A peroxidase gene family and gene trees inHeterobasidionand related genera

Heterobasidion annosum and H. parviporum in the Southern Rocky Mountains and Adjoining States

Collections of Heterobasidion spp. from Nebraska, Colorado, Arizona, and New Mexico were identified based on the sequence of the internal transcribed spacer region of the ribosomal DNA. The North American variant of Heterobasidion annosum sensu stricto was found on Pinus ponderosa and Juniperus virginiana in central Nebraska, southern Colorado, central Arizona, and southern New Mexico. The North American variant of H. parviporum was found on Abies concolor and Picea engelmannii in southern Colorado and central New Mexico. The pathogens were not found in a survey of conifer forests in Wyoming and the Black Hills of South Dakota. Historical records of annosus root disease are reviewed by host group to gain more insight into the potential distributions of the respective pathogens. An apparent lack of overlap in host range suggests that substitution of tree species may be a useful management approach in some cases.

On the mechanism of apoplastic H2O2 production during lignin formation and elicitation in cultured spruce cells--peroxidases after elicitation

A cell culture of Picea abies (L.) Karst. was used for studies of H(2)O(2) generation during constitutive extracellular lignin formation and after elicitation by cell wall fragments of a pathogenic fungus, Heterobasidium parviporum. Stable, micromolar levels of H(2)O(2) were present in the culture medium during lignin formation. Elicitation induced a burst of H(2)O(2), peaking at ca. 90 min after elicitation. Of exogenous reducing substrates that may be responsible for the synthesis of H(2)O(2) from O(2), NADH stimulated H(2)O(2) production irrespective of elicitation. Cysteine (Cys) and glutathione (GSH) partially scavenged the constitutive H(2)O(2), but usually increased or prolonged elicitor-induced H(2)O(2) formation. Culture medium peroxidases were not able to generate H(2)O(2) in vitro with Cys or GSH as reductants. These thiols, however, generated H(2)O(2) non-enzymically at pH 4.5. [(35)S]Sulphate feeding to spruce cells showed that endogenous sulphur-containing compounds (including GSH, GSSG and cysteic acid) existed in the culture medium. The apoplastic levels of these were, however, undetectable by the monobromobimane method suggesting that their contribution to apoplastic H(2)O(2) formation is probably minor. Azide, an inhibitor of haem-containing enzymes, slightly inhibited constitutive H(2)O(2) generation but strongly delayed the elicitor-induced H(2)O(2) accumulation. Diphenylene iodonium, an inhibitor of flavin-containing enzymes, efficiently inhibited H(2)O(2) production irrespective of elicitation. Elicitation led to downregulation of the expression of several peroxidase genes, and peroxidase activity in the culture medium was slightly reduced. Expression of three other peroxidase genes and a respiratory burst oxidase homologue (rboh) gene were upregulated. These data suggest that both peroxidases and rboh may contribute to H(2)O(2) generation.

ClassII peroxidase-encoding genes are present in a phylogenetically wide range of ectomycorrhizal fungi

Fungal peroxidases (ClassII) have a key role in degrading recalcitrant polyphenolic compounds in boreal forest wood, litter and humus. To date, their occurrence and activity have mainly been studied in a small number of white-rot wood decomposers. However, peroxidase activity is commonly measured in boreal forest humus and mineral soils, in which ectomycorrhizal fungi predominate. Here, we used degenerate PCR primers to investigate whether peroxidase-encoding genes are present in the genomes of a wide phylogenetic range of ectomycorrhizal taxa. Cloning and sequencing of PCR products showed that ectomycorrhizal fungi from several different genera possess peroxidase genes. The new sequences represent four major homobasidiomycete lineages, but the majority is derived from Cortinarius, Russula and Lactarius. These genera are ecologically important, but consist mainly of non-culturable species from which little ecophysiological information is available. The amplified sequences contain conserved active sites, both for folding and substrate oxidation. In some Cortinarius spp., there is evidence for gene duplications during the evolution of the genus. ClassII peroxidases seem to be an ancient and a common feature of most homobasidiomycetes, including ectomycorrhizal fungi. Production of extracellular peroxidases may provide ectomycorrhizal fungi with access to nitrogen sequestered in complex polyphenolic sources.

Molecular evolution and diversity of lignin degrading heme peroxidases in the Agaricomycetes

The plant and microbial peroxidase superfamily encompasses three classes of related protein families. Class I includes intracellular peroxidases of prokaryotic origin, class II includes secretory fungal peroxidases, including the lignin degrading enzymes manganese peroxidase (MnP), lignin peroxidase (LiP), and versatile peroxidase (VP), and class III includes the secretory plant peroxidases. Here, we present phylogenetic analyses using maximum parsimony and Bayesian methods that address the origin and diversification of class II peroxidases. Higher-level analyses used published full-length sequences from all members of the plant and microbial peroxidase superfamily, while lower-level analyses used class II sequences only, including 43 new sequences generated from Agaricomycetes (mushroom-forming fungi and relatives). The distribution of confirmed and proposed catalytic sites for manganese and aromatic compounds in class II peroxidases, including residues supposedly involved in three different long range electron transfer pathways, was interpreted in the context of phylogenies from the lower-level analyses. The higher-level analyses suggest that class II sequences constitute a monophyletic gene family within the plant and microbial peroxidase superfamily, and that they have diversified extensively in the basidiomycetes. Peroxidases of unknown function from the ascomycete Magnaporthe grisea were found to be the closest relatives of class II sequences and were selected to root class II sequences in the lower-level analyses. LiPs evidently arose only once in the Polyporales, which harbors many white-rot taxa, whereas MnPs and VPs are more widespread and may have multiple origins. Our study includes the first reports of partial sequences for MnPs in the Hymenochaetales and Corticiales.

Ecological constraints limit the fitness of fungal hybrids in the Heterobasidion annosum species complex

The ability of two closely related species to maintain species boundaries in spite of retained interfertility between them is a documented driving force of speciation. Experimental evidence to support possible interspecific postzygotic isolation mechanisms for organisms belonging to the kingdom Fungi is still missing. Here we report on the outcome of a series of controlled comparative inoculation experiments of parental wild genotypes and F(1) hybrid genotypes between closely related and interfertile taxa within the Heterobasidion annosum fungal species complex. Results indicated that these fungal hybrids are not genetically unfit but can fare as well as parental genotypes when inoculated on substrates favorable to both parents. However, when placed in substrates favoring one of the parents, hybrids are less competitive than the parental genotypes specialized on that substrate. Furthermore, in some but not all fungus x plant combinations, a clear asymmetry in fitness was observed between hybrids carrying identical nuclear genomes but different cytoplasms. This work provides some of the first experimental evidence of ecologically driven postzygotic reinforcement of isolation between closely related fungal species characterized by marked host specificity. Host specialization is one of the most striking traits of a large number of symbiotic and parasitic fungi; thus, we suggest the ecological mechanism proven here to reinforce isolation among Heterobasidion spp. may be generally valid for host-specialized fungi. The validity of this generalization is supported by the low number of known fungal hybrids and by their distinctive feature of being found in substrates different from those colonized by parental species.

Pseudomonas aeruginosa inducing rice resistance against Rhizoctonia solani: production of salicylic acid and peroxidases

Three isolates of Pseudomonas aeruginosa were used for seed treatment of rice; all showed plant growth promoting activity and induced systemic resistance in rice against Rhizoctonia solani G5 and increased seed yield. Production of salicylic acid (Sal) by P. aeruginosa both in vitro and in vivo was quantified with high performance liquid chromatography. All three isolates produced more Sal in King's B broth than in induced roots. Using a split root system, more Sal accumulated in root tissues of bacterized site than in distant roots on the opposite site of the root system after 1 d, but this difference decreased after 3 d. Sal concentration 0-200 g/L showed no inhibition of mycelial growth of R. solani in vitro, while at > or =300 g/L it inhibited it. P. aeruginosa-pretreated rice plants challenged inoculation with R. solani (as pathogen), an additional increase in the accumulation of peroxidase was observed. Three pathogenesis-related peroxidases in induced rice plants were detected; molar mass of these purified peroxidases was 28, 36 and 47 kDa. Purified peroxidase showed antifungal activity against phytopathogenic fungi R. solani, Pyricularia oryzae and Helminthosporium oryzae.

Differential regulation of manganese peroxidases and characterization of two variable MnP encoding genes in the white-rot fungus Physisporinus rivulosus

Manganese peroxidase (MnP) production in the white-rot basidiomycete Physisporinus rivulosus T241i was studied. Separate MnP isoforms were produced in carbon-limited liquid media supplemented with Mn(2+), veratryl alcohol, or sawdust. The isoforms had different pH ranges for the oxidation of Mn(2+) and 2,6-dimethoxyphenol. Although lignin degradation by white-rot fungi is often triggered by nitrogen depletion, MnPs of P. rivulosus were efficiently produced also in the presence of high-nutrient nitrogen, especially in cultures supplemented with veratryl alcohol. Two MnP encoding genes, mnpA and mnpB, were identified, and their corresponding cDNAs were characterized. Structurally, the genes showed marked dissimilarity, and the expression of the two genes implicated quantitative variation and differential regulation in response to manganese, veratryl alcohol, or sawdust. The variability in regulation and properties of the isoforms may widen the operating range for efficient lignin degradation by P. rivulosus.

Conifer root and butt rot caused by Heterobasidion annosum (Fr.) Bref. s.l

SUMMARY The root and butt rot caused by Heterobasidon annosum is one of the most destructive diseases of conifers in the northern temperate regions of the world, particularly in Europe. Economic losses attributable to Heterobasidion infection in Europe are estimated at 800 million euros annually. The fungus has been classified into three separate European intersterile species P (H. annosum), S (H. parviporum) and F (H. abietinum) based on their main host preferences: pine, spruce and fir, respectively. In North America, two intersterile groups are present, P and S/F, but these have not been given scientific names. The ecology of the disease spread has been intensively studied but the genetics, biochemistry and molecular aspects of pathogen virulence have been relatively little examined. Recent advances in transcript profiling, molecular characterization of pathogenicity factors and establishment of DNA-transformation systems have paved the way for future advances in our understanding of this pathosystem.

TAXONOMY

Heterobasidion annosum (Fr.) Bref., H. parviporum Niemelä & Korhonen and H. abietinum Niemelä & Korhonen; kingdom Fungi; class Basidiomycotina; order Aphyllophorales; family Bondarzewiaceae; genus Heterobasidion.

IDENTIFICATION

presence of the fungus fruit bodies, basidiocarps whitish in the margins, upper surface is tan to dark brown, usually irregular shaped, 3.5 (-7) cm thick and up to 40 cm in diameter; pores 5-19, 7-22 and 13-26 mm(2) for the P, F and S groups, respectively. Small brownish non-sporulating postules develop on the outside of infected roots. Asexual spores (conidiospores) are 3.8-6.6 x 2.8-5.0 microm in size. Mating tests are necessary for identification of intersterility groups.

HOST RANGE

The fungus attacks many coniferous tree species. In Europe, particularly trees of the genera Pinus and Juniperus (P), Picea (S), Abies (F) and in North America Pinus (P) and Picea, Tsuga and Abies (S/F). To a lesser extent it causes root rot on some decidous trees (Betula and Quercus). Disease symptoms: symptoms (e.g. exhudation of resin, crown deterioration) due to Heterobasidion root rot in living trees are not particularly characteristic and in most cases cannot be distinguished from those caused by other root pathogens. Heterobasidion annosum s.l. is a white rot fungus. Initial growth in wood causes a stain that varies in colour depending on host tree species. Incipient decay is normally pale yellow and it develops into a light brown decay to become a white pocket rot with black flecks in its advanced stage.

CONTROL

silvicultural methods (e.g. stump removal), chemicals (urea, borates) and biological control agent (Phlebiopsis gigantea, marketed as PG Suspension(R) in the UK, PG IBL(R) in Poland and Rotstop(R) in Fennoscandia) are commonly used approaches for minimizing the disease spread.

Manganese peroxidase of Agaricus bisporus: grain bran-promoted production and gene characterization

The main manganese peroxidase (MnP) isoenzyme of Agaricus bisporus ATCC 62459 produced in lignocellulose-containing cultures was isolated, cloned and sequenced. In liquid medium, where MnP was previously detected only in trace amounts, the production of MnP was enhanced by rye and wheat bran supplements. The pI (3.25) and N-terminal amino acid sequence (25 aa) of the enzyme from bran-containing cultures were identical to those reported from compost-isolated MnP1. MnP1 is a 328-aa long polypeptide preceded by a 26-aa leader peptide. The nucleotide sequence and putative amino acid sequence of MnP1 reveal its similarity to Pleurotus ostreatus MnP3 (62.5%), Lepista irina versatile peroxidase (VP) (61.8%) and Pleurotus eryngii VPs VPL2 and VPL1 (61.9% and 61.2%, respectively). The intron-exon structure resembles that of P. ostreatus MnP1 and P. eryngii VPL1. Despite the sequence similarity to VPs, in the A. bisporus MnP1 sequence, alanine (A163) is present instead of tryptophane (W164), distinguishing it from the veratryl alcohol oxidising P. eryngii VPLs. The MnP sequence can be used as a tool to examine the pattern of ligninolytic gene expression during the growth and fruiting of A. bisporus to optimise compost composition, fungal growth and mushroom production.