New high-tech spray gives hope to native trees

About ten years ago, an invasive fungal disease called myrtle rust reached Australia and began spreading like a plague to some plants. The disease affects plants in the Myrtaceae family, which includes eucalyptus, paperbark and lilly pillies, and accounts for 10% of Australia’s plant biodiversity.

In just a few years, myrtle rust has altered ecosystems, destroying trees and their canopies, wiping out entire species in some areas, and economically impacting industries that grow trees such as lemon-scented myrtle and the tea tree.

The disease is a slow-moving ecological wrecking ball: surveys suggest it could lead to the extinction of at least 16 rainforest tree species in the wild within a generation, and 20 other species are endangered.

We used RNA technology similar to COVID vaccines to create a highly targeted treatment for myrtle rust: a spray that can restore the health of even severely infected trees in about six weeks. Our study was published in Communication biology.

Species at risk in remote locations

Current approaches to controlling tree diseases are limited. We can apply fungicides as part of a scorched earth policy to kill all the fungi, or we can create plants that are resistant to the pathogen.

None of these strategies are effective against myrtle rust. There are too many species to defend, located in some of the most remote places imaginable.

For example, a tree species on the verge of extinction due to myrtle rust is called Lenwebbia sp. Main range. It grows only on the cliffs of the Nightcap Range in northern New South Wales.

Additionally, many culturally important and iconic trees in Australia and Aotearoa (New Zealand) are long-lived and cannot be seasonally exchanged for resistant genotypes.

In the absence of treatment for myrtle rust, the protective measure is to store seeds to preserve the genetics for future generations.

RNA interference

Our myrtle rust treatment uses a molecular mechanism possessed by almost all plants, animals and fungi called “RNA interference”.

RNA is an essential molecule for life, similar to DNA, that is usually in single-stranded form. When a cell detects double-stranded RNA (which in nature usually represents a virus or other threat), it triggers RNA interference to destroy the intruder.

The RNA interference system learns to recognize the threat and will then also destroy any single-stranded messenger RNA that matches. This natural mechanism can be used to defend plants and humans against pathogens, including fungi.

We designed double-stranded RNA matching the essential genes of the fungus that causes myrtle rust and sprayed it onto the leaves of infected plants. This triggered the fungus’ RNA interference mechanism, sabotaging the action of the genes it needs to survive.

Treating serious infections

Rust fungi produce microscopic ram structures called appressoria that are used to forcefully penetrate the host’s leaves. Most fungal spores treated with double-stranded RNA could not germinate to produce their rams, and those that did were withered and impotent.

Double-stranded RNA can also be sprayed on plants in advance to inhibit spore germination and completely prevent disease.

We then tested whether our RNA spray could stop and cure diseases in severely infected plants. We found that this inhibited disease progression, and after six weeks, even severely infected plants had returned to a healthy state.

Targeted treatment

We wanted to make sure our treatment wouldn’t accidentally affect anything except the myrtle rust fungus, so we designed it using “barcode genes” that uniquely identify the species.

Barcode genes are excellent targets for RNA interference. They are generally identical among all members of a species, differ between closely related species, and usually control an essential cellular function.

The rust fungus most closely related to the myrtle rust pathogen is found on a naturalized street tree in Australia called Albizzia lebbeck, but it is different enough that it is not affected by our treatment. It is extremely unlikely that an unrelated organism would have a genetic barcode sequence identical to that of the myrtle rust pathogen, so we would not expect off-target effects.

Another benefit of targeting a barcode gene is that our treatment has a lasting impact. Unlike other genes, barcode genes cannot change through mutation without risking the survival of the organism.

This means the pathogen is less likely to develop resistance. And if dsRNA resistance evolves, the target sequence can be changed to match rust again within days.

An integrated approach

There is no silver bullet for managing pathogens in native ecosystems and agriculture. The Australian Government’s Myrtle Rust Action Plan recommends an integrated approach to controlling this destructive disease.

In the coming years, double-stranded RNA may be incorporated to manage the myrtle rust epidemic in Australia. We hope this will be particularly useful in the conservation, industry and treatment of individual trees, particularly those of cultural significance.

This article is republished from The Conversation under a Creative Commons license. Read the original article.