## Transcriptome Analysis Reveals Putative Genes Involved in  
**Iridoid Biosynthesis in _Rehmannia glutinosa_**  
**[PERSON] 1, [PERSON] 2, [PERSON] 1, [PERSON] 2, [PERSON] 1 and [PERSON] 1,*  
* Institute of Medicinal Plant Development, Peking Union Medical College, Chinese Academy of Medical Sciences, Beijing 100193, China; E-Mails: pengsun(r)126.com (P.S. ); llzhou(r)implad.ac.cn (L.Z. ); jjqi(r)implad.ac.cn (J.Q.) * CAS Key Laboratory of Genome Sciences & Information, Beijing Institute of Genomics, Chinese Academy of Sciences, Beijing 100029, China; E-Mails: songshh(r)big.ac.cn (S.S.); zhangbing(r)big.ac.cn (B.Z.) * Author to whom correspondence should be addressed; E-Mail: xeli(r)implad.ac.cn; Tel./Fax: +86-10-6281-0019. _Received: 6 August 2012; in revised form: 16 October 2012 / Accepted: 17 October 2012 / Published: 23 October 2012_  
_Introduction_  
_Rehmannia glutinosa_, a well-known medicinal plant from the _Scrophulariaceae_ family, has been extensively used in clinics in the Orient to replenish vitality, to treat a variety of ailments such as diabetes, constipation, urinary tract problems, anemia, dizziness, and for regulating menstrual flow [1]. _R. glutinosa_ is rich in iridoids. Previous phytochemical investigations on _R. glutinosa_ have led to the isolation of diverse compounds, the majority of which are iridoids such as catalpol, aucubin, rehmannioside A, B, C and D; more than 30 in total [2]. The main active principal of _R. glutinosa_, catalpol, was reported to make up 2.7%-10.6%, with an average of 6.3% of the fresh root content of _R. glutinosa_[3]. Intensive studies revealed that iridoids exhibit a wide range of bioactivity such as neuroprotective, antitumor, antiinflammatory and antioxidant effects, _etc._[4]. Catalpol has hypoglycemic, diuretic, laxative effects [2], and recently also demonstrated neuroprotective effects [5; 6; 7]. Other iridoids such as aucubin and geniposide in _R. glutinosa_ have also been shown to possess biological activities [4]. Iridoids represent a large group of monoterpenoids characterized by a functionalized cis-fused cyclopentan-[c]-pyran skeleton. Oxidative cleavage of the cyclopentane ring gives rise to a subclass known as secoriridoids, which are often intermediates in the biosynthesis of terpenoid indole alkaloids (TIA). The biosynthesis of iridoids has been fairly well studied through precursor feeding experiments [8; 9; 10; 11]. It is known that two main routes exist for iridoid biosynthesis: route I is from iridodial via deoxyloganic acid and loganin to secologanin, which is the precursor of the derived secoriridoids and complex TIAs; route II involves 8-epi-iridodial, 8-epi-iridodrial and 8-epi-deoxyloganic acid, which are precursors of the decarboxylated carbocyclic iridoids, such as aucubin and catalpol (Figure 1) [8; 12]. Over the past two decades, _Catharanthus roseus_ has been the best investigated species in the field of iridoid biosynthesis providing more than one hundred medicinally important TIAs, such as the anti-cancer compounds vinblastine and vincristine. The iridoid moiety of TIA belongs to the secoiridoid subclass that is derived from iridoid biosynthetic route I, which is regarded as rate limiting for TIA production [13; 14; 15]. The secoiridoid pathway (sometimes also called \"iridoid pathway\") encompasses the biosynthetic route I and several upper steps that involve GPP, geraniol, 10-hydroxygeraniol and 10-oxogeraniol [14; 15]. Intensively biochemical and genetic studies around the _C. roseus_ TIA biosynthesis have established the molecular basis of the secoiridoid pathway, and a set of genes involved in this pathway have been cloned and characterized [16; 17; 18; 19; 20; 21; 22; 23; 24]. The secoiridoid pathway in _C. roseus_ has been well reviewed [14; 15]. However, no genetic or molecular study on iridoid biosynthesis has been reported in non-TIA producing plants, and no molecular information about the biosynthesis of route II iridoids is presently available. _R.