Dynamics of reactive oxygen species and lignin biosynthesis during leaf spot disease of Withania somnifera (L.) Dunal.

Withania somnifera is an important medicinal plant, however, its cultivation and quality are compromised due to infestation by leaf spot disease caused by the fungus Alternaria alternata. To decipher suitable strategies against the disease, studies on post-infectional changes are important. Reactive oxygen species (ROS) are critical as they cross-talk with other defense signaling pathways. Our study involved the analysis of ROS-generating and scavenging systems in the healthy and diseased leaf samples of W. somnifera and ROS-driven downstream defence pathways. DAB and NBT assays for ROS detection, spectrophotometric and in gel assays for ROS scavenging enzymes, thioglycolic acid (TGA) based assay and histochemical staining for lignin content and qRT-PCR for transcript-level expression studies were performed. Leaf spot infection in W. somnifera led to increased NADPH oxidase activity and ROS accumulation in the infected leaves which were supported by the enhanced antioxidant enzyme activities. Leaf spot infected leaves exhibited increased lignin content with higher expression of lignin biosynthesis genes. In addition, the transcript levels of defense-related genes, NPR1 and PR genes were also upregulated. The present work provides insights into the response of leaf spot disease in invoking the defense-related signaling in W. somnifera. It demonstrates the cross-talk between ROS and lignin biosynthesis. This work identifies potential targets for developing future strategies to confer disease resistance against leaf spot pathogen in W. somnifera.