Signaling with Ions: The Keystone for Apical Cell Growth and Morphogenesis in Pollen Tubes

Pollen tubes (PTs) are one of the best characterized plant cell types in many respects. The identification of key players involved in tube growth offers the perspective of an integrative understanding of cell morphogenesis processes. One outstanding feature of PTs is their prominent dependence on ion dynamics to promote and regulate growth. Many reports have identified and characterized membrane transport proteins, such as channels, transporters, and pumps, as well as their regulatory mechanisms, some of which themselves are dependent on ions such as Ca 2+ and H + . The signaling network that governs growth is based on a strict spatial distribution of signaling molecules, including apical gradients of Ca 2+ , H + , and reactive oxygen species. A central role for ion homeostasis, and more generally membrane transport systems, is proposed to underlie the spatiotemporal establishment of the signaling network that controls the PT self-organization and morphogenesis. Here, we review the latest progress on understanding tube growth from the perspective of membrane transporters and ion homeostasis. The ongoing molecular characterization of the Ca 2+ -signaling pathways, as well as the recent identification of female external cues and corresponding receptors on the pollen that control growth orientation, offer a firm biological context to boost the field even further.