Why plants destroy a key immune protein

Plants control the activity of the protein that coordinates their immune system by regulating its destruction.
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Schematic showing a diseased (left) and healthy plant (right) that express dramatically different genes. Image credit: Michael Skelly and Steven Spoel (CC BY 4.0)

Plant diseases cause devastating crop losses around the world and threaten the food supply of millions of people. Over time, plants have developed various mechanisms for fighting off infections caused by pests and other pathogens such as viruses and bacteria. When plants become infected they kick their immune system into action by rapidly switching on and off certain genes. They do this by activating the protein NPR1 which regulates the plant’s immune genes. NPR1 is essential for fighting off infections and plants that do not have this protein are highly susceptible to disease.

Peculiarly, once the plant has detected an infection it builds resistance by destroying the NPR1 protein. A previous study suggested that plants do this to replace old ‘inactive’ NPR1 with newer versions that can activate the genes needed to stop the disease developing. This process, however, requires a lot of energy that could be re-directed to other aspects of the immune response. Now, Skelly et al. – including one of the researchers involved in the previous study – have explored whether there may be other reasons for why plants destroy the NPR1 protein.

Plant cells target NPR1 for destruction by repeatedly tagging it with molecules called ubiquitin to form ubiquitin chains. The length of these chains determines whether a protein is stable and ready for action, or whether it is ready to be destroyed. In experiments with a commonly studied plant known as Arabidopsis thaliana, Skelly et al. found that the length of ubiquitin chains attached to the NPR1 protein could fine-tune its level of activity: short ubiquitin chains activate NPR1, while longer chains lead to its destruction and shut down the protein. This suggests that the steps leading to the destruction of NPR1 regulate the immune genes needed to fight off disease.

This work has uncovered important new components of how plants defend themselves from infection. If these findings translate to crop plants they could inform future agricultural strategies for enhancing the plant’s own defences to increase crop yields, which would provide more food for a rapidly growing population.