Plant Protection by Tannins Depends on Defence-Related Phytohormones

Abstract

Plants produce a wide variety of secondary metabolites to sustain and protect themselves against a wide range of stresses. Among these metabolites, tannins are one of the most abundant polyphenolic compounds, accounting for 25% of the dry weight of leaves, roots and bark of woody plants, but are also abundant in flowering and seed-producing plants. The presence of tannins in these organs serves to protect plants against herbivorous and pathogenic attack through their antidigestive and antimicrobial properties. In addition, tannins play a role in regulating plant growth and development by inhibiting the consumption of unripe fruits due to their astringency. In addition, several studies have also revealed various roles of them under environmental stresses. Tannins can be classified into condensed tannins (CTs), hydrolysable tannins (HTs) and phlorotannins. They are synthesised in plants via the acetate–malonate and shikimic acid pathways. Their accumulation is regulated by several transcription factors during normal development and under different stress conditions. Despite their multiple roles in plant life, information on the regulation of tannin metabolism by defence-related phytohormones is very limited. To cope with biotic and abiotic stresses, plant responses are regulated by defence-related phytohormones such as salicylic acid (SA), jasmonic acid (JA), ethylene (ET) and abscisic acid (ABA), which act as regulators of tannin production under adverse conditions. This review focuses on tannin production, moreover its occurrence, defence potential and regulation by phytohormones under different environmental and biotic stresses, based on the most recent and relevant data. Graphical Abstract