Signalling Pinpointed to the Tip: The Complex Regulatory Network That Allows Pollen Tube Growth

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.

Implications of ionizing radiation on pollen performance in comparison with diverse models of polar cell growth

“…In addition to rapidly expanding pollen tubes, tip growth, which is limited to the apical dome of the cell, has correspondingly been documented for several other cell types of diverse plant species, namely root hairs (Jones & Dolan, 2012; Mendrinna & Persson, 2015), moss protonemata (caulonema and chloronema) (Menand, Calder, & Dolan, 2007; Prigge & Bezanilla, 2010; Vidali & Bezanilla, 2012), fern rhizoids (Bushart & Roux, 2007; Cooke & Racusen, 1986; Jones & Dolan, 2012), algal rhizoids (Hable, 2014; Hable, 2018; Hable & Hart, 2010), and additionally for hyphae of filamentous fungi (Riquelme et al, 2018; Takeshita, 2016). It is well known that the polarized growth process of these cells requires a highly specialized subcellular organization with similar functional characteristics that act in concert (Bascom, Hepler, & Bezanilla, 2018; Hable & Hart, 2010; Hepler & Winship, 2015; Scholz, Anstatt, Krawczyk, & Ischebeck, 2020; Stephan, 2017). Their cytoplasmic organization has related requirements, such as a specialized dynamic cytoskeleton which comprises prominent distinct features particularly in the cellular tube tip: Longitudinally oriented massive actin bundles and actin patches in the tube shank region.…”

Imaging the living plant cell: From probes to quantification

“…While live imaging of cytoskeleton components has been carried out in many different cell types, it is worth mentioning the few model systems that have been recurrently used over the years by different groups. For example, root hairs and pollen tubes have extensively been used to study cytoskeleton dynamics in tip growing cells (Ketelaar, 2013;Scholz et al, 2020;Xu and Huang, 2020). The tobacco (Nicotiana tabacum) pollen tube is, in particular, an excellent model for live imaging studies of tip growth because they are big cells that are easy to transform and to image (Kost et al, 1998;Klahre and Kost, 2006;Scholz et al, 2020;Xu and Huang, 2020;Fratini et al, 2021).…”

“…For example, root hairs and pollen tubes have extensively been used to study cytoskeleton dynamics in tip growing cells (Ketelaar, 2013;Scholz et al, 2020;Xu and Huang, 2020). The tobacco (Nicotiana tabacum) pollen tube is, in particular, an excellent model for live imaging studies of tip growth because they are big cells that are easy to transform and to image (Kost et al, 1998;Klahre and Kost, 2006;Scholz et al, 2020;Xu and Huang, 2020;Fratini et al, 2021). Microtubules are critical for anisotropic growth, which has been extensively studied in the hypocotyl (Shaw, 2013;Lenarcic et al, 2017).…”

Heat stress leads to rapid lipid remodelling and transcriptional adaptations in Nicotiana tabacum pollen tubes

“…It can form nanodomains in the plasma membrane that are important for Rho signalling . These small G proteins have also been shown to be important for the regulation of pollen tube growth (Scholz et al, 2020), but it remains to be studied if they also depend on PS, how PS is distributed in the pollen tube, and if this distribution changes under HS.…”