Role of Phenylpropanoids and Flavonoids in Plant Defense Mechanism

Phenylpropanoids and flavonoids are specialized metabolites that play a crucial role in plant defenses against biotic and abiotic stresses. Secondary metabolites in plants, such as phenylpropanoids, flavonoids, lignins, monolignols, phenolic acids, stilbenes and coumarins, play a significant role in biotic and abiotic stress responses and interactions with their environment. Under stress-free conditions, flavonoids influence pollen tube growth, seed maturation, dormancy, and the longevity of plant reproductive organs and seeds. Under adverse abiotic conditions, flavonoids can mediate defense responses, such as the accumulation of flavonoids in leaves and glandular trichomes. Metabolomics has identified specialized PPP metabolites correlated with plant resistance to fungi and oomycetes. However, the correlation can be in opposite directions depending on the pathogen and a systemic approach is required to achieve increased resistance without decreasing resistance to another pathogen. The PAL gene is located upstream in the PPP pathway and phenylpropanoids and flavonoids have antifungal activity against Fusarium oxysporum, Rhizoctonia solani and Alternaria alternata. Phenylpropanoids have been studied in vitro to develop biological alternatives to synthetic phytoprotectants but the host's defense system is the most effective mode of action. Post-inoculation transcriptomic shifts reveal that some genes encoding enzymes in the PPP are rapidly activated, while others are induced later in the infection response. Insects are the most problematic macroscopic pests on cultivated plants and plants' natural defense mechanisms against herbivores include releasing volatile organic compounds, attracting enemies and producing phenylpropanoids and flavonoids. Increasing the content of flavonoids in infested plants can reduce the population of insects that cause damage to plants and spread viruses. Plant-environment interaction, mediated by secondary metabolites like phenylpropanoids and flavonoids, leads to diversity in gene structures. Systemic biology approach will be realized through sequencing, structural elucidation, and analytical tools. Model plants and crops can be used for exploration.