Inactivation of the Phosphatase CheZ Alters Cell-Surface Properties of Azorhizobium caulinodans ORS571 and Symbiotic Association with Sesbania rostrata

Abstract

Azorhizobium caulinodans can form root and stem nodules with the host plant Sesbania rostrata. The role of the CheZ phosphatase in the A. caulinodans chemotaxis pathway was previously explored using the nonchemotactic cheZ mutant strain (AC601). This mutant displayed stronger attachment to the root surface, enhancing early colonization; however, this did not result in increased nodulation efficiency. In this study, we further investigated the role of CheZ in the interaction between strain ORS571 and the roots of its host plant. By tracking long-term colonization dynamic of cheZ mutant marked with LacZ, we found a decrease of colonization of the cheZ mutant during this process. Furthermore, the cheZ mutant could not spread on the root surface freely and was gradually outcompeted by the wild type in original colonization sites. Quantitative reverse-transcription PCR analyses showed that exp genes encoding exopolysaccharides synthesis, including oac3, were highly expressed in the cheZ mutant. Construction of a strain carrying a deletion of both cheZ and oac3 resulted in a mutant strain defective in the colonization process to the same extent as found with the oac3 single-mutant strain. This result suggested that the enhanced colonization of the cheZ mutant may be achieved through regulating the formation of exopolysaccharides. This shows the importance of the chemotactic proteins in the interaction between rhizobia and host plants, and expands our understanding of the symbiosis interaction between rhizobium and host plant.

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).…”