Reference nodule transcriptomes for Melilotus officinalis and Medicago sativa cv. Algonquin

Abstract

Host/symbiont compatibility is a hallmark of the symbiotic nitrogen-fixing interaction between rhizobia and legumes, mediated in part by plant produced nodule-specific cysteine-rich (NCR) peptides and the bacterial BacA membrane protein that can act as a NCR peptide transporter. In addition, the genetic and metabolic properties supporting symbiotic nitrogen fixation often differ between compatible partners, including those sharing a common partner, highlighting the need for multiple study systems. Here, we report high quality nodule transcriptome assemblies for Medicago sativa cv. Algonquin and Melilotus officinalis, two legumes able to form compatible symbioses with Sinorhizobium meliloti. The compressed M. sativa and M. officinalis assemblies consisted of 79,978 and 64,593 contigs, respectively, of which 33,341 and 28,278 were assigned putative annotations, respectively. As expected, the two transcriptomes showed broad similarity at a global level. We were particularly interested in the NCR peptide profiles of these plants, as these peptides drive bacterial differentiation during the symbiosis. A total of 412 and 308 NCR peptides were predicted from the M. sativa and M. officinalis transcriptomes, respectively, with approximately 9% of the transcriptome of both species consisting of NCR transcripts. Notably, transcripts encoding highly-cationic NCR peptides (isoelectric point > 9.5), which are known to have antimicrobial properties, were ~2-fold more abundant in M. sativa than in M. officinalis, and ~27-fold more abundant when considering only NCR peptides in the six-cysteine class. We hypothesize that the difference in abundance of highly-cationic NCR peptides explains our previous observation that some rhizobial bacA alleles which can support symbiosis with M. officinalis are unable to support symbiosis with M. sativa.