Dear Colleagues,

Pollen and pollen tubes are the carrier through which male gametes are transported to the ovule. Thus, they play a fundamental role in the reproduction of seed plants, both because they provide the transfer of genes and because they represent an important evolutionary factor in the conquest of dry lands. In angiosperms, pollen tubes grow inside the style of receptive flowers, with which they continuously exchange signals and information that control (negatively or positively) its growth. The existing signaling between pollen tubes and female reproductive structures is extremely critical because it promotes and guides the growth of pollen tubes, but also because it regulates the crossbreeding ability through the recognition and rejection of self-incompatible pollen. Therefore, the pollen tube is also a model system for the study of cell–cell interactions. Key genes and molecules involved in guiding the pollen tube have been partially found, but a broader view of how they orchestrate pollen tube growth is still lacking.

Pollen tubes grow in a narrow region, the tip, where many secretory vesicles accumulate, providing supplementary material for the cell wall and plasma membrane. It is in this region that external signals are perceived and used to control growth. In recent years, identification of receptor systems, of GTPases that regulate signal transduction, of the mechanism by which they are recruited into the cell membrane and activated in response to external signals, has become progressively clearer, allowing the mechanism of signal perception and transduction as well as the regulation of different cellular processes to be deciphered. Other important components of the signal transduction mechanism are the transmembrane ion flux, the gradient and dynamics of intracellular ions, the very precise use of reactive oxygen species (ROS), the involvement of polyamines, and the functioning of protein channels that finely regulate the dynamics. All these factors are critical for the polarization of pollen tubes and to maintain growth in the apical region. Understanding the integration of all these processes and how they relate to signal transduction is an extremely critical challenge.

Cell-to-cell signaling interfaces with the mechanisms of exocytosis and endocytosis, whose precise balance regulates pollen tube growth at the apex and whose perturbation necessarily causes significant changes in tube morphology. Exo- and endocytosis consequently regulate the assembly and deposition of the cell wall, the texture of which is important for the correct growth of pollen tubes and, more generally, for the morphogenesis of plant cells. Although several experimental investigations have described the composition of the pollen tube cell wall, little is known of the molecular mechanism that controls cell wall deposition. Cell wall deposition and accumulation of secretory vesicles also depend on the dynamics of the cytoskeleton and on a set of motor and nonmotor proteins. Dynamics of the cytoskeletal filaments also relate the signal transduction pathway and ion dynamics. Pollen and pollen tubes are known to release nanovesicles that are important for correct tube growth, thereby favoring fertilization. Nanovesicles contain different proteins, many of them displaying well-known roles in metabolism and signaling, protein synthesis and processing, cell wall expansion, and cytoskeleton and membrane transport. Some of these proteins are also allergens, and they represent a naturally occurring phenomenon that contributes to pollen sensitization.

In view of this surprising interaction between different molecular processes (from ion dynamics to membrane transport and from cytoskeleton to cell wall synthesis), the pollen tube is considered an excellent model by which to study cell shape and how external signals relate to mechanical requirements and their biological control. Moreover, it should not be underestimated that the pollen tube is also the target of abiotic stressful conditions (such as heat shock, but also environmental pollutants) that can negatively affect the reproductive capacity of plants and, in the case of crops, their productivity. This is another good reason for studying pollen and the pollen tube.

This Special Issue aims to collect contributions to understand the molecular mechanisms that regulate pollen tube growth, from the perception and transduction of extracellular signals to the cytoskeleton-based distribution of cell wall components that, in addition to the integration of these mechanisms into the overarching process, determines the shape of the pollen tube, its growth, and finally the success of plant fertilization.

Prof. Dr. Giampiero Cai
Prof. Dr. Stefano Del Duca
Guest Editors

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• fertilization in higher plants
• signal transduction
• intracellular ion gradients
• exocytosis and endocytosis
• cell wall synthesis
• cytoskeleton dynamics
• pollen nanovesicles
• environmental stress