Plant Roots Avoid Rot Through Saprotropism

Plant roots can detect decaying plant matter in nearby soil and redirect their growth away from it without ever touching it, according to a new study.

They identify the decaying material from a chemical trail left in the soil around it.

This behaviour represents a previously unrecognised type of root navigation, protecting a plant’s most vulnerable tissue from the microbes responsible for rot.

It also demonstrates that the chemical by-products of decay can influence the direction in which a plant grows.

Why roots avoid rot

Decaying plant material creates a highly active zone in soil. Fungi and bacteria gather around it, and the microbes that decompose dead tissue can also infect living roots that grow too close.

Yuzhou Zhang, the plant biologist who led the research at Northwest A&F University (NWAFU) in China, and his colleagues investigated whether roots deliberately avoid such areas.

When Earth.com asked how a root could respond to decay without contact, Zhang said, “Roots do not need to touch the decaying plant material to react to it.”

To examine this possibility, the researchers positioned a rotting apple next to the root tips of thale cress (Arabidopsis), a small plant widely used in plant research.

After two days, roots directed towards the decaying matter had slowed and then ceased growing, unable to enter the area, whereas roots in uncontaminated soil continued to extend as usual.

The researchers saw the same response in crop roots, including canola, tomatoes and wheat, showing that it was not unique to one species. They called this rot-avoiding growth response saprotropism, using a Greek-derived term meaning rotten.

How roots read chemical trails

In a separate experiment, the team placed decaying material less than 1.27 cm to one side of a root, while ensuring it did not touch the root. Instead of growing directly downwards, the roots curved away from the material.

This indicated that roots were detecting a substance moving through the soil, rather than responding to physical contact. The researchers identified acidity as the signal.

As fungi broke down the dead plant material, they released organic acids that lowered the pH of the surrounding soil.

A pH-sensitive dye changed colour in a ring around the rotting apple, revealing a stable pH gradient: soil was more acidic close to the decaying material and less acidic further away.

To establish whether acidity alone caused the response, the scientists created an equivalent gradient using a weak acid, with no decaying matter present.

The roots turned away in the same way as they did from genuine rot. When a buffer neutralised the acid, the bending response stopped.

Roots ignore false alarms

The chemical cue also proved selective. When the researchers used rotting chicken, which makes the soil slightly alkaline rather than acidic, the roots did not respond.

Rather than reacting to decay broadly, roots respond to the acidic chemical signature produced as plant tissue decomposes.

This selectivity was unexpected for the team. Study co-author Jiří Friml is a plant biologist at the Institute of Science and Technology Austria (ISTA).

“This tells us that saprotropism is not a general reaction to decay, but a specific response to the chemical environment created by decomposing plant matter,” Friml told Earth.com.

Saprotropism uses a different hormone pathway

Most recognised tropisms depend on a single hormone. The best-known example is gravitropism, in which roots grow downwards; for roughly a century, this has been linked to auxin, a growth hormone that accumulates along one side of a root and causes it to bend.

Saprotropism did not follow that model. Roots genetically engineered without the system that transports auxin still turned away from decay, and often curved more sharply than ordinary roots. The standard auxin pathway therefore could not explain the response.

Instead, receptors that detect pH on the root’s outermost cells appear to sense the acid gradient.

In turn, abscisic acid - a hormone plants usually use during stress - accumulates more strongly on the side of the root facing the rot.

Turning away from danger

A fluorescent sensor recorded this accumulation directly: within hours of exposure to decay, hormone levels rose in the root tip and gathered towards the acidic side. This uneven distribution determines the direction of bending.

The unequal hormone signal then reorganises microtubules, internal fibres that control the direction in which cells elongate.

Cells on one side grow more than those on the other, causing the root to twist away from the acid.

This hormone-and-fibre system is not entirely unfamiliar in plants. Earlier research showed that roots rely on the same abscisic acid signal and microtubule twisting to grow away from saline soil, a response known as halotropism.

Helping crops avoid disease

The study shows that roots can interpret the chemistry of nearby decay and grow away from it through a hormone signal distinct from that responsible for most tropisms.

It identifies decay-generated acidity as a true directional signal, rather than simply a background source of growth stress.

The result also supports the idea that an individual root assesses several signals at the same time.

In an earlier paper, the same research group described a tug-of-war between a root’s attraction to gravity and its attraction to water. Saprotropism seems to form part of this same process of balancing competing cues.

For agriculture, the findings suggest that crop roots may be able to navigate around patches of decaying matter and the disease-causing microbes concentrated there.

Plants that can avoid infection

Zhang thinks the discovery may ultimately have applications in plant breeding.

“This would not replace classical disease resistance, but it could complement it by helping roots avoid pathogen-rich zones before an infection happens,” he told Earth.com.

This could provide a subtle form of underground self-defence.

Plants already alter soil life through the substances released by their roots. In this case, the relationship works in reverse, as the chemistry created by microbial decay redirects the plant.

Evolutionary mystery remains

Zhang’s team believes saprotropism may have emerged after plants colonised land, because stable acid gradients of this kind would be difficult to maintain underwater.

It remains unclear whether the response occurs throughout the plant kingdom and how the receptor transfers its signal to the hormone.

These issues form part of the laboratory’s wider work on understanding how roots first evolved to find their way through soil.

Image credit: Bao et al. / Science