qBS7 Confers Tobacco Resistance to Phytophthora nicotianae Through Temporal Coordination of Growth-Defense Balance and Metabolic Reprogramming

Background: Black shank, caused by Phytophthora nicotianae, is a major disease of tobacco. The quantitative trait locus qBS7 confers broad-spectrum resistance, but its underlying regulatory mechanisms remain unclear. Methods: We performed comparative RNA-seq of roots from a qBS7-introgressed near-isogenic line (qBS7-NILs) and its susceptible recurrent parent Xiaohuangjin1025 before and at multiple time points after P. nicotianae inoculation. Results: During early infection, qBS7-NILs showed broad suppression of primary metabolic pathways, including glutathione, nitrogen, and sulfur metabolism, together with activation of defense-related genes and transcriptional regulators, indicating a transient adjustment of the growth-defense balance toward immunity. During middle-to-late infection, nitrogen assimilation and amino acid metabolism were reactivated, suggesting metabolic recovery to support sustained defense. Meanwhile, key ethylene biosynthesis genes, including ACO and ACS, were markedly repressed, whereas systemic acquired resistance-related genes remained highly expressed. Specific bZIP and MYB transcription factors also showed stronger induction in the resistant line. qRT-PCR further confirmed the RNA-seq expression patterns. Conclusions: These results indicate that qBS7-mediated resistance involves temporal coordination of early metabolic restraint, subsequent metabolic recovery, and defense-associated hormonal and transcriptional regulation, providing new insights and candidate genes for molecular resistance breeding.