Researchers discover how a protein is regulated between a plant’s healthy state and one in which it is under threat
At the University of California, Riverside, a research team has discovered a regulatory, genetic mechanism that helps plants fight bacterial infections.
Microbial pathogens can cause detrimental or even deadly human, animal and plant diseases and can lead to severe losses in crop yields. Understanding not only the pathogen impact but the immune responses of plants has taken a big step forward with the discovery of how a protein is regulated between a plant’s healthy state and one in which a plant is under threat from infection.
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Using Arabidopsis thaliana, or thale cress, a small flowering plant of the mustard family widely used in plant biology, the team at UCR led by Hailing Jin, professor of microbiology and plant pathology, found that a major core protein called the Argonaute (AGO) protein is controlled by a process called “post-translational modification” when the plant has a bacterial infection.
Normally, when the plant is healthy, the Argonaute protein is regulated, or turned down. It is induced into activity by a bacterial infection when it needs to contribute to the plant’s immunity. The higher the level of the protein, the higher the level of immunity. But the downside of this is that a high level of protein can limit a plant’s growth.
“During normal growing conditions, plants need to save all their energy for growth,” said Jin. “They don’t want to waste any energy on immunity when it’s not needed. Otherwise it will have autoimmune responses like humans. Your immune responses are always turned off. You only want them to turn on when you are attacked by pathogens. So that is why you need to keep the immunity under control during normal growth. That is why this mechanism (in the plant) is provided to control immunity response.”
The Argonaute protein and its associated small RNAs are controlled by the mechanism called post-translational modification, also known as arginine methylation, which regulates the protein and stops it from accumulating to high levels. It is a cellular mechanism that regulates gene expression, or gene activity. Basically, it involves turning off genes, or gene silencing.
Jin said all plants possess the RNAi machinery, as well as an equivalent plant-immunity related Argonaute protein, and gene silencing is seen in mammals and all plants. Until Jin’s study, it was unclear exactly how the protein was controlled during a pathogen attack and how the plant’s immunity was regulated by the RNAi mechanism.
This study was the first to show that arginine methylation regulated the plant’s immune responses so that, when not needed and when the plant was not under attack, all energy could go into growth. But, when the plant became infected, the arginine methylation was suppressed, allowing the protein and its associated small RNAs to accumulate and contribute to the effectiveness of the plant’s immunity. Those two functions, suppression and accumulation, kept the plant alive and able to defend itself.
“The two regulations for the plant provide double security with not just the protein but the protein associated with RNAs to ensure they are under control,” said Jin.
According to Jin’s report, among the 10 Arabidopsis AGOs, one called AGO2 is highly induced by bacterial infection and positively regulates a defence response, especially against virulent strains of the bacterium Pseudomonas syringae.
Climate change is expected to bring greater stresses to plants and greater opportunities for pathogens. With a clearer understanding of the molecular methods of regulation, Jin said that a variety of crops’ immune response mechanisms can be modified to enhance plant immunity.
“We are continuing to work on all aspects of RNAs and RNAi machinery from the plant host perspective as well as the pathogens,” she said. “A lot of pathogens like warm, humid temperatures and a lot of plant immune responses like high temperatures.”
The study results were published in Nature Communications.
