Squeeze like a fungus

As icky as it sounds, some species of fungus can literally squeeze themselves between plant or animal cells and nestle in the space there.

A team led by Norio Takeshita of the University of Tsukuba, Japan, along with collaborators at Nagoya University, Mexico, found key features of the squeeze, showing that the fungi has to compromise between speed and malleability.

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Fast Fact: Fungus

• Fungi are neither an animal nor a plant – they are their own kingdom.
• 5.1 million fungus species have been identified.
• The mycelium of the fungus is made of non-reproductive filaments called hyphae.
• Mycelium is important for decomposing plant matter.

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Before some fungi reach maturity, they start off as a threadlike web while the fungal colony forms. These threads are called hyphae and help the fungus spread along a surface, including penetrating the gaps between cells in plants or animals. But until now, it was unclear how they did this – and why some species could while others couldn’t.

The team tested seven different fungal species by dying them with a fluorescent green dye and chanelling them through gaps that were only one micron thick (one millionth of a metre). This represents the space that hypha – which are normally 2–5 micron thick – would need to penetrate between cells.

Some fungi sped through, but this caused the cell components to bunch up at the tip – similar to what happens when you squeeze a long balloon. This would disrupt the cellular processes, meaning the hyphae would not be able to continue growing in the same direction, or would stop growing altogether.

On the other hand, the team found that some hyphae grew slowly and evenly through the gap, so that the cellular organs remained balanced. This meant that the fungus had to compromise speed in order to balance the change in shape needed to squeeze through the tiny space, the authors suggest in their paper, published in mBio.

“For the first time, we have shown that there appears to be a trade-off between cell plasticity and growth rate,” says Takeshita. “When a fast-growing hypha passes through a narrow channel, a massive number of vesicles congregate at the point of constriction, rather than passing along to the growing tip. This results in depolarized growth: the tip swells when it exits the channel and no longer extends.

“In contrast, a slower growth rate allows hyphae to maintain correct positioning of the cell polarity machinery, permitting growth to continue through the confined space.”