Fatty acid transcription in Cassava is increased and differently targeted by the genomic forms of a Mycorrhizal Fungus

Arbuscular mycorrhizal fungi (AMF) form a symbiosis with most land plant, based on the exchange of soil nutrients against plant hexoses [1]. This view is about to change, as several recent studies suggested that the fungal partner could receive a complex diet including plant lipid [2-4]. This assumption is based on four observations; the fungal type I fatty acid synthase (FAS-I) gene was found missing in AMF genomes [5,6]; in the plant, the concentration of numerous fatty acid increase during AM symbiosis [5], key lipid gene homologues are conserved in plant involved in AMF symbiosis and are necessary for the synthesis and transfer of lipids [7,8] and some genes of the plant lipid biosynthesis were found to be up regulated during symbiosis [9]. In this study, we showed that the up regulation of the fatty acid pathway of the cassava including the conserved key lipid plant genes (RAM1, WRI, RAD1, DIS, FatM, RAM2, STR, STR2) is a common process during symbiosis with all four described genomics forms of Rhizophagus irregularis. We confirm the central role played by RAM1, and showed that it is probably the activator, coupled with RAD1 and WRI5 of the expression of central genes in the fatty acid biosynthesis such as EAR, KASII, MAT and DLD. More importantly we found that the variation in several up regulated key lipid related genes including RAD1, several WRI, DIS and FatM could be link to the phylogeny of R. irregularis, implying that AM fungi have evolved a control of this important plant pathway.