Characterization of UDP-glucose dehydrogenase isoforms in the medicinal legume Glycyrrhiza uralensis

Uridine 5′-diphosphate (UDP)-glucose dehydrogenase (UGD) produces UDP-glucuronic acid from UDP-glucose as a precursor of plant cell wall polysaccharides. UDP-glucuronic acid is also a sugar donor for the glycosylation of various plant specialized metabolites. Nevertheless, the roles of UGDs in plant specialized metabolism remain poorly understood. Glycyrrhiza species (licorice), which are medicinal legumes, biosynthesize triterpenoid saponins, soyasaponins and glycyrrhizin, commonly glucuronosylated at the C-3 position of the triterpenoid scaffold. Often, several different UGD isoforms are present in plants. To gain insight into potential functional differences among UGD isoforms in triterpenoid saponin biosynthesis in relation to cell wall component biosynthesis, we identified and characterized Glycyrrhiza uralensis UGDs (GuUGDs), which were discovered to comprise five isoforms, four of which (GuUGD1–4) showed UGD activity in vitro. GuUGD1–4 had different biochemical properties, including their affinity for UDP-glucose, catalytic constant, and sensitivity to feedback inhibitors. GuUGD2 had the highest catalytic constant and highest gene expression level among the GuUGDs, suggesting that it is the major isoform contributing to the transition from UDP-glucose to UDP-glucuronic acid in planta. To evaluate the contribution of GuUGD isoforms to saponin biosynthesis, we compared the expression patterns of GuUGDs with those of saponin biosynthetic genes in methyl jasmonate (MeJA)-treated cultured stolons. GuUGD1–4 showed delayed responses to MeJA compared to those of saponin biosynthetic genes, suggesting that MeJA-responsive expression of GuUGDs compensates for the decreased UDP-glucuronic acid pool due to consumption during saponin biosynthesis.     

三叶木通转录组分析及三萜皂苷生物合成途径关键酶基因的发掘

为了解三叶木通(Akebia trifoliata(Thunb.)Koidz.)的三萜皂苷合成途径及其关键酶,本研究对其花、叶、根、茎进行转录组测序,组装获得了57.25 Gb数据,含140 859个unigenes,序列平均长度为1350 bp。KEGG代谢通路富集结果显示,517个unigenes参与三萜皂苷合成相关的3条代谢途径,其中415个unigenes编码三萜皂苷生物合成途径的19个关键酶。对三萜皂苷生物合成过程中的关键酶角鲨烯环氧酶(SE)进行序列分析和同源建模,发现其具有保守的底物结合结构域。将三叶木通茎与花、叶、根的基因表达水平进行比较,发现茎与根相比较其上调基因数目最多,其中295个差异表达基因(DEGs)与三萜皂苷生物合成途径相关。     

走马胎三萜皂苷合成相关基因表达分析北大核心CSCD

走马胎是我国华南地区重要的民族药物,其体内的次生代谢物三萜皂苷具有多种药用功效。为了解三萜皂苷含量变化以及生物合成通路相关基因的调控规律,该研究对走马胎在不同组织部位以及不同外源激素处理下所呈现的表达模式进行了比较分析。结果表明:(1)三萜皂苷含量在不同组织部位间和外源激素处理下均存在显著差异,具体表现为根部组织的含量显著高于叶部组织的,而用外源水杨酸(SA)与茉莉酸甲酯(MeJA)喷施处理后其含量低于对照组(CK)。(2)对这两个差异比较组进行qRT-PCR分析结果显示,相比叶部组织,在根部组织中AkPMD、AkHDS、AkSS、AkSM以及与三萜皂苷合成相关的走马胎P450家族、UGT家族基因皆有不同程度的上调;外源SA与MeJA处理会导致合成途径上游的AkPMD、AkHDS、AkSS、AkSM上调,而下游的P450家族、UGT家族基因则有所下调。因此,通过对不同组织部位和不同外源激素处理这两种差异表达模式的比较研究,可推测在这两种表达模式下走马胎三萜皂苷的合成更多地与下游特异化修饰的酶相关。     

三萜皂苷生物合成相关糖基转移酶研究进展

三萜皂苷具有独特的化学性质和丰富的药理活性,在医药、保健品、化妆品、食品添加剂、农业等方面被广泛应用.尿苷二磷酸(UDP)依赖的糖基转移酶(UGTs)是催化三萜皂苷生成的关键酶,对三萜皂苷的结构及其药理活性多样性的形成有重要作用.文中基于UGTs来源及受体底物结构类型对参与植物三萜皂苷生物合成的UGTs进行了综述,并展...     

Molecular activities, biosynthesis and evolution of triterpenoid saponins

Saponins are bioactive compounds generally considered to be produced by plants to counteract pathogens and herbivores. Besides their role in plant defense, saponins are of growing interest for drug research as they are active constituents of several folk medicines and provide valuable pharmacological properties. Accordingly, much effort has been put into unraveling the modes of action of saponins, as well as in exploration of their potential for industrial processes and pharmacology. However, the exploitation of saponins for bioengineering crop plants with improved resistances against pests as well as circumvention of laborious and uneconomical extraction procedures for industrial production from plants is hampered by the lack of knowledge and availability of genes in saponin biosynthesis. Although the ability to produce saponins is rather widespread among plants, a complete synthetic pathway has not been elucidated in any single species. Current conceptions consider saponins to be derived from intermediates of the phytosterol pathway, and predominantly enzymes belonging to the multigene families of oxidosqualene cyclases (OSCs), cytochromes P450 (P450s) and family 1 UDP-glycosyltransferases (UGTs) are thought to be involved in their biosynthesis. Formation of unique structural features involves additional biosynthetical enzymes of diverse phylogenetic background. As an example of this, a serine carboxypeptidase-like acyltransferase (SCPL) was recently found to be involved in synthesis of triterpenoid saponins in oats. However, the total number of identified genes in saponin biosynthesis remains low as the complexity and diversity of these multigene families impede gene discovery based on sequence analysis and phylogeny.This review summarizes current knowledge of triterpenoid saponin biosynthesis in plants, molecular activities, evolutionary aspects and perspectives for further gene discovery.     

Genomic analysis of Asian honeybee populations in China reveals evolutionary relationships and adaptation to abiotic stress

The geographic and biological diversity of China has resulted in the differential adaptation of the eastern honeybee, Apis cerana, to these varied habitats. A. cerana were collected from 14 locations in China. Their genomes were sequenced, and nucleotide polymorphisms were identified at more than 9 million sites. Both STRUCTURE and principal component analysis placed the bees into seven groups. Phylogenomic analysis groups the honeybees into many of the same clusters with high bootstrap values (91%–100%). Populations from Tibet and South Yunnan are sister taxa and together represent the earliest diverging lineage included in this study. We propose that the evolutionary origin of A. cerana in China was in the southern region of Yunnan Province and expanded from there into the southeastern regions and into the northeastern mountain regions. The Cold‐Temperate West Sichuan Plateau and Tropical Diannan populations were compared to identify genes under adaptive selection in these two habitats. Pathway enrichment analysis showing genes under selection, including the Hippo signaling pathway, GABAergic pathway, and trehalose‐phosphate synthase, indicates that most genes under selection pressure are involved in the process of signal transduction and energy metabolism. qRT‐PCR analysis reveals that one gene under selection, the AcVIAAT gene, involved in the GABAergic pathway, is responding to cold temperature stress. Through homologous recombination, we show that the AcVIAAT gene is able to replace the CNAG_01904 gene in the fungus Cryptococcus neoformans and that it makes the fungus less sensitive to conditions of oxidative stress and variations in temperature. Our results contribute to our understanding of the evolutionary origin of A. cerana in China and the molecular basis of environmental adaptation.     

Saponins in cereals

Saponins are a diverse family of secondary metabolites that are produced by many plant species, particularly dicots. These molecules commonly have potent antifungal activity and their natural role in plants is likely to be in protection against attack by pathogenic microbes. They also have a variety of commercial applications including use as drugs and medicines. The enzymes, genes and biochemical pathways involved in the synthesis of these complex molecules are largely uncharacterized for any plant species. Cereals and grasses appear to be generally deficient in saponins with the exception of oats, which produce both steroidal and triterpenoid saponins. The isolation of genes for saponin biosynthesis from oats is now providing tools for the analysis of the evolution and regulation of saponin biosynthesis in monocots. These genes may also have potential for the development of improved disease resistance in cultivated cereals.