Differences of rhizosphere and root nodule microorganisms between early and late maturing varieties of Phaseolus vulgaris in northeast China and nitrogen-fixing capacity of rhizobia

Abstract:

Phaseolus vulgaris L. is one of the key edible legumes in the world. Rhizosphere microorganisms have mutually beneficial interactions with plants, being important factors promoting crop growth and health. However, studies are limited regarding how to utilize the microbiomes of legumes to promote crop growth.Objective To investigate the structural and functional differences of microbial communities in the rhizosphere and root nodules between two varieties (‘Ziguan’ and ‘Juguan’) of P. vulgaris, screen rhizobial strains, and evaluate their nitrogen-fixing and growth-promoting properties.Methods We employed 16S rRNA gene sequencing to analyze the bacterial community structures in the rhizosphere and root nodules of the two varieties. The rhizobial strains were screened by the plate streaking method. Pot experiments with nitrogen-free vermiculite were carried out to evaluate the nitrogen-fixing performance of the 11 rhizobial strains screened out.Results The bacterial diversity in the rhizosphere soil of ‘Juguan’ was significantly lower than that of ‘Ziguan’, and the bacterial diversity in the rhizosphere soil samples of both varieties was significantly higher than that in the root nodule samples. In addition, the rhizosphere of P. vulgaris harbored beneficial bacterial genera such as Rhizobium, Sphingomonas, Burkholderia, among which Rhizobium was dominant in the root nodules of both varieties. Gephi network analysis showed that bacterial communities in the rhizosphere and root nodules had positive correlations, with the relative abundance of 75.52% and 86.67%, respectively. PICRUSt2 function prediction indicated that the bacteria in the rhizosphere mainly had the functions related to carbohydrate, amino acid, and lipid metabolism, with abundant genes involved in nitrogen metabolism. Pot experiments showed that Rhizobium lusitanum NZ5 and R. etli GLJ10 increased the underground dry weight of ‘Ziguan’ by 43.21% and 48.15%, respectively. R. lusitanum NZ5, R. etli GLZ1, and R. changzhiense GLJ12 increased the underground dry weight of ‘Juguan’ by 77.37%, 68.42%, and 67.37%, respectively.Conclusion P. vulgaris possesses ability to selectively enrich a variety of microorganisms in the soil, establishing a closely coordinated and highly functional microbial community in the rhizosphere. Moreover, different rhizobial strains exerted varied growth-promoting effects on P. vulgaris.