The phenylpropanoid pathway serves as a source of metabolites in plants and is required for biosynthesis of lignin thus serving as starting point for the production of many other compounds such as flavonoids, coumarins, lignans and hydrolysable tannins (Vogt 2010; Fraser and Chapple 2011) and are involved in plant defense and stability of cell wall. Compound with plant defense role or no known roles are often referred to as secondary metabolites. Secondary metabolites do not have direct involvement in basic cellular processes such as photosynthesis, respiration, protein or nucleic acid synthesis, these processes are the domain of primary metabolites (Fraser and Chapple 2011). The role of secondary metabolites in defense may involve deterrence/ anti-feedant activity, toxicity or acting as precursors to physical defense system (Bennett and Wallsgrove 1994).
Lignans: Lignans are plant secondary metabolites (Durazzo et al., 2018), biosynthesized in woody plants and are responsible for wood’s natural resistance against insect, fungi and bacteria (Cespedes et al., 2006). Lignans play a role in the defense of plants against insects and act as a regulator of insect feeding (Harmatha and Dinan 2003). They may also have antioxidant activity (Durazzo et al., 2013). Plant dirigent (DIR) protein have a role in biosynthesis of lignan or lignin and are important for defense responses and pathogen resistance (Li et al.,2017; Paniagua et al., 2017). Lignan could serve as a storage pool of monolignols for lignification (Paniagua et al., 2017). C(6)-C(3) i.e. propenyl-phenols and allyl-phenols are named as phenyl propanoids. The dimerization of C(6)-C(3) precursors give rise to three classes of secondary metabolites: Lignan, neo-lignans and nor-lignans. The nor-lignans show biological activities such as antioxidant, antifungal, antibacterial, cytotoxic, phytotoxic, inhibitory of enzymes and proteins (Frezza et al., 2020). Lignans are known to inhibit certain enzymes (MacRae and Towers 1984). Lignans and neo-lignans are potential new bioinsecticides against aphids (Saguez et al., 2013). nor-Lignans isolated from the leaves of Styrax ferrugineus showed antifungal and antibacterial activities (Pauleti et al., 2000). Virgatusin a tetra-substituted tetrahydrofuran lignan showed high antifungal activity against Colletotrichum lagenarium (Akiyama et al., 2007). The in vitro and in vivo antifungal activities of lignan against plant pathogenic fungi have been demonstrated by Cho et al. (2007).
Tannins: Tannins exhibit antimicrobial activity and may provide protection against insects and other herbivores. Their functionality is described by their ability to precipitate protein and be oxidatively active (Constabel et al., 2014). The two main groups are: the hydrolysable tannins and the condensed tannins. Condensed tannins are oligomeric or polymeric flavonoids, also known as proanthocyanidins (War et al., 2012). Hydrolyzable tannins are derived from the shikimic acid derived gallic acid. Condensed tannins are derived from anthocyanin that together with other flavonoid such as flavones, flavonols and isoflavones are the result of condensation of phenyl alanine derived compounds with malonyl CoA (Furstenberg-Hagg et al., 2013). Tannins are astringent bitter polyphenols that act as a feeding deterrents to many insect pest. Proanthocyanidins provides protection against predation (Dixon et al., 2005). Procyanidin, a condensed tannin in groundnut is a feeding deterrent to aphid Aphis craccivora (Grayer et al., 1992). Reduced digestibility acts as a barrier that defend plant from insect attack. When ingested tannins reduce the digestibility of the proteins thereby decreases the nutritive value of plants and plant parts to insect (Belete 2018). Quercus serrata acorns contains considerable amounts of tannins in the cotyledons. The infection success of Ciboria batschiana (a fructicolous Discomycete is a pathogen of Quercus serrata acorns) differs with tannin content of individual acorns (Takahashi et al., 2010). The flower and the seed coat containing proanthocyanidins can act as a protective barrier (Lattanzio et al.,2005; Panjehkeh et al., 2010). Tannins are involved in resistance to disease and pests and reducing tannins concentration to improve nutritional quality of common bean (Phaseolus vulgaris) may have a negative effect on plant resistance (Islam et al., 2003).
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References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 