= Amyloplast
An \b[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.<Gravitropism in a starchless mutant of Arabidopsis: implications for the starch-statolith theory of gravity sensing by T. Caspar and B. G. Pickard>[Abstract: microscopy, inversion and five-minute induction/clinostat experiment.]{ref}
The ratio $10/14\approx0.71$ 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.
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