A new study from the University of Kentucky Martin-Gatton College of Agriculture, Food and Environment (CAFE) helps explain how plants can lose track of their own disease warnings.
Plants do not have blood, nerves or immune cells like people do, but they still have ways to protect themselves. When one leaf is attacked by a pathogen, the plant can send warning signals to other leaves and stems. That gives the rest of the plant time to prepare for another attack.
The Martin-Gatton CAFE study shows how that warning system can break down when a molecule called nitric oxide builds up too much.
The study, titled "Excess nitric oxide alters cellular pH to restrict salicylic acid movement and systemic immunity," was published in Science Advances.
Nitric oxide (NO) is a tiny molecule found in both plants and animals. In people, it helps blood vessels relax and plays a role in immune defense. In plants, it helps control growth, stress responses and disease resistance. But the new study found that plants need the right amount. Too little or too much NO can weaken plants' defenses.
Systemic immunity is like plant memory
Researchers studied Arabidopsis, a small plant often used in laboratory research. They focused on specimens with a mutation in a gene called GSNOR1, which is conserved in both plants and humans. These mutant plants build high levels of NO and struggle to activate systemic acquired resistance (SAR), a whole-plant immune response: Once a single part of a plant becomes infected, SAR helps warn the rest of the plant to prepare for another attack.
A key messenger in that process is salicylic acid, which is chemically related to aspirin. In plants, it helps carry immune signals from infected leaves to other parts of the plant.
"Systemic immunity is like plant memory," said Huazhen Liu, Ph.D., a postdoctoral scholar and first author of the work. "After one leaf survives an attack, the plant needs to warn the rest of its body. Salicylic acid helps carry that alert signal."
The research team found that high levels of NO changed the pH balance inside and outside plant cells.
In the mutant plants, the space outside the cell became too acidic and the inside of the cell became too alkaline. That shift made it harder for salicylic acid to move into the plant's transport system.
Liu described the problem as a kind of "pH traffic jam."
"When nitric oxide levels become too high or uneven, they change the acidity around the cell," Liu said. "That creates a barrier for salicylic acid. The signal gets trapped, and the rest of the plant does not get the warning."
The team also tested whether the plant could still respond if salicylic acid entered via another avenue.
When salicylic acid was sprayed on leaves, the mutant plants still struggled to respond. However, when salicylic acid was delivered through the roots, the plants regained immune signaling and systemic acquired resistance. That result helped show that the plant's immune system was not fully broken. The problem was delivery. The signal could work, but it had to reach the right place.
What this research means for One Health
This work connects to UK's One Health Initiative, which brings together research across disciplines to better understand how the health of plants, animals, people and the environment are linked.
"For crops, this gives us a new way to think about disease resistance," said co-author Pradeep Kachroo, Ph.D., professor in the Department of Plant Pathology and a Fellow of the American Association for the Advancement of Science and the American Society of Plant Biologists. "It is not enough for a plant to make a defense signal. That signal also has to move."
The finding could help researchers better understand how plants move chemical signals during disease, drought, heat and other stress. That knowledge may matter for agriculture, where crops face constant pressure from pathogens and changing growing conditions.
It may also help scientists study how similar signaling systems work across living organisms. Nitric oxide affects transport and communication in animals, too, suggesting that plants and people may share some basic rules for moving chemical messages through living tissue.
Visit the Department of Plant Pathology at Martin-Gatton CAFE to learn more.
Research reported in this publication was supported by the U.S. National Science Foundation under Award No. 2131400. The opinions, findings, and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the U.S. National Science Foundation.
This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award numbers 20246701442593 and 20247041043700. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the Department of Agriculture.
