## 3 Discussion  
Terpene metabolite diversification has allowed plant adaptation to environmental conditions. Accordingly, plant PTs for supplying linear _trans_-isoprenyl-PP backbones have been evolutionarily fine-tuned to result in structural and functional diversity in plant species-specific manners with complexity [40]. To understand the function of rice _trans_-PTs, we comprehensively integrated an extended phylogenetic tree (Figure 1 and Supplementary Table S3), core variation of amino acid residues in conserved motifs and a color-based HeatMap display applied to the \"three floors\" model (Figures 2 and 3) and expression patterns (Figures 4 and 5) and subcellular localizations (Figure 6). In our phylogenetic tree, GGPS/GGPS/GFPS family (Group I) was subdivided into six subgroups (Figure 1). Rice has relatively simpler system in Group I, with only three PTs, compared to _Arabidopsis_ with twelve PTs. Previously in rice, OsGGPS1 (LSU) was reported to have multiple functions as GGPS (67%), GPS (27.2%) and FPS (5.5%) as a homomeric complex and OsGRP (SSU-II) strongly enhanced GGPS function as heteromeric complex with OsGGPS1 [10]. In previous reports, the heteromeric LSU-SSU complexes mediated the enhancement of GPP-biosynthetic activity to produce both GPP and GGPP using DMAPP, GPP and FPP in _Arabidopsis_[15] and hops (_Humulus lupulus_; [PERSON] and [PERSON] [11]) or enabled GGPS_LSU to biosynthesize only GPP without any GGPP-production, even if the homomeric LSU complexes had GGPP-biosynthetic function, as in snapdragon (_Antirhinum majus_, [PERSON] et al. [41]) or no enzymatic function, as in peppermint (_Mentha x piperita_, [PERSON] et al. [23]) and in those plant system, any homomeric GPS has been not reported yet. Considered together, we supposed that the heteromeric LSU-SSU complexes might have evolved to have varied enzymatic preferences for the biosynthesis of GPP and GGPP, depending on the environmental conditions of corresponding plant species and in rice with homomeric GPS, the heteromeric OsGGPS1-OsGRP complex has evolved to enhance GGPP-biosynthetic activity, instead of acquiring GPP-biosynthetic activity. Previously, OsGPS was reported to have the dual functions of both a GPS using DMAPP and a GGPS using FPP [10] (Table 1). However, our phylogenic tree showed that OsGPS was categorized into a G(G)PS subgroup (C) that shows a close relationship with AtGGPS1, 3, 4 and 8, rather than into a representative homomeric GPS subgroup (D) composed of several well-characterized GPSs from the gymnosperm trees Grand fir and Norway spruce (Figure 1). AtGGPS1 and AtGGPS4 have GGPS activities using FPP exclusively in mitochondria and in ER, respectively, and AtGGPS3 using GPP and FPP and AtGGPS8 using all allyic substrates of DMAPP, GPP, FPP and GGPP contain the PPS activities in ER and chloroplasts, respectively [13]. Considered together with the functions of OsGPS, the subgroup (C) families are suggested to have the common feature of using FPP as an allyic substrate, rather than their final product lengths. As expected, the structural analysis following the \"three floors\" model showed that all residues of \"three floors\" were quite varied in a subgroup (C) (Figure 3A), but interestingly, OsGPS had the same residues of Floor 1 as in homomeric GPSs in subgroup D, even if its Floor 2 residues did not match at all. These results support that OsGPS has both structural properties of subgroup C and D, as a new type of a homomeric GPS family. Actually, the expression levels of _OsGPS_ were significantly lower than those of _OsGGPS1_ or _OsGRP_ in all tissues of all developmental stages (Figure 4), but were largely induced by the meJA treatment, with patterns similar to those of _OsTPS19_ followed by _OsGGPS1_ (Figure 5). Taken together, OsGPS is suggested to have functions on monoterpene/ditrepene metabolism as a homomeric GPS using DMAPP or a GGPS using FPP under the condition of JA-related responses.