Flagella Mediate Endophytic Competence Rather Than Act as MAMPS in Rice–Azoarcus sp. Strain BH72 Interactions

Abstract

Azoarcus sp. strain BH72 is an endophytic betaproteobacterium able to colonize rice roots without induction of visible disease symptoms. BH72 possesses one polar flagellum. The genome harbors three copies of putative fliC genes, generally encoding the major structural protein flagellin. It is not clear whether, in endophytic interactions, flagella mediate endophytic competence or act as MAMPs (microbe-associated molecular patterns) inducing plant defense responses. Therefore, possible functions of the three FliC proteins were investigated. Only fliC3 was found to be highly expressed in pure culture and in association with rice roots and to be required for bacterial motility, suggesting that it encodes the major flagellin. Endophytic colonization of rice roots was significantly reduced in the in-frame deletion mutant, while the establishment of microcolonies on the root surface was not affected. Moreover, an elicitation of defense responses related to FliC3 was not observed. In conclusion, our data support the hypothesis that FliC3 does not play a major role as a MAMP but is required for endophytic colonization in the Azoarcus-rice interaction, most likely for spreading inside the plant.

Transcriptional Profiling of Nitrogen Fixation and the Role of NifA in the Diazotrophic Endophyte Azoarcus sp. Strain BH72

“…strain BH72 [35], [36], [37]. Enhancement of expression of pilH , pilY1B and fliF under diazotrophic growth (Table S1) speaks in favor of a common mode of gene regulation.…”

Roots Shaping Their Microbiome: Global Hotspots for Microbial Activity

“…For example, the lack of flagella might decrease the defense response and improve endophytic colonization, an observation reported for a mutant of Salmonella in A. thaliana (53). Contradictory studies showed that in the rice-Azoarcus system, flagella are required for efficient colonization rather than act as MAMPs (21). The so-far limited studies suggest that endophytes induce only low levels of defense responses in compatible interactions (71), mostly from the JA pathway (1, 71), and they might even trigger specialized signaling cascades, but the mechanisms are still obscure (17,26,85).…”

“…A pilT mutant of Azoarcus sp. BH72, which still produced pili but was not capable of twitching motility, was particularly impaired in endophytic colonization but still colonized the root surface (16); likewise, a mutant lacking flagella was retarded only in endophytic colonization (21). Thus, some traits for rhizoplane and endorhizosphere colonization may be different from each other.…”

“…A minor component of type IV pili, PilX, is involved in twitching motility and colonization (96). Other motility machineries that are important for endophytic establishment are flagella, which appear to act as colonization factors rather than as defense elicitors, or MAMPS, in the Azoarcus-rice interaction (21). Also, on the one hand, the flagellin of the beneficial endophyte Burkholderia phytofirmans appears to escape the grapevine immune system mediated by the FLS2 receptor (105).…”

Effects of strategic tillage and plant hormone treatments on wheat-associated microbial communities

“…strain BH72 to establish inside rice roots but this was not important for the colonization on the root surface (Böhm et al, 2007). For beneficial endophytes, the bacterial flagella that typically act as a potent microbe-associated molecular pattern (MAMP) for recognition by the innate immune system may instead mediate endophytic competence by enabling bacterial chemotactic movement and anchoring to plant surfaces (Buschart et al, 2012). The five endophytic bacteria examined by Straub et al (2013) all contain the entire flagella machinery and a flagella-deficient mutant was hampered in colonization efficiency of wheat roots (Croes et al, 1993).…”