Evolutionarily Conserved nodE, nodO, T1SS, and Hydrogenase System in Rhizobia of Astragalus membranaceus and Caragana intermedia

Hui Yan,Zhao Jun Ji,Chang Fu Tian,Wen Xin Chen,Wen Feng Chen,Shou Kun Ji,Liang Zhong,Na Yuan,En Tao Wang,Zhong Yu Wu,Jian Bo Xie,Zheng Lin Du

doi:10.3389/fmicb.2017.02282

Abstract

Mesorhizobium species are the main microsymbionts associated with the medicinal or sand-fixation plants Astragalus membranaceus and Caragana intermedia (AC) in temperate regions of China, while all the Mesorhizobium strains isolated from each of these plants could nodulate both of them. However, Rhizobium yanglingense strain CCBAU01603 could nodulate AC plants and it's a high efficiency symbiotic and competitive strain with Caragana. Therefore, the common features shared by these symbiotic rhizobia in genera of Mesorhizobium and Rhizobium still remained undiscovered. In order to study the genomic background influencing the host preference of these AC symbiotic strains, the whole genomes of two (M. silamurunense CCBAU01550, M. silamurunense CCBAU45272) and five representative strains (M. septentrionale CCBAU01583, M. amorphae CCBAU01570, M. caraganae CCBAU01502, M. temperatum CCBAU01399, and R. yanglingense CCBAU01603) originally isolated from AC plants were sequenced, respectively. As results, type III secretion systems (T3SS) of AC rhizobia evolved in an irregular pattern, while an evolutionarily specific region including nodE, nodO, T1SS, and a hydrogenase system was detected to be conserved in all these AC rhizobia. Moreover, nodO was verified to be prevalently distributed in other AC rhizobia and was presumed as a factor affecting the nodule formation process. In conclusion, this research interpreted the multifactorial features of the AC rhizobia that may be associated with their host specificity at cross-nodulation group, including nodE, nodZ, T1SS as the possible main determinants; and nodO, hydrogenase system, and T3SS as factors regulating the bacteroid formation or nitrogen fixation efficiency.

Highlights

IntroductionBased on their tremendously important medicinal values and remarkable sand-fixing effects, Astragalus and Caragana species (belonging to Tribe Galegeae and Tribe Hydesareae, respectively) are widely cultivated in the northwest region of China
Based on their tremendously important medicinal values and remarkable sand-fixing effects, Astragalus and Caragana species are widely cultivated in the northwest region of China
M. silamurunense CCBAU01550 and M. silamurunense CCBAU45272 originally isolated from Astragalus membranaceus (Zhao et al, 2008), and M. temperatum CCBAU01399 (Lu et al, 2009), M. amorphae CCBAU01570 (Li et al, 2012), M. septentrionale CCBAU01583 (Li et al, 2012), M. caraganae CCBAU01502 (Guan et al, 2008), and R. yanglingense CCBAU01603 (Li et al, 2012) originally isolated from Caragana intermedia were used for whole genome sequencing in this study

Summary

Introduction

Based on their tremendously important medicinal values and remarkable sand-fixing effects, Astragalus and Caragana species (belonging to Tribe Galegeae and Tribe Hydesareae, respectively) are widely cultivated in the northwest region of China. An exception was found in a previous study in our laboratory that strain R. yanglingense CCBAU01603 has a more competitive nodulation ability than the representatives of Mesorhizobium species (M. silamurunense CCBAU01550, M. silamurunense CCBAU45272, M. septentrionale CCBAU01583, M. amorphae CCBAU01570, M. caraganae CCBAU01502) when Caragana plants were grown in sterile vermiculite (Ji et al, 2017). From these results, it could be seen that the nodulation specificity and competence of rhizobial strains on AC plants was different among different strains. The genetic characteristics that influence the nodulation phenotype of AC rhizobia is still unknown

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Discussion

Conclusion