The team, led by a plant pathologist at the University of California, found a regulatory, genetic mechanism in plants that could help fight bacterial infection.
"With a better understanding of this molecular mechanism of regulation, we can change or treat plants in order to stimulate their immune response to bacterial pathogens," said Hailing Jin, a professor of microbiology and plant pathology.
Work on Arabidopsis thalianaA small flowering plant, which is often used by biologists as a model species, the Jin Research Group found that the Argonaut protein, the main core protein in the RNA interfering machine, is controlled by a procedure called "post-translational modification" during bacterial infection.
This process controls the level of the Argonaut protein and its associated small RNA molecules that regulate biological processes by interfering with the expression of genes. This ensures dual safety when regulating machines for RNA disorders. RNA interference, or RNAi, is an important cellular mechanism that many organisms use to regulate gene expression. It includes the exclusion of genes, also known as "gene suppression".
An earlier study at Jin's Laboratory showed that one of the 10 argonavtainous proteins in Arabidopsis is induced by bacterial infection and contributes to immune resistance of the plants – the higher the level of protein, the higher the immunity of the plants. A high level of protein can however limit the growth of plants.
Under normal conditions of plant growth, the Argonaut protein and the associated small RNA are well controlled by arginine methylation – a series of post-translational modification of Argonaut's protein. It regulates the Argonaut's protein and prevents it from accumulating at high levels. Small RNAs associated with Argonaut's protein also prevent accumulation to higher levels, which allows the plant to save energy for growth.
During bacterial infection, arginine methylation of Argonaute protein is inhibited, leading to the accumulation of Argonaut protein and its associated small RNA, which contribute to the immunity of plants. These two changes together enable the plant to survive and defend itself.
"If Argonaute protein and the associated small RNA remain at such a high level, after normal conditions returned, it would be harmful for plant growth," Jin said. "But posttranslational modification of Argonaut proteins, restored under normal conditions, reduces these levels to promote plant growth."
The results of the study appear in Nature Communications.
Jin explained that all plants have an RNAi machine, as well as equivalent Argonaut proteins associated with plant immunity. Silencing RNA is visible to all mammals, plants and most eukaryotes.
"It was not clear to our study how the activity of Argonaut proteins was controlled in the attack, and how the immune response of the plants was regulated by the RNAi machine was mostly a mystery," said Jin, who has President Cy Mouradick at UCR and is a member UCR Institute of Integrative Biology of the Genome. "Our first study, which showed that posttranslational change is governed by RNAi mechanisms in response to plant immune systems."
Jin was joined by Po Hu, Hongwei Zhao, Pei Zhu, Yongsheng Xiao, Weili Miao and Yinsheng Wang.
The work was financed by the National Institute of Health, the National Science Foundation and the Agricultural Experimental Station UCR – extending cooperation.