Hydropowered Pollen

A tension-sensing ion channel regulates hydration and germination in pollen.

Feb 1, 2016
Karen Zusi

ADJUST OR BUST: Pollen grains (green) and their tubes (white) that lack the MSL8 ion channel leak their cellular contents (blue) during germination.GREG JENSEN AND ERIC HAMILTON

EDITOR'S CHOICE IN PLANT BIOLOGY

The paper
E.S. Hamilton et al., “Mechanosensitive channel MSL8 regulates osmotic forces during pollen hydration and germination,” Science, 350:438-41, 2015.

Getting hydrated
When a dry pollen grain first lands on a flower’s stigma, secretions from the flower rehydrate the grain. Eventually, enough inner pressure builds up for the pollen to grow a tube towards the flower’s ovaries. Elizabeth Haswell, a plant molecular biologist at Washington University in Saint Louis, wanted to know how the grain’s cells sense and manage these mechanical changes.

Electrophysiology
Haswell suspected an ion channel was involved, given that membrane tension-sensing channels protect Escherichia coli from taking in too much water. Fluorescent tagging revealed that a related gene, MSL8, was active in Arabidopsis thaliana pollen grains and tubes. When Haswell’s team expressed the gene in Xenopus laevis eggs, the ion channel’s conductance changed in response to membrane tension.

Under pressure
When Haswell’s group grew A. thaliana strains with reduced levels of MSL8 and immersed the pollen in a germination solution, the grains burst, along with the pollen tubes if they had begun growing. When pollen grains overexpressed MSL8 instead, they were unable to form tubes. The team concluded that MSL8 controls the osmotic potential for the cell. “There’s a balance, between having too little or too much, that the pollen grain needs so it can survive hydration but build up enough turgor to germinate,” says Haswell.

Balancing out
“The main component of pollen rehydration is how to control the water going into the pollen grain,” says José Feijó, a plant cell biologist at the University of Maryland, College Park. “This is a mechanism to accommodate the changes in volume and allow them not to be too abrupt or too drastic.”