The C24-methyl/ethyl sterol ratio is increased by Rhizophagus irregularis colonization.

Plant-microorganism interactions underlie many ecosystem roles, in particular the enhancement of plant nutrition through mutualistic relationships, such as the arbuscular mycorrhizal symbiosis that affects a large proportion of land plants. The establishment of this interaction induces a wide range of signaling pathways in which lipids, and particularly sterols, may play a central role. However, their supported functions are poorly known. We performed a study on eleven model plants (banana, barrelclover, flax, grapevine, maize, pea, poplar, potato, rice, sorghum and tomato) to measure the sterol content and characterize the sterol composition of roots that were either non-colonized or colonized by the arbuscular mycorrhizal fungal model Rhizophagus irregularis DAOM197198. Our results reveal a systematic increase in the content of C24-methyl sterols in crude extracts of colonized roots as compared to non-colonized roots. In addition, the transcripts of SMT1 and SMT2 (which encode enzymes that produce C24-methyl and C24-ethyl sterols, respectively) were differentially accumulated in colonized plant roots. No common regulation pattern was observed among plants. The phylogenetic relationship of members of the SMT1 and SMT2 families in more than 100 fully sequenced genomes of plants, ferns, mosses, algae and fungi has allowed the identification of unambiguous clades. Our results therefore highlight a conserved arbuscular mycorrhizal symbiosis-dependent regulation of the root sterol composition in angiosperms, with some plant specificities.