Bioelectricity and the Control of Apical Growth in Pollen Tubes

Abstract

Bioelectricity has been studied since the 18th century in every branch of the tree of life. Bioelectricity is involved in cell growth, proliferation, and behavior, which at a tissue-scale level translates in dramatic tissue rearrangements during embryology and somatic development. Although ion fluxes can be measured in single cells, it is not unanimous that a single cell may create and sustain different bioelectrical states within itself by means of electrochemical nonequilibrium phenomena. We address this possibility, with a focus on the pollen tube as a biological model. Pollen tubes are the subject of intense research given its unique properties, evolutionary streamlined to very fast apical growth and sensitivity to external cues that affect chemotropic responses. Pollen tube's functions rely on conspicuous tip-focused ions dynamics, involving the formation of steep ion gradients, with ion concentration differences over an order of magnitude when compared with the shank cytosol. These gradients are thought to be based on the spatial segregation of ion transporters, channels, and pumps along the cell, creating distinct electrochemical environments at the tip and the shank. But how polarity is generated and maintained is still a matter of debate. In the past we hypothesized that opposing electrochemical forces of depolarization at the tip and hyperpolarization in the shank could create a membrane potential gradient spanning from the tip to the shank that would be part of feedback mechanisms essential to cell polarity in these and likely other cell types. In this study, we review the latest progress on understanding apical growth from the perspective of bioelectricity-driven morphogenesis.