Flavonoids are small molecular secondary metabolites synthesized by plants with various biological activities. Due to their physical and biochemical properties, they are capable of participating in plants’ interactions with other organisms (microorganisms, animals and other plants) and their reactions to environmental stresses. The majority of their functions result from their strong antioxidative properties. Although an increasing number of studies focus on the application of flavonoids in medicine or the food industry, their relevance for the plants themselves also deserves extensive investigations. This review summarizes the current knowledge on the functions of flavonoids in the physiology of plants and their relations with the environment.

Diet bioactive components, in the concept of nutrigenetics and nutrigenomics, consist of food constituents, which can transfer information from the external environment and influence gene expression in the cell and thus the function of the whole organism. It is crucial to regard food not only as the source of energy and basic nutriments, crucial for living and organism development, but also as the factor influencing health/disease, biochemical mechanisms, and activation of biochemical pathways. Bioactive components of the diet regulate gene expression through changes in the chromatin structure (including DNA methylation and histone modification), non-coding RNA, activation of transcription factors by signalling cascades, or direct ligand binding to the nuclear receptors. Analysis of interactions between diet components and human genome structure and gene activity is a modern approach that will help to better understand these relations and will allow designing dietary guidances, which can help maintain good health.

Flavonoids are a large group of secondary plant metabolites with many important functions; they play a role in fruit, flower and seed pigmentation and are involved in multiple protective mechanisms. They are very active natural antioxidants, acting as antimicrobial compounds in defense against pathogens, and they protect the plant against various stress factors, including excessive solar radiation and temperature. They are also an animal deterrent. Flax is already a very useful crop plant with nutritional and biomedical applications. With increased phenylpropanoid content, flax plants could be used in the production of improved dietary supplements and antimicrobial agents. The main aim of this study was to engineer a flax variety with increased flavonoid content by crossing two transgenic flax varieties that have heightened flavonoid levels. A mother plant that over expresses genes encoding the flavonoid biosynthesis pathway enzymes chalcone synthase, chalcone isomerase and dihydroflavonol reductase was crossed with plants overexpressing the glucosyltransferase (GT) gene. It was expected that the progeny would display better properties thanks to the simultaneous increases in flavonoid synthesis and stability. In comparison to the control and parental plants, plants of the selected flax lines were found to have increased contents of flavonoids and other phenylpropanoids, including phenolic acids, in their stems and seeds. A significant increase in the secoisolariciresinol diglucoside content was found in the seeds. The antioxidative properties of extracts from W92 × GT crossbreed plants were higher than the control (non-transgenic) and parental plants. These results correlated with the increase in the susceptibility of the crossbreeds to Fusarium infection. The increased flavonoid content did not cause any negative phenotypic changes or reduce the yield of seeds.