Over the years，scientists have reported that different types of plants，from trees to tomatoes，release compounds(化合物) into the air to help neighboring plants.These chemical warnings all have the same purpose — to spread information about one plant’s disease or infestation so other plants can defend themselves.But exactly how plants receive and act on many of these signals is still mysterious.

　　In this week’s Proceedings of the National Academy of Sciences，researchers in Japan offered some explanations.They had identified one chemical message and traced it all the way from release to action.

　　The scientists looked at tomato plants infested by a common pest，the cutworm.The researchers studied leaves from exposed and unexposed plants.They found one compound showed up more often in the exposed plants.The substance is called HexVic.When the scientists fed HexVic to cutworms，it knocked down their survival rate by 17%.The scientists identified the source of HexVic，and sprayed it lightly over healthy plants.Those plants were then able to start producing the cutworm­killing HexVic.Researchers confirmed that uninfected plants have to build their own weapons to fight off bugs and disease.How do they know when to play defense? They are warned first by their friendly plant neighbors.

　　It is a complex tale，and it may be happening in more plant species than tomatoes.It may also be happening with more chemical signals that are still unknown to us.Further studies are needed to increase agricultural efficiency.

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