Multiple Genes of Symbiotic Plasmid and Chromosome in Type II Peanut Bradyrhizobium Strains Corresponding to the Incompatible Symbiosis With Vigna radiata.

Yue Wu,Bin Huo,La Chen,Wen Xin Chen,Yong Hua Li,En Tao Wang,Wen Feng Chen,Chang Fu Tian,Jiao Ying Shang,Xin Hua Sui

doi:10.3389/fmicb.2020.01175

Yue Wu, Bin Huo + Show 8 more

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https://doi.org/10.3389/fmicb.2020.01175

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Abstract

Rhizobia are capable of establishing compatible symbiosis with their hosts of origin and plants in the cross-nodulation group that the hosts of origin belonged to. However, different from the normal peanut Bradyrhizobium (Type I strains), the Type II strains showed incompatible symbiosis with Vigna radiata. Here, we employed transposon mutagenesis to identify the genetic loci related to this incompatibility in Type II strain CCBAU 53363. As results, seven Tn5 transposon insertion mutants resulted in an increase in nodule number on V. radiata. By sequencing analysis of the sequence flanking Tn5 insertion, six mutants were located in the chromosome of CCBAU 53363, respectively encoding acyltransferase (L265) and hypothetical protein (L615)—unique to CCBAU 53363, two hypothetical proteins (L4 and L82), tripartite tricarboxylate transporter substrate binding protein (L373), and sulfur oxidation c-type cytochrome SoxA (L646), while one mutant was in symbiotic plasmid encoding alanine dehydrogenase (L147). Significant differences were observed in L147 gene sequences and the deduced protein 3D structures between the Type II (in symbiotic plasmid) and Type I strains (in chromosome). Conversely, strains in both types shared high homologies in the chromosome genes L373 and L646 and in their protein 3D structures. These data indicated that the symbiotic plasmid gene in Type II strains might have directly affected their symbiosis incompatibility, whereas the chromosome genes might be indirectly involved in this process by regulating the plasmid symbiosis genes. The seven genes may initially explain the complication associated with symbiotic incompatibility.

Highlights

Symbiotic relationships between legume plants and soil bacteria, collectively termed rhizobia, are characterized by the formation of root nodules, a specialized plant organ, in which rhizobia differentiate into nitrogen-fixing bacteroids and reduce nitrogen to ammonia as nutrient for plant
The knockout mutants of the 53 genes were further constructed through triparental conjugation method, and 7 of the 53 genes were demonstrated to be responsible for the incompatible symbiosis with V. radiata by nodulation tests
A previous study demonstrated that Type II peanut bradyrhizobia strains possessed incompatible symbiotic phenotypes with V. radiata, a plant belonging to the A. hypogaea cross-nodulation group (Li, 2019)

Summary

Introduction

Symbiotic relationships between legume plants and soil bacteria, collectively termed rhizobia, are characterized by the formation of root nodules, a specialized plant organ, in which rhizobia differentiate into nitrogen-fixing bacteroids and reduce nitrogen to ammonia as nutrient for plant. Rhizobial specificity-related factors, such as NodD, exopolysaccharides, lipopolysaccharides, secreted proteins, Nod-factors, and so on, have been reported to affect the nodulation and host specificity (Radutoiu et al, 2007; Okazaki et al, 2013) Mutations in these related genes can cause incompatible symbiosis between rhizobia and legumes with the phenomenon that a rhizobium is unable to nodulate a particular host plant or forms nodules that are incapable of fixing nitrogen (Faruque et al, 2015; Wang et al, 2018). Cell surface exopolysaccharides (EPS) in Sinorhizobium meliloti and lipopolysaccharide (LPS) in Mesorhizobium loti 2231 were reported to affect the incompatible symbiosis with Medicago sativa (Barnett and Long, 2018) and Lotus corniculatus (TurskaSzewczuk et al, 2008), respectively

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