The moment a plant is injured or infected, an energy molecule leaps into action and becomes a signal, triggering the plant’s immune response.
The energy molecule is adenosine 5-triphosapate (ATP). It is part of the DNA and is the energy currency of a cell. It is needed for all normal functions of cells and is normally retained exclusively within cells. But when cells are damaged by perhaps being broken open or sustaining membrane damage, ATP leaks out and becomes, essentially, the first responder. Once outside, neighbouring cells have a sensor or a receptor on their exterior surfaces to detect its presence.
Scientists learned about the receptor receiving the ATP damage signal in 2014 but what was still a mystery until now was how this signal caused the actual immune response.
At Washington State University, researchers have found the pathways that connect ATP to plant cell responses that actually protect the plant.
Researchers are excited about the finding because the pathways are a sort of blueprint for how a plant’s immune system works and that blueprint can accelerate effective breeding programs.
“The alarm goes off and then neighbouring cells recognize that alarm and turn on defence mechanisms to stop infection, for example, from spreading,” said David Gang, WSU professor in the Institute of Biological Chemistry. “They then know that something bad is nearby and that they should prepare to fight off the intruder. That can be a pathogen of bacterial or fungal origin, or it could be an insect chewing on the plant.”
The receptor responding to the ATP outside the cells is called P2K1and it binds to the molecule. It then turns on the signaling pathway within the cells, which in turn fires up the immune system. The actual immune pathway comes downstream from the receptor once it is turned on by the ATP signal.
The response system is not necessarily fast. While the initial detection may take milliseconds, it may take several seconds to several minutes for the first changes in the gene expression to happen, such as in genes involved in defence against an insect attack, bacteria, fungi or other invasion. Gang said it may take many minutes to hours for a full-scale defence to be established.
Understanding how the immune system is turned on and how the different parts of the system are regulated in relation to each other gives researchers essential information about which genes are important in that process and how they function.
“We can then use that information to, for example, pre-prime the plant so that it will respond quicker to attack,” said Gang. “Most pathogens are successful not because they completely wipe out the plant but because they attack faster than the plant can respond. If we can get the plants to respond faster, they will be able to better fight off many pathogens. Right now, we are trying to figure out how plants do respond, and the tools we have are traditional genetic traits without necessarily knowing what the actual genes are that confer those traits.”
He pointed out that there are wild species of tomato that are resistant to fusarium and other fungal species that are, however, damaging to cultivated tomato varieties. Breeders know that, based on genetic crosses, those resistance traits can be integrated into cultivated tomatoes.
“This has been done,” he said. “And some of the genes involved in that resistance have been identified. But it took many years of work — on the order of a couple of decades — to figure out what some of those genes are. It took that long because we didn’t have a very good understanding of the resistance pathways and genes. By increasing our understanding of how they interact, identification of genes involved in conferring specific resistance will be much faster because we can study the whole pathway.”
The importance of understanding these pathways and the roles genes play comes into greater focus in the face of climate change.
“To put this into a global ecological context, we know that plant defence responses do not exist in a bubble,” said Gang. “They are part of the whole plant physiology and function. They are affected by drought stress, temperature stress (too hot or too cold), flooding, nutrient availability, and so forth. If a plant is stressed from lack of water, it cannot photosynthesize properly, becomes energy starved, stunted, and can’t mount good responses to bacterial or fungal attack. We know that disease outbreaks in crop fields are much worse during drought years, confounding the whole problem of reduced yield. So, as the global environment changes, plants face combined stresses that together are much worse than any individual stress alone.”
He said that, over millions of years, plants have faced climate change many times. But those changes have occurred over slower time scales. Plants evolved, adapted, and changed their distribution.
“However, if that change occurs very rapidly, then plants have a hard time dealing with it. They are stuck where their roots hold them down. But, as desertification increases, as arable land decreases, we face issues with being able to grow crops to feed ourselves. This is a serious issue that must be prepared for, dealt with, and preferably prevented.”
The research findings led by Gang were recently published in Plant Physiology.