Following the publication of this article [1] the authors noted that the image in Fig. 1 was incorrect.

The arbuscular mycorrhizal (AM) symbiosis is a beneficial association established between land plants and the members of a subphylum of fungi, the Glomeromycotina. How the two symbiotic partners regulate their association is still enigmatic. Secreted fungal peptides are candidates for regulating this interaction. We searched for fungal peptides with similarities with known plant signalling peptides. We identified CLAVATA (CLV)/EMBRYO SURROUNDING REGION (ESR)-RELATED PROTEIN (CLE) genes in phylogenetically distant AM fungi: four Rhizophagus species and one Gigaspora species. These CLE genes encode a signal peptide for secretion and the conserved CLE C-terminal motif. They seem to be absent in the other fungal clades. Rhizophagusirregularis and Gigasporarosea CLE genes (RiCLE1 and GrCLE1) are transcriptionally induced in symbiotic vs asymbiotic conditions. Exogenous application of synthetic RiCLE1 peptide on Medicago truncatula affects root architecture, by slowing the apical growth of primary roots and stimulating the formation of lateral roots. In addition, pretreatment of seedlings with RiCLE1 peptide stimulates mycorrhization. Our findings demonstrate for the first time that in addition to plants and nematodes, AM fungi also possess CLE genes. These results pave the way for deciphering new mechanisms by which AM fungi modulate plant cellular responses during the establishment of AM symbiosis.

Arbuscular mycorrhizal fungi (AMF), belonging to the fungal phylum Glomeromycota, form mutualistic symbioses with roots of almost 80% of land plants. The release of genomic data from the ubiquitous AMF Rhizophagus irregularis revealed that this species possesses a large set of putative secreted proteins (RiSPs) that could be of major importance for establishing the symbiosis. In the present study, we aimed to identify SPs involved in the establishment of AM symbiosis based on comparative gene expression analyses. We first curated the secretome of the R. irregularis DAOM 197198 strain based on two available genomic assemblies. Then we analyzed the expression patterns of the putative RiSPs obtained from the fungus in symbiotic association with three phylogenetically distant host plants—a monocot, a dicot and a liverwort—in comparison with non-symbiotic stages. We found that 33 out of 84 RiSPs induced in planta were commonly up-regulated in these three hosts. Most of these common RiSPs are small proteins of unknown function that may represent putative host non-specific effector proteins. We further investigated the expressed secretome of Gigaspora rosea, an AM fungal species phylogenetically distant from R. irregularis. G. rosea also presents original symbiotic features, a narrower host spectrum and a restrictive geographic distribution compared to R. irregularis. Interestingly, when analyzing up-regulated G. rosea SPs (GrSPs) in different hosts, a higher ratio of host-specific GrSPs was found compared to RiSPs. Such difference of expression patterns may mirror the restrained host spectrum of G. rosea compared to R. irregularis. Finally, we identified a set of conserved SPs, commonly up-regulated by both fungi in all hosts tested, that could correspond to common keys of AMF to colonize host plants. Our data thus highlight the specificities of two distant AM fungi and help in understanding their conserved and specific strategies to invade different hosts.

[This corrects the article on p. 124 in vol. 8, PMID: 28223991.].