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

Weeping forsythia is an important ornamental, ecological, and medicinal plant. Brown leaf spots reduce the large-scale production of weeping forsythia as a medicinal crop. Alternaria alternata is a pathogen causing brown leaf spots in weeping forsythia; however, its pathogenesis and the immune response mechanisms of weeping forsythia remain unclear. In this study, we identified the two mechanisms based on morphological anatomy, physiological indexes, and gene expression analyses. Our results showed that A. alternata induced leaf stomata to open, invaded the mesophyll, dissolved the cell wall, destroyed the cell membrane, and decreased the number of chloroplasts by up-regulating the expression of auxin-activated signaling pathway genes. A. alternata also down-regulated iron ion homeostasis and binding-related genes, which caused an increase in the levels of iron ions and reactive oxygen species in leaves. These processes eventually led to programmed cell death, destroying palisade and spongy tissues and causing the formation of iron rust spots. A. alternata also caused defense and hypersensitive responses in weeping forsythia through signaling pathways mediated by flg22-like and elf18-like polypeptides, ethylene, H2O2, and bacterial secretion systems. Our study provides a theoretical basis for the control of brown leaf spots in weeping forsythia.