Phenylpropanoids are a numerous group of the secondary metabolites. The pathway of phenolic biosynthesis is induced in plants under the treatment of various unfavorable factors. Phenylpropanoid compounds act twofold: they can be toxic for plant, inhibiting their growth and development, and, on the other side, they protect plants from stress effect. In the paper the most important phenolics, their properties and influence on plant metabolism, the typical reactions and application in pharmacy were discussed. The molecular explanation of oxidation reactions, lignin polymerization, tannin condensation, UV absorbtion and decomposition and the production of reactive oxygen species were demonstrated. In plant physiology phenylpropanoid compounds are grouped into simple and composed phenylpropanoids. Simple pheylpropanoid compounds involve mainly phenolic acids and alcohols, vanilin and coumarins. Chlorogenic acid demonstrates antibiotic properties, while salicylic acid (SA) is a plant growth and development regulator, playing also a signal role in plant defence response to numerous stresses. SA initiates synthesis of PR (pathogenesis-related) proteins, hydrogen peroxide production and controls systemic acquired resistance (SAR). Phenolic alcohols polymerize to lignin, which strengths cell wall and builds natural barrier against pathogen attack. Compounds like vanilin, strong aromatic, attract insects and are used in cosmetic and food industry. Coumarins show phototoxic effect and also demonstrate a growth inhibitor action. Composed phenylpropanoids involve tannins, flavonoids and isoflavonoids. Tannins protect plants from pathogens and deter preying insects. Flavonoids are pigments of flowers and leaves, and can protect cell structures and organic compounds from cold, UV radiation and free radicals. Isoflavonoids are characterized mainly by insecticide feature. Many isoflavonoids belong to phytoalexins, specific compounds synthesized within defence mechanism against stresses. They inhibit fungal spore germination and act osmotically to penetrating hyphae. Moreover, these compounds may imitate steroid molecules joining to specific steroid receptors disturbing numerous metabolic processes. Among the best known phytoalexins pisatin, phaseolin and medicarpin are mentioned. Phenylpropanoids also play an allelopathic role secreted by roots into soil, and inhibiting germinating and growth of other plants.
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