Gum Tree Guardians's Journal

As myrtle rust March kicks off, you may have noticed some extra media attention recently with ALA kicking off the myrtle rust citizen science campaign. Thanks to the campaign, we have some new members joining our myrtle rust monitoring community. I want to welcome all the new members of Gum Tree Guardians project, and hope you find the journal posts insightful.

In the previous journal post, I introduced the initiation of the infection process when myrtle rust meets a Myrtaceae plant. We discussed the process from spore germination, to infiltration of the plant material, and how differences in the plant surfaces can influence this process. But what happens underneath the surface of the plant? How do plants defend themselves once the fungus is inside the plant? The answer lies in the plant's immune system.

When we think about immunity, we often think about our own immune system. Animals have two types of immune systems: innate and adaptive. The innate immune system can be thought of as the surveillance system, monitoring for signs and patterns of invasion. The adaptive immune system on the other hand "learns" from previous infection to respond to the same pathogen in the future.

It might be surprising to learn that it's not just animals that have a sophisticated immune system. However, unlike animals, plants lack an adaptive immune system and instead rely only on their innate immune system to fight off the great number of pests and pathogens they are exposed to throughout their lives. The plant innate immune system can be divided into two branches, PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI).

Fungal pathogens such as myrtle rust have hardy cell walls which provide support and protection to the fungus. This cell wall can also alert the plant to the presence of the pathogen. During PTI, receptors on the surface of plant cells recognise and bind molecules common to many different pathogens including the components of fungal cell wall. Recognition and binding of these molecules triggers a series of defence responses from the plant to stop infection.

Some pathogens have evolved to evade this line of defence by generating small molecules that interfere with the plant's defence mechanisms. These molecules are called effectors and they target different lines of the plants defence systems allowing the pathogen to go undetected in susceptible plants. Plants have developed mechanisms to fight back with some plants evolving to recognise the presence of these effectors. During ETI, receptors within the plant cells detect the presence of these effectors and trigger a response called the hypersensitive response. This incredible response involves the plant killing off the myrtle rust infected plant cells. As myrtle rust needs a living plant host to survive, this process takes away the nutrient source myrtle rust feeds off which stops it from completing it's life cycle.

The figure below sums up our previous couple of Journal posts is a great diagram!

This process helps us to understand why we may observe a plant very heavily infected with myrtle rust, while the plant next to it shows no signs of infection. When you observe a heavily infected plant in the bush or your home garden, that plant lacks the immune system to be able to fight off the infection.

Scientists are working to understand how the the plant immune system differs between resistant and susceptible Myrtaceae species to work towards determining which species may be most at risk from myrtle rust. It can provide an opportunity in the future to safeguard some of our vulnerable Myrtaceae species through breeding efforts. Work is also underway to identify the effectors which myrtle rust releases into the plant to better understand how the fungus is able to go undetected by the plant immune system.

I hope you've enjoyed reading some of the science of myrtle rust infection and immunity! As always, throw your questions or comments into the comments section below. Please let me know if there are any other myrtle rust topics you'd like a write up on! And as always, thank you for your contributions to the project.

Alyssa