Light and nitrogen are crucial factors regulating plant energy metabolism, carbon-nitrogen balance, and stress resistance. In this study, tea plant ‘Zhongming 7’ was subjected to dark treatment and normal light treatment under nitrogen-deficient conditions. The results indicate that dark treatment resulted in a decrease in SPAD values and a noticeable lightening of leaf color. Transcriptome analysis identifies 3 235 differentially expressed genes (DEGs), with 1 492 upregulated and 1 743 downregulated. KEGG enrichment analysis reveals significant enrichment of DEGs in pathways related to nitrogen metabolism, plant circadian rhythms and phytohormone signaling. Among the identified DEGs, an Elongated Hypocotyl 5 (CsHY5) gene was screened for further investigation. Protein-protein interaction network analysis reveals that CsHY5 primarily interacts with circadian rhythm-related genes. The expression level of the CsHY5 gene in new shoot tissues of three tea cultivars was analyzed using quantitative real-time PCR (RT-qPCR). The results reveals that the expression level of CsHY5 gene in the leaves of ‘Zhongming 7’ was higher than those observed in ‘Yunkang 10’ and ‘Hongya Foshou’. Transcriptome sequencing and RT-qPCR analysis indicate that the CsHY5 gene was downregulated under dark treatment. These findings suggest that CsHY5 may serve as a key node in the cross-regulation between light signaling and nitrogen metabolism, playing a crucial role in tea plants' response to combined dark treatment and nitrogen deficiency stress.
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