By Georgia Barrington-Smith & Dr Rebecca Duncan
The agricultural industry is constantly under threat from fungal pathogens that infect important plant crops like tomatoes, bananas, and cotton. In response, plants have developed new defence mechanisms, fuelling an ongoing arms race against these invaders as they, in turn, develop new ways to circumvent these defences.
How the fungus fights…
During infection, the fungi secrete hundreds of proteins, called SIX effectors, into the plant’s vascular system. These effectors damage the plant’s immunity, physiology, and structural integrity. In response, plants have evolved immune receptors that recognise a subset of these SIX effectors, called Avr effectors, and then trigger an immune response.
While many fungal effectors have been studied, to date only one has had its structure fully determined: an effector from the tomato pathogen Fusarium oxysporum f. sp. lycopersici (FOL), known for causing tomato vascular wilt. Understanding how plants recognise Avr effectors is critical for developing crops resistant to fungal disease. However, understanding how these effectors work has been difficult, as they don’t resemble proteins with known functions.
Daniel’s contribution to the arms race
As the function of a protein is highly dependent on its 3D structure, Daniel Yu, an AINSE PGRA scholar, with his collaborators at ANSTO and the Australian National University, used X-ray crystallography on the MX1 and MX2 beamlines at ANSTO’s Australian Synchrotron to determine the 3D crystal structure of three significant fungal effector proteins: Avr1 (SIX4), Avr3 (SIX1), and SIX6.
While investigating the crystalline structures, the team discovered that although these proteins share less than 20% similarity in their amino acid sequences, they have a similar overall structural shape (or “fold”). These three proteins are the first known examples of fungal effectors that are made up of two distinct domains, which are functional regions of the protein. The discovery of these structures is significant because they represent a new group of fungal effectors called the FOLD (F. oxysporum f. sp. lycopersici dual domain) effector family.
Understanding these structures has provided insights into how plant immune receptors recognise fungal effectors. For example, Avr1 is recognised by the ‘I’ receptor in tomato plants, which triggers a defence response that prevents the fungus from spreading. There are two forms of the ‘I’ receptor in different tomato varieties: one that is resistant to the pathogen and one that is not. Interestingly, both receptor types can recognise a similar effector from a different strain of Fusarium oxysporum that affects watermelons.
To understand the details of how the ‘I’ receptor recognises Avr1, researchers created mixed proteins by swapping parts of Avr1 with similar proteins from the watermelon pathogen. Analysis of these mixed proteins identified one key part of the protein that was crucial for effector recognition. This insight could help design modified plants with enhanced resistance to fungal pathogens.
The ongoing fight against fungi!
Unfortunately, in the ongoing arms race between fungi and plants, certain fungal pathogens have gained the upper hand by changing their amino acids to evade recognition by a plant’s immune receptors. This means that plants with the evolved immune receptors, which had previously helped them resist pathogens, are now at risk once again from fungal infections.
Thankfully, Daniel’s new understanding of plant response to Avr effectors can give plants the upper hand in the arms race. By using gene editing approaches, scientists may be able to modify the defeated immune receptors to restore their Avr recognition and resistance. This will help in the development of disease resistant crops to proactively protect from pathogenic threats, with enormous benefits for our food supply and the agricultural industry.
AINSE are proud to spotlight Daniel Yu for his scientific contribution!
To read more research spotlights visit ainse.edu.au/research-spotlight.
And that’s a wrap on our Fungi February series, but don’t worry we will be back with some exciting research in Medical March as we uncover the marvellous mysteries in nuclear medicine.
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