Enhanced auxin signalling, increased andrographolide biosynthesis, and antioxidant defence in gold nanoparticles-elicited Andrographis paniculata adventitious root cultures.

Gold nanoparticles (AuNPs) were used for the first time in this study to enhance andrographolide biosynthesis and auxin regulation in Andrographis paniculata adventitious root cultures (ARCs). ARCs were established from leaf explants in Murashige and Skoog liquid medium with 0.5 mg L⁻¹ naphthalene acetic acid. Pectin-based biogenic AuNPs were synthesized and characterized using multiple analytical techniques. The synthesized AuNPs (15, 30, 45, and 60 mg L⁻¹) were applied to 30-day-old ARCs for 24, 48, and 72 h. Treatment with 30 mg L⁻¹ AuNPs for 48 h resulted in a 7.5-fold increase in andrographolide content compared with the control, as quantified by high-performance liquid chromatography. Real-time quantitative polymerase chain reaction revealed a 20.49-fold upregulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, a rate-limiting gene in the mevalonate pathway, indicating its dominance over 1-deoxy-D-xylulose-5-phosphate synthase from the MEP pathway. Auxin-responsive transcription factors ARF1 and ARF3 were also significantly upregulated (3.13- and 4.69-fold, respectively). Antioxidant enzyme activities, particularly catalase and ascorbate peroxidase, were enhanced by 9.58- and 2.14-fold, respectively. These results demonstrate that AuNPs promote andrographolide accumulation through the coordinated regulation of phytohormone signalling, biosynthetic gene expression, and antioxidant defences. This study highlights the potential of AuNPs as promising abiotic elicitors for boosting the production of secondary metabolites in A. paniculata ARCs.