Functions of Flavonoids in Plant, Pathogen, and Opportunistic Fungal Interactions

Flavonoids are multi-functionary secondary metabolites which have anti-pathogenic properties. As flavonoids might play a potent role in plant defence, it becomes crucial to understand all aspects of the interaction between the plant and beneficial microbes, its pathogens, and the functionality of flavonoids that might influence them. Flavonoids are well known for building resistance in plants to various pests, pathogens, and nematodes, using different biochemical strategies like hypersensitive response mediated cell death and altering iron availability. In addition, they also regulate the soil microbiota of the plant and help in establishing symbiotic relationships. Flavonoids act as key signalling compounds and chemo-attractants in various plant-microbe interactions viz., Fusarium spp., Rhizobium, arbuscular mycorrhizal fungi, or induce virulence genes as seen in Agrobacterium tumefaciens, and Pseudomonas syringae. Interactions of flavonoids with different pathogenic fungi are still understudied and are diverse, subject to the species of the fungal pathogen and host plant. In some cases, flavonoids in root exudates have had a stimulatory effect on the microconidia germination of pathogenic fungi like Fusarium solani on legumes. Similar cases of microconidia germination due to root exudates have also been observed in bean, barley, tomato, tobacco, and cucumber. However, certain flavonoids such as kaempferide triglycoside have induced resistance against F. oxysporum in carnation. The role of flavonoids in disease management is seen in the case of partial resistance towards charcoal rot disease or when plants are engineered with inducible expression of effective flavonoid phytoalexins. Although flavonoids prove increasingly useful, it is imperative to study if their ectopic expression causes harm to beneficial rhizosphere organisms.