Special Issue "Pollen Tube Growth"
A special issue of Plants (ISSN 2223-7747).
Deadline for manuscript submissions: closed (20 December 2012)
Pollen and pollen tubes are known to be the vector by which male gametes are delivered to the egg cell for fertilization in higher plants. Consequently, they have a fundamental role in plant reproduction, ensuring gene transfer and propagation of seed plants. In angiosperms, pollen tubes grow within the style of receptive flowers, with which they exchange continuously signals and information that regulate (either negatively or positively) the growth rate of pollen tubes. Signaling between pollen tubes and female reproductive structures is extremely critical to promote and guide pollen tube growth but also to avoid inbreeding and outcrossing through the recognition and rejection of self- or incompatible pollen. Therefore, the pollen tube is a favorite 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 view of how they orchestrate tube growth is still missing.
Growth of pollen tubes occurs essentially in a limited region, the tip, where a consistent number of secretory vesicles accumulate providing additional material for the cell wall and the 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 the cell membrane and activation in response to external signals are becoming progressively clear allowing to decipher the mechanism of signal perception, transduction and regulation of diverse cellular processes (such as growing within the female organs and delivering of sperms to the female gametophyte). An important component 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 tube tip. Understanding how these features are generated and how they 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 in the tube morphology. Exo- and endocytosis regulates the assembly and deposition of the cell wall, which is important for pollen tube growth and, more generally, for the 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 a clear 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 in 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 (ranging from ion dynamics to membrane transport), the pollen tube is now considered as an excellent model in which to investigate the problem of shape generation and the interaction between a mechanical problem and its biological control, which leads to the generation of complex growth processes. This issue is focused on research aimed at improving our current knowledge of the molecular mechanisms governing pollen tube growth, from the perception of extracellular signals to the cytoskeleton-based delivery of cell wall components to the integration of such mechanisms into a global process that determines the shape of the pollen tube, its growth and ultimately fertilization in plants.
Prof. Dr. Giampiero Cai
- pollen tube growth
- fertilization in higher plants
- signal transduction
- intracellular ion gradients
- exocytosis and endocytosis
- cell wall synthesis
- cytoskeleton dynamics
- system tip growth model