Transcriptome Profiling Revealed Candidate Genes, Pathways and Transcription Factors Related to Nitrogen Utilization and Excessive Nitrogen Stress in Perennial Ryegrass

Ryegrass (Lolium perenne L.) is a type of the high quality forage grasses, which can be a good nutritional source for herbivorous livestock. However, improving the nitrogen utilization efficiency and avoiding the nitrate toxicity caused by excessive nitrogen have long been the challenging tasks in ryegrass. The molecular response mechanism of ryegrass to nitrogen, especially under the condition of excessive nitrogen, remains unclear. In this study, the transcription of perennial ryegrass at different nitrogen levels was identified by high-throughput next-generation RNA sequencing. Phenotypic characterizations showed that ryegrass in treatment N0.5 had a better growth state than the other three groups. Treatments N1 and N10 contained excessive nitrogen, which had a stress effect on plant growth. Analysis of differentially expressed genes indicated that 345 and 104 genes are involved in the regulation of nitrogen utilization and excessive nitrogen stress, respectively. GO enrichment analysis revealed that the plant response to nitrogen was mainly enriched in two categories, including “biological process” and “molecular function”. KEGG enrichment analysis suggested that “Photosynthesis-antenna proteins” may respond positively to nitrogen under appropriate nitrogen conditions, whereas “steroid biosynthesis”, “carotenoid biosynthesis” and “C5-branched dibasic acid metabolism” were identified as the top significantly enriched pathways in response to excessive nitrogen. Transcription factor analysis showed that 21 TFs related to nitrogen utilization were classified into 10 transcription factor families, especially the AP2-EREBP and MYB TF families. Four TFs related to excessive nitrogen stress were identified, including LOB, NAC, AP2-EREBP and HB. The expression patterns of these selected genes were also analyzed.