Pollen Tubes and Tip Growth: of Biophysics and Tipomics

Abstract

Pollen is the male gametophyte of higher plants and is responsible for the successful transport of the sperm cells to the ovules. After landing on a stigma, pollen grains will germinate and grow a pollen tube through the stigma and style tissue towards the ovules, where fertilization takes place. In terms of cell biology, the elongation of the pollen tube is characterized by a dramatically polarized growth process, tip growth, which is common to root hairs, fungal hyphae and some developing neurites. Due to their simple morphology and function, growing pollen tubes became the most well-established model system to study tip growth. Both the editors of this book have been enthusiastic paladins of this trend, even since they first met in 1994 during the 13th International Congress on Sexual Plant Reproduction in Vienna. Our intellectual enthusiasm was first materialized in an essay in which a naive and simple yet forward theoretical model was set forth, implying a set of electrochemistry rules to be at the core of a minimal set of mechanism underlying cell polarity establishment and maintenance during pollen tube growth (Feijó et al. ). In short, subcellular biophysical processes, like ion transport, endogenous electrical fields and a tip-focussed Ca²⁺ gradient, would regulate the shape and growth rate in an essentially self-organizing process. Ever since then, the multitude of nuts and bolts and genes and pathways and their biological consequences have amounted to a vast literature in practically every aspect of the biology of pollen. And yet, some of the essential parts of this naive biophysical view of the pollen tube remained elusive, namely the existence and features of the channels responsible for the unique ion biology of pollen (Michard et al.).