Dynamic 13C-pulse-chase experiments under photorespiratory CO2 concentration have demonstrated that the recycling mechanisms in the CB cycle is also linked to fluxes of C1intermediates metabolites [48,106]. wild-type plants growing in a medium supplemented with sucrose [72,73]. It can be deduced from such reports that the loss of function of and pea (mutant) when compared to wild type plants . This effect was similar to the changes in ME-cDP pool size in fosmidomycin/bisphosphonate treatment in hybrid aspen (see above and  for a detailed discussion). Despite the accumulation of ME-cDP, in fosmidomycin-inhibited plants, the 13C incorporation into ME-cDP in plants is only 25% compared to 70% in wild type plants . Similarly, 13CO2-labelling of ME-cDP in the DXS-upregulation lines is significantly lower than in the wild-type . These facts suggest an alternative pool of ME-cDP different from the MEP pathway. This hypothesis is supported by an observed residual pool of ME-cDP Valdecoxib following dark treatment, and a build-up of ME-cDP due to downregulation of activity of MEP pathway enzymes [88,90]. However, it is generally regarded that phosphorylated intermediates such as ME-cDP are not readily taken up by chloroplasts; thus, a cytosolic flux of non-phosphorylated pentose intermediates into chloroplasts, followed by their conversion into ME-cDP might be responsible for the alternative a substrate for MEP pathway [12,40] (Figure 2). Analysis of leaves from plants grown under nutrient deprivation or subject to other stresses, such as root oxidative stress, root wounding or biotic stress, revealed a close relationship between the levels of MEP intermediates, e.g., DXP, ME-cDP and HMBDP, and the production of hemiterpene glycosides [95,96]. In addition, significant increases in the levels of hemiterpene glycosides were found in fosmidomycin-treated mutant plants compared to untreated wild type and no 13C label was Cd24a detected at these metabolites during labeling experiments. Previous results suggest that under conditions that restrict the Valdecoxib MEP pathway activity, the ME-cDP or HMBDP can be exported out of the plastid and then converted to hemiterpene glycosides in cytosol [95,96]. Due to the lack of a specific carrier capable of transporting these phosphorylated intermediates through the chloroplast membrane, it is likely that a dephosphorylation occurs within the chloroplasts and the glycosylation in the cytosol . In addition, experimental evidence has shown that the accumulation of ME-cDP in plastids can elicit stress-signaling pathway, including changes in nuclear gene expression linked to plant defense signaling [90,93]. However, the exact nature of the signaling mechanisms coupled to ME-cDP content and gene Valdecoxib expression as well as the mode of transport of this plastid metabolite still need to be elucidated. Apart from ME-cDP, there is conclusive evidence of a certain bidirectional Valdecoxib exchange of intermediates between cytosolic and plastidic isoprenoid biosynthetic pathways [78,80,97]. In particular, a plastidic membrane transporter involved in the export of phosphorylated intermediates of isoprenoid synthesis has been characterized . This transporter efficiently Valdecoxib carries IDP and GDP, but lower transport rates were observed for the substrates FDP and DMADP [78,80]. However, the way this transporter operates is not fully clear. The transport of IDP seems to occur via a proton symport mechanism driven by transmembrane pH gradient and membrane potential, and the transport rate is regulated by Ca2+ concentration [78,80]. It was further demonstrated that the transport mechanisms are different from those of the known plastidic phosphate translocator family (PT) [80,97]. The transport does not appear to be antiport in exchange of other phosphorylated compounds (e.g., inorganic phosphate) at the other side of the membrane . Nevertheless, transport of IDP is strongly dependent on the presence of inorganic phosphate or small phosphorylated molecules on the.