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Fungus-farming termites have enormous enzyme potential

Gepubliceerd op
3 oktober 2014

DNA research into fungus-farming termites, their gut bacteria and the fungi that the termites cultivate show that collaboration between these organisms involves an enormous range of enzymes. The fungus farmers are hugely successful in decomposing plant biomass. This success can be explained by the division of labour between the farmed fungus and the gut bacteria. Mostly, the fungi decompose complex components, whereas the gut bacteria deal with simple components. An international research team, including researchers from Wageningen UR, has published these results in the scientific journal PNAS.

Fungus-farming termites play an important role in the carbon cycle

Fungus-farming termites are the principal decomposers of plant material in tropical and subtropical Africa and south-east Asia. In some savannah ecosystems, they are responsible for breaking down up to 90% of dead plant material. They manage to achieve an almost total breakdown of the material because they use both the Termitomyces fungus and their own gut bacteria. Duur Aanen, a researcher at Wageningen UR: “We discovered this interaction by analysing the complete genome sequences of a termite, its symbiotic fungus and its gut bacteria, all from one particular colony. This enormous amount of DNA information enabled us to very precisely determine which enzymes can be made  by the termites, fungi and bacteria.”

Termites are ruminants

Termite fungus-farming necessitates a complex interaction between older and younger workers (see figure 2). Older workers collect plant material and bring it to the nest. Young workers consume the material which then, in their guts, is mixed with fungal spores. The mixture forms a fresh layer in the underground cultivation beds when the workers deposit it there (Figure 3). Once the spores germinate, the Termitomyces fungus grows rapidly through the plant material, digesting it as it does so. Older workers then consume the half-digested plant material. Subsequently in their guts, bacteria make use of the parts that have not yet been digested by the fungus. By then, practically all the organic material has been digested.
Termites are ruminants: they chew their food the first time as young workers, consuming raw plant material, and the second time as old workers, eating up digested plant material. The young workers also eat fungal balls which contain proteins the workers themselves can exploit. From the fungal balls, they extract spores as well, using them for constructing new cultivation beds.

Large numbers of enzyme genes

The genome sequences show that the Termitomyces fungus has practically all the available enzyme genes needed to decompose the large, complex molecules of which plant cells are composed. The bacteria, by contrast, possess enzyme genes suitable for decomposing simple organic molecules. In total, at least 86% of all known families of hydrocarbon-decomposing enzymes are represented in the symbiosis between termites, fungi and gut bacteria.

Further research

Although the genome sequences are now known, a number of questions still need to be answered. One such important question is: when exactly are the various enzymes produced? Thanks to the available genome sequence, expression studies will make it possible to look in more detail at the division of labour between fungi and bacteria when decomposing plant material.

Extraordinary colonies

Termite colonies are set up when, once a season, a queen and king fly out of the existing colony along with thousands of others. As soon as an individual has found a partner, both shed their wings and shut themselves up in what is called a 'royal cell' (figure 1). The pair starts to lay eggs from which workers hatch out. As the colony grows, the rear end of the queen swells up to enormous proportions and she becomes a sort of egg-laying machine. The royal pair may live for several decades and, during that time, maintain a colony comprising hundreds of thousands of short-lived workers and soldiers that look after the fungus garden. The royal pair are mainly fed with fungal material. The researchers discovered in fact that the gut of the queen contains hardly any plant-decomposing bacteria.