Root-cap columella cell

A root-cap columella cell is a cell in the central columella of a plant's root cap. In Arabidopsis thaliana, these cells form recognizable tiers and files and contain starch-bearing amyloplasts. The organelles can sediment after the root changes orientation relative to gravity, making these cells useful for investigating gravitropism.^[ref]

An especially informative experiment removes selected cells, rather than the entire growing tip. In 1998, investigators used laser ablation and then measured curvature after reorientation. Inner central columella cells contributed more strongly to the response than the other tested regions. Measuring extension alongside bending helped distinguish damage to gravity-associated signaling from a general failure of growth.^[ref]

The distinction between sensing and response is important: a cap cell associated with gravity perception need not be the cell whose elongation produces the visible bend. Much of that differential growth occurs farther back in the root elongation zone. Likewise, observing an amyloplast settle supplies a candidate physical mechanism, while a targeted perturbation tests the functional importance of its cellular setting.