J Chem Ecol (2013) 39:283–297 DOI 10.1007/s10886-013-0248-5 REVIEW ARTICLE Flavonoids: Their Structure, Biosynthesis and Role in the Rhizosphere, Including Allelopathy Leslie A. Weston & Ulrike Mathesius Received: 29 December 2012 /Revised: 18 January 2013 /Accepted: 23 January 2013 /Published online: 9 February 2013 # Springer Science+Business Media New York 2013 Abstract Flavonoids are biologically active low molecular Keywords Plant interference . Roots . Exudation . weight secondary metabolites that are produced by plants, Rhizosphere . Secondary metabolites . Phenolics with over 10,000 structural variants now reported. Due to their physical and biochemical properties, they interact with many diverse targets in subcellular locations to elicit various Introduction activities in microbes, plants, and animals. In plants, flavo- noids play important roles in transport of auxin, root and Flavonoids are low molecular weight secondary metabolites shoot development, pollination, modulation of reactive ox- that are produced by plants, and generally are described as ygen species, and signalling of symbiotic bacteria in the non- essential for plant survival, unlike primary metabolites. legume Rhizobium symbiosis. In addition, they possess an- Secondary products are biologically active in many ways, tibacterial, antifungal, antiviral, and anticancer activities. In and over 10,000 structural variants of flavonoids have been the plant, flavonoids are transported within and between reported (Williams and Grayer, 2004; Ferrer et al., 2008); plant tissues and cells, and are specifically released into their synthesis appears to be ubiquitous in plants and the rhizosphere by roots where they are involved in plant/- evolved early during land plant evolution, aiding in plant plant interactions or allelopathy. Released by root exudation protection and signalling (Pollastri and Tattini, 2011; Delaux or tissue degradation over time, both aglycones and glyco- et al., 2012). Due to their physical and biochemical proper- sides of flavonoids are found in soil solutions and root ties, flavonoids also are able to interact with many diverse exudates. Although the relative role of flavonoids in allelo- targets in subcellular locations to elicit various activities in pathic interference has been less well-characterized than that microbes, plants and animals (Taylor and Grotewold, 2005; of some secondary metabolites, we present classic examples Buer et al., 2010). Although flavonoids have many roles in of their involvement in autotoxicity and allelopathy. We also plants, including their influence on the transport of auxin describe their activity and fate in the soil rhizosphere in (Brown et al., 2001; Wasson et al., 2006; Peer and Murphy, selected examples involving pasture legumes, cereal crops, 2007), they also play important roles in modulating the and ferns. Potential research directions for further elucida- levels of reactive oxygen species (ROS) in plant tissues tion of the specific role of flavonoids in soil rhizosphere (Taylor and Grotewold, 2005; Agati et al., 2012), and pro- interactions are considered. vide colouring to various tissues including flowers (Davies et al., 2012). In addition, they are required for signalling symbiotic bacteria in the legume rhizobium symbiosis (Djordjevic et al., 1987; Zhang et al., 2009), and are impor- * L. A. Weston ( ) tant in root and shoot development (Buer and Djordjevic, EH Graham Centre, Charles Sturt University, Wagga Wagga, NSW 2678, Australia 2009). e-mail: [email protected] In relation to their role in allelopathy and the inhibition of seedling root growth, the activity of flavonoids as regulators U. Mathesius of auxin transport and degradation is likely to be of partic- Division of Plant Science, Research School of Biology, Australian National University, Canberra, ACT 0200, Australia ular importance. Depending on their structure, flavonoids e-mail: [email protected] can impact the breakdown of auxin by IAA oxidases and 284 J Chem Ecol (2013) 39:283–297 peroxidases (Furuya et al., 1962; Stenlid, 1963; Mathesius, elucidated, compared to biosynthetic pathways of other 2001) and also affect polar auxin transport (Stenlid, 1976; secondary products (Dixon and Steele, 1999; Winkel- Jacobs and Rubery, 1988; Peer and Murphy, 2007), thereby Shirley, 2001). Flavonoids are synthesized through the impacting root growth of target species. Some isoflavonoid phenylpropanoid or acetate-malonate metabolic pathway, phytoalexins act as cofactors to auxin in adventitious root which also is well-described in Arabidopsis. Interestingly, development, although the mode of action of these mole- unlike legumes, Arabidopsis lacks chalcone reductase and cules remains unknown (Yoshikawa et al., 1986). In addi- isoflavone synthase enzymes, so therefore it cannot pro- tion, flavonoids show affinity for many enzymes and other duce one subset of flavonoids, the isoflavonoids (Buer et proteins in plants and animals, including those required for al., 2007, 2010,). mitochondrial respiration. In this case, certain flavonoids Arabidopsis mutants (Peer et al., 2001) and transgenic contribute to inhibition of NADH oxidase and the balance legumes with modified branches of the flavonoid pathway of reactive oxygen species (Hodnick et al., 1994, 1988), (Yu et al., 2003; Subramanian et al., 2005, 2006; Wasson et thereby impacting respiration. al., 2006) now are available and provide a unique tool for In animal systems, plant-produced flavonoids are impor- studying the role of flavonoids in rhizosphere interactions. tant dietary components, and are known to possess a broad Interestingly, flavonoids have similar precursors to those range of properties including antibacterial, antifungal, anti- utilized for lignin biosynthesis but exhibit a number of basal viral, and anticancer activity (Taylor and Grotewold, 2005; structures that result in generation of diverse structures Soto-Vaca et al., 2012). Many flavonoids also have served including flavones, flavonols, flavan-3-ols, flavanones, iso- as templates in the development of new pharmaceuticals flavanones, isoflavans, and pterocarpans (Fig. 1). (Cutler et al., 2007). Interestingly, flavonoids in planta can Substitution by glycosylation, malonylation, methylation, be transported within and between tissues and cells, and hydroxylation, acylation, prenylation, or polymerization often are released into the rhizosphere where they are in- leads to diversity in this family and has important impact volved in plant to plant interactions, specifically allelopathic upon function, solubility, and degradation (Dixon and interference (Hassan and Mathesius, 2012). They can be Steele, 1999; Winkel-Shirley, 2001; Zhang et al., 2009). released by root exudation or through tissue degradation In higher plants, flavonoid synthesis begins when en- over time, and although both aglycones and glycosides of zyme complexes form on the cytosolic side of the endoplas- flavonoids are found in root exudates, their relative role in mic reticulum (Jorgensen et al., 2005),whichthenmay allelopathic interference, specific activity and selectivity, localize to the tonoplast for subsequent glycosylation and and mode(s) of action remain less well-characterised storage in the vacuole (Winkel, 2004). In specific tissues, (Berhow and Vaughn, 1999;WestonandDuke,2003; flavonoid synthesis and accumulation often is located in Levizou et al., 2004; Hassan and Mathesius, 2012). This distinct cells (Fig. 2). Subcellularly, flavonoids have been review describes the diversity of flavonoids produced by found in the nucleus, the vacuole, cell wall, cell membranes higher plants, their biosynthesis and transport, their roles in and the cytoplasm (Hutzler et al., 1998; Erlejman et al., the rhizosphere, and gives particular emphasis to their re- 2004; Saslowsky et al., 2005; Naoumkina and Dixon, cently described roles in allelopathic interference with other 2008). While flavonoid glycosides stored in the vacuole plants. We also outline potential research directions for the probably do not generally have active roles, their released future to further elucidate the specific role of flavonoids in aglycone counterparts could have functions in the plant soil-rhizosphere interactions. cytoplasm, e.g., in regulation of enzyme activity, formation of reactive oxygen species, and auxin transport (Taylor and Grotewold, 2005; Naoumkina and Dixon, 2008). In some Flavonoid Structure, Function, and Biosynthesis studies, flavonoid glycosides also have been found to have in Plants active roles, e.g., in regulation of IAA oxidase, which could lead to changes in auxin accumulation (Furuya et al., 1962; Flavone ring structures are found in fruits, vegetables, Stenlid, 1968). Accumulation of flavanols (catechins) often grains, nuts, stems, leaves, flowers and roots and are ubiq- has been observed in nuclei, especially in gymnosperm uitous throughout nature, playing an integral role in plant species. Their roles could include the regulation of gene growth and development (Harborne, 1973). The term flavo- expression through chromatin remodelling and effects on noid generally is used to describe a broad collection of enzymes and protein complexes that regulate gene expres- natural products that possess a C6-C3-C6 skeleton, or more sion (Feucht et al., 2012). specifically a phenylbenzopyran function (Marais et al., In root tissues, flavonoids can accumulate at the root tip 2007). The typical flavone ring is the backbone of flavonoid and in root cap cells from