| Symbiotic nitrogen fixation(SNF)is an important strategy to ensure soybean survival under nitrogen deficiency condition.However,once the nitrogen nutrition is sufficient,the establishment of symbiotic relationship between soybean and rhizobia and the symbiotic nitrogen fixation in mature nodules are strictly inhibited due to the high energy consumption of SNF.This phenomenon is also called "nitrogen inhibition" of SNF.Recently,although some research progress has been made on the molecular mechanism of "nitrogen inhibition" in mature nodules,the question has not been clearly answered about whether there also has a "nitrogen inhibition" mechanism at the early stage after rhizobia interaction.It is very important and interest to analyze whether nod factor(NF)receptor NFR1 is also involved in the core regulatory network of "nitrogen inhibition" at the early stages.In this study,non-nodulation mutant nod49 carrying a point mutation of Gm NFR1αin soybean and its background cultivar Bragg were used as materials.Soybean plants were inoculated with rhizobial at 10 days after germination under low nitrogen(LN)and high nitrogen(HN)conditions.The samples used for transcriptome and metabolomic analysis analysis were taken at different time points after rhizobial infection.The transcriptome analysis results showed that:(1)Transcriptome profiling revealed diverse gene expression patterns in soybean at different time points after rhizobial infection under LN and HN condition.(2)The subclustering module 8(1430 genes)showed to be induced at 3 HAI under LN,while the induction expression pattern was significantly inhibited under HN.(3)GO enrichment analysis of genes in module 8 revealed that these genes were significantly enriched in microbial recognition,regulation of defense and immune responses,nitrogen response and metabolism,and other metabolic pathways in cells.(4)At 6 and 72 HAI,although the expression pattern of identified genes appeared to be similar with that of 3 HAI,the result of GO enrichment was clearly different.(4)Interestingly,a series of genes revealed to insensitive to external nitrogen levels in Bragg,but its expression was significantly sensitive to nitrogen levels in nod49.The metabolomic and further intgegration analysis showed that:(1)the metabolic level was significantly changed in response to different nitrogen condition.The content of amino acids,lipids,sugars and organic acids showed to be accumulated in the soybean roots under LN condition compared with that of HN.(2)The accumulation of nucleotides,amino acids,sphingolipids and glucosinolates in soybean cultivar Bragg was significantly changed after rhizobial inoculation under LN.Some of metabolites could be significantly accumulated in roots after rhizobial infection under LN,but it showed to be clearly decreased in roots under HN condition.(3)The accumulation pattern of methionine and arachidonic acid in soybean roots was significantly depended on nod factor receptor Gm NFR1αunder different nitrogen condition.(4)High consistency and correlation in transcription level and metabolic level was appeared at the early stages after rhizobial infection,which mainly included metabolite changes,the regulation of metabolic pathways and biological processes,such as the accumulation of methionine,proline,glucosinoside and arachidonic acid.(5)Different gene expression pattern was appeared in the samples under different nitrogen condition in response to rhizobial infection,which was also dependent on Gm NFR1α.In summary,these data preliminarily suggest that soybean might have developed complex reprogramming networks for regulating a rapid and precise "nitrogen inhibition" mechanism in response to different external nitrogen environment,rhizobial infection,included the regulation of the internal NF-signaling pathway.The results provide an important reference for the in-depth analysis of the core molecular regulatory network of transcription-protein-metabolic reprogramming in the early stage of soybean-rhizobial interaction. |
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