New tropism discovered: Saprotropism

'Saprotropism' helps roots avoid decaying plant matter—but not animal decay

Decaying matter shapes life in soil, but it can also create hostile zones for growing roots.
Researchers have now identified "saprotropism," a root response that guides plants away from decaying plant-derived matter—but not animal-derived decay.
The study published in Science, reveals how roots adapt their growth direction by sensing local pH gradients around rot.

Plants cannot run away from danger or toward something they desire. Instead, they adjust the direction in which they grow. Shoots bend toward light (a well-known phenomenon called phototropism), roots and shoots use gravity to grow downward and upward, respectively (gravitropism), and roots can also bend toward water (hydrotropism).

These directional growth responses, known as plant tropisms, help plants navigate changing environments. Now, researchers from China and Austria describe a new member of this family: saprotropism, from "sapro," meaning rotten or decaying.

The researchers first showed that direct contact with decaying plant tissue strongly inhibited root growth and activated defense pathways linked to immunity and pathogens. In other words, roots treated these decay zones as biologically threatening environments.

Animals instinctively avoid rotten food because it often harbours harmful microbes.
The newly identified tropism enables roots to actively bend away from decaying plant matter. In experiments, roots avoided decay zones made from "fleshy" matter such as apples or leaves, and—contrary to initial assumptions—also from woody material such as sawdust.
However, when the researchers tested animal-derived decay, such as small pieces of chicken meat, the roots showed no directional growth response.

One of the striking findings was therefore that the roots did not simply avoid anything rotten.
They responded specifically to decomposing plant material. This tells us that saprotropism is not a general reaction to rot, but a dedicated response to plant-derived decay.
The response was observed not only in the model plant Arabidopsis thaliana, but also in crop species including rapeseed, tomato and wheat—suggesting that saprotropism is widespread among plants.

The team found that a key signal comes from microorganisms, especially fungi, as they break down dead plant material. During decomposition, fungi release acidic metabolites, including organic and phenolic acids. These compounds diffuse into the surrounding soil and create stable local pH gradients around the decaying material.

Roots can detect this acidity pattern even before direct contact and use it as directional information, bending away from the more acidic side. However, the "plant graveyard" does not send a permanent warning signal—it stops automatically after the matter has turned into soil. "Once the plant material had almost fully broken down, the acidic warning signal faded—and the roots stopped bending away.

Fungi and other microbes break down dead plant material in the soil. By doing so, they release acidic chemicals that spread outward like a warning signal. Nearby roots sense this acidic zone before touching the decay and bend away from it. Credit: Bao et al. / Science

How this happens

Within the root, an external signal is transformed into a growth decision: Cells on the root surface detect that one side of the root is exposed to stronger acidity than the other. This uneven signal changes the distribution of the plant hormone abscisic acid, or ABA, across the root tip.

As a result, the internal framework of root cells is rearranged, causing one side of the root to grow differently from the other. The root then bends away from the decaying plant material.
Saprotropism shows how plants interpret microbial activity in the soil and make growth decisions accordingly.
The discovery of saprotropism—a term coined by the study authors—opens new research avenues, such as how roots interpret microbial activity in soil. In the long term, a better understanding of such root behaviors could help inform approaches in agriculture, soil management and crop resilience.
Understanding the molecular basis of saprotropism opens new opportunities to develop crops with an enhanced ability to detect and avoid pathogen-rich environments.

Zhulatai Bao et al, Roots navigate around decay regions by sensing local pH gradients, Science (2026). DOI: 10.1126/science.adw6568. www.science.org/doi/10.1126/science.adw6568