Mushrooms on the defensive: UV enzyme clues

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  • Published: May 1, 2017
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Mushrooms on the defensive: UV enzyme clues

Shroom weapons

Ultraviolet-visible (UV/Vis) spectroscopy and various other techniques have been used to help explain how mushrooms get defensive. Credit: Wiley/Angewandte Chemie/Hoffmeister  et al

Ultraviolet-visible (UV/Vis) spectroscopy and various other techniques, including MALDI mass spectrometry and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy have been used to help explain how mushrooms get defensive.

According to work by Dirk Hoffmeister of the Friedrich-Schiller-Universität Jena, Germany, and colleagues just a single enzyme induces the chemical defence of a mushroom against larvae that would otherwise eat the fungi. Writing in the journal Angewandte Chemie, the team explains how some mushrooms produce long-chain unsaturated carboxylic acids as their chemical defence against insect larvae. These polyenes are biosynthesised by one enzyme, the team reports. They highlight the unprecedented multiple double-bond-shifting activity of the enzyme a polyketide synthase. While they now know a lot about this biological catalyst it remains a representative of a yet uncharacterized phylogenetic group of such enzymes.

Mushrooms and other fungi consume dead plant and animal matter as well as faeces and other biological waste products. As such, they fulfil a critical role in ecosystems and ultimately the carbon cycle. Of course, fungi are in turn eaten, whether that is by a diner in a fine restaurant nibbling on haute cuisine wild mushrooms or insect larvae or other animals also gorging themselves on the wild side of the forest. As has been said many times before nature is red in tooth and claw. But, if you have no teeth and no claws as is the case of most plants and fungi you have to turn to chemical weapons to avoid being eaten alive.

Fruiting bodies

For instance, when the mycelium, the vegetative part of a fungus, such as the false turkey-tail mushroom is injured by the bite of a larva, it quickly biosynthesises various polyenes that then interfere with larval pupation. The biosynthesis of these polyenes takes a unique and previously unknown route, as Hoffmeister and his team have shown.

Indeed, the basidiomycete, those mushroom that produce vegetative bodies, of which one, called BY1(a member of the Stereaceae family), is of particular interest. This species produces two different branched-chain unsaturated carboxylic acids when it is damaged. Unlike the majority of other known polyenes, the BY1 defence polyenes do not comprise the more familiar isoprene building blocks that form the molecular skeleton of common polyenes such as natural rubber. Instead, these mushroom polyenes have a polyketide body, which is based on a well-known class of secondary natural products, having an array of conjugated double bonds. The double bonds in these molecules are shifted along by one carbon atom relative to the acetate units from which the molecule is built. It was not known how biosynthesis could achieve the requisite large number of double-bond shifts in a single molecule without a sophisticated array of biosynthetic transformations. Turns out that in the BY1 mushroom, all those shifts are coordinated by a single enzymic reaction. The polyketides synthases are well known for their ability to generate a vast molecular diversity in natural products across the plant and fungi realms. Nevertheless, the mechanism observed in this system is rather unusual. Hoffmeister and his colleagues argue that it was "the first observation of injury-induced PKS gene expression and the unprecedented shift of multiple double bonds, catalysed by a single PKS." This single enzyme is essentially the control system for the mushroom's arsenal of chemical weapons.

Genetic proof

To prove that the enzyme belongs to a previously uncharacterised clade, the team also reconstituted the BY1 PKS gene into a model fungus Aspergillus niger. They found that this common black mould was then able to biosynthesise both of the BY1 polyene carboxylic acids as if it were the native mushroom. The team suggests that this chemical defence strategy based on unusual polyenes may be widespread in mushrooms.

Related Links

Angew Chem Int Edn 2017, online: "Induced Chemical Defense of a Mushroom by a Double-Bond-Shifting Polyene Synthase"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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