Physiological mechanism and transcriptome analysis of sweet potato overgrowth under high-nitrogen conditions

Sweet potato is an important food and economic crop in China; however, excessive vegetative growth under high nitrogen (N) conditions often leads to reduced yield and quality. In this study, we used two contrasting varieties—Xushu 32 (non-overgrowth) and Xuzishu 8 (prone to overgrowth) to investigate phenotypic responses and changes in carbon and nitrogen content under different nitrogen application levels (0, 80, and 200 kg hm-2). Transcriptome sequencing was performed to identify metabolic pathways and candidate genes associated with stem overgrowth, with subsequent validation by RT-qPCR. The results showed that under high-N conditions, Xuzishu 8 exhibited significantly enhanced shoot growth, a marked reduction in storage root yield, and more severe overgrowth compared to Xushu 32. High nitrogen treatment increased nitrogen content and decreased soluble sugar content in stem tips of both varieties, along with significant upregulation of genes involved in carbon and nitrogen metabolism pathways. These effects were more pronounced in Xuzishu 8, indicating greater sensitivity to high-N-induced overgrowth. Transcriptomic analysis identified 3852 differentially expressed genes (DEGs) between normal and high-N treatments. GO and KEGG enrichment analyses of 1174 DEGs shared between the two varieties revealed significant enrichment in pathways related to wound response, jasmonic acid signaling, amino acid metabolism, and zeatin biosynthesis, suggesting conserved mechanisms under high-N stress. Additionally, 6998 DEGs were identified between the two varieties under the same nitrogen treatments. Enrichment analysis of 1186 high-N-specific DEGs highlighted pathways associated with isoprenoid metabolism, hormone signaling, cytochrome P450 activity, nitrogen metabolism, and zeatin biosynthesis. Six candidate genes (g9506.t1, g14945.t1, g37255.t1, g25587.t1, g7821.t1, and g18076.t1) were further implicated in high-N-induced overgrowth. This study provides theoretical insight into the physiological mechanisms underlying stem overgrowth in sweet potato and offers a foundation for breeding varieties with improved tolerance to high nitrogen.