AUTHOR=Xu Yuan-Yuan , Yang Jin-Shui , Liu Cong , Wang En-Tao , Wang Ruo-Nan , Qiu Xiao-Qian , Li Bao-Zhen , Chen Wen-Feng , Yuan Hong-Li 

TITLE=Water-Soluble Humic Materials Regulate Quorum Sensing in Sinorhizobium meliloti Through a Novel Repressor of expR

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.03194

DOI=10.3389/fmicb.2018.03194

ISSN=1664-302X

ABSTRACT=<p>Quorum sensing (QS) plays an important role in the growth, nodulation, and nitrogen fixation of rhizobia. In this study, we show that water-soluble humic materials (WSHM) repress the expression of the QS related genes <italic>sinI, sinR</italic>, and <italic>expR</italic> in <italic>Sinorhizobium meliloti.</italic> This decreased the production of <italic>N</italic>-acetyl homoserine lactones (AHL) and exopolysaccharides (EPS), and ultimately increased <italic>S. meliloti</italic> cell density. We also identified a novel regulator, SMc03890 (renamed QsrR), which binds directly to the <italic>expR</italic> promoter. Deletion of <italic>qsrR</italic> increased <italic>expR</italic> expression. WSHM repressed the expression of <italic>expR</italic> by augmenting the interaction between QsrR and the <italic>expR</italic> promoter; this was determined by a bacterial-one-hybrid assay. These effects of WSHM on the QS system in <italic>S. meliloti</italic> may be the underlying mechanism by which WSHM increase the symbiotic nitrogen fixation of <italic>Medicago sativa</italic> inoculated with <italic>S. meliloti</italic>. This study provides the first evidence that humic acids regulate the QS of rhizobia and suggests that WSHM could be used as fertilizers to improve the efficiency of symbiotic nitrogen fixation.</p>