Karyological characterization of meiosis, post-meiotic mitosis and nuclear migration in the ectomycorrhizal fungus Rhizopogon roseolus (= R. rubescens)

Basidium formation of Rhizopogon roseolus with Pinus thunbergii in agar medium

The edible ectomycorrhizal (ECM) mushroom Rhizopogon roseolus usually develops basidium and basidiospores in the gleba of its basidiomata. Here, we report a novel production of basidia in laboratory cultures of the edible ECM mushroom. The basidium with sterigma was observed on the old mantle structure (> six months) of the ECM between R. roseolus and Pinus thunbergii in a modified Melin-Norkrans medium that was subjected to a temperature shock from 25 °C to 4 °C. The basidia were cylindrical to clavate, with prominent sterigmata and no basidiospores. The absence of basidiospores might indicate partial development of the basidium structure as a response to environmental stress and incomplete life cycle of R. roseolus. In addition, branched cystidia were evident in two or three clavate-to-ovoid cells. This study suggests the possibility of obtaining the primary mycelium of R. roseolus from pure cultures and may be an alternative genetic source for cultivation purposes. Further observations are required to induce basidiosporogenesis of R. roseolus basidia in an agar medium focusing on manipulation of temperature.

The impact of host plant (Pinus thunbergii) on the mycelial features of the ectomycorrhizal fungus Rhizopogon roseolus

The impact of host plant on the mycelial features of mycorrhizal symbiont and its characteristics has been poorly investigated. This study aimed to compare and quantify (statistically tested) some of the mycelial features of an ectomycorrhiza (ECM)-forming fungus with and without the ECM host. The ECM-forming fungus, Rhizopogon roseolus, inoculated with or without Pinus thunbergii on both rich and poor nutrient media, was observed under laboratory conditions. On rich medium, fungi with the host had the highest colony diameter and consistently produced the highest hyphal length relative to fungi on other media. Thus, the host had a significant impact on the mycelium production of R. roseolus in both rich and poor media. Further, fungi without the host had a higher number of hyphal anastomoses per hyphal length on both poor and rich media than fungi with the host in the same medium. Anastomosis, which refers to the fusion of two parallel hyphae, was evident in all experiments. However, there was no significant impact of the host on the number of hyphal anastomoses. In addition, fungi without the host had more frequent hyphal branching on both rich and poor media than fungi with the host. The occurrence of a host only had a significant impact on the formation of the hyphal branch on poor medium. Further, a chlamydospore-like structure was identified, which had a higher diameter when formed with the host on both rich and poor media. The present data provide new insights into the host plant's impact on the mycelial features of ECM-forming fungi.

Effect of sodium chloride on basidiospore germination and vegetative mycelial growth of the ectomycorrhizal fungus <i>Rhizopogon roseolus</i>

Rhizopogon roseolus is a basidiomycetous ectomycorrhizal fungus that inhabits mainly coastal areas. Understanding the response of this fungus to salinity at each stage of its life cycle will lead to elucidation of the strategies for its propagation. This study examined the effect of sodium chloride (NaCl) on basidiospore germination and mycelial growth of both homokaryotic and heterokaryotic strains of R. roseolus, on nutrient agar media with varying concentrations of NaCl (0, 50, 150, and 300 mM). Regardless of the presence of NaCl, R. roseolus basidiospores germinated and the germlings grew, forming compatible fusions. In addition, all multispore strains, including homokaryons and heterokaryons, grew under these NaCl conditions. Most of these strains had an effective concentration inhibiting mycelial growth by 50% value greater than 300 mM of NaCl. These results indicate that R. roseolus can germinate, grow, and mate in the presence of NaCl, allowing it to propagate in saline habitats.

Multinuclei Occurred Under Cryopreservation and Enhanced the Pathogenicity of Melampsora larici-populina

Melampsora larici-populina is a macrocyclic rust, and the haploid stage with two nuclei and the diploid of mononuclear sequentially occur annually. During the preservation of dry urediniospores at −80°C, we found that one isolate, ΔTs₀₆, was different from the usual wild-type isolate Ts₀₆ at −20°C because it has mixed polykaryotic urediniospores. However, the other spores, including the 0, I, III, and IV stages of a life cycle, were the same as Ts₀₆. After five generations of successive inoculation and harvest of urediniospores from the compatible host Populus purdomii, the isolate ΔTs₀₆ steadily maintained more than 20% multiple nucleus spores. To test the pathogenesis variation of ΔTs₀₆, an assay of host poplars was applied to evaluate the differences between ΔTs₀₆ and Ts₀₆. After ΔTs₀₆ and Ts₀₆ inoculation, leaves of P. purdomii were used to detect the expression of small secreted proteins (SSPs) and fungal biomasses using quantitative real-time PCR (qRT-PCR) and trypan blue staining. ΔTs₀₆ displayed stronger expression of five SSPs and had a shorter latent period, a higher density of uredinia, and higher DNA mass. A transcriptomic comparison between ΔTs₀₆ and Ts₀₆ revealed that 3,224 were differentially expressed genes (DEGs), 55 of which were related to reactive oxygen species metabolism, the Mitogen-activated protein kinase (MAPK) signaling pathway, and the meiosis pathway. Ten genes in the mitotic and meiotic pathways and another two genes associated with the “response to DNA damage stimulus” all had an upward expression, which were detected by qRT-PCR in ΔTs₀₆ during cryopreservation. Gas chromatography–mass spectrometry (GC-MS) confirmed that the amounts of hexadecanoic acid and octadecadienoic acid were much more in ΔTs₀₆ than in Ts₀₆. In addition, using spectrophotometry, hydrogen peroxide (H₂O₂) was also present in greater quantities in ΔTs₀₆ compared with those found in Ts₀₆. Increased fatty acids metabolism could prevent damage to urediniospores in super-low temperatures, but oxidant species that involved H₂O₂ may destroy tube proteins of mitosis and meiosis, which could cause abnormal nuclear division and lead to multinucleation, which has a different genotype. Therefore, the multinuclear isolate is different from the wild-type isolate in terms of phenotype and genotype; this multinucleation phenomenon in urediniospores improves the pathogenesis and environmental fitness of M. larici-populina.