The legacy of kinesins in the pollen tube 30 years later

Abstract

The pollen tube is fundamental in the reproduction of seed plants. Particularly in angiosperms, we now have much information about how it grows, how it senses extracellular signals, and how it converts them into a directional growth mechanism. The expansion of the pollen tube is also related to dynamic cytoplasmic processes based on the cytoskeleton (such as polymerization/depolymerization of microtubules and actin filaments) or motor activity along with the two cytoskeletal systems and is dependent on motor proteins. While a considerable amount of information is available for the actomyosin system in the pollen tube, the role of microtubules in the transport of organelles or macromolecular structures is still quite uncertain despite that 30 years ago the first work on the presence of kinesins in the pollen tube was published. Since then, progress has been made in elucidating the role of kinesins in plant cells. However, their role within the pollen tube is still enigmatic. In this review, I will postulate some roles of kinesins in the pollen tube 30 years after their initial discovery based on information obtained in other plant cells in the meantime. The most concrete hypotheses predict that kinesins in the pollen tube enable the short movement of specific organelles or contribute to generative cell or sperm cell transport, as well as mediate specific steps in the process of endocytosis.

Heat stress at the bicellular stage inhibits sperm cell development and transport into pollen tubes

“…Notably, a decline in the number of functional proteins, particularly those involved in cytoskeletal organization, was previously also shown in mature HS tomato ( Solanum lycopersicum ) pollen ( Keller et al 2018 ). While transport of sperm cells and vesicles in pollen tubes is largely actin-dependent, microtubules and their kinesin motors play among others an important role of linking the sperm cells with the vegetative nucleus forming the MGU ( McCue et al 2011 ; Cai 2022 ). Their misexpression can well explain the observed transport defect.…”

Heat stress at the bicellular stage inhibits sperm cell development and their transport into pollen tubes

“…Notably, a decline in the number of functional proteins, particularly those involved in cytoskeletal organization was previously also shown in mature HS tomato pollen (Keller et al, 2018). While transport of sperm cells and vesicles in pollen tubes is largely actin-dependent, microtubules and their kinesin motors play among others an important role of linking the sperm cells with the vegetative nucleus forming the male germ unit (McCue et al, 2011; Cai, 2022). Their mis-expression can well explain the observed transport defect.…”

Microtubules ensure transport of vegetative nuclei and sperm cells by fine-tuning their home positions

“…Immunostaining analysis of tubulins and the GFP-tagged plus-end binding protein AtEB1 showed that pollen tubes contain longitudinally aligned microtubules in the shank region and shorter cables in the subapical region (Cheung et al, 2008; Cai and Cresti, 2010). Kinesins with the calponin homology domain (KCH), which belong to the kinesin 14 subfamily expressed in pollen tubes, are candidates for motor proteins that drive SCs, in a “tug-of-war” model (Cai et al, 1997; Schattner et al 2021; Cai, 2022). In vitro experiments have demonstrated that rice Os KCH1 interacts with both microtubules and actin fibers to drive fast (average 69.17 ± 14.77 nm s −1 ) and slow (average 24.2 ± 13.42 nm s −1 ) sliding movements (Schattner et al, 2021; Walter et al, 2015).…”