SBG1 gene reveals how plants build stronger suberin root barriers

Cork has a distinctive feel: compact, mildly springy and resistant to water. That same material is also produced within plant roots.

The amount a plant deposits, and the location of those deposits, can differ enormously.

Researchers in Switzerland have now linked this diversity to one previously unidentified gene.

A cork-like barrier

Roots form the interface between a plant and the surrounding soil. Plants coat an internal layer of root cells with suberin, the same waxy material, to regulate what enters and prevent water from escaping.

When viewed using a fluorescent dye, suberin appears as a pale-yellow sheath around the inner root, immediately outside the vessels that carry water upwards.

Previously, nearly all knowledge of how this layer develops was based on a single greenhouse-maintained laboratory line from one species, used for genetic research.

Its role in wild plants, however, remained much less clear.

Plants from many climates

Marie Barberon, associate professor of plant sciences at the University of Geneva (UNIGE), led the research alongside colleagues from the University of Lausanne (UNIL).

The researchers examined the stained roots of 284 natural varieties of Arabidopsis thaliana, a small flowering weed that has long served as a model organism in plant genetics.

Their findings revealed substantial differences. Some varieties produced thick, unbroken suberin sheaths close to the root tip.

Others formed a more uneven barrier, or located it lower down in the more mature part of the root.

A clear climate pattern emerges

These varieties originated in widely contrasting environments, from warm Mediterranean slopes to colder Scandinavian farmland.

The team compared each suberin pattern with the climate in the plant’s native area, revealing a distinct relationship.

Plants from hotter, drier regions with less predictable rainfall produced the greatest quantities of suberin. The barrier was thickest precisely where plants had the strongest need to retain water.

“Our results suggest that strengthening the barrier is a natural adaptation to water stress, enabling better control of water exchange with the soil,” said Jian-Pu Han, first author of the study.

Identifying the SBG1 genetic mechanism

A genome-wide analysis of all 284 varieties uncovered a small gene not previously known to researchers. The team called it SUBER GENE1, or SBG1.

This gene encodes an exceptionally small protein comprising just 129 building blocks, making it far shorter than most proteins.

Varieties that formed thicker barriers possessed more active forms of the gene, whereas plants with patchier suberin carried less active copies.

The association was sufficiently strong for the researchers to investigate the protein’s function.

Before this research, the gene had no established role in any plant. It had not been associated with root barriers, hormone signalling or any other process, remaining in the Arabidopsis genome like an unread paragraph.

A recent tomato study had associated suberin with drought tolerance, yet the underlying genetic control remained unknown. By pairing natural variation in wild plants with genome mapping, the team has now identified one such control point.

How the gene works

“This gene acts as a key regulator of suberin: when it is more active, the barrier becomes stronger; when it is disrupted, it forms less efficiently,” said Han.

To understand how SBG1 operates, the team identified the other proteins it binds to. They found that it attaches to a family of plant enzymes involved in regulating stress responses.

When those enzymes were removed, the barrier became even thicker - the reverse of the effect seen when SBG1 itself was knocked out.

The two systems therefore act against one another within the root.

The hormone link

At the heart of this process is abscisic acid, a hormone released by plants when they detect water stress.

Previous studies had suggested a relationship between this hormone and suberin, but the precise mechanism had not been established.

The new findings provide a missing connection. SBG1 and the relevant enzymes appear to control how strongly the abscisic acid signal reaches the machinery responsible for building the barrier.

In their absence, the hormone has a weaker effect on suberin.

“Our results show that modulation of hormonal responses affecting suberin deposition is a central element of plants’ adaptation strategy to climate,” said Barberon.

Tougher crops of the future

Put simply, plants living in more demanding climates have evolved thicker protective root sheaths, and a previously unknown gene helps determine their thickness.

This discovery could offer an opportunity for crop breeders. Wheat, rice, tomatoes and other staple crops all have their own forms of the suberin barrier.

Manipulating SBG1 or the enzymes with which it interacts could help farmers grow crops that retain water more effectively during dry periods.

As agriculture faces increasingly erratic rainfall, such a mechanism has long been sought. The Geneva team has now brought one closer to use.