How an engineer and a biologist could help with kauri dieback and myrtle rust

Wednesday, 12 June 2019

Novel techniques being developed by an engineer and biologist might protect New Zealand from kauri die-back and myrtle rust infection. Will Harvie reports.

The organisms responsible for kauri dieback and myrtle rust have at least one trait in common.

They grow long arms, a bit like octopus tentacles. And these arms – called hyphae – generate enormous force for their size.

They are a bit like 'pressurised drill bits', says Dr Ashley Garrill, an associate professor of biology at the University of Canterbury. 

'The hyphae use the force to burrow their way into host tissue, which they have also weakened with enzymes, and then colonise the host organism,' says Garrill.

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Those hyphae keep going, using their 'penetrative force, a protrusive force, to actually burrow their way in,' says Garrill.

It's the start of an invasive infection that can, in the case of an adult kauri tree, lead to death in as little as two years.

Bear in mind that these hyphae are much smaller than the diameter of a human hair, while kauri trees can grow more than 50 metres high, their trunk girths can be 16m, and some live for 2000 years. The infection strikes kauri roots. 

Biologists have understood that force plays a key role in pathogenic and parasitic infections for decades, Garrill says, but there's no consensus on how they regulate such power.

A few years ago, a new opportunity to understand hyphae force came from an unexpected place – the engineering department at Canterbury. 

Senior lecturer Dr Volker Nock gave a talk in the biology department about how he and colleagues measured the force applied by a 1 millimetre long nematode, an extraordinarily common worm.  

Nock and Garrill got talking and realised their combined expertise – engineering and biology – could unlock the hyphae force and perhaps blunt it.

If they can, then it's not just kauri dieback and myrtle rust infections in New Zealand that might be resolved, it could be of global importance.

Fungi and oomycetes​ (water moulds) are for the most part harmless and often beneficial. 

'There are estimated to be millions of species out there that are essential for ecosystems, for nutrient recycling,' says Garrill. Others do wonders for humans, including penicillins and other antibiotics. Humans also rely on these organisms to make bread, beer, cheese and a huge delicatessen of other foods. 

But there are also some bad ones, including Phytophthora infestans, a water mould that caused the Irish potato famine of the mid 1840s. It's thought a million Irish died and another million fled the country. 

P. infestans and its forceful hyphae turned potato tubers to mush. And that sort of threat hangs over other species.  

'It's frightening … we are in a very precarious situation,' says Garrill. 'I think we're just seeing the tip of the iceberg.'

Rice, wheat, bananas and other foods could be hit, especially where they are grown in monocultures.

Chestnuts, elms and oaks have already seen significant declines in numbers, as have many amphibians, bats and snakes.

Blame the usual suspects – globalisation, climate change, and over-reliance on broad spectrum drugs and anti-fungals.

'You name it, there's a pathogen out there that can create havoc,' says Garrill.

In Aotearoa, the fungus myrtle rust attacks pōhutukawa, mānuka, rātā, kānuka, as well as commercial species such as eucalyptus.

Kauri dieback is caused by Phytophthora agathidicida, a type of water mould.

Both, as we've seen, rely on forceful hyphae to burst through host tissue and Nock was learning to measure biological force at microscopic scale.

He earned his masters degree in microsystems engineering at a university in Freiburg, Germany, one of the pioneering centres for this branch of engineering. Microsystems makes important contributions to airbags, autopilots, smartphones and many other devices and uses.

It was recognised early on that microsystems could play a role in life sciences and Nock has pursued that idea into another emerging field called 'lab on a chip'.

In his case, the chip is not a computer chip, but the simple microscope glass slides found in science labs the world over.

On top of the glass, Nock and colleagues have learned to apply a polymer coating. Various three-dimensional channels formed by light penetrate the polymer. This is something of a technical feat but beyond this article.

Into these channels, Nock, Garrill and their students have been inputting spores of fungi and moulds that are similar to the organisms that cause kauri dieback and myrtle rust. 

The channels are fine enough that a single spore can be isolated and inputted. It is encouraged to grow, creating the all-important hyphae – that growing penetrative, forceful arm.

In time, the growing hyphae knock into a vertical tube called a pillar that's been placed in a channel. The pillar is made of polymer and its properties are very well understood, Nock says.

The force with which the hyphae hits the pillar determines how much the pillar deflects or bends. With simple calculations (to an engineer), the force applied by the hyphae can be calculated. Yes, there is plenty of force, at this microscopic level. 

But what if that force can be blunted or slowed down or somehow made less effective at hitting the pillar – or bursting through a host tissue? 

Nock and Garrill's lab on a chip allows single compounds to be added. They call this compartmentalisation and if they can perfect it, they have created a situation with a single hyphae, a single pillar, and a single compound, and nothing else. That's new and promising.

Does the compound affect the hyphae or the force with which it strikes the pillar? If the answer is no, then they can try another compound. And another. And another. 

Theoretically, they could test 100,000 compounds to find those that change the hyphae's force, Nock says. Once they isolated those that affect the hyphae, they could vary the doses or strengths of the compounds. How much is enough?

Moreover, once the technique is perfected, they could run hundreds or thousands of tests in parallel. This sort of replication is important in biology.

Or they could test compound cocktails. Or they could add an electrical field (the roots of kauri trees seem to produce electrical fields and these may help attract the water mould that cause dieback).

Landcare Research and Plant and Food, the Crown Research Institutes heavily involved in the effort to understand and resolve myrtle rust, already have some compounds in mind and are searching for others, Nock says. 

He and Garrill are pushing their lab-on-a-chip research forward in the hopes of helping out. 

There are, of course, complications to be resolved. The pathogens aren't simply creating force. They may be using enzymes to help break through host tissues. 

And the organisms being tested at Canterbury aren't the ones actually behind kauri dieback and myrtle rust infections. Those pathogens are contained in high-security facilities. That won't be a problem down the road, Nock says, because chip labs can be taken into those facilities. 

'The main advantage with the chips is really compartmentalisation,' says Nock. 'The fact that you can look at individual cells, which is difficult to do otherwise. And then you can do this in a high throughput. So you can test multiples of these within short periods of time.'​