Apocarotenoid biosynthesis in arbuscular mycorrhizal roots: Contributions from methylerythritol phosphate pathway isogenes and tools for its manipulation

The accumulation of 1-deoxy- d-xylulose 5-phosphate reductoisomerase (DXR) protein involved in apocarotenoid biosynthesis accompanies the degeneration of fungal arbuscules in mycorrhizal roots. The DXR protein is visualized by immunolocalization (green channel) overlaid to the red staining of the fungal structure, which represents a collapsing arbuscule. During colonization by arbuscular mycorrhizal (AM) fungi plant roots frequently accumulate two types of apocarotenoids (carotenoid cleavage products). Both compounds, C 14 mycorradicin and C 13 cyclohexenone derivatives, are predicted to originate from a common C 40 carotenoid precursor. Mycorradicin is the chromophore of the “yellow pigment” responsible for the long-known yellow discoloration of colonized roots. The biosynthesis of apocarotenoids has been investigated with a focus on the two first steps of the methylerythritol phosphate (MEP) pathway catalyzed by 1-deoxy- d-xylulose 5-phosphate synthase (DXS) and 1-deoxy- d-xylulose 5-phosphate reductoisomerase (DXR). In Medicago truncatula and other plants the DXS2 isogene appears to be specifically involved in the AM-mediated accumulation of apocarotenoids, whereas in the case of DXR a single gene contributes to both housekeeping and mycorrhizal (apo)carotenoid biosynthesis. Immunolocalization of DXR in mycorrhizal maize roots indicated an arbuscule-associated protein deposition, which occurs late in arbuscule development and accompanies arbuscule degeneration and breakdown. The DXS2 isogene is being developed as a tool to knock-down apocarotenoid biosynthesis in mycorrhizal roots by an RNAi strategy. Preliminary results from this approach provide starting points to suggest a new kind of function for apocarotenoids in mycorrhizal roots.