How do plants act fast to fight off infections?

Close-up of stemrust on wheat, which is caused by a fungal pathogen. Credit: Yue Jin/USDA
Kangmei Zhao of Carnegie and Sue Rhee have discovered a new way plants can activate their defenses against bacterial infection. This knowledge could be used to increase crop yields and fight global hunger.

Rhee stated, "Understanding the plant's response to stress environments is crucial for developing strategies to safeguard important food crops and biofuel crops from climate change."

In eLife, Zhao and Rhee published new research along with Benjamin Jin from Carnegie and Christina Smolke, both of Stanford University. They investigated how the genetic level activates production of a plant defence compound called camalexin.

Zhao explained that plants can't escape predators and pathogens because they grow in one place. They have evolved compounds to help them repel invaders.

Camalexin is made by enzymes, a group of specialized worker proteins that perform many functional functions in the cell. The genes responsible for these enzymes are activated when the plant is subject to environmental stress. Researchers set out to discover how plants can respond quickly to threats and external conditions.

The cell's genetic material contains the instructions for making the camalexin-producing enzymes as well as all the proteins the cell might need to perform its functions in a variety conditions. It is quite a bit of information. This is why it is important to organize the genetic code within the cell.

Rhee stated, "Imagine that a cell's genome can be described as a huge library and each gene as a book. Each chromosome is a large shelf." "The cell has many mechanisms to quickly find the gene it requires in this huge array of information. This allows it to be translated and transcribed to make the encoded proteins and respond to threats and stress.

These strategies include adding or subtracting tags or marks to the packaging of all genes and associated materialcollectively known as chromatin. This can either enhance or inhibit the expression of specific genes. Bivalent chromatin is a phenomenon where both activating and suppressing elements can be present at the same time.

Zhao, Rhee, and their colleagues were able to elucidate the existence of a never-before-characterized type of bivalent chromatinthey termed it a kairostat, from the Greek "kairos," meaning at the right moment, and "stat," meaning devicewhich keeps the biosynthesis pathway for camalexin inactive until there is a pathogen signal. Both elements are required to regulate the plant's response when exposed to stress.

Camalexin and other defensive compounds can be very costly and toxic to produce. Zhao stated that it is not beneficial for plants to produce them all the time. Plant scientists have known for years that defense compounds are produced just at the right time to protect plants from pathogens and pests. Now we have a better understanding of the molecular mechanism that allows us to precisely time camalexin production. This discovery could help in strategies to combat climate change and global hunger or the synthesis plant-derived medicines.

The group plans to study all proteins involved in the establishment and removal of epigenetic marks in order to identify more kairostats, and better understand their roles in other functions such as environmental responses.

Kangmei Zhao and colleagues, A novel bivalent DNA associates with rapid camalexin biosynthesis gene induction in response to an Arabidopsis pathogen signal, eLife (2021). Information from the Journal: eLife Kangmei Zhao et. al., A novel bivalent chromatin associated with rapid camalexin biosynthesis gene induction in response to a Pathogen Signal in Arabidopsis (2021). DOI: 10.7554/eLife.69508