Biological functions of flavonoids in plants

Protection against abiotic stress

Flavonoids are key secondary metabolites that enable plants to overcome abiotic stress through their structural diversity and antioxidant properties. Abiotic stresses activate cellular signaling pathways that influence plant reactions to ultraviolet (UV) radiation, drought, salinity, extreme temperatures, heavy metal toxicity and nutrient deficiency [51-55]. Under high salinity and drought conditions, plants enhance flavonoid accumulation to mitigate oxidative damage. For example, NaCl treatment increases flavonoid and flavonoid glycoside levels in Ginkgo biloba seeds and activates flavonoid-based antioxidant systems in Casuarina glauca, improving salt tolerance [56,57]. Cold stress also induces flavonoid production; increased phenolics and flavonoids have been reported in tomato, sorghum, Haberlea rhodopensis and Arabidopsis thaliana, where flavonols and anthocyanins protect membranes and proteins from freezing damage [58-60].

Excessive UV radiation produces oxidative stress and DNA damage in plants. As a defence, plants accumulate UV-absorbing flavonoids such as quercetin and luteolin glycosides, which act as effective antioxidants and UV screens [61,62]. UV-B exposure strongly induces flavonoid biosynthesis and upregulates key genes such as chalcone synthase (CHS), as demonstrated in apple, tomato, Ligustrum vulgare, Isatis tinctoria and soybean [63-67]. Heavy metal stress stimulates flavonoid production, which contributes to metal chelation and reduces oxidative damage. Increased flavonoid accumulation and enhanced expression of phenylpropanoid-related enzymes have been observed in plants exposed to Cu, Cd, Cr, Pb and Se [68,69].

Nutrient imbalance is another major abiotic stress. Under nutrient-deficient conditions, plants release flavonoids into the rhizosphere via ABC transporters to improve nutrient availability [70]. Isoflavones and flavonols facilitate iron and phosphate uptake by reducing Fe3+ to Fe2+ and chelating metal ions [71,72]. Deficiencies in nitrogen and phosphorus also induce the expression of flavonoid and anthocyanin biosynthetic genes, leading to increased antioxidant capacity in crops such as red and green cabbage [73,74].

Pigmentation and pollinator attraction

Insect pollination (entomophily) is the primary reproductive strategy of flowering plants, in which pollinators identify flowers mainly through visual cues such as colour and, to a lesser extent, the floral scent [75]. Flower colour signals the reproductive status of plants and provides information on nectar and pollen availability, thereby acting as a key visual attractant for pollinators [76]. The structural diversity of flavonoids enables the production of a wide range of flower colours that match pollinator visual perception [77]. Pollinators exhibit species-specific preferences influenced by colour, scent and reward availability, with some insects showing strong attraction to yellow hues due to their dichromatic visual systems [78,79]. Anthocyanins are the dominant flavonoid pigments involved in pollinator attraction and accumulate in flowers and fruits. Six anthocyanidins, delphinidin, pelargonidin, cyanidin, petunidin, peonidin and malvidin, are widely distributed in edible plants [80]. Differences in hydroxylation patterns and pH conditions generate colours ranging from red to blue, with increased hydroxylation and higher pH favouring bluish hues [81]. Floral colour evolution is closely linked to pollinator perception of specific pigment combinations. Anthocyanins create distinct colour loci that are consistently perceived across diverse pollinators, including bees, butterflies, mosquitoes and birds [82].

Regulation of plant growth and development

In sexual reproduction of flowering plants, male gametes contained in pollen grains are delivered to the female gametophyte through pollen tube formation, which grows through the pistil to fertilize the ovule [83]. Studies in Petunia hybrida showed that transgenic plants with blocked flavonol biosynthesis (via CHS inhibition) lacked flavonols in the stigma, ovules, pollen and pollen tubes, resulting in male sterility [84]. Although pollen initially germinated normally, in vitro assays revealed impaired pollen tube growth, protoplasmic damage, and eventual pollen death. Similar results were reported in maize and petunia mutants deficient in CHS, where pleiotropic effects disrupted pollen fertility and flavonoid biosynthesis [85]. Supplementation with kaempferol restored pollen germination and pollen tube growth in vitro and increased seed set in vivo. Likewise, anthocyanin-deficient tomato mutants exhibited reduced seed production due to decreased pollen quantity, viability, germination and pollen tube elongation [86]. This phenotype was associated with elevated ROS and H2O2 accumulation in pollen and pollen tubes, reflecting the loss of flavonols as ROS scavengers in reproductive tissues. In rice, mutant studies demonstrated that multiple flavonoid classes, including flavanones, flavonols, flavones, and their glycosides, are required for anther fertility and male reproduction, in contrast to other plant species that rely predominantly on flavonols alone [87].

Flavonoids also play a role in the regulation of the plant hormone auxin, where auxin and cytokinin act synergistically or antagonistically to control development and modify growth in response to environmental cues [88]. The activity of P-glycoproteins that transport auxin is modulated by flavonoids, and they influence phosphatases and kinases as regulatory proteins [89]. Auxin activity increases in response to reduced water availability and suboptimal soil mineral nutrient levels. The evolution of auxin transport mechanisms is closely associated with the production of flavonoids, which function to counteract reactive oxygen species and provide defence against herbivores and pathogens [90].

Symbiotic signalling and defence

The rhizosphere serves as a dynamic habitat for soil microorganisms that suppress pathogen invasion and enhance plant nutrient acquisition [91]. Flavonoids play a key role in rhizosphere signalling by stimulating rhizobial chemotaxis, promoting bacterial colonization and inducing nodulation (nod) gene expression [92]. Under unfavourable environmental conditions, plants actively release flavonoids and other metabolites into the rhizosphere through root exudation to initiate beneficial plant-microbe interactions [93]. Once released, flavonoid activity depends on soil properties and structural modifications. Flavonoids may bind to soil particles and become inactive, while glycosylated forms are rapidly deglycosylated by microbes to yield more hydrophobic aglycones [94]. In addition, several flavonoids act as quorum-sensing inhibitors (QSI), influencing bacterial biofilm formation, nitrogen fixation and motility; examples include catechin and naringenin, which also function as nod gene inducers in legumes [95,96].

Allelopathy refers to a form of plant interference in which defence metabolites (allelochemicals) are produced and released to negatively affect neighbouring plants [97]. Flavonoids function as allelochemicals exuded by roots, inhibiting seed germination by inducing reactive oxygen species (ROS). For example, isoschaftoside, a C-glycosyl flavone isolated from Desmodium uncinatum, suppresses the growth of the hemiparasitic weed Striga hermonthica that damages maize [98]. Similarly, catechin released from the roots of Centaurea maculosa inhibits germination and growth of native Montana plant species, where older plants may be more resistant [99].