## 1 Introduction  
Camptothecin (CPT) is a well-known terpene indole alkaloid (TIA), which was isolated for the first time by [PERSON] and [PERSON] in 1966 from _Campotheca acuminata_[1]. CPT has been found in some distantly related plants, including _C. acuminata_, _Erautamia heyneana_, _Merrilliodendron megacarpum_, _Nothapodytes foetida_, _N. nimmoniana_, and several _Ophiorrhiza_ species [2; 3; 4; 5]. CPT exhibits excellent anti-tumor activity by inhibiting the activity of topoisomerase I [1; 6]. Its two semisynthetic water-soluble derivatives, irinothecan and topothecan, were approved by the US Food and Drug Administration (FDA) in 1994 and are used extensively for the treatment of ovarian cancer, lung cancer, metastatic colorectal cancer, and cervical cancer throughout the world [7; 8; 9]. Like many TIAs, CPT is derived from the shikimate pathway and iridoids biosynthesis pathway, which supply the precursor's tryptamine and secologanin for CPT synthesis [5; 10]. The terpene part of CPT is received from the universal precursor's isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP), which were derived through the methylerythitol phosphate (MEP) and mevalonate (MVA) pathways. Geranyl pyrophosphate (GPP) was catalyzed by a series of enzymes, including geraniol synthase (GES), geraniol-8-oxidase (G8O), 8-hydroxy-geraniol oxidoreductase (8-HGO), iridoid synthase (IS), iridoid oxidase (IO), 7-deoxyloganetic acid UDP-glucosyltransferaseanferase (7-DLGT), 7-deoxyloganic acid hydroxylase (7-DLH), loganic acid \(O\)-methyltransferaseasferase (LAMT), geraniol-10-hydroxylase (G10H), 10-hydroxy-geraniol oxidoreductase (10-HGO), and secologanin synthase (SLS), finally forming secologanin [5,11]. In this pathway, the enzyme nicotinamide adenine dinucleotide phosphate (NADPH): cytochrome P450 reductase (CPR) is essential for the activity of G10H and SLS, which plays a vital role in electron transfer from NADPH to cytochrome P450 [12]. Subsequently, tryptamine and secologanin catalyzes into strictosidine by strictosidine synthase (STR), and strictosidine was converted via strictosidine beta-glucosidase (SGD) into strictosamide [1,5]. The later specific stages of how strictosamide was metabolized to CPT biosynthesis remain unknown [10,13]. At present, camptothecin is mainly extracted from plants; however, the low accumulation of CPT in plants cannot meet the expanding demand of the market [14, 15, 16]. \(O\)_pumila_ is a member of _Rubiaceae_, a medically herbaceous plant that provides an excellent platform for metabolic engineering of CPT biosynthesis [3]. So far, several genes were overexpressed or repressed in the hairy roots of \(O\)_pumila_, confirming the function of the genes in camptothecin biosynthesis. For instance, suppression of \(OpTDC\) or \(OpSLS\) resulted in decreased production of CPT and some other metabolites in \(O\)_pumila_ hairy roots, revealing that \(OpTDC\) and \(OpSLS\) catalyzed the rate-limiting steps in CPT biosynthesis [17]. \(STR\) or/and \(G10H\) from _Cathananthus roseus_ introduced in \(O\)_pumila_ hairy roots led to a significant promotion of CPT compared with the control line [18]. Nevertheless, limited information is available about the regulatory mechanism of camptothecin biosynthesis in \(O\)_pumila_. As a large family of plant-specific transcription factors, WRKY proteins contain the highly conserved tryptophan-arginine-lysine-tyrosine (WRKY) domain, which is approximately 60 amino acids long with the highly conserved amino acid sequence WRKYGQK at its \(N\)-terminus and a C2H2 or C2 HC zinc finger motif at its C-terminus [19,20]. WRKYs can be separated into three major groups in accordance with the number of WRKY domains and the features of their zinc finger-like motifs [21,22]. Based on the primary amino acid sequence, group II can be further categorized into five subgroups (IIa, IIb, IIc, IId, and IIe) [21,23].