Exploring the functional roles of secondary metabolites in Brassica defense networks.

Plants produce a diverse array of secondary metabolites (SMs) that play critical roles in adaptation to environmental challenges and defense against pathogens and herbivores. Brassica species accumulate a wide range of SMs, including glucosinolates and their hydrolysis products (isothiocyanates, nitriles, thiocyanates, epithionitriles, and oxazolidinethiones), phenolics, tannins, cyanogenic glycosides, alkaloids, flavonoids, and terpenoids. These compounds function as defense molecules, antioxidants, signaling mediators, and regulators of plant growth and stress responses. This review summarizes current knowledge on the biosynthesis, regulation, and functional roles of Brassica secondary metabolites in plant defense networks. The review highlights the glucosinolate-myrosinase system as a central defense mechanism against insect pests, herbivores, and phytopathogens. The roles of phenolics, tannins, alkaloids, and other nitrogen-containing metabolites in mitigating biotic stress and enhancing resilience are also discussed. Particular emphasis is placed on the molecular regulation of SM biosynthesis by transcription factors including MYBs, MYCs, WRKYs, ERFs, and NAC proteins, as well as by phytohormones such as jasmonic acid, salicylic acid, and brassinosteroids. These signaling pathways coordinate defense responses through activation of genes involved in secondary metabolism, antioxidant activity, and pathogen resistance. Advances in molecular breeding, transgenic approaches, genome editing, metabolomics, and bioinformatics provide opportunities to manipulate secondary metabolite pathways for the development of disease-resistant, stress-tolerant, and nutritionally improved Brassica cultivars. Enhanced understanding of the interaction between secondary metabolites, transcriptional regulators, and phytohormone networks will facilitate sustainable crop improvement strategies while reducing dependence on chemical pesticides and promoting environmentally friendly agriculture.