Role of secondary metabolites in plant defense mechanisms: a molecular and biotechnological insights

The plants produce secondary metabolites (SMs) as defence compounds against both abiotic and biotic stresses. These stresses instigate the secretion and release of SMs by up or down-regulating the concerned genes involved in their synthesis. The secretion of SMs varies with the plant's genetic constitution and accordingly-they are susceptible or resistant. These metabolites mostly act as deterrents or antifeedants, allelochemicals, toxins or precursors of other metabolites that defend plants from stresses. However, some pathogens use these metabolites as a signal for host recognition or nutrition rather than using them as toxins or deterrents. The SMs activate different signalling pathways e.g. terpenoids modulate the calcineurin pathway, sesquiterpenoids modulate the jasmonic acid and salicylic acid pathway, polyphenols activate the jasmonic acid and phenylpropanoid pathway, and alkaloids activate the salicylic acid pathway to protect against pathogens and herbivores. Polyphenolic compounds provide resistance to different microbes by expressing different pathogenesis-proteins and hypersensitive reaction-mediated cell death and eliminate pathogens by altering the membrane permeability (inhibiting efflux pump), cell wall integrity, suppressing enzyme activity, free radicals' generation, inhibiting protein biosynthesis, damaging DNA and reducing the expression of virulent genes. Flavonoids help plants sustain pathogen stresses through the changes in the auxin transport process. The pathogen exposure upregulate genes of alkaloid synthesis pathways such as tyrosine decarboxylase (TyDC), S-norcoclurine synthase (NCS), codeinone reductase 2-like (COR-2), and StWRKY8 transcription factors which in turn accumulate alkaloids in large amounts. Plant exposure to pathogens leads to hypersensitivity reactions and phytoalexin accumulation. The plant's treatment of salicylic acid and jasmonic acid upregulated downstream transcription factors, increased the expression of defence proteins, triggered the synthesis of SMs, and provided resistance against multiple pathogens. Pathogens and herbivores have also coevolved to cope with defence metabolites by detoxifying the toxic metabolites, converting toxins into useful products, evolving their food choice, fast digestive system, expulsion of toxins, and down-regulation of the gene-producing secondary metabolites. This review article gives a molecular insight into the genes and regulatory proteins controlling the synthesis of SMs, which may help decipher the role of the biosynthetic pathway intermediates and thereby scoring genes providing resistance to various stresses. The article comprehensively describes the roles of different SMs in plant defence and their molecular mechanisms of action.