Amyloplasts are necessary for full gravitropism in thallus of Marchantia polymorpha

Gravitropism is a plant response to gravity that directs organ growth and development, playing a key role in the adaptation of land plants. While its molecular basis has been extensively studied in flowering plants, much less is known about this process in other plant lineages. Here, we investigated the gravitropic response of the liverwort Marchantia polymorpha, a model for early land plant evolution. In darkness, the thallus tips extended upward, forming several straight, narrow structures whose growth directions was consistently opposite to gravity and disrupted by clinostat treatment. These structures contained amyloplasts in parenchymatous cells; their sedimentation preceded gravitropic curvature, suggesting a role as statoliths. Amyloplast sedimentation started near the tip, slowed with distance, and in more distal regions, both the size and number of amyloplasts decreased. In starchless mutants (Mppgm1 and Mpaps1), the narrow structures displayed abnormal growth directions, although they still tended to elongate upward. These results indicate that while amyloplasts are required for proper gravitropism, M. polymorpha retain the ability to sense gravity even without well-developed amyloplasts. Our findings suggest that land plants use amyloplasts as statoliths but also possess amyloplast-independent mechanisms for gravitropic sensing.