Summary: Ectomycorrhizal fungi play a key role in forests by establishing mutualistic symbioses with woody plants. Genome analyses have identified conserved symbiosis‐related traits among ectomycorrhizal fungal species, but the molecular mechanisms underlying host specificity remain poorly known. We sequenced and compared the genomes of seven species of milk‐cap fungi (Lactarius, Russulales) with contrasting host specificity. We also compared these genomes with those of symbiotic and saprotrophic Russulales species, aiming to identify genes involved in their ecology and host specificity. The size of Lactarius genomes is significantly larger than other Russulales species, owing to a massive accumulation of transposable elements and duplication of dispensable genes. As expected, their repertoire of genes coding for plant cell wall‐degrading enzymes is restricted, but they retained a substantial set of genes involved in microbial cell wall degradation. Notably, Lactarius species showed a striking expansion of genes encoding proteases, such as secreted ectomycorrhiza‐induced sedolisins. A high copy number of genes coding for small secreted LysM proteins and Lactarius‐specific lectins were detected, which may be linked to host specificity. This study revealed a large diversity in the genome landscapes and gene repertoires within Russulaceae. The known host specificity of Lactarius symbionts may be related to mycorrhiza‐induced species‐specific genes, including secreted sedolisins.

Arbuscular mycorrhizal (AM) fungi form a mutualistic symbiosis with a majority of terrestrial vascular plants. To achieve an efficient nutrient trade with their hosts, AM fungi sense external and internal nutrients, and integrate different hierarchic regulations to optimize nutrient acquisition and homeostasis during mycorrhization. However, the underlying molecular networks in AM fungi orchestrating the nutrient sensing and signaling remain elusive. Based on homology search, we here found that at least 72 gene components involved in four nutrient sensing and signaling pathways, including cAMP-dependent protein kinase A (cAMP-PKA), sucrose non-fermenting 1 (SNF1) protein kinase, target of rapamycin kinase (TOR) and phosphate (PHO) signaling cascades, are well conserved in AM fungi. Based on the knowledge known in model yeast and filamentous fungi, we outlined the possible gene networks functioning in AM fungi. These pathways may regulate the expression of downstream genes involved in nutrient transport, lipid metabolism, trehalase activity, stress resistance and autophagy. The RNA-seq analysis and qRT-PCR results of some core genes further indicate that these pathways may play important roles in spore germination, appressorium formation, arbuscule longevity and sporulation of AM fungi. We hope to inspire further studies on the roles of these candidate genes involved in these nutrient sensing and signaling pathways in AM fungi and AM symbiosis.

Lactarius hatsudake is a common ectomycorrhizal edible mushroom in Pinus massoniana forests, and has important ecological and potential economic values. However, there are only a few reports on the establishment of Pinus massoniana–Lactarius hatsudake symbiosis. Here, we isolated a new strain of L. Lactarius hatsudake (GX01) from a local masson pine forest and established its ectomycorrhizal symbiosis with the P. massoniana. Potato dextrose agar (PDA) medium was optimal for the growth of L. hatsudake GX01. The saffron-to-brown ectomycorrhiza formed by L. hatsudake GX01 are usually bifurcated or coralloid shape, with a rod and a smooth surface, without emanating hyphae. The characteristic mantle and Hartig net structures of ectomycorrhizae were confirmed by microscope and scanning electron microscope (SEM). L. hatsudake GX01 can significantly promote the formation and development of lateral roots of P. massoniana seedlings during the early interaction. This study thus lays the foundation for subsequent study of the symbiotic molecular mechanism and application of P. massoniana–L. hatsudake symbiosis.