Tuberculina/Helicobasidium: A fungal Jekyll and Hyde

By Nathan Smith

Taxonomy can often present itself as fixed fact; a sturdy rock in the uncertain storm of science. However this is not always the case, especially in understudied groups such as fungi. For example, Tuberculina a genus of fungus that parasitizes rust fungi. Rusts, such as Coffee leaf rust, Asian soybean rust, and wheat stem rust, are plant pathogens with major economic impact and Tuberculina was seen as a potential biocontrol agent for their management.

Jekyll and Hyde, or Helicobasidium and Tuberculina?

Jekyll and Hyde, or Helicobasidium and Tuberculina?

Helicobasidium, on the other hand, is responsible for violet root rot, causing root rot, yellowing, and in extreme cases death of the host. It has a wide host range including apple, sugar beet, soybean, potato, cotton, peanuts, tea, plum, grape, and carrot. More than 24% of planted acres of sugar beet in the USA have economic damage caused by violet root rot with the losses being as high as 50%.

It appears then that Tuberculina is a genus that can be used beneficially and should be encouraged in crop fields whereas Helicobasidium should be controlled against and excluded where possible. There’s just one problem: they are the same genus.

These ostensibly separate genera actually represent different stages in the fungal life cycle. Tuberculina and Helicobasidium samples were found to have morphological and genetic similarity. Most importantly, inoculation of a host with Helicobasidium spores was capable of causing a Tuberculina infection.

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Helicobasidium on a carrot (credit Rasbak)

Tuberculina is proposed to form an amplification stage, where the fungus produces large amounts of genetically identical conidia. The fungus then enters dormancy and the Helicobasidium stage where sexual reproduction takes place, allowing the fungus to remain genetically diverse.

That Tuberculina and Helicobasidium are one and the same is strong evidence for the argument against the use of the fungus as a biocontrol. However, for some Tuberculina species, an equivalent Helicobasidium-stage could not be found. It’s possible that some Tuberculina could have completely abandoned the sexual Helicobasidium stage. If this is the case, Tuberculina may still have potential as a biocontrol agent, although this would require extreme caution.

Fungi are critically under-studied as a kingdom and basic research into their various life-cycles is much needed if we are to effectively control fungal diseases and manipulate fungi for our own benefit. The Jekyll and Hyde characteristics of Tuberculina/Helicobasidium show this clearly and, without fundamental fungal research, we could all too easily still be supporting the traitor in our midst.

See the original paper on Tuberculina and Helicobasidium here.

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Prototaxites: Fungal Obelisks or World’s Oldest Carpet?

By Nathan Smith

A long time ago in a galaxy far far aw…..well actually in our galaxy, indeed on our planet, there existed tree-like organisms up to 8.8m tall (that’s approximately 4.5 doors tall or, for a more enjoyable measurement, roughly the size of two T-Rexs standing one on top of the other with the one on top wearing a party hat) and 1.36m wide. Found between 420 and 370 million years ago, its internal structure consisted of tiny intertwining tubes less than 50 micrometres in diameter.

Prototaxites_Dawson1888Prototaxites existed well before the developments of trees; its surrounding environment consisted of mats of moss and liverworts, populated by giant invertebrates, such as an ancient form of scorpion that could reach up to a metre long. The historical views of these structures are that they are the fossilised remains of huge external fungal structures; organic statues standing defiant in an otherwise flat landscape.  There are also suggestions that these giant structures may have had algal symbionts, and therefore should be classified as a lichen. Amongst other things, this suggestion would give good reason for why such structures became ‘extinct’ —this being that they were outcompeted by the emergence of vascular land plants in terms of being able to access the light they required.

But maybe Prototaxites aren’t fungi. Maybe they aren’t even a unique organism. There is an alternative theory; one that suggests that these giant structures weren’t signs of fungal domination but rather the results of an epic battle between nature and itself. Specifically, it suggests that the giant structures are the result of mats of liverworts being pulled up from the ground and rolled up by means of wind, water, or gravity, with algae and fungi possibly being caught up in the mix. Its evidence for this suggestion comes from the similarity in microstructure between fossilised Prototaxites and artificially created liverwort rolls, the paper being published in the American Journal of Botany in 2009.

Whether tree-like fungi or sections of liverwort torn out of the ground, Prototaxites remain a fossilised oddity of a time long since forgotten.

Three in the bed make for a hot relationship

By Nathan Smith

A plant, a fungus, and a virus live together in an environment inhospitable to each partner on their own. This isn’t an absurdist sitcom that’s been written whilst high in the garden, but a genuine biological phenomenon.

The plant, a type of grass known as Panic Grass (Dichanthelium lanuginosum), can grow at temperatures of up to 65C (for a point of comparison, the lethal temperature for humans is about 40C). It is found growing in Yellowstone National Park but only when it has a fungal symbiont Curvularia protuberata and this in turn is ‘infected’ with Curvularia thermal tolerance virus (CThTV).

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Too close for comfort? Not for Panic Grass, found near geysers in Yellowstone National Park in the United States

The ecological love-triangle was shown to be necessary for the plant-partner’s survival. This was done by infecting the plant with a ‘cured’ fungus (one that lacks the virus) and comparing it to a plant with both fungus and virus and a plant with no symbionts. The plants were then treated to growing conditions of 65C for 10hrs and 37C for 14hrs. At the end of the treatment, only plants with both partners remained healthy. Furthermore, all plants with both symbionts remained alive whereas the majority of the plants with only the fungus or nothing at all died before the experiment was completed.

However, panic grass isn’t that important or useful to us. It’s not eaten, nor is it cultured to produce fibre or biofuels. So is there any point to this knowledge? Well, the tri-kingdom system can be translated into more economically important crops. It has been shown that the fungal symbiont can colonise the roots of tomato plants and provide protection against higher temperatures, though not to the same extent as is provided to panic grass. It also suggests that the adaption to this system is widespread in nature, as panic grass and tomatoes diverged relatively early in the evolution of plants.

This certainly is hot stuff!

You’ve got the wrong (fun)guy!

by Nathan Smith

If you were presented with a plant and a fungus and asked to pick the parasite, chances are you’d pick the fungus. Whilst this is often the case, there are significant and widespread cases of the relationship being the other way around. Enter Orchids.

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Common Spotted Orchid

Orchids are family of plants distinct in physical appearance and are renowned for their sweet scent and aesthetic beauty, despite the fact they more closely related to rice than they are to roses. Their seeds contain rather small reserves of nutrients and they are unable to photosynthesise immediately after germination, instead going through an achlorophyllic stage; in fact some orchids are not capable of photosynthesis during their lifespan. Usually small reserves and an initial inability to photosynthesise would be considered a bad strategy for a plant, but Orchids are still thriving and there is a good reason why.

Throughout their non-photosynthesising stage, and indeed throughout their entire existence, they are the dominant partners in what can best be described as an uneven symbiotic relationship with a fungal partner.  In fact, a fungal partner is required by the orchid for them to germinate ‘in the wild’. Orchids can be germinated in sterile conditions; however this requires exposure to the ‘fungal sugar’ trehalose.  So what is the trade between the orchid and the fungus?  The fungus supplies the plant with organic carbon, a source of nitrogen, phosphorous, and other minerals and nutrients, and in return, gets… well, not much really. This uneven relationship continues once the plant gains the ability to photosynthesise and there is little evidence that the fungus gains a significant amount of reduced carbon from its photosynthetic symbiont. The fact that the fungus enters into a symbiosis with the plant in the first place, and continues this relationship throughout the plant’s life, suggests the fungus gains something from the relationship or that the plant emits a strong attractant, however there is little to no evidence for this and so these hypothesises remain little more than speculation.

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Tway-blade Orchid

What about the orchids that never photosynthesise? These plants, for instance the Bird’s-nest Orchid, have a habit of forming symbioses with fungi that also associate with tree roots. This allows them to use the fungus like a straw and indirectly parasitise what they need from the unsuspecting trees. Clever stuff.

Orchids are beautiful and interesting plants and deserve to be admired, but it doesn’t mean it’s the good guy. Next time spare a thought for the poor little fungus.

Photography by Leanne Massie