Plants move to get away from their decaying brethren

Plants seem stationary: they don’t skip, fly or walk around. But that doesn’t mean they don’t move. The organisms bend their roots and shoots in response to stimuli such as water, light, salt and gravity. These responsive movements, called tropisms, are crucial for plants’ survival.

Now researchers have discovered a new way plants move, which they call “saprotropism”: the bending of roots to avoid dead or rotting plant tissue. The results were published in Science.

The discovery began with a failed experiment and an unexpected observation. Yuzhou Zhang, a biologist at Northwest A&F University in China, was studying how plants move in response to gravity in a laboratory in Austria when one of his agar plates became contaminated with airborne microbes. Just before he threw it away, he noticed something striking: almost every single one of the plants’ roots in the plate was bent away from the spoilage. The consistency “immediately made me wonder whether this was more than a random response,” Zhang says.

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His team decided to investigate further. The researchers grew the common research plant Arabidopsis thaliana in containers, some of which were dotted with patches of rotting apple. The plants that encountered “decay zones” bent their roots to avoid the bad apple, whereas the ones in containers lacking decay grew straight through. Repeating the experiment with other plants, including tomatoes and wheat, yielded similar results—but an encounter with putrefying animal tissues had no effect. Zhang and his colleagues theorize that this reaction might help plants avoid pathogens, similar to the way unpleasant smells help animals steer clear of rotten food.

Saprotropism seems to operate differently than most other plant tropisms. “It does not follow the classical mechanism,” Zhang says. For example, gravitropism, or movement in response to gravity, and phototropism—in response to light—occur when plants redistribute the hormone auxin in their stems or roots, causing them to change their growth direction.

In contrast, saprotropism relies on a hormone called abscisic acid (ABA). When the plant roots encounter an area acidified by decaying plant matter—a chemical gradient distinct from that of animal matter—ABA builds up on the side facing the rot, encouraging growth in the other direction.

The work’s scope is impressive, says Chris Wolverton, a plant biologist at Ohio Wesleyan University, who was not involved in the study. He adds, however, that “there’s still a lot of open space in the signaling mechanism for more detail to get fleshed out.” At what point, for instance, does decaying plant tissue break down enough to become nutritious compost?

Zhang’s team hopes to fill those gaps in future studies. Ultimately the research could help illuminate how plants sense and interact with the world around them.