Using RNAi to combat myrtle rust

The fungus Austropuccinia psidii is responsible for causing the invasive plant disease commonly known as myrtle rust, which threatens a large range of plants within the Myrtaceae (myrtle) family. In Aotearoa New Zealand, there is a range of culturally, ecologically and economically important species belonging to this family such as mānuka, pōhutukawa, rātā, and introduced species such as guava, feijoa and eucalyptus.

Rights: The University of Waikato Te Whare Wānanga o Waikato

Myrtle family species in New Zealand

A collage of trees from the myrtle family that grow in New Zealand. The bottom three are all exotics. A number of these trees have a variety of species. A full list of myrtle rust susceptible host species can be downloaded from MPI.

All images sourced from iNaturalist and released under CC BY-NC 4.0: pōhutukawa, miro; mānuka, kānuka and rātā (southern), John Barkla; swamp maire and ramarama, rewielliot; feijoa, Nicolas Maine; guava (Chilean), Frits Schouten; eucalyptus (blue gum – Eucalyptus globulus), Jon Sullivan.

The fungus targets the actively developing sections of plants by limiting their ability to regenerate new parts such as shoot tips and leaves and potentially leads to plant death if multiple infections occur. This continued infection weakens the plants’ immune systems, making them more vulnerable to other diseases, and the plant slowly becomes focused on fighting disease instead of growth.

The spread

Rust fungi, such as myrtle rust, produce multiple tiny yellow-orange rust-coloured spores that are easily dispersed by wind, insects and birds or by human activity. This has allowed the disease to travel large distances as it originates from South America.

While myrtle rust has caused harm in various countries, it has been the most severe in Australia. Myrtle rust made its way into Australia in 2010 and reached New Zealand a few years later in 2017. Since myrtle rust was first detected, it has spread rapidly throughout the country, sparking growing concerns among New Zealanders.

Rights: Public domain

Powdery eruptions of myrtle rust

Myrtle rust first appears in bright yellow, powdery eruptions on the underside of young leaves.

Myrtle rust remains a major problem as there are currently no effective methods of control. In New Zealand, it is no longer impossible to eradicate myrtle rust, so the strategy has shifted to managing the disease. Common management strategies involve a focus on the physical removal and burial of plants as well as chemical treatments, including fungicides. Fungicides reduce infection and spore production but must be used with caution as they can cause harm to the surrounding environment and may kill bees and other beneficial insects. Fungicides do not eliminate myrtle rust so they need to be used frequently to be effective.

Scientists from Australia are addressing the myrtle rust threat by developing a treatment using a novel tool – RNA interference, which is also known as gene silencing.

RNA interference

RNA interference (RNAi) is a natural process that occurs in cells where small RNA molecules ‘silence’ or decrease the activity of specific genes by preventing them from making proteins.

RNA interference is also a biotechnology tool in which double-stranded RNA sequences (dsRNA) are synthesised and then trimmed into tiny fragments called small interfering RNA. These tiny fragments match with messenger RNA, which carries the genetic instructions for making a specific protein, and turn off protein production. This silences or prevents translation (gene expression) of the gene.

Rights: Queensland Alliance for Agriculture and Food Innovation

Using an RNAi vaccine to control myrtle rust

University of Queensland scientists have created an RNA-based spray to deliver dsRNA to the leaves of myrtle plants. Lab results show positive results in slowing the spread of myrtle rust.

Diagram by Dr Anne Sawyer.

RNAi delivery

Scientists have designed an environmentally friendly spray to deliver this RNAi treatment for plants to prevent infection of myrtle rust. Specific dsRNA sequences have been constructed to match up with genes necessary for the survival of myrtle rust and are added to water in a spray bottle. When this water-based dsRNA spray is applied onto the plant, it can be left unprocessed on the leaf surface – it is only taken up by the fungus if it appears. When myrtle rust is present and takes up the dsRNA, the RNAi pathway is activated. The essential myrtle rust survival genes specifically targeted with dsRNA begin a turning down process, which reduces spore germination and infection, reducing disease symptoms in the plant. This research has been possible through the collaboration of scientists from New Zealand and Australia.

Myrtle rust is a really destructive pathogen that both countries are trying to deal with. I see the ongoing collaboration between Australia and New Zealand to fight this common enemy as one of the best examples of collaboration that I’ve ever been involved with.

Grant Smith, Plant & Food Research plant pathologist and Beyond Myrtle Rust (BMR) leader

Rights: Queensland Alliance for Agriculture and Food Innovation

Field trials in Australia

In early field trials, scientists used a spray to deliver RNAi treatment to plants to prevent infection of myrtle rust.

Photo by Megan Pope for QAAFI.

Potential tool

Early field trials have shown plants treated with the RNAi spray showed significantly less disease coverage compared to their control group treated with a water spray only. The next steps for researchers involve investigating whether RNAi can help reduce myrtle rust symptoms on plants that have already been infected. RNAi has the potential to be a valuable addition to myrtle rust management strategies especially because it is eco-friendly as it can degrade easily with low environmental impacts. RNAi has also been proposed as a tool to control invertebrate pests such as varroa mite.

Rights: New Zealand’s Biological Heritage National Science Challenge

New Zealand’s Biological Heritage National Science Challenge

The BioHeritage Challenge is one of 11 National Science Challenges funded by MBIE.