Transcriptomic analysis reveals genetic factors underlying impaired symbiotic nitrogen fixation in lines derived from crosses between cultivated peanut (Arachis hypogaea L.)  and its wild ancestors
Symbiotic nitrogen fixation (SNF) is a complex process regulated by numerous genes extensively studied in legumes that undergo intracellular infection, such as Lotus japonicus, Medicago truncatula, and Glycine max. However, the molecular and genetic mechanisms of SNF in legumes that rely on the intercellular infection pathway, such as peanut (Arachis hypogaea L.), remain poorly understood. In a previous study, we identified two chromosome segment substitution lines (CSSLs), 12CS_051 and 12CS_044, each contains a wild segment on homeologous regions of chromosomes A02 and B02 respectively, that are severely impaired in nitrogen fixation. In this study, we have compared the transcriptomes of those lines with that of their recurrent parent, Fleur11, in roots inoculated with the effective Bradyrhizobium vignae strain ISRA400 to identify candidate genes associated with the reduced nitrogen fixation observed in these CSSLs. A comparative analysis of the transcriptome profiles of the CSSLs and Fleur11 revealed significant changes in the expression of genes involved in plant immune signaling and key symbiotic genes, such as NIN, EFD, FEN1 or SNF-related transporters. These results align with the phenotypic differences observed during the symbiotic process in the CSSLs. When focusing on each QTL region, we found that only the orthologs of the symbiotic gene FEN1, which is responsible for the failure in the enlargement of infected cells in L. japonicus, exhibited a lack of expression in the two CSSLs compared to Fleur11. FEN1 encodes a homocitrate synthase that is essential for the nitrogenase activity. We hypothesize that changes in the expression of FEN1 could affect the nitrogenase activity, potentially leading to the unfair SNF observed in these lines. In this study, we analyzed the expression profiles of two ineffective nitrogen-fixing chromosome segment substitution lines and identified FEN1 as a suitable candidate gene involved in peanut symbiosis. This research provides valuable insights into understanding and improving SNF in peanut. Overall design: RNA-Seq profiling of two CSSLs (12CS_051 and 12CS_044) and their recurent parent Fleur11 at 0, 12, and 21 days post-inoculation