Pisolithus arhizus which is also referred to as Pisolithus tinctorius in older texts, is not the most aesthetically pleasing fruiting body in the fungal kingdom. Let’s face it, it’s commonly referred to as the dog turd fungus. Though, let me assure you that this species has some quite incredible traits worth being appreciated by the masses. After yesterday’s blog post revealing the different hyphal formations riddled across the forest floor, I became most interested in structures I knew least about; sclerotia. After some research, I came across a family of fungus aptly named the Sclerodermataceae. Here, I found the genus Pisolithus and came across a species that is as hardy as it is helpful. Pisolithus arhizus really is amazing. Here’s why.
P. arhizus is a strong ectomycorrhizal species that forms symbiotic relationships with a wide array of plants species. For these reasons, it is widely used as a mycorrhizal inoculant for species rich restoration efforts. Although it can be found forming symbiotic relationships all across the Northern Hemisphere, it has been proposed that the center of evolution for the genus is Australia. This argument is strengthened based on this species prominence with more dry habitats, and strong associations with Eucalyptus species. Like its family’s name suggests, this species form survival structures called sclerotia, ultimately adding to its ability to survive in habitats not normally suited for water loving fungi.
Not only does this species survive and thrive in arid habitats, but exists in areas that would be toxic to most other fungi. Like I said earlier, P. arhizus is a hardy species, and can withstand high levels of heavy metals without losing the capability so carry out its life cycle. Even more interestingly, when this species is in a mycorrhizal union with another plant within a polluted area, it helps increase the plant growth several-fold, compared to a plant without P. arhizus living around its roots. Smith and Read in 1997 showed that these fungi actually filter the heavy metals that would otherwise enter the plant in polluted areas.
Like most higher fungi, the spores of P. arhizus are haploid. If its haploid spore lands at the base of a compatible tree, and suitable environmental conditions are present, the spore will germinate into a monokaryotic hyphae. The hyphae which may develop into a more branched network of mycelium, will be attracted by plant exudates, and grow towards plant roots. Monokaryons have the ability to pair with the plant and exchange nutrients for plant sugars, but this mutualism doesn’t hit the ground running. Nutrients and plant sugars really start to flow when another sexually compatible monokaryotic hyphal arrangement genetically combines with the initial colonizer. They soon genetically combine, and form a more voracious, effective, nutrient scavenging network of dikaryotic mycelium.
It is only when a dikaryotic ectomycorrhizal network forms that a sexual fruiting body may develop. With enough nutrients and plant sugars, the formation of Pisolithus arhizus fruiting bodies are triggered by some, largely unknown abiotic or biotic factors. These spherical fungi have a thin outer skin, that hold densely packed structures called peridioles. Each peridiole contain countless spore producing basidia that mature first at the uppermost layer. The gelatinous matrix within the peridioles soon dries into a spore packed powder where wind ultimately picks up these spores, filling the role of disperser.
Evolving in Australia really does equip species with extraordinary traits. Many pictures circulate online mushroom forums with P. arhizus popping up through blacktop, or showing themselves in bone dry, polluted habitats. Well, this again is a lesson that not every fungus evolved in a moist, nutrient rich environment. There most definitely are fungal bad asses out there. Pisolithus arhizus, although not the prettiest specimen, is definitely one of them.