Amyloplasts are necessary for full gravitropism in thallus of Marchantia polymorpha

Gravitropism is a response in which plants sense gravity and determine the direction of organ growth and development. This trait is important for adaptation in land plants. The molecular mechanisms of gravitropism have been studied mainly in flowering plants, but there is limited research on other organisms. In this study, we examined the gravitropic response of the liverwort Marchantia polymorpha, a model for investigating the evolution of land plants. We found the tips of the thallus extend upward and form several straight narrow structures in the dark. These growth directions were always in the opposite direction of gravity, and clinostat treatment disrupted them. The parenchymatous cells in the narrow structures contained amyloplasts, and the sedimentation of the amyloplasts preceded the gravitropic curvature, suggesting their role as statoliths. The starchless mutants, Mppgm1 and Mpaps1 were generated, and an abnormal direction of growth was observed in the narrow structures, but they tended to elongate upward. These observations indicate that although amyloplasts are required for complete gravitropism, plants can sense gravity without well-developed amyloplasts. These results suggest that land plants use amyloplasts as statoliths but also have amyloplast-independent mechanisms of gravitropism. These results suggest that land plants use amyloplasts as statoliths but also have amyloplast-independent mechanisms of gravitropism. HighlightIn M. polymorpha, amyloplasts act as statoliths in parenchyma cells and are important in gravitropism but it was not completely lost without starch granules.