Unsaturated fatty acids play important roles in Sinorhizobium meliloti adaptation to external environment stress and symbiotic nodulation. The symbiotic plasmid A of S. meliloti encodes a DesA homologue, which has 44% identity to Bacillus subtilis DesA. However, whether S. meliloti DesA is able to play functions in fatty acid synthesis was not determined. We report that though SmdesA gene could not restore the growth of E. coli fabA temperature sensitive mutant CY57 under non-permissive conditions, argentation thin layer chromatography assay show that CY57 carrying desA encoded plasmid could produce palmitoleic acid and octadecadienoic acid. This indicate that SmdesA encodes acyl-lipid desaturase. In addition, deletion of SmdesA gene have led to a little effects on the fatty acid composition of S. meliloti, but show a significant impact on growth rate in low temperature and high salt conditions, and the ability of alfalfa symbiotic nodulation. DesA is not the key enzyme of unsaturated fatty acid synthesis of S. meliloti, but it plays important biological roles in S. meliloti dealing with environment stress and nodular process.