in both Al tolerance and pathogen resistance (Rudrappa et al, 2008;Lakshmanan et al, 2012;Kobayashi et al, 2013). In our previous study, a pathogenesis-related gene was found to be up-regulated by Al stress in rice bean .…”

Deciphering the Responses of Root Border-Like Cells of Arabidopsis and Flax to Pathogen-Derived Elicitors

“…Whereas mechanisms of plant defense involving the aerial parts are well documented, there is little knowledge of root responses to soil-borne pathogens (Attard et al, 2010;Millet et al, 2010;Lakshmanan et al, 2012;Wang et al, 2012). Extrapolation of defense responses from the leaves to the root system should be used carefully, since growing evidence suggests that molecular mechanisms are quite different between roots and aerial organs (Attard et al, 2010;Millet et al, 2010).…”

The ALMT Gene Family Performs Multiple Functions in Plants

“…The authors found that the malate release helped recruit the beneficial bacterium Bacillus FB17 around the roots [16,31]. Furthermore, AtALMT1 expression was induced by flg22, a kind of MAMP (microbe-associated molecular pattern) in a manner that was independent of the salicylic acid and jasmonic acid pathways [55]. A recent study with Arabidopsis, which also relies on malate efflux via a ALMT1 channel, proposes that the Al:malate complex is taken up by the root cells and transported to shoot by NIP1;2 to maintain low Al concentrations in the apoplast shoots [56].…”