## 2 Triterpenoids  
### Biosynthesis of Triterpenoids  
Triterpenoids belong to a large and structurally diverse class of natural products [125]. Basidiomycetes are considered a main source for triterpenoids. Compared to plants, very few types of triterpenoid skeletons have been reported in basidiomycetes, thus more research is needed [126]. Triterpenoids extracted from _Ganoderma_ spp. are named as _Ganoderma_ triterpenoids (GTs) [125]. _Ganoderma_ triterpenenoids (GTs) isolated from the fruiting bodies, cultured mycelia and basidiospores of _Ganoderma_[127; 128; 129] belong to the lanostane-type triterpenoids and are one of the major chemical constituents in _Ganoderma_. Studies have confirmed that GTs are biosynthesized using isoprenoid pathways [130; 131]. This pathway was considered to start from acetyl-coenzyme A and termed as a Mevalonate pathway (MVA) [132]. The MVA pathway (Figure 1) is one of the important metabolic pathways that can be divided into four main processes: conversion, construction, condensation, and postmodification [35]. The initial step is the transformation of acetyl-coenzyme A to isopentenyl pyrophosphate (IPP). Then the activities of different prenyltransferase produce farnesyl pyrophosphate (FPP), geranyl pyrophosphate (GPP) and geranyl glyceranyl pyrophosphate (GGPP), which are higher-order terpenoid building blocks from this IPP precursor. Then, these junction mediators can self-condense, and are also used in alkylation processes to produce prenyl side chains (for a range of nonterpenoids) or create a ring (to build the basic skeletons of triterpenoids). Ultimately, oxidation and reduction reactions, conjugation, isomerization or other secondary reactions magnify the distinctive characteristics of the triterpenenoids [133; 134; 35]. The relationship between the content of _Ganoderma_ triterpenoids and the expression levels of major genes has been evaluated in several studies. These kinds of studies may provide valuable data for studying the role of key genes and, finally, for raising the production of triterpenoids. According to Wang et al. [126], the most comprehensive studies on _Ganoderma_ triterpenenoids were concentrated on _G. lucidum_. A considerable number of essential enzyme genes are involved in the production of _G. lucidum_ triterpenoids during the biosynthesis. Liu et al. [135] inspected the _G. lucidum_ genes in the \"terpenoid backbone biosynthesis (map09900)\" pathway and discovered that the genes are solely spread in  
Figure 1: Outline of the MVA and lanostane-type triterpenenoids biosynthesis. Enzymes involved in the pathway are: 1. Acetyl-CoA acetyltransferase, AACT; 2. 3-Hydroxy-3-methylglutaryl-CoA synthase, HMGS; 3. 3-Hydroxy-3-methylglutaryl-CoA reductase, HMGR; 4. Mevalonate kinase, MK; 5. Phosphomevalonate kinase, MPK; 6. Phosphomevalonate decarboxylase, MVD; 7. Isopenetrapl diphosphate isomerase, IDI; 8. Farnesyl diphosphate synthase, FPPs; 9. Squalene synthase, SQS; 10. Squalene monooxygenase, SE; 11. 2,3-Oxidosqualene-lanosterol cyclase, OSC (The structures were redrawn in ACD/ChemSketch: Freeware: 2012). the MVA pathway, not the MEP/DOXP (methylerythnitol 4-phosphate/deoxyxylulose 5-phosphate) pathway. Meanwhile, several genes in this pathway have been cloned in _G. lucidum_, including 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) [136], Farnesyl diphosphate synthase (FPPs) [137], squalene synthase (SQS) [138], and lanosterol synthase (also namely 2,3-oxidosqualene lanosterol cyclase, OSC) [135]. These observations demonstrated that triterpenoid backbone biosynthesis could only be accomplished via the MVA pathway at the genome level in fungi. Scientists have performed a large number of studies to find out key enzyme genes involved in increasing the yield of ganoderic acid (GA). Ye et al. [139] examined the effect of salicylic acid (SA) and calcium ions on the biosynthesis of triterpenoids in _G. lucidum_ through spraying during the fruiting stage.