The Symbiotic Requirements of Different Medicago Spp. Suggest the Evolution of Sinorhizobium Meliloti and S. Medicae with Hosts Differentially Adapted to Soil pH

Abstract

Nitrogen fixing rhizobia associated with the Medicago L. genus belong to two closely related species Sinorhizobium medicae and S. meliloti. To investigate the symbiotic requirements of different Medicago species for the two microsymbionts, 39 bacterial isolates from nodules of eleven Medicago species growing in their natural habitats in the Mediterranean basin plus six historical Australian commercial inocula were symbiotically characterized with Medicago hosts. The bacterial species allocation was first assigned on the basis of symbiotic proficiency with M. polymorpha. PCR primers specific for 16S rDNA were then designed to distinguish S. medicae and S. meliloti. PCR amplification results confirmed the species allocation acquired in the glasshouse. PCR fingerprints generated from ERIC, BOXA1R and nif-directed RPO1 primers revealed that the Mediterranean strains were genetically heterogenous. Moreover PCR fingerprints with ERIC and BOX primers showed that these repetitive DNA elements were specifically distributed and conserved in S. meliloti and S. medicae, clustering the strains into two divergent groups according to their species. Linking the Sinorhizobium species with the plant species of origin we have found that S. medicae was mostly associated with medics well adapted to moderately acid soils such as M. polymorpha, M. arabica and M. murex whereas S. meliloti was predominantly isolated from plants naturally growing on alkaline or neutral pH soils such as M. littoralis and M. tornata. Moreover in glasshouse experiments the S. medicae strains were able to induce well-developed nodules on M. murex whilst S. meliloti was not infective on this species. This feature provides a very distinguishing characteristic for S. medicae. Results from the symbiotic, genotypic and cultural characterization suggest that S. meliloti and S. medicae have adapted to different Medicago species according to the niches these medics usually occupy in their natural habitats.

Stress Tolerance and Symbiotic and Phylogenic Features of Root Nodule Bacteria Associated with Medicago Species in Different Bioclimatic Regions of Tunisia

“…Payakapong et al (25) isolated a highly salt tolerant Ensifer species (Ensifer sp.BL3), and Hung et al (14) described strains of Rhizobium, Bradyrhizobium and Agrobacterium that could grow in a medium containing 4.5% NaCl and also grew well at 45°C. The diversity of rhizobia nodulating Medicago species has been widely reported (3,4,10,38) including in Tunisia (1,16,(44)(45)(46). Three different species, Ensifer meliloti (17), Ensifer medicae (27) and Rhizobium mongolense (37), were reported as microbial symbionts of Medicago species.…”

Enhanced nodulation and symbiotic effectiveness of Medicago truncatula when co-inoculated with Pseudomonas fluorescens WSM3457 and Ensifer (Sinorhizobium) medicae WSM419

“…Nodules were then individually crushed in 100 μl of sterile 0.89% (w/v) NaCl 2 and the resulting milky solution streaked onto TY, ½ LA and NA plates, incubated at 28 C, and checked daily for growth. WSM419 was identified based on its typical donut morphology (Garau et al 2005). Bacteria other than WSM419 were picked and streaked onto both TY and NA until pure cultures were obtained.…”

Cyclopropane fatty acyl synthase in Sinorhizobium meliloti

“…In the last two decades, extensive research has been conducted to understand the biochemistry and genetics of nitrogenfixing bacteria and their symbiosis with leguminous plants, given the importance of this process for sustainable agriculture (Cowie et al, 2006;Galibert et al, 2001;MacLean et al, 2007;Weidner et al, 2003). Soil bacteria encounter numerous adverse conditions in the environment, including increased soil salinity, acidic pH conditions and limited inorganic phosphate (P i ) (Cheng et al, 2005;Garau et al, 2005;Ibragimova et al, 2006).…”