Making Eden by David Beerling

Author:David Beerling [Beerling, David]
Language: eng
Format: epub, pdf
ISBN: 9780192519221
Publisher: OUP Oxford
Published: 2019-01-25T00:00:00+00:00

Figure 20 Haplomitrium gibbsiae (upper picture, between arrows) and Treubia pygmaeaone (lower picture), two of the most primitive liverworts on Earth.

Back in the UK, and cultured under environmental conditions mimicking those in New Zealand, our collected specimens of Haplomitrium and Treubia thrived, making it possible for us to conduct experiments designed to answer key questions about the nature of their alliance with pea truffle fungi.34 We fed the plants radioactive carbon dioxide that was converted subsequently into radioactively labelled organic compounds via photosynthesis. This meant we could determine whether the plants delivered these sugars to the fungi. Fungi were, in turn, supplied with specific sources of phosphorus and organic nitrogen, each labelled with radioactive forms to allow traceability. From this work, we discovered the fungi were adept at capturing and transferring nutrients from soil to their host liverworts. When we conducted the experiments under the simulated carbon dioxide-rich atmosphere of the Palaeozoic, the fungi gained greater supplies of carbon from their hosts. Could this be a clue to explaining why they switched from a saprotrophic lifestyle, involving feeding on dead tissue, to a mutualistic relationship with living plants? By forming associations with plants, they could obtain abundant and reliable supplies of photosynthate in return for assisting the uptake by the plant of growth-limiting nutrients. Complicated experiments like these ultimately provided evidence that Haplomitriopsida liverworts and pea truffle fungi are engaged in a mutually beneficial symbiosis.

Still, mycologists are a difficult bunch to satisfy and demand a high burden of proof if you want to demonstrate that the two groups of organisms function in a symbiosis. For them, a prerequisite to establishing the functional basis of any relationship between a fungus (and indeed any microorganism) and its host plant is that the putative partners be grown separately, then brought together to determine whether the naturally occurring symptoms and structures of the association are reproduced. This procedure is one of Koch’s Postulates, after the German physician Robert Koch (1843–1910), who first proposed what has become central to the diagnosis of pathogenesis and the aetiology of disease. Working with specimens collected from New Zealand, we successfully cultured Haplomitrium and Treubia and the pea truffle fungi separately in sterile petri dishes laced with sugary agar-gel. Without their fungal associates, liverworts failed to develop normal leafless axes and failed to produce copious mucilage, suggesting fungal partners induce this feature of their biology. Without their plant partners providing them with a supply of photosynthate energy, the fungi grow miserably (if at all) relying on their saprotrophic capabilities to obtain energy. Re-introducing them to each other induces substantial anatomical changes in the host plants. Liverworts in combination with their fungal partners reproduced the typical symptoms of colonization seen in the specimens collected from field sites on South Island.35

How might these findings from laboratory experiments fit in with remote events back in the Ordovician? A curious feature of the biology of pea truffle fungi is that some of them can live a dual lifestyle, functioning either as free-living saprotrophs or as symbionts teamed up with plants.

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