www.landesbioscience.com Plant Signaling & Behavior 636 Plant Signaling & Behavior 7:6, 636-641; June 2012; © 2012 Landes Bioscience MINI-REVIEW Introduction Flavonoids are secondary metabolites derived from the phenyl- propanoid pathway and >9,000 flavonoids have been character- ized in plants. 1 Major subgroups of flavonoids that are found in most higher plants include chalcones, flavones, flavonols, anthocyanins, proanthocyanidins (condensed tannins) and aurones. 2,3 Diverse functions of flavonoids include UV protec- tion, sexual reproduction process, defense and flower coloring. Even if complex, the biosynthesis of these molecules has been well described and numerous enzymatic steps are involved (Fig. 1). 4,5 In Arabidopsis most enzymes implicated in flavonoid bio- synthesis are encoded by single copy genes, whereas in most plant species most genes occur in multigene families. 6 Flavonoids are synthesized in the cytosol, 7 stored in vacuoles, 8 and they are also known to be present in external rhizosphere through exudation. 9 Moreover, flavonoid synthesis is organ and tissue-dependent, and is affected by environmental conditions, such as light intensity, temperature and nitrogen. 10-12 Flavonoids accumulate specifi- cally in dividing cells in the root, for example in the lateral root *Correspondence to: Valérie Hocher; Email: valerie.hocher@ird.fr Submitted: 03/13/12; Accepted: 03/15/12 http://dx.doi.org/10.4161/psb.20039 Flavonoids are a group of secondary metabolites derived from the phenylpropanoid pathway. They are ubiquitous in the plant kingdom and have many diverse functions including key roles at diferent levels of root endosymbioses. While there is a lot of information on the role of particular lavonoids in the Rhizobium-legume symbiosis, yet their exact role during the establishment of arbuscular mycorrhiza and actinorhizal symbioses still remains unclear. Within the context of the latest data suggesting a common symbiotic signaling pathway for both plant-fungal and plant bacterial endosymbioses between legumes and actinorhiza-forming fagales, this mini-review highlights some of the recent studies on the three major types of root endosymbioses. Implication of the molecular knowledge of endosymbioses signaling and genetic manipulation of lavonoid biosynthetic pathway on the development of strategies for the transfer and optimization of nodulation are also discussed. The role of lavonoids in the establishment of plant roots endosymbioses with arbuscular mycorrhiza fungi, rhizobia and Frankia bacteria Khalid Abdel-Lateif, Didier Bogusz and Valérie Hocher* Equipe Rhizogenèse; UMR DIADE (IRD, UM2); Institut de Recherche pour le Développement (IRD); Montpellier, France Key words: flavonoids, endosymbiosis, legume-rhizobium symbioses, actinorhizal symbioses, arbuscular mycorrhiza, signaling and nodule primordia of subterranean clover ( Trifolium subter- raneum), 13,14 in the root tip and the lateral root primordia of Arabidopsis. 15,16 Plant roots form three major types of intracellular endosymbi- oses in symbiotic relationships with arbuscular mycorrhizal (AM) fungi of the group of Glomeromycecota and with nitrogen fixing bacteria of the genus Rhizobium and Frankia. At least 80% of all angiosperms are able to participate in AM symbiosis while only ten families of angiosperms are known to form symbiotic asso- ciation with nitrogen-fixing bacteria in root nodules. 17 In addi- tion to occurring with rhizobia in the legumes and Ulmaceae, nitrogen-fixing symbioses involving root nodules also occur with Frankia in some members of Betulaceae, Casuarinaceae, Coriariaceae, Datiscaceae, Elaeagnaceae, Myricaceae, Rhamnaceae and Rosaceae. Molecular phylogeny of plant groups that engage in root nodule symbiosis shows that they all belong to a single clade, the Fabid (Eurosid 1). 18 Some features of root nodule endosym- biosis could have been recruited from the more ancient AM sym- biosis. 19,20 Hence, the question of shared mechanisms including the control of early signaling events is raised. 21,22 Orchestration of these early events requires the exchange of signaling mol- ecules. The rhizobial signal molecule, the so-called nodulation (Nod) factors are lipochitooligosaccharides (LCOs), consisting of an N-acetylglucosamine backbone, N-acylated on the termi- nal non-reducing sugar and bearing different substitutions on the oligosaccharidic backbone that is symbiosis specific. 23 In the case of actinorhizal symbiosis, Frankia signals are unknown. 24,25 However, it has been previously shown that Frankia alni (ACN14a) produces a root hair deforming factor (RHDF) in culture supernatant that reacts with Alnus glutinosa root hair cells inducing branching and curling of root hair cells. 26 This factor was shown to have a molecular weight below 3,000 da, to be heat-stable (similar to Rhizobium Nod factor) but also to be hydrophilic and to resist to chitinases (contrary to Rhizobium Nod factor). In AM symbiosis, it has been suggested that LCOs produced by the AM fungus could have a role in the establish- ment of AM symbiosis. 27 The work on the diversity and the role in discriminating specificity of LCOs produced by AM fungus is still in progress. 27 The recent availability of genomic resources in AM fungi is opening new possibilities to characterize the genes involved in the synthesis of signaling molecules. 28