C13 α-Ionol (Blumenol) Glycosides and C14 Mycorradicin: Apocarotenoids Accumulating in Roots during the Arbuscular Mycorrhizal Symbiosis

The arbuscular mycorrhizal (AM) symbiosis is a widespread underground interaction of mutual benefit between plant roots and a specialized clade of fungi, which helps plants in the acquisition of minerals from nutrient-poor soils. Arbuscules are tiny fungal organs growing inside root cortex cells to release minerals collected from soils. A yellow colouration of roots strongly colonized by AM fungi has been attributed to the accumulation of a linear C14 apocarotenoid called mycorradicin. It is accompanied by the formation of cyclic C13 α-ionol apocarotenoids, which are usually glycosylated. Both apocarotenoids originate from the enzymatic cleavage of a common carotenoid precursor, presumably zeaxanthin carried out in two consecutive steps by CCD7 and CCD1. These apocarotenoids are formed specifically in root cells hosting fungal arbuscules, and there are now arguments that at least one of them may be involved in the control of the arbuscule life span by the plant. The universal presence of C13 α-ionol derivatives, but not of C14 mycorradicin, and other findings, argue for a role of C13 compounds in such processes. Certain C13 derivatives appear to be transported from mycorrhizal roots to leaves, which can be exploited for an aboveground monitoring of the underground mycorrhization status of plants.