The pollen tube quest to find the ovule: spatio-temporal ion regulation as a mechanism for apical growth and guidance in single cells

Abstract

Ionic regulation is central for cell development in all biological systems, with pollen tubes being an outstanding model for cell polarity and directional growth. Sperm cell delivery to the ovule relies on their fast growth and sensitivity to external guidance cues, as to target the ovule. Pollen tubes grow exclusively at the apex, which is thought to be sustained by tip-focused ion gradients generated by the anisotropic spatial distribution of ion channels, transporters and pumps. Synchronized oscillations between diverse cellular processes are observed, including growth rate, flux and cytosolic concentrations of H+, Ca2+ and Cl-, although their roles in pollen tube guidance are still unknown. Curiously, the double mutant chx21 chx23 (Cation/H+ exchangers) has a dramatic fertilization defect due to loss of pollen tube guidance, leading to almost complete sterility. My preliminary results on chx21 chx23 show a significant reduction in H+ influx at the tip, with a slower and more regular oscillatory pattern compared to wild type. Mutant pollen tubes show no guidance response to the attractant LURE1, and a decreased response to the repellent hormone Nitric Oxide. Although wild-type pollen tubes show chemotropism, they fail to respond when showing a similar oscillatory signature to chx21 chx23. These striking results provide the first clue of a biological function of oscillations in pollen tubes, while favoring an ion-based mechanism responsible for attraction. The aim of this project is to perform experiments and modeling to establish a novel mechanism of apical growth and guidance regulated by H+ and membrane potential. This research will create an unique experimental unit in the country, capable of non-invasive quantitative spatio-temporal measurements of multiple ions in diverse biological systems, fostering interdisciplinary research in an international level. (AU)