Basipetal propagation of gravity-induced surface pH changes along primary roots of Lepidium sativum L.

While there is ample evidence for a role of auxin in root gravitropism, the seeming rapidity of gravi-induced changes in electrical parameters has so far been an argument against auxin being a primary signal in gravitropic signal transmission. To address this problem, we re-investigated the effect of gravistimulation on membrane voltages of Lepidium sativum L. and Vigna mungo L. root cells. In our hands, gravistimulation did not induce changes in membrane voltage in cells of the root cap statenchyma, root meristem or apical elongation zone that can be correlated with the orientation of the cells relative to the gravity vector. While these results challenge a model of rapid electrically based signal transmission, there is evidence for a slower signal propagation along gravistimulated L. sativum roots. Using multiple proton-selective microelectrodes to simultaneously measure surface pH on opposite root flanks at different distances from the root tip, we observed gravi-induced asymmetric pH changes at the surface of all investigated root zones. Upon gravistimulation, the surface pH decreased on the physically upper root flank and increased on the lower flank. The pH asymmetry appeared first [2.1±0.4 min (mean ± SD) after tilting] at the root cap and then - with incrementing lag times - at the meristem (after 2.5±0.3 min at 300 μm from root tip; after 3.7±0.4 min at 700 μm) and apical elongation zone (4.8±0.5 min at 1,000 μm), suggesting a basipetal progression of differential surface acidification at a rate of 250-350 μm min^-1, consistent with reported auxin transport rates.