**1. Introduction**

*Dendrobium o*ffi*cinale* Kimura et Migo (also known as *D. catenatum*) is a perennial herb that is commonly used as a valuable Chinese herbal medicine and has a long evolutionary history among orchids. *D. o*ffi*cinale* is rich in alkaloids [1,2], and its genome, transcriptome, and metabolome indicate that *D. o*ffi*cinale* may also contain terpenoid indole alkaloids (TIAs) [3–5]. The common precursor of TIAs is strictosidine, which is formed by the combination of tryptamine and secologanin [6,7]. Tryptophan decarboxylase (TDC), which catalyses the formation of tryptamine, belongs to the type II pyridoxal phosphate-dependent decarboxylase (PLP\_deC) family [8]. To date, the actual roles of many PLP\_deCs in plants are still unknown due to a lack of relevant protein sequences and information about the biochemical properties. In particular, the role of PLP\_deC in the alkaloid synthesis pathway of *D. o*ffi*cinale* has not been reported.

Pyridoxal 5 -phosphate (PLP) is the active form of vitamin B6 and is used by a variety of enzymes in all organisms [8]. Previously, we classified all PLP-dependent enzymes into at least five structural groups based on their protein structures [8,9]. Among them, the type I group is the most common and contains aminotransferases, decarboxylases, and an enzyme that catalyzes α-, β-, or γ-eliminations. Type II encodes the enzymes involved in β-elimination reactions. Type III is primarily alanine-racemase

specific, while type IV enzymes typically include D-alanine aminotransferases. Type V enzymes are the most diverse, including glycogen and starch phosphorylases. One important group of PLP-dependent enzymes belongs to the PLP\_deC family, which includes aromatic-L-amino acid decarboxylase (AAD), glutamic acid decarboxylase (GAD), and histidine decarboxylase (HDC) [10]. The biological functions of plant and animal AADs are closely related to their corresponding substrate selectivity and catalytic reactions; thus, some AADs, such as tyrosine decarboxylase (TYDC) and tryptophan decarboxylase (TDC), are further annotated based on their principal substrates [8,11]. These enzymes catalyze the decarboxylation of aromatic L-amino acids and are primarily involved in the synthesis of secondary metabolites in plants [12,13].

Numerous data indicate that *PLP\_deC*exhibits tissue-specific and inducible transcript accumulation during plant development. In addition, several roles of *PLP\_deC* in plant development have been identified. For example, TDC is a key enzyme that links primary and secondary metabolism with high substrate specificity [14,15]. In addition, the transcript levels of *PLP\_deC* genes are affected by abscisic acid (ABA), methyl jasmonate (MeJA), salicylic acid (SA), and abiotic stress [16,17].

In 2015, Chinese scientists announced that they had completed the genome sequence of the orchid *Phalaenopsis equestris* [18]. *D. o*ffi*cinale* and *P. equestris* (Schauer) Rchb.f. are epiphytes in the family Orchidaceae. The draft of the *D. o*ffi*cinale* genome sequence was reported recently [4,19]. To further understand the *PLP\_deC* gene family in orchids, we identified 8 and 6 *PLP\_deC* genes from the genomes of *D. o*ffi*cinale* and *P. equestris,* respectively, and analyzed their phylogenetic relationships, gene structures, cis-regulatory elements, tissue expression patterns, and expression profiles under MeJA, ABA, and SA treatments. Our results may not only improve the current understanding of the evolutionary expansion, sequence conservation, and functional differentiation of *PLP\_deC* genes but also provide in-depth basic biological information for further studies of the evolution of these genes in Orchidaceae.
