Statoliths function in gravity perception in plants: yes, no, yes!

The starch-statolith theory was established science for a century when the existence of gravitropic, starchless mutants questioned its premise. However, detailed kinetic studies support a statolith-based mechanism for graviperception. Gravitropism is the directed growth of plants in response to gravity, and the starch-statolith hypothesis has had a consensus among scientists as the accepted model for gravity perception. However, in the late 1980s, with the isolation of a starchless mutant (lacking phosphoglucomutase, pgm) of Arabidopsis thaliana that was gravitropic, a statolith-based hypothesis for graviperception was questioned. Two groups studied the physiology and gravitropism kinetics of this pgm mutant, and these papers were published side-by-side in Planta. Based on the observation that the starchless mutant was responsive to gravity, Tim Caspar and colleagues (Caspar and Pickard, Planta 177:185-197, 1989) suggested that their results negated the starch-statolith hypothesis. In contrast, John Z. Kiss (Kiss et al., Planta 177:198-206, 1989) and colleagues turned the argument around 180 degrees and concluded that since a full complement of starch is required for full gravitropic sensitivity, in fact, their pgm studies provided strong support for a statolith-based model for gravity perception. Kiss and coworkers also provided evidence that the starchless plastids were relatively dense and proposed that these organelles function as statoliths in the pgm mutant plants. These two publications stimulated novel approaches (e.g., magnetophoresis, optical tweezers, spaceflight experiments, and laser ablation) to the study of gravity perception in plants. The controversy regarding the starch-statolith hypothesis remained for about a decade or so, but the current consensus supports a statolith-based model for graviperception in plants.