Plants use two distinct isoprenoid biosynthesis pathways: the methylerythritol phosphate (MEP) pathway and the mevalonic acid (MVA) pathway. 1-deoxy-D-xylulose5-phosphate synthase (DXS) and 1-deoxy-D-xylulose5-phosphate reductoisomerase (DXR) are the rate-limiting enzymes in the MEP pathway, and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) is a key regulatory enzyme in the MVA pathway. Previously, overexpression of Populus trichocarpa PtDXR in Nanlin 895 ( Populus × euramericana cv. ‘Nanlin 895’) poplar was found to upregulate resistance to salt and drought stresses, and the transgenic poplars showed improved growth. In the present study, PtHMGR overexpressors (OEs) exhibited higher expression levels of DXS, DXR , 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate synthase ( HDS ), and 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase ( HDR ) and lower expression levels of 2-C-methyl-d-erythritol4-phosphate cytidylyltransferase ( MCT ), 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase ( CMK ), and 3-hydroxy-3-methylglutaryl-coenzyme A synthase ( HMGS ) than non-transgenic poplars (NT). However, the poplar PtDXR- OEs showed upregulated expression levels of MEP-related genes and downregulated expression of MVA-related genes. Moreover, overexpression of PtDXR and PtHMGR in poplars caused changes in MVA-derived trans-zeatin-riboside (TZR), isopentenyl adenosine (IPA), castasterone (CS) and 6-deoxocastasterone (DCS), as well as MEP-derived carotenoids, gibberellins (GAs), and abscisic acid (ABA). In PtHMGR-OE s, greater accumulation of geranyl diphosphate synthase ( GPS ) and geranyl pyrophosphate synthase ( GPPS) transcript levels in the MEP pathway led to an accumulation of MEP-derived isoprenoids, while upregulation of farnesyl diphosphate synthase ( FPS ) expression in the MVA pathway contributed to increased levels of MVA-derived isoprenoids. Similarly, in PtDXR -OEs, increased GPS and GPPS transcript levels in the MEP pathway boosted MEP-derived isoprenoid levels and changes in FPS expression affected MVA-derived isoprenoid yields. From these results, we can conclude that cross-talk exists between the MVA and MEP pathways. ### Competing Interest Statement The authors have declared no competing interest.
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