Non-Additive Transcriptomic Responses to Inoculation with Rhizobia in a Young Allopolyploid Compared with Its Diploid Progenitors.

Root nodule symbioses (nodulation) and whole genome duplication (WGD, polyploidy) are both important phenomena in the legume family (Leguminosae). Recently, it has been proposed that polyploidy may have played a critical role in the origin or refinement of nodulation. However, while nodulation and polyploidy have been studied independently, there have been no direct studies of mechanisms affecting the interactions between these phenomena in symbiotic, nodule-forming species. Here, we examined the transcriptome-level responses to inoculation in the young allopolyploid (T2) and its diploid progenitor species to identify underlying processes leading to the enhanced nodulation responses previously identified in T2. We assessed the differential expression of genes and, using weighted gene co-expression network analysis (WGCNA), identified modules associated with nodulation and compared their expression between species. These transcriptomic analyses revealed patterns of non-additive expression in T2, with evidence of transcriptional responses to inoculation that were distinct from one or both progenitors. These differential responses elucidate mechanisms underlying the nodulation-related differences observed between T2 and the diploid progenitors. Our results indicate that T2 has reduced stress-related transcription, coupled with enhanced transcription of modules and genes implicated in hormonal signaling, both of which are important for nodulation.