Identification of allelochemicals and investigation of the mechanism of action of Andrographis paniculata.

Andrographis paniculata (Burm. f.) Nees, recognized by the World Health Organization as an essential medicinal plant, is widely cultivated in South and Southeast Asia. However, continuous cropping significantly reduces its growth and quality, likely as a result of allelopathic effects. A comparative analysis of plants grown in soils from a three-year continuous cropping system and blank controls revealed significant reductions in plant height, fresh weight, chlorophyll content, and root length, underscoring the adverse effects of continuous cropping. Potential allelochemicals were identified through soil metabolomics. The result of PCA and OPLS-DA showed that substantial differences in metabolite profiles between continuous cropping and blank soils. The flavonoids were the most prominent among the upregulated differential metabolites in the continuous cropping group. Four flavonoids were extracted, isolated, and purified from the roots of A. paniculata, among which 5-hydroxy-7,8-dimethoxyflavone showed detectable allelopathic activity. Transcriptomic analysis revealed high expression of key enzyme genes in the flavonoid biosynthesis pathway within root tissues, indicating that flavonoids are synthesized in the roots and subsequently secreted into the soil. Metagenomic analysis further showed that allelochemicals introduced into the soil reduced soil microbial richness. These findings enhance the understanding of allelopathic mechanisms in A. paniculata and provide a foundation for mitigating continuous cropping challenges.