Plant-fungi interactions

Kew publishes its State of the World’s Fungi report

Kew’s report highlights how ignorant we are of an astonishingly successful Kingdom of life.

State of the World's FungiKew’s State of the World’s Fungi has been published. The aim of the report is to establish a baseline for fungal conservation and research. The report states there are 144,000 species of fungi known, with 2000 being added to the list each year. However, that baseline is rather fuzzy. They also estimate there are between 2.2 and 3.8 million fungi out there. That means we know of about 5% of the species at the moment. And at that rate it’ll take about another thousand years to find them all.

The reason biologists think there are so many fungi is down to advances in DNA. Putting together sequences of DNA and comparing them, it’s possible to see how closely related fungi are, and how diverse fungal species are. This kind of analysis is revealing that very similar looking fungi, for example, pathogens like rusts, are actually surprisingly diverse. They are also hints of fungi with distinctive chemical signatures that we haven’t identified yet – which are called dark taxa.

These new techniques are controversial. Kew reports: “The challenge for the future will be to reach consensus across the community of scientists who work on fungi on how these exciting new discoveries of fungal diversity, based purely on DNA sequence data, are incorporated into existing fungal classification systems. Only then will it be possible to reach a truly comprehensive understanding of the full extent of global fungal diversity.”

It might seem odd that Kew, a botanic garden, is studying fungi – which are definitely not plants. They differ in a few important ways.

One of the differences for fungi is how they eat. Plants make their food internally (usually). Fungi exude enzymes externally that digest what they’re in contact with and then reabsorb the goo back into their cells. The cells walls are not made of cellulose, but chitin, the same material that insects use to make their shells. They also store food as glycogen and lipids, instead of starch. So in the way they eat, they way their cells work and the way they store energy they are very different from plants.

The Kew report notes that this different biology means they play a very important role in the environment. The report states: “Fungi are also the most significant organisms that break down cellulose, hemicellulose and lignin. These are the tough polymers in plant cell walls that give wood its great strength and durability. Their decomposition by wood-decaying fungi releases key plant nutrients back into the soil, thereby allowing the next generation of seedlings to grow. Without nutrient cycling, life on Earth as we know it would not exist; nutrients would be in such short supply that biological growth would be severely limited right across the globe.”

Plant-fungi interactions
Plant-fungi interactions. Source: Kew State of the World’s Fungi Report 2018.

Despite fungi being Not Plants, plant scientists still have a keen interest in them. The report highlights the positive relationships between fungi and plants, with a large proportion of plants thought to live in symbiosis with fungi of one sort or another.

Endophytes are fungi that live between cells in a plant, in the shoots or roots. Kew reports: “[E]ndophytic fungi such as Trichoderma (Ascomycota), used as a seed treatment in agriculture, can induce plant resistance to diseases, water deficits, salinity and also heat stress. They do this by altering the expression of the genes involved in root growth, nutrient uptake or protection against oxidative damage.”

The other popular fungal organisms are Mycorrhizas. Kew says: “It is estimated that around 90% of living plant species have mycorrhizal fungi associated with their roots. In contrast, less than 2% of fungal species enter into mycorrhizal partnerships. Through becoming specialised to co-exist with plants, the fungi involved rely on their plant hosts
for their supply of carbon, having lost the ability of their ancestors to decompose dead organic matter . In return for their photosynthetic carbon, the plants receive water and mineral nutrients from the soil via the fungi. Most mycorrhizal fungi are dependent on their hosts for survival, just as many plants are dependent on their fungal partners.”

Kew also describes some of the fungi as fungal ‘bodyguards’ for plants, changing how plant genes react and helping boost defences against herbivores. Applying fungi to seeds gives the potential for crops to be planted with their own guardians. The report states even treatments as low as 500mg per hectare can have an effect.

In light of this, conservation is important and that is another hole in our knowledge. Kew notes that only 56 species of fungi have had their conservation status globally evaluated. For comparison, the same can be said for 25,452 plants and 68,054 animals.

There are clear problems in creating conservation plans. Because fungi are largely microscopic, there’s a lot of gaps in our understanding of where on the planet they are. They can also change form, or look like other fungi. Kew says that: “Despite the eye- catching and prolific displays of spore-bearing structures (e.g. mushrooms) produced by some species, they are generally regarded as difficult to detect and count because, when not sporulating, most are composed of nothing more substantial than a wispy network of mycelium.”

This lack of knowledge has an impact on discussion in conservation. The report states: “It was noted from a review of papers published in the top twelve mainstream conservation journals that only around 3% discussed fungi.” and Kew estimates that at least 10% of European macrofungi, the fungi that produce bodies big enough to see, are threatened with extinction.

Given the peculiar chemistries of fungi, it would seem to be a bad idea to lose many of them before they could be studied. This report and the associated conference will, over the next few days, give some impetus to studying this neglected Kingdom.

Alun Salt

Alun (he/him) is the Producer for Botany One. It's his job to keep the server running. He's not a botanist, but started running into them on a regular basis while working on writing modules for an Interdisciplinary Science course and, later, helping teach mathematics to Biologists. His degrees are in archaeology and ancient history.

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