Comparative transcriptomics provides insights into the pathogenic immune response of brown leaf spots in weeping forsythia.

This article focuses on elucidating the pathogenesis of brown leaf spots in weeping forsythia (Forsythia suspensa), a medicinal and ornamental plant, caused by the fungal pathogen Alternaria alternata, as well as the plant's immune response mechanisms. Using morphological anatomy, physiological measurements, and transcriptome sequencing, the study found that A. alternata induces stomatal opening via up-regulation of auxin-activated signaling pathway genes, enabling invasion of leaf mesophyll, cell wall degradation, chloroplast reduction, and increased iron ion levels that elevate reactive oxygen species, leading to programmed cell death and tissue damage. The immune response of weeping forsythia involves differential expression of WRKY and NBS-LRR gene families and activation of signaling pathways mediated by flg22-like and elf18-like polypeptides, ethylene, hydrogen peroxide (H₂O₂), and bacterial secretion systems, which contribute to defense and hypersensitive responses. These findings provide a molecular and physiological basis for understanding and potentially controlling brown leaf spot disease in cultivated weeping forsythia.