The Composite Genome of the Legume Symbiont <i>Sinorhizobium meliloti</i>

TLDR

Nitrogen scarcity limits plant growth, but legumes acquire nitrogen through rhizobial bacteria that reduce atmospheric N₂ to ammonium. The study provides the annotated genome sequence of Sinorhizobium meliloti, the alfalfa symbiont. Analysis of the 6.7‑Mb genome shows that the chromosome and two megaplasmids each contribute to symbiosis and suggests evolutionary pathways for genome assembly. The genome consists of a 3.65‑Mb chromosome and 1.35‑ and 1.68‑Mb megaplasmids, offering a resource for studying interkingdom interactions and soil ecology.

Abstract

The scarcity of usable nitrogen frequently limits plant growth. A tight metabolic association with rhizobial bacteria allows legumes to obtain nitrogen compounds by bacterial reduction of dinitrogen (N 2 ) to ammonium (NH 4 + ). We present here the annotated DNA sequence of the α-proteobacterium Sinorhizobium meliloti , the symbiont of alfalfa. The tripartite 6.7-megabase (Mb) genome comprises a 3.65-Mb chromosome, and 1.35-Mb pSymA and 1.68-Mb pSymB megaplasmids. Genome sequence analysis indicates that all three elements contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in understanding the dynamics of interkingdom associations and of life in soil environments.