## 2 Results and Discussion
### Identification of Putative Genes Related to Iridoid Biosynthesis
The first enzymatic step in the secoiridoid pathway is catalyzed by GES, which converts GPP into geraniol. The enzyme and its encoding gene were first characterized from the peltate glands of sweet basil [33]. In the next step, G10H hydroxylated geraniol to form 10-hydroxygeraniol. G10H is a cytochrome P450 monooxygenase belonging to the CYP76B superfamily, which controls the first committed step in the biosynthesis of TIAs in _C. roseus_[20, 21]. CPR which is essential for the G10H catalyzed reaction presumably acts as the electron donor for G10H [18]. The gene expression profiles of the CPR and G10H of _C. roseus_ are similar and they respond to jasmonic acid induction with similar kinetics [20]. The conversion of 10-hydroxygeraniol to 10-oxogeraniol is carried out by 10 HGO, a NADP+ oxidoreductase. This enzyme was first purified and characterized in _Rauwolfia serpentina_[34]. So far, the early steps of the iridoid pathway have not been detailed, though they were suggested to be similar to those of the secoiridoid pathway [8]. As we know, no secoiridoid compound has been identified in _R. glutinosa_, oppositely, more than 30 iridoids have been isolated from this species. The presence of the high homologues of secoiridoid pathway genes in non-TIA producing plant _R. glutinosa_ implied that they are most likely to be implicated in the biosynthesis of route II iridoids and built the molecular base in support of the previous suggestion. So it would be reasonable to conclude that the biosynthesis of iridoids and secoiridoids shares the common enzyme steps before the formation of iridane skeleton (Figure 1). A comparison of our data with that reported by [PERSON] _et al._ dealing with the pyrosequencing of the transcriptome of _Campototheca acumninate_[35], an important plant producing TIAs, indicated that our 454 dataset contains more genes involved in iridoid biosynthesis, including the genes _4-diphosphocytidyl-2-C-methyl-b-erythitol synthase_ (_CMS_), _geranyl diphosphate synthase_ (_GPPS_) and _geraniol synthase_ (_GES_) that had not been discovered in [PERSON] _et al._'s research. In this study, a total of 550 EST sequences representing 21 gene families putatively involved in iridoid biosynthesis were discovered (Table 2). These results demonstrated that our 454 data pool is a good resource for studying \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Enzyme** & \
\begin{tabular}{c} **Number of** \\ **unigenes in 454** \\ **library** \\ \end{tabular} &
\begin{tabular}{c} **Number of** \\ **reads in 454** \\ **library** \\ \end{tabular} \\ \hline acetoacetyl-coenzyme A thiolase (AACT) & 2.3.1.9 & 5 & 9 \\
3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) & 2.3.3.10 & 5 & 32 \\
3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) & 1.1.1.34 & 10 & 185 \\ mexulonate kinase (MK) & 2.7.1.36 & 3 & 14 \\ phosphomevelonate kinase (PMK) & 2.7.4.2 & 2 & 2 \\ diphosphomevalonate decarboxylase (DPMDC) & 4.1.1.33 & 4 & 19 \\
1-deoxy-d-xylulose-5-phosphate synthase (DXS) & 2.2.1.7 & 6 & 13 \\
1-deoxy-d-xylulose-5-phosphate reductioomerase (DXR) & 1.1.1.267 & 4 & 36 \\
4-diphosphocytidyl-2-C-methyl-b-erythitol synthase (CMS) & 2.7.7.60 & 1 & 1 \\
4-diphosphocytidyl-2-C-methyl-b-erythnit kinase (CMK) & 2.7.1.148 & 1 & 2 \\
2-C-methyl-b-erythnitol 2,4-cyclodiphosphate synthase (MECS) & 4.6.1.12 & 4 & 4 \\
1-hydroxy-2-methyl-butenyl 4-diphosphate synthase (HDS) & 1.17.7.1 & 4 & 7 \\
1-hydroxy-2-methyl-butenyl 4-diphosphate reductase (HDR) & 1.17.1.2 & 3 & 7 \\ isopentenyl diphosphate isomerase (IDI) & 5.3.3.2 & 6 & 17 \\ farnesyl diphosphate synthase (FPP) & 2.2.1.10 & 4 & 19 \\ geranyl diphosphate synthase (GPPS) & 2.5.1.1 & 1 & 3 \\ geranylgeranyl pyrophosphate synthase (GGPS) & 1.3.1.74 & 9 & 21 \\ geraniol synthase (GES) & 3.1.7.3 & 1 & 2 \\ geraniol 10-hydroxylase (G10H) & 1.14.14.1 & 1 & 2 \\ cytochrome P450 reductase (CPR) & 1.6.2.4 & 5 & 58 \\
10-hydroxygeraniol oxidoreductaseuctase (10 HGO) & 1.1.1.255 & 6 & 97 \\ Total number & & 85 & 550 \\ \hline \hline \end{tabular}
\end{table}