NrcR, a New Transcriptional Regulator of Rhizobium tropici CIAT 899 Involved in the Legume Root-Nodule Symbiosis
April
NrcR, a New Transcriptional Regulator of Rhizobium tropici CIAT 899 Involved in the Legume Root-Nodule Symbiosis
Pablo del Cerro 0 1
Amanda A. P. Rolla-Santos 1
Rocío Valderrama-Fernández 1
Antonio Gil- Serrano 1
Ramón A. Bellogín 0 1
Douglas Fabiano Gomes 1
Francisco Pérez-Montaño 0 1
Manuel Megías 0 1
Mariangela Hungría 1
Francisco Javier Ollero 0 1
0 Departamento de Microbiología, Facultad de Biología, Universidad de Sevilla. Sevilla , Spain, 2 Embrapa Soja, Cx. Postal 231, 86001-970, Londrina, Paraná, Brazil , 3 Departamento de Química Orgánica, Facultad de Química, Universidad de Sevilla. Sevilla , Spain
1 Editor: Francisco Martinez-Abarca, Estacion Experimental del Zaidin-CSIC , SPAIN
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 wildtype 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.
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OPEN ACCESS
Data Availability Statement: All relevant data are in
the body of the manuscript and in the supporting
information.
Funding: This work was supported by the Project
AGL2012-38831 of the Ministerio de Economía y
Competitividad (Spanish Government, www.mineco.
gob.es), CNPq (National Council for Scientific and
Technological Development, http://cnpq.br/, Science
without Borders 400205/2012-5). AAPR-S
acknowledges a postdoctoral fellowship from CNPq
(National Council for Scientific and Technological
Development, http://cnpq.br/, Science without
Borders 400205/2012-5). MH a research fellow from
CNPq (NPq (National Council for Scientific and
Technological Development, http://cnpq.br/). FPM a
postdoctoral fellowship from VPP (University of
Seville,
https://investigacion.us.es/secretariadoinvestigacion/plan-propio). PdC a FPU fellowship
from Ministerio de Economía y Competitividad
(FPU14/00160, Spanish Government, www.mineco.
gob.es). The funders had no role in study design,
data collection and interpretation, or the decision to
submit the work for publication.
Introduction
The establishment of the symbiosis between rhizobia and their specific host legumes involves
highly complex events that culminate in the formation of nodules and in the establishment of
the nitrogen fixation process. Nodulation requires a molecular dialogue between the bacteria
and the host plants. The dialogue begins with the exudation of molecules from the legume
roots, mostly flavonoids, which are recognized by the specific rhizobium, inducing the
transcription of a set of nodulation genes [
1, 2
]. These genes are responsible for the biosynthesis
and secretion of Nod factors (NFs), identified as lipochitooligosaccharides, required for
launching the nodulation and the nitrogen-fixation process [
3–7
].
A large number of bacterial transcriptional regulators actuate nodulation, the most
important of which are the NodD proteins, which belong to the LysR-type transcriptional-regulator
family. The nodD genes are constitutively expressed and their cognate proteins are responsible
for the recognition of suitable flavonoids, which start the transcription of the nodulation genes
[
8, 9
]. Another important group of transcriptional regulators is the family of metal-sensing
regulatory proteins. In this case, a specific metal-ion union regulates the protein’s allosteric
conformation and modulates the expression of several target ge (...truncated)