NrcR, a New Transcriptional Regulator of <i>Rhizobium tropici</i> CIAT 899 Involved in the Legume Root-Nodule Symbiosis

<div><p>The establishment of nitrogen-fixing rhizobium-legume symbioses requires a highly complex cascade of events. In this molecular dialogue the bacterial NodD transcriptional regulators in conjunction with plant inducers, mostly flavonoids, are responsible for the biosynthesis and secretion of Nod factors which are key molecules for successful nodulation. Other transcriptional regulators related to the symbiotic process have been identified in rhizobial genomes, including negative regulators such as NolR. <i>Rhizobium tropici</i> CIAT 899 is an important symbiont of common bean (<i>Phaseolus vulgaris</i> L.), and its genome encompasses intriguing features such as five copies of <i>nodD</i> genes, as well as other possible transcriptional regulators including the NolR protein. Here we describe and characterize a new regulatory gene located in the non-symbiotic plasmid pRtrCIAT899c, that shows homology (46% identity) with the <i>nolR</i> gene located in the chromosome of CIAT 899. The mutation of this gene, named <i>nrcR</i> (<u><i>n</i></u><i>ol</i><u><i>R</i></u>-like plasmid <u>c R</u>egulator), enhanced motility and exopolysaccharide production in comparison to the wild-type strain. Interestingly, the number and decoration of Nod Factors produced by this mutant were higher than those detected in the wild-type strain, especially under salinity stress. The <i>nrcR</i> mutant showed delayed nodulation and reduced competitiveness with <i>P</i>. <i>vulgaris</i>, and reduction in nodule number and shoot dry weight in both <i>P</i>. <i>vulgaris</i> and <i>Leucaena leucocephala</i>. Moreover, the mutant exhibited reduced capacity to induce the <i>nodC</i> gene in comparison to the wild-type CIAT 899. The finding of a new <i>nod</i>-gene regulator located in a non-symbiotic plasmid may reveal the existence of even more complex mechanisms of regulation of nodulation genes in <i>R</i>. <i>tropici</i> CIAT 899 that may be applicable to other rhizobial species.</p>