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The Sweet Side of Plant-Specialized Metabolism

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The Sweet Side of Plant-Specialized Metabolism Downloaded from http://cshperspectives.cshlp.org/ on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press The Sweet Side of Plant-Specialized Metabolism Thomas Louveau and Anne Osbourn Department of Metabolic Biology, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom Correspondence: [email protected] Glycosylation plays a major role in the structural diversification of plant natural products. It influences the properties of molecules by modifying the reactivity and solubility of the corresponding aglycones, so influencing cellular localization and bioactivity. Glycosylation of plant natural products is usually carried out by uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs) belonging to the carbohydrate-active en- zyme glycosyltransferase 1 (GT1) family. These enzymes transfer sugars from UDP-activated sugar moieties to small hydrophobic acceptor molecules. Plant GT1s generally show high specificity for their sugar donors and recognize a single UDP sugar as their substrate. In contrast, they are generally promiscuous with regard to acceptors, making them attractive biotechnological tools for small molecule glycodiversification. Although microbial hosts have traditionally been used for heterologous engineering of plant-derived glycosides, tran- sient plant expression technology offers a potentially disruptive platform for rapid character- ization of new plant glycosyltransferases and biosynthesis of complex glycosides. ollectively, plants synthesize a diverse array wealth of chemical diversity, with around one Cof chemicals, most likely as a means of sur- million specialized metabolites being reported vival in diverse ecological niches. These mole- from plants so far (Afendi et al. 2012). Among cules have been variously associated with abiotic these modifications, glycosylation has a pro- stress resistance (Trossat et al. 1998; Nuccio et al. found impact on the physicochemical and bio- 1999), defense against herbivores and pathogens active properties of phytochemicals, affecting (Osbourn 1996; Vetter 2000; Howe and Jander solubility, cellular localization, and bioactivity. 2008), establishment of symbiotic interactions Here we review recent developments in un- (Oldroyd 2013), allelopathy (Weston and Ma- derstanding the enzymes involved in plant nat- thesius 2013), and attraction of pollinators ural product glycosylation. We also consider the (Ogata et al. 2005; Theis and Raguso 2005; Yu various heterologous expression hosts used for and Utsumi 2009). Plant-specialized metabolites metabolic engineering of plant glycosides. Fi- are derived from a repertoire of different types of nally, we discuss the potential for harnessing scaffolds. These scaffolds commonly undergo enzymes from plants for glycodiversification of further modification (e.g., oxidation, glycosyla- small molecules for medicinal, agronomic, and tion, methylation, and acylation) to generate a industrial applications. Editor: Pamela C. Ronald Additional Perspectives on Engineering Plants for Agriculture available at www.cshperspectives.org Copyright © 2019 Cold Spring Harbor Laboratory Press; all rights reserved Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a034744 1 Downloaded from http://cshperspectives.cshlp.org/ on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press T. Louveau and A. Osbourn GLYCOSYLATION OF PLANT-SPECIALIZED (Augustin et al. 2011; Sawai and Saito 2011). METABOLITES Two examples, avenacin A-1 from oat and α-tomatine from tomato, are shown in Figure The Impact of Glycosylation on Bioactivity 1A. Glycosylation is critical for the ability of Many plant-specialized metabolites accumulate triterpenoid saponins and steroidal glycoalka- as glycosides. Glycosylation is often a means of loids to integrate into and permeabilize plasma storage of bioactive molecules (e.g., endogenous membranes, and also for bioactivity (Bowyer phytoanticipins or xenobiotics). While some et al. 1995). Glycosylation is also important for plant glycosides are biologically active in their flavonoid pigment stability, enabling intra- and glycosylated form, in other cases partial or com- intermolecular association of anthocyanins with plete hydrolysis of sugars may activate or en- other flavonoids, metal ions (metalloanthocya- hance bioactivity (Fig. 1). In scenarios where nins), or aromatic organic acids under condu- glycosylation is required for bioactivity, complex cive pH conditions. The central role of flavonoid glycosylation patterns are often observed. For glycosidic moieties in formation of such hydro- example, triterpenoid saponins and steroidal phobic stacking structures has recently been glycoalkaloids often contain oligosaccharide demonstrated by the engineering of blue chry- chains consisting of different types of sugar santhemums (Noda et al. 2017). Ternatin D3, units (Vincken et al. 2007). Such compounds one of the anthocyanins responsible for the protect plants against attack by pathogenic mi- blue coloration of Clitoria ternatea, is also crobes, herbivores, and competing plant species shown in Figure 1A. O Hydrolysis AB Triterpenoid saponin O Glucosinolate O O OH HN Sulforaphane HO O OH S HO S S S C N O HO O OH O HO O OH OH N O O Glucoraphanin S O Avena sativa Brassica oleracea N C O HO – + OH SO3 K Antifungal OH Sulforaphane nitrile HO O HO O Avenacin A-1 OH Antifeedant Cyanogenic glucosides OH Linamarin Steroidal glycoalkaloid HO O HO O OH Insecticidal NH Manihot HCN N esculenta OH O O HO Hydrogen cyanide OH H O H HO HO O HO O H OH HO O O OH H H OH OH Inhibitor of respiration O Solanum Lotaustralin N HO O O O HO H HO OH lycopersicum OH α-Tomatine Monoterpenoid OH HO Anthocyanin HO O HO O OH O Geraniol Blue pigment Rosa × odorata O Geranyl glucoside HO O Pollinator attractant HO O OH OH O OH O + HO O O Benzoxazinoid O OH HO OH OH OOOH HO OO O OH OOO DIMBOA O OH O Zea mays HO OH OH Clitoria N O O OH ternatea N O OH OH HO OH Ternatin D3 2,4-Dihydroxy-7-methoxy Antimicrobial -1,4-benzoxazine-3-one (DIMBOA) glucoside Figure 1. Examples of glycosylated plant-specialized metabolites. Compounds that are active in their fully glycosylated forms (A) or upon hydrolysis (B) are shown. 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a034744 Downloaded from http://cshperspectives.cshlp.org/ on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press Plant Natural Product Glycosylation Glucosinolates and cyanogenic glucosides GT1 enzymes make major contributions (Fig. 1B) are examples of situations where gly- to the glycodiversification of plant-specialized cosylated plant natural products are activated metabolites (Vogt and Jones 2000; Bowles et by removal of sugars to give deterrent or toxic al. 2006). They are involved in the production breakdown products that provide protection of important defense compounds such as terpe- against pest and pathogen attack. The enzymes noid glycosides, glucosinolates, cyanogenic gly- that catalyze deglucosylation (thiospecific glu- cosides, and flavonoid glycosides (Sønderby cohydrolases myrosinases or specific β-glycosi- et al. 2010; Sawai and Saito 2011; Gleadow and dases, respectively) are normally stored away Møller 2014). The stability of pigments (e.g., from their substrates in separate subcellular anthocyanins), the taste of fruit (e.g., flavonoids, compartments or cell types but are released on diterpenoids), and the retention of aromas in tissue damage (Hopkins et al. 2009; Gleadow flowers or fruits (e.g., monoterpenes, phenyl al- and Møller 2014). Other examples of activation cohols) are also modulated via GT1-mediated of plant natural products by hydrolysis of glycosylation (Noda 2018; Song et al. 2018). sugars include glycoside-bound volatiles (e.g., GT1 enzymes are also involved in regulation of monoterpenes, sesquiterpenes, phenolics) with plant growth and development via modulation various roles in pollinator attraction, biotic/ of phytohormone homeostasis (Piotrowska and abiotic stress tolerance, and communication Bajguz 2011). They further enable xenobiotic between plants (e.g., geranyl glucoside) and detoxification through glucoconjugation, a step benzoxazinoid defense compounds (e.g., 2,4-di- that precedes transfer and subsequent storage hydroxy-7-methoxy-1,4-benzoxazinone-3-one of the modified xenobiotic in the vacuole (Bra- (DIMBOA) glucoside) (Fig. 1B; de Bruijn et al. zier-Hicks et al. 2018). 2018; Song et al. 2018). Those compounds that are activated by hydrolysis, in general, have rel- GT1 PHYLOGENY atively simple glycosidic moieties and are usual- ly mono- or diglycosides, typically containing The family 1 GTs are one of the largest groups of D-glucose. natural product-decorating enzymes in plants. The expansion of this family in higher plants likely reflects chemical diversification during ad- Glycosyl Transferase Family 1 (GT1): aptation to life on land (Yonekura-Sakakibara An Engine for Glycodiversification and Hanada 2011; Caputi et al. 2012). Mining Enzymes that build or break down the con- of the complete genome sequence of Arabidopsis siderable diversity of glycosylated structures thaliana has identified 107 predicted GT1 genes. found in living organisms (e.g., proteins, These GT1 enzymes have been classified into lipids, polysaccharides) are collectively referred 14 monophyletic groups (groups A to N) (Ross to as carbohydrate-active enzymes (CAZymes) et al. 2001). Three new phylogenetic groups (Cantarel et al. 2009). CAZymes
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