Surface charge on isolated maize-coleoptile amyloplasts

Whole amyloplasts were isolated from Zea mays L. coleoptiles. Electron microscopy confirmed that their envelopes consisted of two intact membranes. Their surface charge was quantified through electrokinetic measurements employing a vertically oriented cataphoresis cell. Amyloplasts from coleoptiles of different ages (5–9 d) and degree of exposure to light (0–9 d) all had negative zeta potentials (mean of -19.4 mV). Isolated starch granules had comparable values. The net negative surface charge was confirmed ultrastructurally by the binding of cationised ferritin to both amyloplasts and starch grains. Cationised ferritin tagged with a fluorescent label (fluorescein isothiocyanate) showed binding to some amyloplasts but not to starch grains. These results are discussed in the context of a hypothesized role for statolith charge in plant graviperception.