Isoprenoid metabolism and plastid reorganization in arbuscular mycorrhizal roots

Plant root-colonizing arbuscular mycorrhizal (AM) fungi activate the methylerythritol phosphate pathway, carotenoid biosynthesis and oxidative carotenoid cleavage in roots, leading to C-13 and C-14 apocarotenoids, that is, cyclohexenone and mycorradicin derivatives. Mycorradicin causes the characteristic yellow coloration of many AM roots accumulating within a complex mixture of unknown components. The accumulating C-13 cyclohexenones exhibit various ring substitutions and different glycosyl moieties. Transcript levels of the first two enzymes of the MEP pathway, 1-deoxy-D-xylulose 5-phosphate synthase and 1-deoxy-D-xylulose 5-phosphate reductoisomerase, and of the carotenoid pathway, phytoene desaturase and zeta-carotene desaturase, along with a carotenoid-cleaving dioxygenase, are markedly increased in AM roots. This correlates with proliferation and reorganization of root plastids. These results allow at this point only speculation about the significance of apocarotenoid accumulation: participation in the production of signaling molecules and control of fungal colonization or protection against soil-borne pathogens; protection of root cells against oxidative damage of membranes by reactive oxygen species; and promotion of the symbiotic interactions between plant roots and AM fungi.