Abstract

Symbiosis with arbuscular mycorrhizal (AM) fungi of the Glomeromycota phylum is very ancient and widespread in the plant kingdom. Its establishment is mediated through a molecular dialogue involving release and perception of multiple signals from both symbionts. Its earliest stages while being the most critical are also the least well understood. Mutational analyses led to identification of multiple genes essential for mycorrhizal colonization, including the so called Sym genes also required in nodulation, but to date the mutation of none of them results in the suppression of all plant responses to AM fungi. Here we describe the rice iho mutant, which is completely unresponsive to AM fungi as monitored microscopically and by early marker genes expression. Normal susceptibility and gene induction in response to both pathogenic and endophytic fungi shows the specific contribution of IHO to AM fungal perception. Genetic complementation experiments demonstrated that the IHO gene encodes for an alpha-beta hydrolase, which also mediates developmental responses in rice. The previous identification of multiple alpha beta hydrolases as receptors (e.g. GID1 and SABP3 ) makes IHO a very strong candidate receptor of AM signals while its redundancy in legume genomes but not in rice explains the failure in its identification in previous screenings. Global gene profiling experiments in response to developmental and AM fungal signals define the signaling events mediated by IHO and generate novel information on the earliest molecular events of AM symbiosis.