SBG1 Gene Explains Thicker Suberin Barriers in Plants

Cork is instantly recognisable: compact, mildly springy and waterproof. The compound responsible for these properties also performs a discreet role within plant roots.

The amount of this material that plants deposit, as well as its position, can differ greatly.

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

A cork-like barrier

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

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

Previously, nearly all scientific understanding of how this layer develops came from one greenhouse-maintained laboratory line of a single species used for genetic research.

Its role in plants growing in natural environments, however, remained a separate question.

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 stained and closely examined the roots of 284 natural varieties of Arabidopsis thaliana, a small flowering weed that has long served as a mainstay of plant genetics.

Their findings showed substantial differences. Certain lines produced thick, unbroken suberin sheaths close to the root tip.

In other varieties, the barrier remained uneven or was formed further down, nearer the older part of the root.

A clear climate pattern emerges

These varieties originated in highly contrasting environments, from warm Mediterranean slopes to cold Scandinavian fields.

The team compared each suberin pattern with the climate in the plant’s native region and found a distinct trend.

Plants from hotter, drier areas with unreliable rainfall deposited the greatest amount of suberin. Their barriers were thickest precisely where retaining water mattered most.

“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 genetic mechanism

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

This gene encodes a very small protein containing only 129 building blocks, making it far shorter than most proteins.

Varieties that produced thicker barriers had more active forms of the gene, whereas plants with more patchy suberin carried less active copies.

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

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

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

How the SBG1 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 determine how it functions, the researchers examined the other proteins to which SBG1 binds. They found that it attaches to a family of plant enzymes involved in regulating stress responses.

Removing those enzymes caused the barrier to become still thicker, the reverse of the effect seen when SBG1 itself is 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 problems with water availability.

Previous studies had suggested a relationship between this hormone and suberin, although the precise mechanism was not known.

The new research supplies a missing connection. SBG1 and these enzymes appear to govern how strongly the abscisic acid signal is transmitted to the machinery that builds the barrier.

In their absence, the hormone’s influence on suberin is weakened.

“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 adapted to tougher climates have developed thicker root sheaths, and a newly discovered gene helps determine their thickness.

This could create opportunities for crop breeders. Wheat, rice, tomatoes and other major crops possess their own forms of the suberin barrier.

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

As agriculture confronts increasingly erratic rainfall, this type of genetic control has long been sought. The Geneva team has now brought one within reach.