Identification of type III effectors modulating the symbiotic properties of Bradyrhizobium vignae strain ORS3257 with various Vigna species

Pongpan Songwattana,Pongdet Piromyou,Jenjira Wongdee,Joel Fardoux,Djamel Gully,Albin Teulet,Alicia Camuel,Clémence Chaintreuil,Eric Giraud,Panlada Tittabutr,Alexandre Mahougnon Aurel Zoumman,Mamadou Mbaye,Neung Teaumroong

doi:10.1038/s41598-021-84205-w

Abstract

The Bradyrhizobium vignae strain ORS3257 is an elite strain recommended for cowpea inoculation in Senegal. This strain was recently shown to establish symbioses on some Aeschynomene species using a cocktail of Type III effectors (T3Es) secreted by the T3SS machinery. In this study, using a collection of mutants in different T3Es genes, we sought to identify the effectors that modulate the symbiotic properties of ORS3257 in three Vigna species (V. unguiculata, V. radiata and V. mungo). While the T3SS had a positive impact on the symbiotic efficiency of the strain in V. unguiculata and V. mungo, it blocked symbiosis with V. radiata. The combination of effectors promoting nodulation in V. unguiculata and V. mungo differed, in both cases, NopT and NopAB were involved, suggesting they are key determinants for nodulation, and to a lesser extent, NopM1 and NopP1, which are additionally required for optimal symbiosis with V. mungo. In contrast, only one effector, NopP2, was identified as the cause of the incompatibility between ORS3257 and V. radiata. The identification of key effectors which promote symbiotic efficiency or render the interaction incompatible is important for the development of inoculation strategies to improve the growth of Vigna species cultivated in Africa and Asia.

Highlights

The Bradyrhizobium vignae strain ORS3257 is an elite strain recommended for cowpea inoculation in Senegal
A recent phylogenomic analysis conducted on a large number of Bradyrhizobium genomes showed that a T3SS is almost systematically found in all the strains nodulating and containing nod genes, suggesting that T3SS is an overriding factor in determining the symbiotic properties of the rhizobia that belong to this g enus[10]
Our study conducted on B. vignae ORS3257 strain supports this hypothesis as we observed drastic impacts of the mutation of the T3SS machinery on the symbiosis of the bacteria on the three Vigna species tested

Summary

Introduction

The Bradyrhizobium vignae strain ORS3257 is an elite strain recommended for cowpea inoculation in Senegal This strain was recently shown to establish symbioses on some Aeschynomene species using a cocktail of Type III effectors (T3Es) secreted by the T3SS machinery. Thanks to the ability of legume plants to interact symbiotically with nitrogen fixing rhizobia, they play a major agronomical and ecological role. Beyond the NF signals, other bacterial components are important for the successful establishment of the symbiosis, including exopolysaccharides (EPS), lipopolysaccharides (LPS), capsular polysaccharides (KPS), and cyclic β-glucans[3,4]. Polysaccharides can act directly as a symbiotic signal or may be modified to avoid the induction of plant triggered immunity (PTI) which normally occurs when the plant recognises a PAMP s ignal[5,6]

Methods

Results

Conclusion