## 3 Discussion
As several possibilities, GPP/FFP could be supplied by GPS activity (27.2%) and FPS activity (5.5%) of OsGGPS1 within chloroplasts or the flux of FPPs from cytosol could be utilized by OsSPS1 and OsGPS, as shown in Figure 7. Moreover, the biosynthetic activity of a plastidial OsSPS2 requires the existence of GPP as an allylic substrate [20], but OsGPS producing GPP in chloroplasts is expressed at a very low level in all rice tissue under the normal conditions (Figure 4). Considered with the expression levels of OsSPS2, the absolute substrate shortage is expected, and OsGGPS1 is possibly supposed to supply GPP to OsSPS2 through its GPS activity (27.2%) for the biosynthesis of SPPs, essential to produce plastoquinone in chloroplasts (Figure 7). In summary, all rice _trans_-PT genes through a phylogenetic tree using 136 plant _trans_-PT genes were collected, and their biochemical functions were predicted by matching sequences with known conserved motifs and the \"three floors\" model, taken together with their expression patterns and subcellular localizations. Collectively, we propose a predicted topology-based working model of rice _trans_-PTs to biosynthesize the linear _trans_-isoprenyl-PP backbones in Figure 7, by the following points: Where rice PTs are distributed in rice cells, what kinds of allylic substrates they utilize and what kinds of terpene metabolites are produced in rice. As the major cellular-compartmentations of _trans_-isoprenyl PPs, GPPs for mono-terpenoids are produced by OsGPS in plastoglobules or OsGGPS1 in stroma, GGPPs for di-/tetra-terpenoids are produced by a heteromeric complex between OsGGPS1 and OsGRPor a homomeric OsGGPS1 in both plastoglobules and stroma, SPP/PPP are by OsSPS2 in stroma for plastoquinone, by OsSPS1 in mitochondria for ubiquinone/polyprenols and by OsSPS4 in cytosolic vesicles for dolichols/polyprenols and FPPs are by OsFPS1 in cytoplasm for sesqui-/tri-terpenoids (Figure 7). In rice, the diverse terpene metabolites have been reported, which include phytohormones, chlorophyll, tocopherols, carotenoids, plastoquinone, ubiquinone, phytosterols and dolichols to play diverse essential roles for plant development and for adaptation to environmental conditions, antimicrobial monoterenes such as limonene, linalool and terpinene, and other anti-fungal and insect-attractant terpen such as momilactones, phytoalkexins (oryzalezins), bisabolene, caryophyllene, and nerolidol (bactericides) [45; 46; 47; 48]. Our networking model of all predictable rice PTs based on subcellular topology expands our understanding about rice terpenoid metabolism, and if combined with the further identification of enzymatic activity in the near future, it could be very useful to apply to functional terpenoid metabolic engineering in monocot plant systems, including rice. Figure 7: The networking model of rice _trans_-PTs based on subcellular topology. The subcellular organelles are colored: Chloroplasts with plastoglobules (green with dark green), mitochondria (beige) and cytosol with vesicles (light-gray with pink). Rice _trans_-PTs have colored background in yellow (OsGPS), orange (OsGGPS1), pale orange (OsGRP), grey (OsSPSs) and purple (OsFPSs). DMAPP, dimethylallyl pyrophosphate; FPP, farnesyl pyrophosphate; FPS, FPP synthase; GFPP, geranylfarnesyl pyrophosphate; GFPS, GFPP synthase; GPP, geranyl pyrophosphate; GPS, GPP synthase; GGPP, geranylgeranyl pyrophosphate; GGPS, GGPP synthase; PPP, polyprenyl pyrophosphate; PPS, PPP synthase; SPP, solanesyl pyrophosphate; SPS, SPP synthase. Allelic substrates and main actors among PTs with the same enzymatic activity are bolded.