Distribution and Phylogeny of Microsymbionts Associated with Cowpea (Vigna unguiculata) Nodulation in Three Agroecological Regions of Mozambique.

Ifeoma N Chidebe,Sanjay K Jaiswal,Felix D Dakora,Volker Müller

doi:10.1128/aem.01712-17

Ifeoma N Chidebe, Sanjay K Jaiswal + Show 2 more

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https://doi.org/10.1128/aem.01712-17

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Abstract

ABSTRACTCowpea derives most of its N nutrition from biological nitrogen fixation (BNF) via symbiotic bacteroids in root nodules. In Sub-Saharan Africa, the diversity and biogeographic distribution of bacterial microsymbionts nodulating cowpea and other indigenous legumes are not well understood, though needed for increased legume production. The aim of this study was to describe the distribution and phylogenies of rhizobia at different agroecological regions of Mozambique using PCR of the BOX element (BOX-PCR), restriction fragment length polymorphism of the internal transcribed spacer (ITS-RFLP), and sequence analysis of ribosomal, symbiotic, and housekeeping genes. A total of 122 microsymbionts isolated from two cowpea varieties (IT-1263 and IT-18) grouped into 17 clades within the BOX-PCR dendrogram. The PCR-ITS analysis yielded 17 ITS types for the bacterial isolates, while ITS-RFLP analysis placed all test isolates in six distinct clusters (I to VI). BLASTn sequence analysis of 16S rRNA and four housekeeping genes (glnII, gyrB, recA, and rpoB) showed their alignment with Rhizobium and Bradyrhizobium species. The results revealed a group of highly diverse and adapted cowpea-nodulating microsymbionts which included Bradyrhizobium pachyrhizi, Bradyrhizobium arachidis, Bradyrhizobium yuanmingense, and a novel Bradyrhizobium sp., as well as Rhizobium tropici, Rhizobium pusense, and Neorhizobium galegae in Mozambican soils. Discordances observed in single-gene phylogenies could be attributed to horizontal gene transfer and/or subsequent recombinations of the genes. Natural deletion of 60 bp of the gyrB region was observed in isolate TUTVU7; however, this deletion effect on DNA gyrase function still needs to be confirmed. The inconsistency of nifH with core gene phylogenies suggested differences in the evolutionary history of both chromosomal and symbiotic genes.IMPORTANCE A diverse group of both Bradyrhizobium and Rhizobium species responsible for cowpea nodulation in Mozambique was found in this study. Future studies could prove useful in evaluating these bacterial isolates for symbiotic efficiency and strain competitiveness in Mozambican soils.

Highlights

Cowpea derives most of its N nutrition from biological nitrogen fixation (BNF) via symbiotic bacteroids in root nodules
A total of 122 bacterial isolates were obtained from root nodules of both cowpea varieties (IT-1263 and IT-18) planted at Muriaze, Ruace, and Sussundenga in Mozambique
There was no clear distinction based on isolate site of origin, as the rhizobial isolates from the three study sites were distributed across all the clades, except for clade XII, which consisted of bacterial isolates exclusively from the Ruace site (Table 1; Fig. 1)

Summary

Introduction

Cowpea derives most of its N nutrition from biological nitrogen fixation (BNF) via symbiotic bacteroids in root nodules. IMPORTANCE A diverse group of both Bradyrhizobium and Rhizobium species responsible for cowpea nodulation in Mozambique was found in this study. Cowpea grain contains 57% carbohydrates [2], essential amino acids, dietary fiber, and abundant minerals [3] and contributes to a balanced diet. In addition to their nutritional importance, legumes are very highly valued in the agroecosystem as they have the most efficient biological nitrogen fixation (BNF) system known in nature [4]. The distinctive feature of this partnership is the presence of specialized symbiotic organs, or nodules, which are developed on the roots and occasionally stems of legumes following a series of molecular exchanges and morphological modifications in the two partners [5]

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