Physiological and transcriptomic assessment of Arabidopsis identifies two distinct gravity signaling systems

Plants employ multiple sensing and signaling mechanisms to inform their growth. Earth's gravity is a constant force, which plants perceive and use to direct growth. Despite the ubiquity of gravitropism in plants, the mechanism for signal initiation remains a point of debate. The starch statolith hypothesis suggests that dense amyloplasts sediment within perceptive cells to initiate signaling. However, the persistence of gravitropism in starchless pgm-1 mutants of Arabidopsis (Arabidopsis thaliana) suggests that plants still sense gravity even without amyloplast sedimentation, hinting toward a second mechanism by which plants can perceive gravity. In an attempt to isolate this mechanism, we exposed seedlings to a range of fractional gravities from 0.003 to 1 g; this showed that plants without starch require a much larger force to induce gravitropic signaling than those with starch-filled statoliths. We used the difference in final root angle between genotypes after simultaneous application of blue light and gravity to estimate the relative contributions of the two systems to gravity sensing, demonstrating that starchless signaling can produce 51.7% of the wild-type response. Transcriptomics across the gravity gradient showed a distinctive shift in RNA regulation coinciding with the force required for starchless response. Mutants of these highly regulated genes showed gravity-specific defects and were largely involved in cell-to-cell communication and extracellular signaling. These data provide molecular evidence for both starch-dependent and starch-independent gravity signaling within a vascular plant, as well as the molecular components used in the starch-independent response.