Plant (di)terpenoid evolution: from pigments to hormones and beyond

Abstract

Covering: up to 2014-2022.Diterpenoid biosynthesis in plants builds on the necessary production of (E,E,E)-geranylgeranyl diphosphate (GGPP) for photosynthetic pigment production, with diterpenoid biosynthesis arising very early in land plant evolution, enabling stockpiling of the extensive arsenal of (di)terpenoid natural products currently observed in this kingdom. This review will build upon that previously published in the Annual Review of Plant Biology, with a stronger focus on enzyme structure-function relationships, as well as additional insights into the evolution of (di)terpenoid metabolism since generated.

Fig. 1. Examples of diterpenoid biosynthesis from (A) phytohormone (ent-kaurenoic acid) and (B) more specialized (10-oxodepressin phytoalexin) metabolism, including (C) the biosynthetic gene cluster associated with the latter from the rice genome. While both initiated from the general precursor GGPP, these provide examples of labdane-related diterpenoids, as defined by the initial activity of a class II diterpene cyclase (phytohormone) or direct cyclization by a class I diterpene synthase, as well as cytochromes P450 (CYPs) inserting oxygen(s) into the resulting hydrocarbon backbones.

Scheme 1. NNPP and derived diterpenes.

Fig. 2. Irregular terpenoids. (A) Relevant branching, cylopropyl and cyclobutyl reactions. (B) Diterpene cyclization via irregular coupling.

Scheme 2. Basic DTC catalysis, including initial bicyclization of the decalin core and subsequent rearrangement (PP = diphosphate). Also shown are known stereoisomers for the initial decalin bicycle, with derived products from identified DTCs indicated by superscript (ⁿnormal, ^eent, ^ssyn, no ent-syn have yet been identified; note that KPP, kovalenyl diphosphate, is used to distinguish this from the labdane, CPP). In addition, rearrangement of the initially formed decalin bicycle also occurs, with one such (fungal) DTC identified.

Fig. 3. Current view of the evolutionary origin of plant terpene synthases (adapted from ref. 13).

Scheme 3. Examples of class II catalyzed cyclizations with associated catalytic acid motifs.

Fig. 4. KS reaction and associated motifs. (A) Catalyzed reaction and effect of noted threonine for isoleucine substitution, with relative pK_a for carbocations and hydroxyls shown. (B) Conservation of DDxxD (underlined) characteristic of class I TPSs, with extension and PIV motif (with key residues indicated by asterisks) specific to the KSs involved in phytohormone biosynthesis across all land plants shown here by alignment of representative examples spanning land plant evolution (AtKS, Arabidopsis thaliana; OsKS, Oryza sativa; PgKS, Pinus glauca; LjCPSKS, Lygodium japonicum; SmKS, Selaginella moellendorffii; PpCPSKS, Physcomitrella patens; MpKS, Marchantia polymorpha). (C) Approximate phylogenetic tree corresponding to sequences shown in panel B.

Fig. 5. CPS reaction and associated motifs. (A) Catalyzed reaction and effect of noted substitutions. (B) Conservation of motifs (with key residues indicated by asterisks) specific to the CPSs involved in phytohormone biosynthesis across all land plants shown here by alignment of representative examples spanning land plant evolution (AtCPS, Arabidopsis thaliana; OsCPS, Oryza sativa; PgCPS, Pinus glauca; LjCPSKS, Lygodium japonicum; SmCPS, Selaginella moellendorffii; PpCPSKS, Physcomitrella patens; MpCPS, Marchantia polymorpha). (C) Approximate phylogenetic tree corresponding to sequences shown in panel B.

Fig. 6. TPS evolution (derived from ref. 29). (A) Unrooted phylogenetic tree for the TPS family. (B) Model of TPS evolution depicting phylogenetic relationship of land plants, indicated by grey lines, with TPS domain structure and catalytic motifs as defined in text and indicated in legend.

Scheme 4. Reactions catalyzed by bifunctional TPS-d3 group members with key motifs and effects of noted substitutions, as well as efficiently catalyzed heterocyclization.

Scheme 5. Class I TPS catalyzed isomerization and cyclization of GGPP to rhizathalene.

Fig. 7. Rice oryzalexins (antimicrobial phytoalexins).

Fig. 8. Phylogenetic tree for 21 (of 137) CYP71 subfamilies (as well as the related/subsumed CYP99 and CYP726), with members known to function in diterpenoid biosynthesis indicated by asterisks (*). Tribe described here indicated by red dot (•). Assembled using CLUSTAL Omega (neighbor-joining) from 1073 sequences selected to reduce redundancy. Note the intermixing of CYP71D and CYP71BE subfamilies arising from increased sequence coverage since their original designations.

Fig. 9. Analogous reactions catalyzed by members of distinct CYP families in different plants. (A) Momilactone biosynthesis in rice and moss, with the relevant CYPs shown in red or blue text, respectively. (B) TMTT biosynthesis in monocots and eudicots, with the relevant CYPs shown in red or blue text, respectively. (C) Hydroxylation of casbene in rice and Euphorbiaceae, with the relevant CYPs shown in red or blue text, respectively.

Scheme 6. Alternative hydroxylation of ent-sandaracopimaradiene catalyzed by distinct members of the CYP76M sub-family from rice and their relevance to oryzalexin biosynthesis.

Scheme 7. Examples of CYP catalyzed rearrangement and heterocyclization in diterpenoid biosynthesis. (A) Rearrangement in gibberellin (GA) biosynthesis. (B) Cyclic either formation (heterocyclization) in tanshinone biosynthesis.

Scheme 8. 2ODD catalyzed (complex) reaction in gibberellin (GA) biosynthesis.

Scheme 9. SDR catalyzed cyclization from Euphorbiaceae macrocyclic diterpenoid biosynthesis.

Fig. 10. Rice biosynthetic gene clusters (BGCs) associated with labdane-related diterpenoids from chromosome 2 (Os2BGC) and 4 (Os4BGC), along with recently elucidated pathway for momilactones, requiring genes from both BGCs (largely Os4BGC, note that the positioning of MS1 and MS2 has been corrected relative to our other publications, but also CYP76M8 from Os2BGC, as well as CYP701A8 from a tandem gene array).