NEW FRONTIERS IN CONDENSED TANNINS

RUIZ, OSCAR A

PERUGIA

Seminario; SEMINARIO INSTITUCIONAL; 2017

Institute of Biosciences and Bioresources.

Proanthocyanidins(PAs) are secondary metabolites derived from the flavonoid biosynthesispathway. These metabolites areoligomeric and polymeric end-products synthesized through the catechin andepicatechin branches in the most crop species. In planta, these compounds have been related to defence mechanismsagainst pathogens, fungi and herbivores, as well to abiotic stresses tolerance (Dixon & Paiva, 1995). Froma productive point of view, PAs are amongthe compounds that strongly affect the nutritional value of forage legume. Boththe concentration and structure of PAs, also known as condensed tannins, are ofoutstanding relevance (Barry & Mcnabb, 1999). Even though high concentrations of PAs reduce thepalatability and forage intake (Barry & Mcnabb, 1999), moderate quantities of PAs (about 5 mg PAs/g DW)in forage prevent proteolysis during ensiling and rumen fermentation, therebyprotecting ruminants against pasture bloat (Aerts, Barry, & Mcnabb, 1999). In fact, by binding to proteins in the rumen, PAsreduce fermentation rate and increase the protein levels and essential aminoacids passing through the rumen of grazing animals. Therefore, PAs increase theconversion efficiency of plant protein into animal protein and reduce the needto include supplemental protein in the diet (Kingston-Smith & Thomas, 2003). Additionally, decreased protein degradation in therumen diminishes methane production and ammonium excretion in urine. Methaneand nitrous oxide from pastures are potent greenhouse gases (GHG), specificallythe methane produced by ruminants accounts for 17-37% derived fromanthropogenic production (Steinfeld & Wassenaar, 2007). It is therefore, obtaining forage legumes withadequate levels of PAs would contribute largely to improve the bioeconomy ofcattle production system, one of the priorities of forage breeders worldwide.The most valuable forage species, such as alfalfa (Medicago sativa) and clovers (Trifoliumspp.) accumulate PAs in the seedcoat but, unfortunately, they are absent from their leaves (Pang, Peel, Wright, Wang, & Dixon, 2007). Meanwhile, some Lotus species are able to accumulate PAs in their leaves (Gruber, Skadhauge, Yu, Muir, & Richards, 2008; Sivakumaran et al.,2006).Among Lotus spp., L. corniculatus L. is a widelycultivated forage legume with moderate levels of PAs (Blumenthal & McGraw, 1999). Also, L.tenuis is regarded as a ?keystone species? for cattle production inmarginal areas such as the Flooding Pampas in South America (Escaray et al., 2012). Nevertheless, although L. tenuis is phylogenetically closely to L. corniculatus, it is unable to accumulate adequate levels of PAsin leaves. The use of both species, with the recent acquisition in ourlaboratory of an L. tenuis x L. corniculatus hybrid (Escaray et al., 2014), represent a novel genetic pool to study the regulationof PAs trait in legumes.Most structural genes of the PAs pathway have been characterized indifferent model and crop species (Zhao & Dixon, 2009b). Particularly for L. tenuis and L. corniculatushave been described the gene expression pattern of phenylalanine ammonia-lyase (PAL), dihydroflavonol 4-reductase (DFR), chalcone synthase (CHS),anthocyanidin synthase (ANS),anthocyanidin reductase (ANR), leucoanthocyanidinreductase (LAR1 and LAR2) and multidrug and toxic compound extrusionfamily protein (MATE) (Escaray et al., 2014).At transcriptional level, in many plant species it has been described aregulatory complex (MBW) shaped by MYB, bHLH and WD40 class proteins, involvedin flavonoid biosynthesis (reviewed by Xu et al.,2015). In A. thaliana, the members of MBW complex arecodify by TT2 (R2R3-MYB), TT8 (bHLH) and TTG1 (WD40) genes. Within this complex, the R2R3-MYB class memberis the key component regulating PAs biosynthesis (Nesi et al., 2000; Nesi, Jond, Debeaujon, Caboche, & Lepiniec, 2001) and orthologs of these genes have recently been characterizedfrom L. japonicus, Medicago spp. and Trifolium spp. (Hancock et al., 2012; Verdier,Zhao, & Torres-Jerez, 2012; Yoshida, Kume, et al., 2010a).Some authors have suggested that the accumulation of PAs in plants is orchestratedby numerous genes (Xu et al., 2015; Zhou et al., 2015). As a consequence, the attempts to increase leaf PAscontent in Medicago and Trifolium, by ectopic expression of PAsstructural and regulatory genes, have all proved unsuccessful (Hancock et al., 2012; Pang et al., 2007). The most recent hypothesis of transcriptional PAs-regulationsuggest that different genes from R2R3-MYBs class could be actingsynergistically (Liu, Jun, & Dixon, 2014) and/or it would be a balance between activators andrepressors (Xu et al., 2015).Despite the advances in knowledge, in particular for legume species it isnecessary continue the studies to better understand the regulation of thesesecondary compounds. In this work, we address the expression pattern and itscorrelation with PAs accumulation of new putative components of the MBWtranscriptional complex, taking advantage of selected F1 and F2 L. tenuis x L. corniculatus hybrid plants and its parental L. tenuis and L. corniculatus.