Genomic analysis of Sinorhizobium meliloti LPU63, an acid-tolerant and symbiotically efficient alfalfa-nodulating rhizobia

CAFIERO, JUAN HILARIO; SALVETTI, MARÍA; LOZANO, MAURICIO JAVIER; VACCA, CAROLINA; LAGARES, ANTONIO; DEL PAPA, MARÍA FLORENCIA

Congreso; RELAR XXX; 2021

The growth and persistence of alfalfa (Medicago sativa), a perennial legume capable of producing high yields of high-quality forage, are both reduced in moderately acidic soils. The low performance of alfalfa in acidic soils is due to numerous factors that affect the host plant, their rhizobia and the symbiotic interaction. Sinorhizobium meliloti LPU63 was isolated from an acidic soil (Argentina) and it proved to be a highly competitive and efficient N2-fixing rhizobium under neutral and moderately acidic soil conditions. In this study we obtained a draft of the LPU63 genome sequence using Illumina HiSeq4000. The whole genome phylogenetic analysis confirmed the taxonomic position of LPU63 as a S. meliloti strain. A multilocus sequence analysis confirmed that LPU63 is not related to any of the strains currently used in Argentina as inoculants. In addition to the canonical chromosome, pSymA and pSymB, LPU63 bears a putative accessory plasmid which codes for a repABC origin of replication and a conjugative T4SS, suggesting that this plasmid could be self-transmissible. The LPU63 genome codes for the complete denitrification pathway (i.e. the gene clusters nap, nir, nor and nos), including napC and nosZ, that could be used as an alternative respiration route under hypoxic conditions with moderate N2O emissions. Also, genes associated with plant growth promoting activities (pqqBCDE, for phosphate solubilization, and acdS encoding ACC deaminase) were identified. LPU63 is a highly melanogenic strain, a property that could enhance its survival under soil conditions. The data suggest that mcoA, involved in melanin production, is split in two ORFs. The information regarding LPU63 activities that are compatible with plant-growth promotion phenotypes, together with other characteristics mentioned here (melanin production, potential moderate N2O emissions), constitute the basis of future experiments toward the rational design of a novel bioinoculant for an environmentally sustainable production of alfalfa.