Abstract
Proanthocyanidins (PAs) contribute to poplar defense mechanisms against biotic and abiotic stresses. Transcripts of PA biosynthetic genes accumulated rapidly in response to infection by the fungus Marssonina brunnea f.sp. multigermtubi, treatments of salicylic acid (SA) and wounding, resulting in PA accumulation in poplar leaves. Anthocyanidin reductase (ANR) and leucoanthocyanidin reductase (LAR) are two key enzymes of the PA biosynthesis that produce the main subunits: (+)-catechin and (−)-epicatechin required for formation of PA polymers. In Populus, ANR and LAR are encoded by at least two and three highly related genes, respectively. In this study, we isolated and functionally characterized genes PtrANR1 and PtrLAR1 from P. trichocarpa. Phylogenetic analysis shows that Populus ANR1 and LAR1 occurr in two distinct phylogenetic lineages, but both genes have little difference in their tissue distribution, preferentially expressed in roots. Overexpression of PtrANR1 in poplar resulted in a significant increase in PA levels but no impact on catechin levels. Antisense down-regulation of PtrANR1 showed reduced PA accumulation in transgenic lines, but increased levels of anthocyanin content. Ectopic expression of PtrLAR1 in poplar positively regulated the biosynthesis of PAs, whereas the accumulation of anthocyanin and flavonol was significantly reduced (P<0.05) in all transgenic plants compared to the control plants. These results suggest that both PtrANR1 and PtrLAR1 contribute to PA biosynthesis in Populus.
Highlights
IntroductionFlavonoids are a large group of plant secondary metabolites that comprise several classes of compounds (e.g. anthocyanins, flavonols, isoflavones and flavan-3-ols) and accumulate in a wide variety of plant tissues [1]
Flavonoids are a large group of plant secondary metabolites that comprise several classes of compounds and accumulate in a wide variety of plant tissues [1]
PA biosynthesis is often induced by various stresses such as mechanical wounding, pathogen infection and insect herbivory in poplar [4,24,37]
Summary
Flavonoids are a large group of plant secondary metabolites that comprise several classes of compounds (e.g. anthocyanins, flavonols, isoflavones and flavan-3-ols) and accumulate in a wide variety of plant tissues [1]. Proanthocyanidins (PAs), called as condensed tannins, are oligomers or polymers of flavan-3-ols and are among the major flavonoid compounds found in higher plants [2,3]. As a final product of the flavonoid pathway, PAs play an important role in the protection of plants against herbivores and pathogens [1,4,5]. PAs contribute to the astringency and taste of many fruits and the quality of other plant products, such as wine, tea (Camellia sinensis), and cocoa [11]. An understanding of the mechanisms leading to the formation of PA polymers and its regulation is important for regulation of PA biosynthesis in plants
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