Abstract
BackgroundEnsifer alkalisoli YIC4027, a recently characterized nitrogen-fixing bacterium of the genus Ensifer, has been isolated from root nodules of the host plant Sesbania cannabina. This plant is widely used as green manure and for soil remediation. E. alkalisoli YIC4027 can grow in saline-alkaline soils and is a narrow-host-range strain that establishes a symbiotic relationship with S. cannabina. The complete genome of this strain was sequenced to better understand the genetic basis of host specificity and adaptation to saline-alkaline soils.ResultsE. alkalisoli YIC4027 was found to possess a 6.1-Mb genome consisting of three circular replicons: one chromosome (3.7 Mb), a chromid (1.9 Mb) and a plasmid (0.46 Mb). Genome comparisons showed that strain YIC4027 is phylogenetically related to broad-host-range Ensifer fredii strains. Synteny analysis revealed a strong collinearity between chromosomes of E. alkalisoli YIC4027 and those of the E. fredii NGR234 (3.9 Mb), HH103 (4.3 Mb) and USDA257 (6.48 Mb) strains. Notable differences were found for genes required for biosynthesis of nodulation factors and protein secretion systems, suggesting a role of these genes in host-specific nodulation. In addition, the genome analysis led to the identification of YIC4027 genes that are presumably related to adaptation to saline-alkaline soils, rhizosphere colonization and nodulation competitiveness. Analysis of chemotaxis cluster genes and nodulation tests with constructed che gene mutants indicated a role of chemotaxis and flagella-mediated motility in the symbiotic association between YIC4027 and S. cannabina.ConclusionsThis study provides a basis for a better understanding of host specific nodulation and of adaptation to a saline-alkaline rhizosphere. This information offers the perspective to prepare optimal E. alkalisoli inocula for agriculture use and soil remediation.
Highlights
Ensifer alkalisoli YIC4027, a recently characterized nitrogen-fixing bacterium of the genus Ensifer, has been isolated from root nodules of the host plant Sesbania cannabina
Genes involved in adaptation to saline-alkaline soils Since E. alkalisoli YIC4027 was isolated from a root nodule of S. cannabina grown in a saline-alkaline soil, its ability to grow well under saline (4% NaCl) and alkaline conditions corresponds to its environmental adaptation
E. alkalisoli YIC4027 is a predominant symbiont of S. cannabina growing in saline-alkaline soils of the Yellow River Delta (YRD)
Summary
Ensifer alkalisoli YIC4027, a recently characterized nitrogen-fixing bacterium of the genus Ensifer, has been isolated from root nodules of the host plant Sesbania cannabina. This plant is widely used as green manure and for soil remediation. E. alkalisoli YIC4027 can grow in saline-alkaline soils and is a narrow-host-range strain that establishes a symbiotic relationship with S. cannabina. Rhizobia are soil bacteria that can establish a mutualistic symbiosis with leguminous plants by forming nitrogenfixing nodules. Due to its outstanding resistance to salt and flooding stress, S. cannabina is widely cultivated in subtropical and tropical regions of Asia, Africa and Australia for various purposes such as green manure, salinization alleviation, and land remediation [2, 3]. Despite the economic and environmental importance of this plant, little is known of its nitrogen-fixing symbionts which can considerably promote plant
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