Abstract
Plants display a complex life cycle, alternating between haploid and diploid generations. During fertilisation, the haploid sperm cells are delivered to the female gametophyte by pollen tubes, specialised structures elongating by tip growth, which is based on an equilibrium between cell wall-reinforcing processes and turgor-driven expansion. One important factor of this equilibrium is the rate of pectin secretion mediated and regulated by factors including the exocyst complex and small G proteins. Critically important are also non-proteinaceous molecules comprising protons, calcium ions, reactive oxygen species (ROS), and signalling lipids. Among the latter, phosphatidylinositol 4,5-bisphosphate and the kinases involved in its formation have been assigned important functions. The negatively charged headgroup of this lipid serves as an interaction point at the apical plasma membrane for partners such as the exocyst complex, thereby polarising the cell and its secretion processes. Another important signalling lipid is phosphatidic acid (PA), that can either be formed by the combination of phospholipases C and diacylglycerol kinases or by phospholipases D. It further fine-tunes pollen tube growth, for example by regulating ROS formation. How the individual signalling cues are intertwined or how external guidance cues are integrated to facilitate directional growth remain open questions.
Highlights
Pollen are the male microgametophytes of seed plants, part of the short haploid phase in the life cycle of the spermatophytes
The microgametophyte consists of one vegetative cell and two sperm cells formed from a generative cell, either during pollen development or later during fertilisation
Activation of type II pectin methylesterases (PMEs) on the other hand can be achieved by the removal of pectin methylesterase inhibitors (PMEIs) from the apoplast, which is attained by the endocytic uptake of the PMEIs in the subapical region of the pollen tube [32]
Summary
Pollen are the male microgametophytes of seed plants, part of the short haploid phase in the life cycle of the spermatophytes. There, the pollen tube ruptures and releases the sperm cells for double fertilisation, which concludes the gametophytic phase of the angiosperm life cycle [3,4,5]. Growth of the angiosperm pollen tube relies on extreme polarisation of the vegetative cell. Massive secretion of new cell wall material takes place at the very tip of the pollen tube, which exclusively extends into one direction. This extreme form of polarised cell expansion is called tip growth and occurs in plant pollen tubes as well as in plant root hairs [6,7]. It poses significant challenges to cell organisation, cell wall integrity, secretion, and signalling networks regulating the complex process of tip growth
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