The invasive mimosoid legume Dichrostachys cinerea (L.) Wight & Arn is nodulated by diverse strains of Ensifer and Bradyrhizobium in different agroclimatic regions of India

Abstract

Dichrostachys cinerea (L.) Wight & Arn, which belongs to the Mimosoid clade of the legume subfamily Caesalpinioideae, was introduced into India and has since become invasive across wide areas of the country. It is nodulated, and like all other mimosoids it has indeterminate nodules with its microsymbionts housed in membrane-bound symbiosomes rather than within cell wall-enclosed fixation threads. Fifty-eight bacterial strains were isolated from root nodules on plants growing in soils from 13 sampling sites in India with various agroclimatic conditions. Genetic analysis of 36 strains resulted in diverse RAPD genotypes, with equal composition of Ensifer and Bradyrhizobium as its root nodule microsymbionts. Multi locus sequence analysis (MLSA) of 12 strains using the recA, glnII, atpD and 16S rRNA genes revealed significant genetic diversity forming novel clades and lineages and are potential new species. The D. cinerea strains were variants of local symbionts previously described as rhizobia associated with native and exotic mimosoid trees, as well as rhizobia associated with the non-mimosoid Caesalpinioid shrub Chamaecrista pumila and wild Papilionoid legumes from India. The symbiosis essential genes (nodA and nifH) of the D. cinerea strains were diverse and clustered according to geographical origin. Mosaic combinations of core and sym genes were harbored by both Ensifer and Bradyrhizobium suggesting gradual diversification and microevolution of rhizobia under pressure from the host in combination with edaphic and environmental factors. The dominant microsymbionts of native and invasive legumes, including D. cinerea, in alkaline soils of India are essentially of the ‘E. aridi’ and B. yuanmingense types. Dichrostachys cinerea rhizobia were symbiotically efficient on their homologous host, but also have ability to nodulate the crop Vigna radiata, and hence may be good candidates to be used for inoculants on legume crops as well as on Mimosoid trees (P. cineraria, V. nilotica, V. raddiana, S. senegal) used in sustainable agroforestry practices to enhance soil nitrogen content.