Amyloplast

An amyloplast is a nonphotosynthetic plastid specialized for synthesis and accumulation of starch granules. The familiar starch-rich interior of a potato illustrates storage; an amyloplast is an entire membrane-bounded organelle, not merely one starch granule. In certain gravity-sensing plant cells, starch-filled amyloplasts also provide sedimenting material that helps couple orientation to gravitropism.

A revealing test removed starch rather than removing the whole cell. T. Caspar and B. G. Pickard studied an Arabidopsis thaliana mutant lacking plastid phosphoglucomutase activity. Light and electron microscopy confirmed starch deficiency. In normal seedlings, starch-filled plastids moved toward the new lower side after inversion; the mutant's starchless plastids did not show significant sedimentation. Yet the mutant roots still curved in response to gravity. After a five-minute gravity stimulus followed by rotation on a clinostat, the reported root curvature was approximately 10 degrees in the mutant versus 14 degrees in the normal seedlings.^[ref]

The ratio $10/14\approx 0.71$ makes the distinction concrete: the response was reduced under those conditions, but did not disappear. The experiment therefore limits a claim that sedimenting starch-filled amyloplasts are absolutely required for every root gravity response. It does not make starch storage or sedimentation irrelevant; it separates contribution to a response from strict necessity.