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

Abstract

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. Rhizobium tropici CIAT 899 is an important symbiont of common bean (Phaseolus vulgaris L.), and its genome encompasses intriguing features such as five copies of nodD 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 nolR gene located in the chromosome of CIAT 899. The mutation of this gene, named nrcR (nolR-like plasmid c Regulator), 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 nrcR mutant showed delayed nodulation and reduced competitiveness with P. vulgaris, and reduction in nodule number and shoot dry weight in both P. vulgaris and Leucaena leucocephala. Moreover, the mutant exhibited reduced capacity to induce the nodC gene in comparison to the wild-type CIAT 899. The finding of a new nod-gene regulator located in a non-symbiotic plasmid may reveal the existence of even more complex mechanisms of regulation of nodulation genes in R. tropici CIAT 899 that may be applicable to other rhizobial species.

Experimental Evolution of Legume Symbionts: What Have We Learnt?

“…For example, the expression of nodulation genes is regulated by NodD proteins in all rhizobia. However, depending on rhizobium species, the expression of these genes is controlled by either a single NodD regulatory protein [78] or multiple, both positive and negative, regulators organized in intricate networks [99][100][101][102][103]. Likewise, expression of nitrogen fixation (nif -fix) genes is under the control of the conserved NifA protein, but upstream from NifA the regulatory cascade is highly variable among rhizobia, involving different two-component systems (FixLJ, RegSR, NtrBC, ActRS, FixL-FxkR) responding to various signals [104][105][106][107] and specific downstream transcription regulators (FixK1, FixK2, FnrN) [108][109][110].…”

OnfD, an AraC-Type Transcriptional Regulator Encoded by Rhizobium tropici CIAT 899 and Involved in Nod Factor Synthesis and Symbiosis

“…For an in-depth analysis, the chemical identification of the NF produced by the ΔonfD mutant was also obtained by UHPLC-MS/MS in the presence of apigenin (3.7 M) or NaCl (300 mM) (see Table S1 in the supplemental material). In a previous study, 36 and 29 NF were detected in CIAT 899 supernatants in the presence of salt and apigenin, respectively (43). Here, as a positive control of NF production, the supernatant of the CIAT 899 wild-type strain grown in the presence of both apigenin and salt was employed (25 different NF).…”

Identification of a gene encoding a sugar transporter as a potential target of a novel LysR‐type transcriptional regulator rluD from rhizobium required for symbiotic nodulation in soybean (Glycine max Merr.)

“…As the genomes from several rhizobia species have been sequenced, an increasing number of LysR‐type transcriptional regulators (LTTR), as well as nodD regulators, have been annotated (Del Cerro et al., ; Pérez‐Montaño et al., ; Tang, Wang, & Luo, ). More than 90 genes encoding LysR regulatory proteins have been predicted since the genome sequence of S. meliloti 1021 was published in 2001 (Galibert et al., ).…”