Rice stem study points to a gravity-sensing layer beside its protective barrier

A thin layer of cells near the center of a rice stem may help organize its response to gravity while a neighboring layer maintains a protective barrier. That is the model emerging from a study combining microscopy, gene-expression analysis, chemical treatment and mutant seedlings.

The bioRxiv preprint, posted November 2, 2026, has not been peer reviewed. Led by researchers at the Indian Institute of Technology Roorkee with international collaborators, it presents converging evidence for a gravity-associated signaling layer, rather than definitive proof of which cells perceive gravity.

Two neighboring layers, different roles

In Arabidopsis shoots, gravity sensing occurs in starch-rich endodermal cells. Rice stems present a different anatomical setting: their endodermis becomes lignified, forming a boundary around the central vascular region. Crown roots develop within the stem and emerge through surrounding tissues.

The researchers examined rice seedling stem bases using stains that reveal lignified walls and cell outlines, alongside a fluorescent reporter called DR5 that indicates auxin-responsive gene activity. Auxin is a plant hormone involved in coordinating growth.

They found a narrow DR5-positive layer immediately outside the lignified endodermis. They call it the innermost cortical cell layer, or IC, based on its position. During development, the appearance of this localized auxin response coincided with endodermal lignification. IC cells also developed a distinctive shape, elongated around the stem’s circumference rather than predominantly along its radius.

These observations establish a spatial and developmental association. They do not show that lignification causes the neighboring auxin response or cell-shape changes.

Following the response to a sideways turn

The IC contained abundant starch granules, whereas the adjacent endodermis lacked detectable granules. Starch-filled structures that settle under gravity are characteristic of plant gravity-sensing cells. One hour after seedlings were rotated through 90 degrees, the researchers observed realignment of starch granules with the new direction of gravity.

The auxin-response pattern changed too. Within two hours, before visible stem bending, DR5 activity on the lower side extended toward the shoot tip beyond its original alignment with the endodermis. It subsequently spread sideways across additional cortical layers. Later measurements found asymmetric cell shapes alongside the expanded response.

DR5 reports a cellular response to auxin, not a direct measurement of hormone concentration or transport. Its changing pattern therefore supports an asymmetric signaling response without, by itself, tracing auxin movement between cells.

Reanalysis of a published spatial gene-expression atlas provided another line of evidence. Inner radial regions were enriched for genes associated with gravity signaling, auxin transport and growth. RNA localization experiments also detected ring-like expression patterns for selected genes around the vascular region.

However, the atlas did not resolve a distinct endodermal population. The researchers used broader tissue categories and radial bins as approximations, leaving uncertainty about the precise cells producing some transcripts.

What the interventions establish

To examine barrier function, the team used a crown-rootless mutant whose stem retained a continuous endodermis. A fluorescent dye was excluded at the lignified boundary. This supports barrier activity in that experimental setting, but does not comprehensively test water and solute regulation during normal root emergence.

A separate experiment supplied stronger causal evidence for a role of auxin transport in bending. Seedlings treated with the transport inhibitor NPA bent by about 25 degrees after 24 hours, compared with roughly 60 degrees in controls. Expansion of the lower-side DR5 domain was also largely suppressed.

Because the treatment was not restricted to IC cells, it supports a requirement for polar auxin transport in the overall response, not proof that the IC alone initiates it.

Seedlings carrying a disrupted OsWOX12 gene showed delayed bending during the first 12 hours and shorter cells on the responding lower side. By 24 hours, their bending angles were comparable to wild type. This indicates that OsWOX12 contributes to the timing and growth response, rather than being indispensable for eventual bending under these conditions.

A promising model, not a settled identity

The central limitation is the absence of cell-type-specific markers and selective disruption experiments that could establish whether IC cells are necessary for gravity perception. Tissue mechanics were not measured directly, and the work focused on young seedlings under laboratory conditions, with gravity-response assays conducted in darkness.

The results nevertheless support a useful anatomical model: barrier function and gravity-associated signaling occupy neighboring layers in rice stems. Whether that arrangement is an adaptation preserving both protection and growth flexibility remains a hypothesis. The study also does not demonstrate improvements in mature crop architecture, lodging resistance or yield.