Durotropic Growth of Pollen Tubes

Reimann R, Kah DTE, Mark C, Dettmer J, Reimann T, Gerum R, Geitmann A, Fabry B, Dietrich P, Kost B (2020)

Publication Type: Journal article

Publication year: 2020


Book Volume: 183

Pages Range: 558-569

Issue: 2

DOI: 10.1104/PP.19.01505


To reach the female gametophyte, growing pollen tubes must penetrate different tissues within the pistil, the female reproductive organ of a flower. Past research has identified various chemotropic cues that guide pollen tubes through the transmitting tract of the pistil, which represents the longest segment of its growth path. In addition, physical mechanisms also play a role in pollen tube guidance; however, these processes remain poorly understood. Here we show that pollen tubes from plants with solid transmitting tracts actively respond to the stiffness of the environment. We found that pollen tubes from Nicotiana tabacum and other plant species with a solid or semisolid transmitting tract increase their growth rate in response to an increasing matrix stiffness. By contrast, pollen tubes from Lilium longiflorum and other plant species with a hollow transmitting tract decrease their growth rate with increasing matrix stiffness, even though the forces needed to maintain a constant growth rate remain far below the maximum penetration force these pollen tubes are able to generate. Moreover, when confronted with a transition from a softer to a stiffer matrix, pollen tubes from N. tabacum display a greater ability to penetrate into a stiffer matrix compared with pollen tubes from L. longiflorum, even though the maximum force generated by pollen tubes from N. tabacum (11 µN) is smaller than the maximum force generated by pollen tubes from L. longiflorum (36 µN). These findings demonstrate a mechano-sensitive growth behavior, termed here durotropic growth, that is only expressed in pollen tubes from plants with a solid or semisolid transmitting tract and thus may contribute to an effective pollen tube guidance within the pistil.

Animal sperm cells have the ability to freely swim by rhythmic movements of their flagella (Malo et al., 2006). By contrast, sperm cells of angiosperm plants have lost this ability (Dresselhaus et al., 2016) and are contained within the cytoplasm of the vegetative cell of a pollen grain. Upon germination, the vegetative pollen cell forms a long tubular protrusion, the pollen tube, which rapidly elongates through the pistil and transports the enclosed immobile sperm cells toward the egg cell and the central cell for double fertilization (Zhang et al., 2017).

As opposed to cell division, pollen tubes elongate by tip growth. This process depends on a fine-tuned interplay between turgor pressure and vesicle trafficking, which delivers material required for cell wall and plasma membrane extension exclusively to the pollen tube tip (Lord, 2000; Chebli et al., 2013; Hafidh et al., 2016a; Grebnev et al., 2017; Luo et al., 2017). Therefore, only the surface at the pollen tube tip changes its relative position with respect to the environment during cell elongation, giving rise to a low-friction and thus energetically favorable growth process that has also been observed in other invasively growing cell types, such as root hairs, fungal hyphae, and neurons (Palanivelu and Preuss, 2000; Sanati Nezhad and Geitmann, 2013). This mechanism enables pollen tubes of some plant species to grow at rates of more than 300 µm·min−1 (Williams et al., 2016), faster than any other plant cell (Shamsudhin et al., 2016).

To reach the female gametophyte, growing pollen tubes must penetrate different tissues within the pistil. After initial growth on the surface of the stigma, pollen tubes subsequently elongate through the transmitting tract within the style and the ovary, penetrate the septum epidermis to leave the transmitting tract, continue to elongate on the surface of the funiculus and through the micropyle of the ovule, and finally enter a synergid cell, where they burst and discharge their cytoplasm together with the enclosed sperm cells (Hulskamp et al., 1995; Crawford et al., 2007). Over the past two decades, many factors have been identified that are involved in the guidance of pollen tubes along their path toward the female gametophyte, including sugars, calcium ions, nitric oxide, lipids, and secreted peptides (Hulskamp et al., 1995; Ray et al., 1997; Wolters-Arts et al., 1998; Mollet et al., 2000; Higashiyama et al., 2003; Prado et al., 2004; Chae and Lord, 2011; Sanati Nezhad et al., 2014; Qu et al., 2015; Hafidh et al., 2016b; Higashiyama and Yang, 2017; Jiao et al., 2017). Most of these chemical signals guide the pollen tubes toward and inside the ovule following their emergence from the transmitting tract. However, the transmitting tract of the pistil typically represents the longest section of the pollen tube growth path in situ (de Graaf et al., 2003; Crawford and Yanofsky, 2008). Because chemical gradients are more difficult to maintain over longer distances, physical guidance mechanisms are thought to play an important role in directing pollen tube growth within the transmitting tract (Lennon et al., 1998; Lush et al., 2000), but this has so far not been characterized in detail.

Flowers of different plant species display a highly diverse pistil and transmitting tract anatomy, which complicates the investigation of physical pollen tube guidance. Within the transmitting tract of hollow (sometimes called “open”) styles, as observed for example in Lilium longiflorum flowers, pollen tubes grow on the epidermal surface of a cell-free canal filled with a viscous extracellular matrix (Sanders and Lord, 1992; de Graaf et al., 2001; Erbar, 2003). By contrast, the transmitting tract in solid (sometimes called “closed”) styles, as found in Arabidopsis (Arabidopsis thaliana) and Nicotiana tabacum flowers, is filled with tissue composed of cells embedded in an extracellular matrix, which pollen tubes need to penetrate (Lennon et al., 1998; Cheung et al., 2000; Erbar, 2003). The tissue within the transmitting tract imposes a substantial physical resistance on pollen tube growth (Agudelo et al., 2012), in particular as intercellular spaces in this tissue are typically smaller than the pollen tube diameter (Lennon et al., 1998; Roy et al., 1999; Cheung et al., 2000).

The maximum (or stalling) force that a growing pollen tube can generate to overcome the mechanical resistance restricting its expansion within pistil tissues is determined by the product of the hydrostatic turgor pressure and the cross-sectional area of the tube at its tip. Maximum forces generated by L. longiflorum tubes measured with capacitive force sensors (Burri et al., 2018) were found to be in the range of 9.6 ± 1.6 µN, whereas forces generated by Camellia japonica pollen tubes measured with soft microcantilevers (Ghanbari et al., 2018) were found to be around 1.5 µN. However, direct force measurements using capacitive force sensors or microcantilevers are technically highly demanding (Agudelo et al., 2013; Ghanbari et al., 2014; Sanati Nezhad et al., 2014). Moreover, measurements of the maximum stalling force cannot provide information on the penetration force of a pollen tube growing inside a pistil under physiological conditions, as this force depends on the growth rate and the mechanical impedance of the surrounding matrix (Sanati Nezhad et al., 2013).

In this study, we present a method to estimate the penetration force generated by a pollen tube tip during its growth through matrices of different stiffness. With this method, we investigated pollen tubes from plant species, both with hollow and with solid styles, and measured the relationship between penetration force and growth rate for matrices with different mechanical properties. We found that pollen tubes from plants with solid styles, but not those from plants with hollow styles, increase their growth rate in matrices with higher physical resistance, indicating that an active mechanosensory mechanism controls the tropic growth of these pollen tubes, which may help them to navigate through the complex architecture of the pistil to reach the female gametophyte. We propose the term durotropism for this mechanosensory mechanism that guides pollen tubes from plants with solid styles toward stiffer environments, analogous to the term durotaxis that describes the preferential migration of mammalian mesenchymal cells toward regions with higher substrate rigidity (Lo et al., 2000).

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Reimann, R., Kah, D.-T.E., Mark, C., Dettmer, J., Reimann, T., Gerum, R.,... Kost, B. (2020). Durotropic Growth of Pollen Tubes. Plant Physiology, 183, 558-569. https://doi.org/10.1104/PP.19.01505


Reimann, Ronny, et al. "Durotropic Growth of Pollen Tubes." Plant Physiology 183 (2020): 558-569.

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