Mixed nitrogen forms coordinate growth and andrographolide accumulation in <i>Andrographis paniculata</i> via sulfur metabolism and abscisic acid signaling

Mixed nitrogen (N) sources, comprising nitrate (NO3-) and ammonium (NH4+), have been shown to enhance plant growth and secondary metabolite accumulation in medicinal plants; however, the underlying mechanisms remain largely unclear. This study investigated the effects of different nitrogen treatments on Andrographis paniculata, including sole NO3- (NN), a 1:1 ratio of NO3- to NH4+ (NA1), a 1:2 ratio of NO3- to NH4+ (NA2), and sole NH4+ (AN). Treatments containing NH4+ significantly increased the levels of andrographolide, 14-deoxyandrographolide, and dehydroandrographolide, as well as biomass, photosynthetic capacity, and photosynthetic N use efficiency (PNUE). NH4+ addition reprogrammed C and N metabolism by suppressing N assimilation and promoting sugar accumulation, suggesting that mixed N sources coordinate growth and andrographolide accumulation by enhancing PNUE. Transcriptome analysis further revealed upregulation of sulfur (S)-related metabolic pathways correlated with improved plant growth under mixed N conditions, while andrographolide biosynthesis was associated with abscisic acid (ABA) signaling. Specifically, NH4+-containing treatments upregulated key transcription factors (TFs) from the AP2/ERF-ERF and bZIP families and downregulated MYB-related and WRKY TFs. Notably, correlation network analysis revealed that the upregulated bZIP TFs, along with ERF and CAMK_CAMKL-CHK1-related TFs, exhibited strong co-expression with andrographolide biosynthetic genes and are known transducers of ABA signaling. Integrating these findings, we propose an "NH4+-ABA-bZIP" regulatory model that connects NH4+ nutrient to secondary metabolite production, thereby providing a foundation for further mechanistic studies on yield and quality regulation in A. paniculata.