How plants stop growing to survive stress

UC Riverside researchers have identified a mechanism that allows plants to rapidly slow growth in response to extreme environmental stress. The finding could help farmers grow more resilient crops, and one researcher continued the work years into retirement to uncover it.

The rapid response system is based on a process inside plant cells that produces compounds needed for growth, development, and survival. If even one of the key enzymes in this process fails, the plant cannot live.

Under stress conditions such as intense light, this biological pathway behaves in an unexpected manner. Rather than being governed by changes in gene expression, a standard mechanism in biology, it is modulated instantly through direct alterations in enzyme activity.

In most living things, cells adjust their RNA levels to alter protein production, which then changes the balance of other important molecules. But this process takes time that plants may not have when faced with sudden light or heat stress.

In plants, the response is much faster. Stress directly alters the activity of enzymes already present in the cell, allowing leaves to respond immediately without waiting for new proteins to be made."This kind of response has to be immediate," said Katie Dehesh, UCR distinguished professor of molecular biochemistry. "Changing gene expression takes time, but modifying enzyme activity allows the plant to react right away and survive."

Reactive oxygen molecules interfere with the enzymes, reducing their activity and slowing the pathway. At the same time, new compounds build up, blocking earlier steps in the process and preventing some enzymes from working efficiently.

The immediate effect is protective. By limiting the pathway's output, the plant reduces production of growth-related compounds, effectively pausing development while it copes with stress.

Over time, a second phase begins as the plant adjusts its internal machinery to prolonged stress. These longer-term changes help the plant adapt, but often at a cost, redirecting resources away from growth and resulting in smaller or slower development.

There have been many efforts to engineer plants to increase crop yields and drought tolerance as well as produce valuable molecules like carotenoids, which protect against damage. However, these engineering efforts often fail because they did not account for the two-stage response identified by the Dehesh laboratory and described in the Proceedings of the National Academy of Sciences.

The breakthrough was the result of painstaking work led by Mien van de Ven, a former lab manager and research supervisor who continued contributing to the project even after retiring. She systematically measured intermediate compounds at each step of the pathway, even though they are present in extremely small amounts.

"There were both conceptual and experimental challenges," Dehesh said. "The metabolites are at very low levels, and even identifying them required careful, step-by-step work."

The team's progress began with an unexpected clue. A mutation in one enzyme caused plants to grow smaller without dying. Following this lead, the researchers analyzed each step of the pathway and discovered that one downstream compound accumulated at unusually high levels.

They eventually determined why. The compound binds to an upstream enzyme, blocking it and slowing the entire pathway.

Proving this interaction was technically difficult. The team had to isolate delicate enzymes and recreate the right conditions for them to function outside the plant. Even then, the work was challenging. Proteins can become unstable outside their natural environment, and excess materials can interfere with measurements.

"It took a lot of time to get all the components working together under the right conditions," van de Ven said.

The work culminated in a clearer picture of how plants balance survival and growth under stress. Because similar pathways exist in bacteria, the findings may reflect a broader strategy used by living organisms to respond to environmental change.

The research also has practical applications. Enhancing this natural pathway could help scientists develop crops that are more resilient to drought and high light as well as temperature extremes and salinity.

Equally notable is the path to the discovery. Van de Ven continued working on the project for two years after retiring, returning to the lab to complete key experiments.

"She just kept going," Dehesh said. "It shows how much impact one person can have on science through dedication."

For van de Ven, now enjoying baking and line dancing in retirement, the decision was simple: finish what she started.

"I didn't know it would take as long as it did," van de Ven said. "But it was worth continuing to see it through."

(Cover image: Stan Lim/UCR)