RNA-seq analysis of the Rhizobium tropici CIAT 899 transcriptome shows similarities in the activation patterns of symbiotic genes in the presence of apigenin and salt

Abstract

BackgroundRhizobium tropici strain CIAT 899 establishes effective symbioses with several legume species, including Phaseolus vulgaris and Leucaena leucocephala. This bacterium synthesizes a large variety of nodulation factors in response to nod-gene inducing flavonoids and, surprisingly, also under salt stress conditions. The aim of this study was to identify differentially expressed genes in the presence of both inducer molecules, and analyze the promoter regions located upstream of these genes.ResultsResults obtained by RNA-seq analyses of CIAT 899 induced with apigenin, a nod gene-inducing flavonoid for this strain, or salt allowed the identification of 19 and 790 differentially expressed genes, respectively. Fifteen of these genes were up-regulated in both conditions and were involved in the synthesis of both Nod factors and indole-3-acetic acid. Transcription of these genes was presumably activated through binding of at least one of the five NodD proteins present in this strain to specific nod box promoter sequences when the bacterium was induced by both apigenin and salt. Finally, under saline conditions, many other transcriptional responses were detected, including an increase in the transcription of genes involved in trehalose catabolism, chemotaxis and protein secretion, as well as ribosomal genes, and a decrease in the transcription of genes involved in transmembrane transport.ConclusionsTo our knowledge this is the first time that a transcriptomic study shows that salt stress induces the expression of nodulation genes in the absence of flavonoids. Thus, in the presence of both nodulation inducer molecules, apigenin and salt, R. tropici CIAT 899 up-regulated the same set of symbiotic genes. It could be possible that the increases in the transcription levels of several genes related to nodulation under saline conditions could represent a strategy to establish symbiosis under abiotic stressing conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2543-3) contains supplementary material, which is available to authorized users.

High Salt Tolerance of a Bradyrhizobium Strain and Its Promotion of the Growth of Stylosanthes guianensis

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The Rhizobium tropici CIAT 899 NodD2 protein regulates the production of Nod factors under salt stress in a flavonoid-independent manner

“…However, in our previous transcriptomic study of CIAT 899, at least 17 genes located in the symbiotic plasmid and controlled by 5 different NB are over-expressed in the presence of both inducer molecules apigenin and salt (Fig. 2)28. For this reason, to determine whether transcriptional activation of all these nod genes and the symbiotic-related genes under salt stress conditions is mediated by NodD2, 12 RNA-seq libraries were generated from wild-type, nodD1 (RSP82 strain) and nodD2 (nodD2 ::Ω) mutant strains grown in the presence or absence of apigenin (3.7 μM) and salt (NaCl 300 mM) (two independent biological experiments for each condition).…”

Proteins in the periplasmic space and outer membrane vesicles ofRhizobium etliCE3 grown in minimal medium are largely distinct and change with growth phase

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