Poison Pax

Appearance

Resembling a brown wooden top, the epigeous fruit body may be up to 6 cm high. The cap, initially convex then more funnel-shaped with a depressed centre and rolled rim, may be reddish-, yellowish- or olive-brown in colour and typically 4–12 cm wide; the cap diameter does not get larger than 15 cm.

The cap surface is initially downy and later smooth, becoming sticky when wet. The cap and cap margin initially serve to protect the gills of young fruit bodies: this is termed pilangiocarpic development. The narrow brownish yellow gills are decurrent and forked, and can be peeled easily from the flesh. Gills further down toward the stipe become more irregular and anastomose, and can even resemble the pores of bolete-type fungi. The fungus darkens when bruised and older specimens may have darkish patches. The juicy yellowish flesh has a mild to faintly sour or sharp odor and taste, and has been described as well-flavored upon cooking. Of similar colour to the cap, the short stipe can be crooked and tapers toward the base.

The spore print is brown, and the dimensions of the ellipsoid spores are 7.5–9 by 5–6 μm.
The hymenium has cystidia both on the gill edge and face, which are slender and filament-like, typically measuring 40–65 by 8–10.5 μm.

Distribution

"Paxillus involutus" forms ectomycorrhizal relationships with a number of coniferous and deciduous tree species. Because the fungus has somewhat unspecialised nutrient requirements and a relatively broad host specificity, it has been frequently used in research and seedling inoculation programs.

There is evidence of the benefit to trees of this arrangement: in one experiment where "P. involutus" was cultivated on the root exudate of red pine, the root showed markedly increased resistance to pathogenic strains of the ubiquitous soil fungus "Fusarium oxysporum". Seedlings inoculated with "P. involutus" also showed increased resistance to "Fusarium". Thus "P. involutus" may be producing antifungal compounds which protect the host plants from root rot. "Paxillus involutus" also decreases the uptake of certain toxic elements, acting as a buffer against heavy metal toxicity in the host plant. For example, the fungus decreased the toxicity of cadmium and zinc to Scots pine seedlings: even though cadmium itself inhibits ectomycorrhiza formation in seedlings, colonization with "P. involutus" decreases cadmium and zinc transport to the plant shoots and alters the ratio of zinc transported to the roots and shoots, causing more cadmium to be retained in the roots of the seedlings rather than distributed through its entire metabolism. Evidence suggests that the mechanism for this detoxification involves the cadmium binding to the fungal cell walls, as well as accumulating in the vacuolar compartments. Further, ectomycorrhizal hyphae exposed to copper or cadmium drastically increase production of a metallothionein—a low molecular weight protein that binds metals.

The presence of "Paxillus involutus" is related to much reduced numbers of bacteria associated with the roots of "Pinus sylvestris". Instead bacteria are found on the external mycelium. The types of bacteria change as well; a Finnish study published in 1997 found that bacterial communities under "P. sylvestris" without mycorrhizae metabolised organic and amino acids, while communities among "P. involutus" metabolised the sugar fructose. "Paxillus involutus" benefits from the presence of some species of bacteria in the soil it grows in. As the fungus grows it excretes polyphenols, waste products that are toxic to itself and impede its growth, but these compounds are metabolised by some bacteria, resulting in increased fungal growth. Bacteria also produce certain compounds such as citric and malic acid, which stimulate "P. involutus".

Highly abundant, the brown roll-rim is found across the Northern Hemisphere, Europe and Asia, with records from India, China, Japan, Iran, and Turkey's eastern Anatolia. It is equally widely distributed across northern North America, extending north to Alaska, where it has been collected from tundra near Coldfoot in the interior of the state. In southwestern Greenland, "P. involutus" has been recorded under the birch species "Betula nana", "B. pubescens" and "B. glandulosa". The mushroom is more common in coniferous woods in Europe, but is also closely associated with birch. Within woodland, it prefers wet places or boggy ground, and avoids calcareous soils. It has been noted to grow alongside "Boletus badius" in Europe, and "Leccinum scabrum" and "Lactarius plumbeus" in the Pacific Northwest region of North America. There it is found in both deciduous and coniferous woodland, commonly under plantings of white birch in urban areas. It is one of a small number of fungal species which thrive in "Pinus radiata" plantations planted outside their natural range. A study of polluted Scots pine forest around Oulu in northern Finland found that "P. involutus" became more abundant in more polluted areas while other species declined. Emissions from pulp mills, fertiliser, heating and traffic were responsible for the pollution, which was measured by sulfur levels in the pine needles.

"Paxillus involutus" can be found growing on lawns and old meadows throughout its distribution. Fruit bodies are generally terrestrial, though they may be found on woody material around tree stumps. They generally appear in autumn and late summer. In California, David Arora discerned a larger form associated with oak and pine which appears in late autumn and winter, as well as the typical form that is associated with birch plantings and appears in autumn. Several species of flies and beetles have been recorded using the fruit bodies to rear their young. The mushroom can be infected by "Hypomyces chrysospermus", or bolete eater, a mould species that parasitises Boletales members. Infection results in the appearance of a whitish powder that first manifests on the pores, then spreads over the surface of the mushroom, becoming golden yellow to reddish-brown in maturity.

Australian mycologist John Burton Cleland noted it occurring under larch, oak, pine, birch and other introduced trees in South Australia in 1934, and it has subsequently been recorded in New South Wales, Victoria and Western Australia. It has been recorded under introduced birch and hazel in New Zealand. Mycologist Rolf Singer reported a similar situation in South America, with the species recorded under introduced trees in Chile. It is likely to have been transported to those countries in the soil of imported European trees.

Defense

"Paxillus involutus" was widely eaten in Central and Eastern Europe until World War II, although English guidebooks did not recommend it. In Poland, the mushroom was often eaten after pickling or salting. It was known to be a gastrointestinal irritant when ingested raw but had been presumed edible after cooking. Questions were first raised about its toxicity after German mycologist Julius Schäffer died after eating it in October 1944. About an hour after he and his wife ate a meal prepared with the mushrooms, Schäffer developed vomiting, diarrhea, and fever. His condition worsened to the point where he was admitted to hospital the following day and developed renal failure, perishing after 17 days.

In the mid-1980s, Swiss physician René Flammer discovered an antigen within the mushroom that stimulates an autoimmune reaction causing the body's immune cells to consider its own red blood cells as foreign and attack them. Despite this, it was not until 1990 that guidebooks firmly warned against eating "P. involutus", and one Italian guidebook recommended it as edible in 1998. The relatively rare immunohemolytic syndrome occurs following the repeated ingestion of "Paxillus" mushrooms. Most commonly it arises when the person has ingested the mushroom for a long period of time, sometimes for many years, and has shown mild gastrointestinal symptoms on previous occasions. The "Paxillus" syndrome is better classed as a hypersensitivity reaction than a toxicological reaction as it is caused not by a genuinely poisonous substance but by the antigen in the mushroom. The antigen is still of unknown structure but it stimulates the formation of IgG antibodies in the blood serum. In the course of subsequent meals, antigen-antibody complexes are formed; these complexes attach to the surface of blood cells and eventually lead to their breakdown.

Poisoning symptoms are rapid in onset, consisting initially of vomiting, diarrhea, abdominal pain, and associated decreased blood volume. Shortly after these initial symptoms appear, hemolysis develops, resulting in reduced urine output, hemoglobin in the urine or outright absence of urine formation, and anemia. Medical laboratory tests consist of testing for the presence of increasing bilirubin and free hemoglobin, and falling haptoglobins. Hemolysis may lead to numerous complications including acute renal failure, shock, acute respiratory failure, and disseminated intravascular coagulation. These complications can cause significant morbidity with fatalities having been reported.

There is no antidote for poisoning, only supportive treatment consisting of monitoring complete blood count, renal function, blood pressure, and fluid and electrolyte balance and correcting abnormalities. The use of corticosteroids may be a useful adjunct in treatment, as they protect blood cells against hemolysis, thereby reducing complications. Plasmapheresis reduces the circulating immune complexes in the blood which cause the hemolysis, and may be beneficial in improving the outcome. Additionally, hemodialysis can be used for patients with compromised renal function or renal failure.

"Paxillus involutus" also contains agents which appear to damage chromosomes; it is unclear whether these have carcinogenic or mutagenic potential. Two compounds that have been identified are the phenols involutone and involutin; the latter is responsible for the brownish discolouration upon bruising.