Spatial Decoupling of the Endodermal Functions Establishes the Innermost Cortex as the Gravity Sensor in Rice Stems

Gravitropism is a remarkable example of coordinated growth, enabling plants to perceive gravity and generate asymmetric cell expansion that restores organ orientation. In Arabidopsis, gravity sensing in shoots occurs within starch-rich endodermis cells. In rice, however, the stem must accommodate gravitropic bending while preserving integrity of tissues surrounding emerging crown roots, raising the possibility that gravity sensing relies on distinct structural and physiological adaptations. Here, using high-resolution imaging together with auxin-response analyses, we identify a specialized cell layer at the inner boundary of the rice stem cortex, operationally defined by its position adjacent to the lignified endodermis and associated with gravitropic responses. This layer is characterized by circumferentially elongated cells, starch sedimentation, a confined auxin-response domain, and expression of gravity-sensing markers. Following gravistimulation, starch granules realign and the auxin response domain expands asymmetrically within this layer, coinciding with differential growth during stem bending. Our results further indicate that this response depends on polar auxin transport and conserved components of the gravitropic regulatory network. Together, these findings support a model in which gravity sensing and signaling in rice stems are spatially separated from the lignified endodermal barrier. Such tissue organization may represent a developmental adaptation that allows the stem to reconcile barrier integrity with growth flexibility.