Signaling Through Ions is Essential for Chemotropism and Reproduction

Abstract

Chemotropism is essential for many cell types which must orient growth in complex biological spaces, such as neurites, hyphae and pollen tubes. Despite its relevance, chemotropism remains poorly understood and there is no generalized mechanism of either signaling or cellular effectors. We present evidence that two CHX H+/K+ antiporters are necessary for the chemotropic response of pollen tubes towards ovules in flowering plants. Pollen tubes of mutants for these proteins grow normally but are unable to perceive either negative and positive chemotropic stimuli. We determined that this defect is associated in the dynamic regimens of growth and ion dynamics, namely of oscillations of cytosolic pH and extracellular fluxes of H+, with the mutant showing steady oscillations and the wildtype irregular cycles. The phenotype can be rescued by genetics and pharmacology aimed at restoring capacity of H+ extrusion by H+-ATPases, suggesting that pH/H+ fluxes are necessary for sensitization of the intracellular mechanisms that lead to the change of growth axis. We used FHN modeling of plasma membrane potential to explore its role as integrator of these pH changes. Results are suggestive that the existence of standing domains of membrane depolarization could underlie some of these responses, acting upstream of any receptor binding mechanism.