Dear Colleagues,

Pollen and pollen tubes are vectors by which male gametes are delivered to egg cells for fertilization in higher plants. As such, they have a fundamental role in plant reproduction by allowing gene transfer and propagation of seed plants. In angiosperms, pollen tubes grow within the style of receptive flowers and exchange signals and information that regulate (either negatively or positively) the growth rate of pollen tubes. Signaling between pollen tubes and pistils is critical to promote and guide pollen tube growth, but also to avoid inbreeding and outcrossing through recognition and rejection of self- or incompatible pollen. Therefore, pollen tubes are a model system for the study of cell–cell interaction and cell guidance in plants. Key genes and molecules involved in pollen tube guidance have been partially identified but a general model by which they orchestrate tube growth is still missing.

Growth of pollen tubes occurs essentially at the tip, where a number of secretory vesicles accumulate thereby providing material for cell wall and plasma membrane. In this region, external signals are perceived, interpreted and used to regulate the growth rate. In recent years, identification of RAC/ROP GTPases, their recruitment to cell membrane and activation in response to external signals are becoming progressively clear allowing to decipher the mechanism of signal perception and transduction, as well as the regulation of cellular processes (such as growing within pistils and delivering of sperms to the female gametophyte). Important components of the signal transduction mechanism are ion flux, intracellular ion gradients and dynamics, which are critical for the polarization of pollen tubes and for maintaining the growth site at the tip. Understanding how these features are related to the signal transduction pathway is an important challenge.

Signaling is interfaced to the dynamics of exocytosis and endocytosis, whose precise balance regulates pollen tube growth at the apex and whose perturbation causes significant changes to tube morphology. Exo- and endocytosis regulates assembly and deposition of the cell wall, which is critical for pollen tube growth and, more generally, for global morphogenesis of plants. A number of evidences describe the composition of the pollen tube cell wall, but little is known about the molecular mechanism controlling cell wall deposition. Nevertheless, we are progressively appreciating how callose and cellulose are synthetized, deposited, and designed to be load-bearing and resistant to tensile forces. Secretion, modification and dynamics of pectins are also progressively elucidating but an outlook of how the synthesis of cell wall polymers is interplayed is still missing as well as their relationship with the signal transduction pathway. Deposition of cell wall and accumulation of secretory vesicles are both dependent on the dynamics of the cytoskeleton, whose activity is regulated by both motor and non-motor proteins. The precise balance between polymerized and unpolymerized cytoskeletal filaments (coupled to the dynamic interplay between cytoskeleton and motor proteins) supports the continuous supply of secretory vesicles to the tip. Regulation of cytoskeleton activity is likely to be dependent on both signal transduction pathway and ion dynamics.
In view of this amazing interplay between different molecular processes (from ion dynamics to membrane transport), pollen tubes are now considered an excellent model by which investigating the mechanism of cell shaping and the interaction between mechanical problems and their biological control. This Special Issue on “Pollen Tube Growth” (which is a continuation/update of the previous call ended in 2012) is focused on researches aimed at improving our current knowledge of the molecular mechanisms controlling pollen tube growth, from perception of extracellular signals to cytoskeleton-based delivery of cell wall components to integration of such mechanisms into global processes that determine the shape and growth of pollen tubes and, ultimately, fertilization in plants.

Prof. Dr. Giampiero Cai
Guest Editor

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• pollen tube growth
• fertilization in higher plants
• signal transduction
• intracellular ion gradients
• exocytosis and endocytosis
• cell wall synthesis
• cytoskeleton dynamics
• system tip growth model