Gravity sensing in plants

Gravity, a constant force on Earth, fundamentally shapes plant architecture by directing organ growth, a process known as gravitropism. Shoots typically grow upward (negative gravitropism) and roots downward (positive gravitropism). In seed plants, gravity is sensed by specialized cells, including endodermal cells in shoots and columella cells in roots, collectively termed statocytes. The gravitropic response occurs through three sequential steps: gravity sensing/perception, signal transduction, and growth response. For over a century, the starch-statolith hypothesis dominated our conceptual understanding of gravity sensing, yet its molecular mechanism remained elusive. A molecular basis was established in 2023, when sedimenting amyloplasts were shown to repolarize LAZY proteins. This discovery allows us to molecularly define one type of gravity sensing as the process from amyloplast sedimentation (physical susception) to LAZY repolarization (physiological signal conversion). Meanwhile, evidence suggests the existence of alternative gravity-sensing pathways independent of the starch-statolith model, the mechanisms of which remain largely unknown. This review summarizes current knowledge and perspectives on gravity sensing, primarily in vascular plants, while integrating key insights from nonvascular lineages to provide an evolutionary context. Potential agricultural applications of gravity-sensing mechanisms are also discussed.