Metabolite levels are low under healthy conditions, but their production is quickly turned on when the plant is attacked
To combat harmful bacteria and fungi, plants rely on an arsenal of chemicals and compounds that they pull into play at a moment’s notice.
New research from Carnegie Institution for Science in Stanford, California, has shown how production of a plant’s defence compound called camalexin is activated at the genetic level.
Kangmei Zhao, postdoctoral fellow in plant biology at the university, said defence metabolites such as camalexin are in low abundance under healthy conditions, but their production is quickly turned on when the plant is attacked.
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Zhao said that camalexin belongs to a chemical group called indole alkaloids, which are present in plants such as mustard, cabbage, and broccoli as well as the plant arabidopsis used in the study.
Camalexin inhibits bacteria and fungi by disrupting their cell membranes. The aim of the research was to find the mechanism that plants rapidly employ to produce camalexin and fight pathogens.
A cell’s genetic material encodes everything for making all the proteins the cell needs to function effectively.
“Imagine a cell’s genome is a massive library, each gene is a book, and each chromosome is an extremely large shelf,” said Zhao. “The cell has different mechanisms for quickly finding the gene it needs in this vast array of information so that it can be transcribed and translated to make the encoded protein and respond to environmental stresses and threats.”
This organization is even more critical given that camalexin is a toxic substance even to the plant itself. Consequently, plants need to protect themselves and avoid toxicity from the compound. Camalexin in the plant cell is extremely low when the plant is healthy. It increases rapidly in the face of a threat.
To do this, plants must be alert to the very first hint of danger.
Zhao wrote in the report that for the genes that rule the production of camalexin to be converted into proteins, their DNA sequences need to be physically accessible. That availability is controlled by adding or removing chemical groups, or marks.
These precise defence modifications help activate or repress the expression of a gene and are collectively called chromatin. Sometimes these activating and repressing elements are present simultaneously creating what is called a bivalent chromatin domain.
According to the news release, Zhao and her colleagues were able to discover a never-before-characterized type of bivalent chromatin that keeps the biosynthesis pathway for camalexin inactive until there is a pathogen signal.
They called it a “kairostat” from the Greek “kairos” meaning at the right moment, and “stat” meaning device. The finding indicates that both activation and repression elements are needed to control the precise timing of the plant’s response to a threat.
“We can think of kairostat as the balanced state when plants are growing in a healthy environment which is maintained by the co-existence of an activation mark and a repression mark,” she said. “Under this scenario, the biosynthesis of camalexin is paused due to the presence of the repressor. When plants are under attack, the repressor is removed and the activator becomes more abundant to reinforce the rapid activation of camalexin production.”
Zhao said that camalexin and other defence compounds are often difficult and toxic for the plants to make, so it is to the plants’ disadvantage to make them all the time.
“We now have a new handle on the molecular mechanism that enables this precise timing of camalexin production. We think that this respond-at-the-moment strategy can be generalized to other stresses.”
The research team plans to characterize all the proteins involved in establishing and removing these genetic marks to identify more kairostats and more clearly understand their role in environmental responses and plant functions.
The research was recently published in the journal eLife.
