Transform in seconds, ready for fertilization.

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Transform in seconds, ready for fertilization.

A research group directed by Associate Professor Daisuke Maruyama in Kihara Institute for Biological Research, Yokohama City University, revealed dynamic changes of plant sperm cells immediately before fertilization.

Date: 31 Jan 2023

Maruyama and his colleagues including Project Assistant Professor Naoya Sugi and Rie Izumi developed new live-imaging system and captured the moment of sperm cell maturation prior to double fertilization in a model plant Arabidopsis thaliana.

In most flowering plants, a pair of sperm cells cannot swim by themselves, but are transported through actively growing pollen tubes. Sperm cells are enclosed by a single membrane called the inner vegetative plasma membrane (IVPM) throughout this transport. Sperm cells should quickly remove IVPM upon pollen tube discharge, allowing direct plasma membrane contact between exposed sperm cells with their fertilization partner cells. However, morphological change of IVPM and precise timing of its breakdown remains to be shown. Maruyama’s group generated a transgenic plant whose IVPM was specifically visualized with a fluorescent protein. His group cultured pollen tubes from this marker plant with mature ovules and observed fragmentation of IVPM immediately after pollen tube discharge by fluorescent microscopy. This observation of rapid IVPM removal from sperm cells provides new insight on the sophysticated mechanism of double fertilization in flowering plants.

This study was published in "Frontiers in Plant Science" on January 26, 2023.

ー(Video) Modified from Sugi et al., 2023 Front. Plant Sci.

Points of the research results

We successfully captured the moment when the inner plasma membrane fragments and collapses immediately after the release of sperm cells from the pollen tube.
The inner plasma membrane breakdown is inhibited in sperm cells during transport.
The elucidation of the changes in the inner plasma membrane contributes to a comprehensive understanding of the activation mechanism of sperm cells.

FIGURE 1
A fine control of IVPM breakdown prior to gamete interaction.
Upper panel indicates stable IVPM whose integrity is maintained until pollen tube (PT) discharge. Lower panel shows selective elimination of IVPM after PT discharge. Trigger signal(s) of the IVPM breakdown would arise simultaneously with PT discharge, and thus may be caused by osmotic stress, mechanical damage, ROS and/or Ca2+ signal. Due to IVPM breakdown, sperm cells must expose their plasma membranes beforehand to adhere, recognize, and fuse with the egg or central cells.