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Plants Capable of Rapid, Long-Distance Defensive Signaling
Missouri Ag Connection - 10/09/2018

Plants have a built-in wound sensor and rapid communication system that allows them to defend themselves when attacked by insects, new research from the University of Missouri has found.

Insects cause an estimated $470 billion in crop damage worldwide every year. Prior research has shown that plants can defend themselves by producing toxic compounds that incapacitate or kill insects. To learn more about this process, researchers carried out the study using Arabidopsis, a small flowering plant with a fully sequenced genome that is commonly used for plant research.

"We observed that when insects attacked the plant, there was a spike of calcium in distant organs throughout the plant that induced defensive responses," said Abraham Koo, an associate professor of biochemistry in MU's College of Agriculture, Food and Natural Resources. "It turned out that glutamate, upon sensing a wound, triggered rapid, long-distance signals in the form of a wave of calcium. Glutamate, an amino acid, and its receptor-like channels act as a sort of wound sensor that puts the whole process in motion."

Koo worked in conjunction with researchers at MU, the University of Wisconsin, Michigan State University and Saitama University in Japan. Building on earlier research showing that jasmonate, a plant hormone, regulates defensive responses, researchers sought to understand how the process is initiated in remote parts of the plant that are not directly under attack. To that end, Arabidopsis plants were observed when being fed upon by caterpillars. Researchers found that glutamate "receptor-like proteins" acted as sensors that signaled a wound to the rest of the plant, sending a wave of calcium throughout the plant.

To draw a correlation between the calcium wave and plant-wide defense responses, researchers used fluorescent microscopy--which can detect very faint fluorescent lights emitted when calcium is present in cells--and highly sensitive mass spectrometry, which accurately measured trace amounts of the defense hormone jasmonate.

"Glutamate not only helps plants defend against a single attack on a specific leaf, but it also alerts other leaves to the potential danger and induces pre-emptive responses," Koo said. "This signaling process takes only minutes. While that may seem like a long time in human terms, that is a lightning-fast response for plants."

In addition to glutamate and glutamate receptors that have functions in mammalian neurotransmission, the jasmonate hormone also has counterparts in the human body involved in wound healing and immune response. This study will help researchers understand how the fundamental processes of cell-damage sensing and system-wide communication have independently evolved in plants and animals with such drastically contrasting lifestyles, Koo said.

The study, "Glutamate triggers long-distance, calcium-based plant defense signaling," was published in Science. Other researchers involved in the study were Tong Zhang of MU; Masatsugu Toyota and Wang Jiaqi of Saitama University; Dirk Spencer, Satoe Sawai-Toyota and Simon Gilroy of the University of Wisconsin; and Gregg Howe of Michigan State University. The study was supported by grants from the National Science Foundation, the U.S. Department of Energy and the National Aeronautics and Space Administration.

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