A Dual Role of Amino Acids from Sesbania rostrata Seed Exudates in the Chemotaxis Response of Azorhizobium caulinodans ORS571

Abstract

Azorhizobium caulinodans ORS571 can induce nodule formation on the roots and the stems of its host legume, Sesbania rostrata. Plant exudates are essential in the dialogue between microbes and their host plant and, in particular, amino acids can play an important role in the chemotactic response of bacteria. Histidine, arginine, and aspartate, which are the three most abundant amino acids present in S. rostrata seed exudates, behave as chemoattractants toward A. caulinodans. A position-specific-iterated BLAST analysis of the methyl-accepting chemotaxis proteins (MCPs) (chemoreceptors) in the genome of A. caulinodans was performed. Among the 43 MCP homologs, two MCPs harboring a dCache domain were selected as possible cognate amino acid MCPs. After analysis of relative gene expression levels and construction of a gene-deleted mutant strain, one of them, AZC_0821 designed as TlpH, was confirmed to be responsible for the chemotactic response to the three amino acids. In addition, it was found that these three amino acids can also influence chemotaxis of A. caulinodans independently of the chemosensory receptors, by being involved in the increase of the expression level of several che and fla genes involved in the chemotaxis pathway and flagella synthesis. Thus, the contribution of amino acids present in seed exudates is directly related to the role as chemoattractants and indirectly related to the role in the regulation of expression of key genes involved in chemotaxis and motility. This “dual role” is likely to influence the formation of biofilms by A. caulinodans and the host root colonization properties of this bacterium.

Oxalic Acid From Sesbania rostrata Seed Exudates Mediates the Chemotactic Response of Azorhizobium caulinodans ORS571 Using Multiple Strategies

“…Azorhizobium caulinodans ORS571, a symbiont of the host Sesbania rostrata , can fix atmospheric nitrogen in free-living and symbiotic states. As reported for other rhizobia symbiotic associations (Caetano-Anolles et al, 1988; Merritt et al, 2007; Miller et al, 2007; Scharf et al, 2016), the establishment of the A. caulinodans-S. rostrata association involves chemotaxis toward plants roots, colonization of the root surface, infection, and the initiation of nodule organogenesis (Jiang et al, 2016; Liu et al, 2017, 2019a,b; Liu W. et al, 2018; Liu X. et al, 2018). In particular, two chemoreceptors, IcpB and TlpA1, were shown to be involved in the chemotactic response of A. caulinodans to some organic acids, such as succinate, citrate, tartrate, and malate (Jiang et al, 2016; Liu et al, 2017).…”

“…Recently, we have found that aspartate, histidine, and arginine in seed exudates have indirect effects promoting the chemotactic response to succinate (Liu et al, 2019a). Therefore, we investigated whether oxalate also has an indirect effect on response to other organic acids using the soft agar plate assay.…”

“…Sesbania rostrata seeds (40 g) were treated with sulfuric acid to sterilize the surface, as previously described (Liu et al, 2019a). After being soaked in the concentrated sulfuric acid for 30 min, seeds were washed with sterilized water at least five times and transferred to an incubator at 37°C for 48 h. After 48 h, 100 ml of sterile water was added to the seeds to collect the exudates.…”

“…TlpA1 is a transmembrane chemoreceptor, encoded by tlpA1 , that is located upstream of the chemotaxis gene cluster ( che cluster containing cheA , cheW , cheY1 , cheB , and cheR genes) in the A. caulinodans genome (Liu et al, 2017). Recently, we have identified TlpH, a transmembrane protein containing a dCache domain, as the cognate chemoreceptor of histidine, arginine, and aspartic acid, which are the three most abundant amino acids present in the seed exudates of S. rostrata (Liu et al, 2019a). In addition to their role in the chemotactic response of A. caulinodans , it was also shown that amino acids from seed exudates played an indirect role, being involved in the regulation of che genes and flagella synthesis (Liu et al, 2019a).…”

Effects of Calcium and Signal Sensing Systems on Azorhizobium caulinodans Biofilm Formation and Host Colonization

“…Two chemoreceptor mutants, Δ azc_0821 and Δ azc_0660 , were also employed to investigate the role of chemoreceptors on the calcium-mediated biofilm formation. The two chemoreceptors AZC_0821 (also termed as TlpH) and AZC_0660 (also termed as TlpA1) have broad-range ligands including organic acids and amino acids ( Liu et al, 2017b ; Liu et al, 2019b,c ). At 6 or 12 h, they both showed a significant decrease of biofilm biomass compared to wild type ( Figure 4A ).…”