Plant Biotechnology 31, 465–482 (2014) DOI: 10.5511/plantbiotechnology.14.1003a Review Microbial production of plant specialized metabolites Shiro Suzuki1,3,*, Takao Koeduka2, Akifumi Sugiyama1, Kazufumi Yazaki1, Toshiaki Umezawa1,3 1 Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto 611-0011, Japan; 2 Faculty of Agriculture, Yamaguchi University, Yamaguchi 753-8515, Japan; 3 Institute of Sustainability Science, Kyoto University, Uji, Kyoto 611-0011, Japan * E-mail: [email protected] Tel: +81-774-38-3624 Fax: +81-774-38-3682 Received July 22, 2014; accepted October 3, 2014 (Edited by T. Shoji) Abstract Plant specialized metabolites play important roles in human life. These metabolites, however, are often produced in small amounts in particular plant species. Moreover, some of these species are endangered in their natural habitats, thus further limiting the availability of some plant specialized metabolites. Microbial production of these compounds may circumvent this problem. Considerable progress has been made in the microbial production of various plant specialized metabolites over the past decade. Now, the microbial production of these compounds is becoming robust, fine-tuned, and commercially relevant systems using the methods of synthetic biology. This review describes the progress of microbial production of plant specialized metabolites, including phenylpropanoids, flavonoids, stilbenoids, diarylheptanoids, phenylbutanoids, terpenoids, and alkaloids, and discusses future challenges in this field. Key words: Phenylpropanoids, flavonoids, terpenoids, alkaloids, synthetic biology. complex structures on a large-scale and at a low cost Introduction (Misawa 2011). By contrast, metabolic engineering in Plants produce various low-molecular-weight organic microorganisms does not require dangerous chemicals compounds that can be utilized as medicines, cosmetics, and enables the production of very complex metabolites flavors, dyes, seasonings, functional ingredients, and with the aid of enzymes having high substrate industrial chemicals, all of which play important roles in specificities from simple carbon sources such as sugars human life (Facchini et al. 2012). These compounds have synthesized during plant photosynthesis (Keasling 2008). long been referred to as plant secondary metabolites. Over the past decade, there has been considerable Recently, a new term “plant specialized metabolites” progress in the metabolic engineering of microorganisms has been proposed to define these compounds, with and in identifying genes involved in plant specialized emphasizing their function in plants’ adaptations for metabolism. This has enabled the construction of specific ecological situations and their plant lineage- sophisticated and fine-tuned metabolic pathways specific distribution (Pichersky and Lewinsohn 2011), in microorganisms to produce plant specialized although many of these compounds also occur in metabolites (Chemler and Koffas 2008; Du et al. 2011; organisms other than plants. In this review, the term Keasling 2008, 2012; Lee et al. 2009; Marienhagen and “plant specialized metabolites” will be adopted. Bott 2013; Siddiqui et al. 2012). Microbial platforms for Despite the importance to human life, plant the production of plant specialized metabolites can be specialized metabolites are often produced in small beneficial to plant scientists because these systems can amounts by particular plant species. Some of these be used to identify novel enzymes by incorporating the species are endangered in their natural habitats, further coding genes into microbes (Cyr et al. 2007). Moreover, limiting the availability of plant specialized metabolites. the reconstitution and investigation of metabolism Although this problem may be overcome by the total in a simple system like microbial cells facilitate the organic synthesis of these compounds, organic synthesis quantitation of pathway flux without interference from often uses heavy metals, toxic organic solvents, and other related pathways and the characterization of strong acids, many of which have high environmental enzyme-enzyme interactions between pathway enzymes burdens (Du et al. 2011). Furthermore, it is still (Ralston et al. 2005; Ro and Douglas 2004). difficult to synthesize plant specialized metabolites with In this context, this review focuses on the progress This article can be found at http://www.jspcmb.jp/ Published online January 7, 2015 466 Microbial production of plant specialized metabolites Figure 1. Biosynthetic pathways for phenylpropanoids. The bold arrows represent a major pathway towards coniferyl and sinapyl alcohols formation in P. trichocarpa (Wang et al. 2014). 4CL, 4-coumarate:CoA ligase; C3H, coumarate 3-hydroxylase; C4H, cinnamic acid 4-hydroxylase; C4H–C3H, the enzyme complex of C4H and C3H; CAD, cinnamyl alcohol dehydrogenase; CAld5H, coniferaldehyde 5-hydroxylase; CAldOMT, 5-hydroxyconiferaldehyde O-methyltransferase; CCoAOMT, caffeoyl-CoA O-methyltransferase; CCR, cinnamoyl-CoA reductase; CSE, caffeoyl shikimate esterase; HCT, hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyltransferase; PAL, phenylalanine ammonia lyase. * PAL showing strong deaminase activity toward L-tyrosine is often called tyrosine ammonia lyase (TAL). **Note that only chemical structures of the shikimate esters are shown. of microbial production of major plant specialized metabolites, including phenylpropenes, lignans, metabolites, including phenylpropanoids, flavonoids, neolignans, and norlignans. Polymer lignins are stilbenoids, diarylheptanoids, phenylbutanoids, also classified phenylpropanoids (Umezawa 2010). terpenoids, and alkaloids (Limem et al. 2008; Matsumura Monolignols (4-hydroxycinnamyl alcohols) are shared et al. 2013; Putignani et al. 2013; Sato and Kumagai 2013; as representative monomer precursors by these Wang et al. 2011). Additionally, future challenges in this phenylpropanoids. Monolignols are biosynthesized field are discussed. via the cinnamate/monolignol pathway, and major pathways are proposed in angiosperms, with some variation among plant species (Suzuki and Suzuki Phenylpropanoids 2014; Umezawa 2010; Vanholme et al. 2010; Vanholme Phenylpropanoids are composed of C6–C3 units. This et al. 2012) (Figure 1). In a major pathway towards group includes a variety of low-molecular-weight coniferyl alcohol biosynthesis recently proposed in S. Suzuki et al. 467 Populus trichocarpa based on proteomic-based enzyme of L-tyrosine to caffeic and ferulic acids in E. coli reaction and inhibition kinetics (Wang et al. 2014), L- by the coexpression of actinomycete Saccharothrix phenylalanine is deaminated by phenyalanine ammonia espanaensis TAL (sam8), S. espanaensis 4-coumaric lyase (PAL) to produce cinnamic acid, which is converted acid 3-hydroxylase (sam5), and Arabidopsis thaliana to 4-coumaric acid by a cytochrome P450 (CYP) enzyme, 5-hydroxyconiferaldehyde O-methyltransferase cinnamic acid 4-hydroxylase (C4H). 4-Coumaric acid (CAldOMT). Subsequently, caffeic and ferulic acids is then activated by 4-coumarate:CoA ligase (4CL) to were produced by a tyrosine-overproducing strain yield 4-coumaroyl-CoA, which is coupled to shikimic of E. coli in the absence of L-tyrosine supplementation acid or quinic acid to yield 4-coumaroyl shikimate or (Kang et al. 2012). Lin and Yan (2012) reported that a 4-coumaroyl quinate by a hydroxycinnamoyltransferase bacterial PAL from Rhodobacter capsulatus and an E. coli (HCT). The resulting 4-coumaroyl shikimate or hydroxylase (4-hydroxyphenylacetate 3-hydroxylase) quinate is converted to caffeoyl shikimate or quinate by were expressed in E. coli JW1316, a strain in which a coumarate 3-hydroxylase (C3H). These caffeoyl esters chromosomal regulatory gene for tyrosine biosynthesis, are further hydrolyzed by HCT, producing caffeoyl-CoA. tyrR, was deleted to enhance tyrosine biosynthesis. The resulting caffeoyl-CoA is sequentially O-methylated Additionally, they overexpressed tyrA (encoding and reduced by caffeoyl-CoA O-methyltransferase chorismate mutase and prephenate dehydrogenase), (CCoAOMT), cinnamoyl-CoA reductase (CCR), and modified aroG (encoding 3-deoxy-D-arabino- cinnamyl alcohol dehydrogenase (CAD), respectively. heptulosonate-7-phosphate synthase), ppsA (encoding On the other hand, it was proposed that another major phosphoenolpyruvate synthase), and tktA (encoding monolignol, sinapyl alcohol, is mainly synthesized by the transketolase). As a result, caffeic acid was produced, up hydroxylation of coniferyl alcohol and the O-methylation to 50.2 mg L−1 after fermentation for 48 h. Later, Zhang of 5-hydroxyconiferyl alcohol in P. trichocarpa (Wang and Stephanopoulos (2013) optimized the similar system, et al. 2014). However, several by-path pathways were which resulted in higher titer (108 mg L−1) of caffeic acid also described regarding coniferyl and sinapyl alcohols production. biosynthesis in both P. trichocarpa and other plant Very recently, 4-coumaryl alcohol production was species: 1) the conversion of cinnamic acid to caffeic reported in E. coli (Jansen et al. 2014). They incorporated acid by a C4H–C3H complex and subsequent caffeoyl- Rhodobacter sphaeroides TAL, Pteroselinum crispum CoA formation (Chen et al. 2011), 2) the transformation 4CL, Zea mays CCR and CAD into E. coli cells. The final of caffeoyl shikimate to caffeate by caffeoyl shikimate yields of 4-coumaryl alcohol after 30 h fermentation esterase (CSE) and subsequent caffeoyl-CoA formation were 20 mg L−1 without feeding