A study on intersterility groups of Armillaria in China

Armillaria altimontana in North America: Biology and Ecology

Armillaria altimontana is a fungus (Basidiomycota, Agaricomycetes, Agaricales, and Physalacriaceae) that is generally considered as a weak/opportunistic pathogen or saprophyte on many tree hosts. It widely occurs across the northwestern USA to southern British Columbia, Canada, but relatively little is known about its ecological role in the diverse forest ecosystems where it occurs. This review summarizes the biology and ecology of A. altimontana, including its identification, life cycle, distribution, host associations, and bioclimatic models under climate change.

Metabolomics analysis of mycelial exudates provides insights into fungal antagonists of Armillaria

The genus Armillaria has high edible and medical values, with zones of antagonism often occurring when different species are paired in culture on agar media, while the antagonism-induced metabolic alteration remains unclear. Here, the metabolome of mycelial exudates of two Chinese Armillaria biological species, C and G, co-cultured or cultured separately was analysed to discover the candidate biomarkers and the key metabolic pathways involved in Armillaria antagonists. A total of 2,377 metabolites were identified, mainly organic acids and derivatives, lipids and lipid-like molecules, and organoheterocyclic compounds. There were 248 and 142 differentially expressed metabolites between group C-G and C, C-G, and G, respectively, and fourteen common differentially expressed metabolites including malate, uracil, Leu-Gln-Arg, etc. Metabolic pathways like TCA cycle and pyrimidine metabolism were significantly affected by C-G co-culture. Additionally, 156 new metabolites (largely organic acids and derivatives) including 32 potential antifungal compounds, primarily enriched into biosynthesis of secondary metabolites pathways were identified in C-G co-culture mode. We concluded that malate and uracil could be used as the candidate biomarkers, and TCA cycle and pyrimidine metabolism were the key metabolic pathways involved in Armillaria antagonists. The metabolic changes revealed in this study provide insights into the mechanisms underlying fungal antagonists.

An exploration of mechanism of high quality and yield of Gastrodia elata Bl. f. glauca by the isolation, identification and evaluation of Armillaria

BACKGROUND

Gastrodia elata Bl. f. glauca, a perennial herb of G.elata Bl. in Orchidaceae, is one of the most valuable traditional Chinese medicines. G. elata Bl. is a chlorophyll-free myco-heterotrophic plant, which must rely on the symbiotic growth of Armillaria, but not all Armillaria strains can play the symbiotic role. Additionally, Armillaria is easy to degenerate after multiple generations, and the compatibility between the strains from other areas and G. elata Bl. f. glauca in Changbai Mountain is unstable. Therefore, it is incredibly significant to isolate, identify and screen the symbiotic Armillaria suitable for the growth of G. elata Bl. f. glauca in Changbai Mountain, and to explore the mechanism by which Armillaria improves the production performance of G. elata Bl. f. glauca.

RESULTS

Firstly, G. elata Bl. f. glauca tubers, and rhizomorphs and fruiting bodies of Armillaria were used for the isolation and identification of Armillaria. Five Armillaria isolates were obtained in our laboratory and named: JMG, JMA, JMB, JMC and JMD. Secondly, Armillaria was selected based on the yield and the effective component content of G. elata Bl. f. glauca. It was concluded that the yield and quality of G. elata Bl. f. glauca co-planted with JMG is the highest. Finally, the mechanism of its high quality and yield was explored by investigating the effects of different Armillaria strains on the soil, its nutrition element contents and the soil microbial diversity around G. elata Bl. f. glauca in Changbai Mountain.

CONCLUSIONS

Compared with commercial strains, JMG significantly increased the content of Na, Al, Si, Mn, Fe, Zn, Rb and the absorption of C, Na, Mg, Ca, Cr, Cu, Zn and Rb in G. elata Bl. f. glauca; it improved the composition, diversity and metabolic functions of soil microbial communities around G. elata Bl. f. glauca at phylum, class and genus levels; it markedly increased the relative abundance of bacteria such as Chthoniobacter and Armillaria in the dominant populations, and enhanced such functions as Cell motility, amino acid metabolism and Lipid metabolism; it dramatically decreased the relative abundance of Bryobacter and other fungi in the dominant populations, and reduced such functions as microbial energy metabolism, translation and carbohydrate metabolism. This is the main reason why excellent Armillaria strains promote the high quality and yield of G. elata Bl. f. glauca in Changbai Mountain.

Phylogenetic Relationships, Speciation, and Origin of Armillaria in the Northern Hemisphere: A Lesson Based on rRNA and Elongation Factor 1-Alpha

Armillaria species have a global distribution and play various roles in the natural ecosystems, e.g., pathogens, decomposers, and mycorrhizal associates. However, their taxonomic boundaries, speciation processes, and origin are poorly understood. Here, we used a phylogenetic approach with 358 samplings from Europe, East Asia, and North America to delimit the species boundaries and to discern the evolutionary forces underpinning divergence and evolution. Three species delimitation methods indicated multiple unrecognized phylogenetic species, and biological species recognition did not reflect the natural evolutionary relationships within Armillaria; for instance, biological species of A. mellea and D. tabescens are divergent and cryptic species/lineages exist associated with their geographic distributions in Europe, North America, and East Asia. While the species-rich and divergent Gallica superclade might represent three phylogenetic species (PS I, PS II, and A. nabsnona) that undergo speciation. The PS II contained four lineages with cryptic diversity associated with the geographic distribution. The genus Armillaria likely originated from East Asia around 21.8 Mya in early Miocene when Boreotropical flora (56–33.9 Mya) and the Bering land bridge might have facilitated transcontinental dispersal of Armillaria species. The Gallica superclade arose at 9.1 Mya and the concurrent vicariance events of Bering Strait opening and the uplift of the northern Tibetan plateau might be important factors in driving the lineage divergence.

The melanized layer of Armillaria ostoyae rhizomorphs: Its protective role and functions

Armillaria ostoyae (Romagn.) Herink is a highly pathogenic fungus that uses exploratory, cordlike structures called rhizomorphs to seek out new sources of nutrition, posing a parasitic threat to natural stands of trees, orchards, and vineyards. Rhizomorphs are notoriously difficult to destroy, and this resilience is due in large part to a melanized layer that protects the rhizomorph. While this structure has been previously observed, its structural and chemical defenses are yet to be discerned. Research was conducted on both lab-cultured and wild-harvested rhizomorph samples. While both environments produce rhizomorphs, only the wild-harvested rhizomorphs produced the melanized layer, allowing for direct investigation of its structure and properties. Imaging, chemical analysis, mechanical testing, and finite element modeling were used to understand the defense mechanisms provided by the melanized layer. Imaging showed a porous outer layer in both types of rhizomorphs, though the pores were smaller in the harvested melanized layer. This melanized layer contained calcium, which provides chemical defense against both human and natural control methods, but was absent from cultured samples. Nanoindentation resulted in a larger variance of hardness values for cultured rhizomorphs than for wild-harvested. Finite element analysis proved that the smaller pore structure of the melanized porous layer had the best balance between maximum deformation and resulting permanent deformation. These results allow for a better understanding of the defenses of this pathogenic fungus, which may lead to better control methods.

Desarmillaria caespitosa, a North American vicariant of D. tabescens

Desarmillaria caespitosa, a North American vicariant species of European D. tabescens, is redescribed in detail based on recent collections from the USA and Mexico. This species is characterized by morphological features and multilocus phylogenetic analyses using portions of nuc rDNA 28S (28S), translation elongation factor 1-alpha (tef1), the second largest subunit of RNA polymerase II (rpb2), actin (act), and glyceraldehyde-3-phosphate dehydrogenase (gpd). A neotype of D. caespitosa is designated here. Morphological and genetic differences between D. caespitosa and D. tabescens were identified. Morphologically, D. caespitosa differs from D. tabescens by having wider basidiospores, narrower cheilocystidia, which are often irregular or mixed (regular, irregular, or coralloid), and narrower caulocystidia. Phylogenetic analyses of five independent gene regions show that D. caespitosa and D. tabescens are separated by nodes with strong support. The new combination, D. caespitosa, is proposed.

Epidemiology, Biotic Interactions and Biological Control of Armillarioids in the Northern Hemisphere

Armillarioids, including the genera Armillaria, Desarmillaria and Guyanagaster, represent white-rot specific fungal saprotrophs with soilborne pathogenic potentials on woody hosts. They propagate in the soil by root-like rhizomorphs, connecting between susceptible root sections of their hosts, and often forming extended colonies in native forests. Pathogenic abilities of Armillaria and Desarmillaria genets can readily manifest in compromised hosts, or hosts with full vigour can be invaded by virulent mycelia when exposed to a larger number of newly formed genets. Armillaria root rot-related symptoms are indicators of ecological imbalances in native forests and plantations at the rhizosphere levels, often related to abiotic environmental threats, and most likely unfavourable changes in the microbiome compositions in the interactive zone of the roots. The less-studied biotic impacts that contribute to armillarioid host infection include fungi and insects, as well as forest conditions. On the other hand, negative biotic impactors, like bacterial communities, antagonistic fungi, nematodes and plant-derived substances may find applications in the environment-friendly, biological control of armillarioid root diseases, which can be used instead of, or in combination with the classical, but frequently problematic silvicultural and chemical control measures.

Chromosomal assembly and analyses of genome-wide recombination rates in the forest pathogenic fungus Armillaria ostoyae

Recombination shapes the evolutionary trajectory of populations and plays an important role in the faithful transmission of chromosomes during meiosis. Levels of sexual reproduction and recombination are important properties of host-pathogen interactions because the speed of antagonistic co-evolution depends on the ability of hosts and pathogens to generate genetic variation. However, our understanding of the importance of recombination is limited because large taxonomic groups remain poorly investigated. Here, we analyze recombination rate variation in the basidiomycete fungus Armillaria ostoyae, which is an aggressive pathogen on a broad range of conifers and other trees. We analyzed a previously constructed, dense genetic map based on 198 single basidiospore progeny from a cross. Progeny were genotyped at a genome-wide set of single-nucleotide polymorphism (SNP) markers using double digest restriction site associated DNA sequencing. Based on a linkage map of on 11,700 SNPs spanning 1007.5 cM, we assembled genomic scaffolds into 11 putative chromosomes of a total genome size of 56.6 Mb. We identified 1984 crossover events among all progeny and found that recombination rates were highly variable along chromosomes. Recombination hotspots tended to be in regions close to the telomeres and were more gene-poor than the genomic background. Genes in proximity to recombination hotspots were encoding on average shorter proteins and were enriched for pectin degrading enzymes. Our analyses enable more powerful population and genome-scale studies of a major tree pathogen.

Armillaria Root-Rot Pathogens: Species Boundaries and Global Distribution

This review considers current knowledge surrounding species boundaries of the Armillaria root-rot pathogens and their distribution. In addition, a phylogenetic tree using translation elongation factor subunit 1-alpha (tef-1α) from isolates across the globe are used to present a global phylogenetic framework for the genus. Defining species boundaries based on DNA sequence-inferred phylogenies has been a central focus of contemporary mycology. The results of such studies have in many cases resolved the biogeographic history of species, mechanisms involved in dispersal, the taxonomy of species and how certain phenotypic characteristics have evolved throughout lineage diversification. Such advances have also occurred in the case of Armillaria spp. that include important causal agents of tree root rots. This commenced with the first phylogeny for Armillaria that was based on IGS-1 (intergenic spacer region one) DNA sequence data, published in 1992. Since then phylogenies were produced using alternative loci, either as single gene phylogenies or based on concatenated data. Collectively these phylogenies revealed species clusters in Armillaria linked to their geographic distributions and importantly species complexes that warrant further research.

Frequent diploidisation of haploid Armillaria ostoyae strains in an outdoor inoculation experiment

Very little is known about the biology and ecology of haploid Armillaria strains in nature. In this outdoor inoculation experiment, we assessed the virulence of six haploid Armillariaostoyae strains along with their diploid parent towards 2-year-old seedlings and 4-year-old saplings of Norway spruce (Picea abies), and determined their ability to colonise freshly cut stumps. As inoculum source an Armillaria-colonised hazelnut (Corylus avellana) stem segment was inserted into the soil substrate. Re-isolations from mycelial fans at the root collar of infected trees or stumps were made. Surprisingly, not a single haploid re-isolate could be recovered. Microsatellite genotyping of 133 re-isolates suggests that the inoculated haploid strains were diploidised either by mating propagules (basidiospores or haploid mycelia) already present in the soil substrate or naturally disseminated in the course of the experiment from nearby forests. Consequently, no conclusion about the infectious ability of haploid Armillaria mycelia under natural conditions can be drawn. Nonetheless, the diploid half-sib families resulting from the diploidisation showed varying degrees of virulence, with a high correlation between the experiment with 2-year-old seedlings and 4-year-old saplings. Despite extensive genotyping of re-isolates, no evidence for somatic recombination between haploid mating propagules and diploidised mycelia was detected, suggesting that this is an uncommon phenomenon in A. ostoyae.

High-density genetic mapping identifies the genetic basis of a natural colony morphology mutant in the root rot pathogen Armillaria ostoyae

Filamentous fungi exhibit a broad spectrum of heritable growth patterns and morphological variations reflecting the adaptation of the different species to distinct ecological niches. But also within species, isolates show considerable variation in growth rates and other morphological characteristics. The genetic basis of this intraspecific variation in mycelial growth and morphology is currently poorly understood. By chance, a growth mutant in the root rot pathogen Armillaria ostoyae was discovered. The mutant phenotype was characterized by extremely compact and slow growth, as well as shorter aerial hyphae and hyphal compartments in comparison to the wildtype phenotype. Genetic analysis revealed that the abnormal phenotype is caused by a recessive mutation, which segregates asa single locus in sexual crosses. In order to identify the genetic basis of the mutant phenotype, we performed a quantitative trait locus (QTL) analysis. A mapping population of 198 haploid progeny was genotyped at 11,700 genome-wide single nucleotide polymorphisms (SNPs) making use of double digest restriction site associated DNA sequencing (ddRADseq). In accordance with the genetic analysis, a single significant QTL was identified for the abnormal growth phenotype. The QTL confidence interval spans a narrow, gene dense region of 87kb in the A. ostoyae genome which contains 37 genes. Overall, our study reports the first high-density genetic map for an Armillaria species and shows its successful application in forward genetics by resolving the genetic basis of a mutant phenotype with a severe defect in hyphal growth.

Phylogenetic constrains on Polyporus umbellatus-Armillaria associations

It has been well established that some Armillaria species are symbionts of Polyporus umbellatus, However, little is known about the evolutionary history of P. umbellatus-Armillaria associations. In this research, we used an analysis based on the strength of the phylogenetic signal to investigate P. umbellatus-Armillaria associations in 57 sclerotial samples across 11 provinces of China. We isolated Armillaria strains from the invasion cavity inside the sclerotia of P. umbellatus and then phylogenetically analyzed these Armillaria isolates. We also tested the effect of P. umbellatus and Armillaria phylogenies on the P. umbellatus-Armillaria associations. We isolated forty-seven Armillaria strains from 26 P. umbellatus sclerotial samples. All Armillaria isolates were classified into the 5 phylogenetic lineages found in China except for one singleton. Among the 5 phylogenetic lineages, one lineage (lineage 8) was recognized by delimitation of an uncertain phylogenetic lineage in previous study. Results of simple Mantel test implied that phylogenetically related P. umbellatus populations tend to interact with phylogenetically related Armillaria species. Phylogenetic network analyses revealed that the interaction between P. umbellatus and Armillaria is significantly influenced by the phylogenetic relationships between the Armillaria species.

Virulence and Stump Colonization Ability of Armillaria borealis on Norway Spruce Seedlings in Comparison to Sympatric Armillaria Species

Although Armillaria borealis is one of the closest relatives of the aggressive root rot pathogen A. ostoyae, little is known about its ecology. In central and northern Europe, A. borealis often co-occurs with A. ostoyae or A. cepistipes, a weak pathogen, in conifer or mixed-forest stands. In this study, the virulence of 10 A. borealis, nine A. ostoyae, and five A. cepistipes isolates toward 2- and 4-year-old potted Norway spruce (Picea abies) seedlings was assessed. In addition, the ability of all isolates to colonize fresh stumps cut from 6-year-old Norway spruce seedlings was tested. All inoculations were done by insertion of Armillaria-colonized hazelnut stem segments into the soil substrate. On the 2-year-old seedlings, A. borealis and A. ostoyae showed, overall, a similar virulence 29 months after inoculation whereas, on the 4-year-old seedlings, A. ostoyae was more virulent. The third species in the experiment, A. cepistipes, caused almost no seedling mortality. Six months after cutting, the highest percentage of stumps was colonized by A. ostoyae (94.3%), followed by A. borealis (85.2%), and A. cepistipes (78.4%). Our inoculation experiments show that A. borealis has a clear pathogenic potential toward Norway spruce seedlings. However, compared with A. ostoyae, damage caused by A. borealis may decrease more rapidly with increasing tree age. Similar to other Armillaria species, A. borealis seems to be an efficient colonizer of fresh stumps.

Fungal phoenix rising from the ashes?

During May 2010, sporocarps of what appeared to be an Armillaria sp. were found in large clumps in historic Kirstenbosch Botanical Gardens on the foot of Table Mountain, Cape Town, South Africa. These sporocarps could be physically linked to the roots of unidentified dead trees and Protea spp. The aim of this study was to identify the Armillaria sp. found fruiting in Kirstenbosch. To achieve this goal isolates were made from the mycelium under the bark of dead roots linked to sporocarps. The ITS and IGS-1 regions were sequenced and compared to sequences of Armillaria spp. available on GenBank. Cladograms were generated using ITS sequences to determine the phylogenetic relationship of the isolates with other Armillaria spp. Sequence comparisons and phylogenetic analyses showed that the isolates represented A. mellea. They were also identical to isolates of this species previously discovered in the Company Gardens in South Africa and introduced from Europe apparently by the early Dutch Settlers. Armillaria mellea is alien and apparently invasive in Cape Town, fruits profusely and has the potential to spread to sensitive native forests on the foothills of the City.