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Terpenes are important natural products functioning in both the primary and specialized metabolism of plants, bacteria, fungi, and other life forms. Core structural diversity is mainly determined by terpene synthases (TPS), enzymes that convert ubiquitous prenyl diphosphates such as geranyl diphosphate, farnesyl diphosphate, and geranylgeranyl diphosphate into the various terpene backbones.

Terpene synthases in the crop plant maize (Zea mays) have been the subject of active research since the 1990s. A majority of maize TPS enzymes have already been functionally described and characterized, many of them in the laboratories of the authors of this clarification effort (reviewed in [1, 2]; Table 1). A comprehensive analysis of the TPS genes in the maize inbred lines B73 and W22 showed that both contain about 40 TPS genes, although the number varies between the different lines (43 in B73 versus 38 in W22) [3, 4]. These numbers include apparent pseudogenes, as it has been shown that a pseudogene in one maize line can be functional in another line [3, 5, 6].

Table 1 Current existing nomenclature of the terpene synthase gene family in maize. The first column contains the names already assigned to functionally characterized maize TPS genes in the literature (for citations see last column). To fill the remaining gaps in the TPS numbering and to unify the nomenclature, we propose the names listed in the second column, mainly following the original names or the names already given in Ding et al. [4] (third column). Please note that this table also includes obvious pseudogenes, because it has been shown that a pseudogene in one maize line can be functional in another line [3, 5, 6]
Full size table

The amazing quantitative and qualitative plasticity of the maize TPS gene family was confirmed in a recent paper by Sun and coworkers, who analyzed TPS genes in the genomes of 26 inbred lines [7]. However, only 31 gene loci were included in this analysis, resulting in one-third of the already characterized TPS genes being omitted by the authors. Furthermore, the authors did not address the extensive pre-existing literature on maize terpene synthases prior to proposing a new nomenclature that was both incomplete and inconsistent with previously published names. Our concern with this approach is that it could lead to massive confusion in this field as readers will be unable to compare the new names with the original names without extensive sequence comparisons.

With the goal of minimizing confusion, we provide an overview of the existing maize nomenclature(s) and cite the primary literature in which the maize TPSs were first described and enzyme products characterized (Table 1). In addition, following the previously published TPS names, we propose to designate all mono- and sesquiterpene synthase genes with the abbreviation “ZmTPS” and a sequential numbering (ZmTPS1 - ZmTPS36). Further we propose the continued designation of the class I diterpene synthase genes, namely kaurene synthase-like (KSL) genes, as ZmTPS42/KSL1 to ZmTPS47/KSL6. Similarly, the class II diterpene synthase genes, namely the five copalyl diphosphate synthase (CPS) genes, are abbreviated as ZmTPS37/CPS1 to ZmTPS41/CPS5 (Table 1). Those involved in biosynthesis of the gibberellin hormone also have been designated by the original mutant names – i.e., an1/2 and d5, with the latter further modified as KS(L3)/D5 to highlight its activity as an ent-kaurene synthase. Note that this nomenclature includes not only the 43 TPS gene loci found in the B73 reference genomes GRAMENE 4.0 and NAM 5.0 (www.maizegdb.org), but also four additional TPS genes not present in B73 but identified in other maize lines by Sun and coworkers [7]. The improvement of the already sequenced genomes and the sequencing of additional maize lines will lead to continued changes in the absolute number of known maize TPS genes in the future. Therefore, the nomenclature proposed here is itself evolving and merits periodic revision that builds upon existing knowledge.

The overview of the maize TPS gene family presented in this paper, together with the proposed nomenclature that includes all previously published names, is intended to help to minimize confusion about maize TPS names. In addition, the list of uncharacterized TPS genes presented in Table 1 can serve as a reference point to motivate future research on TPSs and their biological roles in maize.