Mechanically gated channels help pollen during plant sex

October 22, 2015 § Leave a comment

Pollen, which consists of grains with the plant male gametes tucked inside, dries as it matures to increase its chances of survival. When carried over to neighboring plants, for example by insects or wind, pollen comes into contact with fluid in the female organ of a plant and then swells rapidly.

This swelling process a sensitive procedure; a pollen grain can die if it is not carefully rehydrated. Until now, the mechanism regulating this fluid uptake was unknown. Researchers writing in the journal Science today report that they have discovered an ion channel that helps pollen grains sense and respond to changes in internal water pressure.

Elizabeth Haswell at Washington University in St. Louis has been studying mechanical signals in plants for more than a decade. In the paper just published, she and her colleagues describe the discovery of ion channels on pollen membranes that monitor and respond to osmotic changes.

If the fluid content inside a membrane becomes too great, pores open to allow ions to leave. Water follows, relieving the pressure. The mechanosensitive ion channel, known as MSL8, senses pressure and makes adjustments as necessary. An incorrect amount of this protein deceases the pollen’s ability to fertilize.

By using RNA analysis, Haswell’s team determined that MSL8 transcripts are found in floral tissue but not in leaf or root tissue. They then fluorescently marked the proteins to show that the proteins were present on the plasma membranes of mature pollen grains.

After rehydrating, pollen grows a tube to carry its sperm cell to the eggs. Haswell and colleagues found that pollen without MSL8 germinated more effectively but generated so much pressure that the tube burst, impairing fertilization. Conversely, pollen that overexpressed MSL8 did not generate enough pressure for the pollen tube to break through the cell wall, rendering the pollen infertile.

This delicate osmotic balance demonstrates mechanical signals aiding in the developmental process. Researchers previously established that bacteria use stretch-activated channels to relieve internal pressure in response to environmental stress signals. The findings by Haswell and colleagues now indicate a previously unknown use for mechanically gated ion channels: reproduction. To cope with “the uncertain and potentially severe conditions of [the] pollen journey, pollen has developed some equally severe compensatory mechanisms, including this fascinating desiccation and rehydration process,” Haswell says. Other strategies include multiple nuclei and a tough cell wall.

While the function of MSL8 seems clear, the mechanism by which it operates — directly, by releasing osmolytes, or indirectly, through regulatory pathways — will be a target for further study. Haswell’s team also is interested in several related ion channels and in studying how membranes survive the dehydration/rehydration process.

 

 This blog post was written by Alexandra Taylor who is a science writing intern at the American Society for Biochemistry and Molecular Biology.

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