Researchers in the Department of Plant and Microbial Biology (PMB) have uncovered the intricate molecular processes that precede reproduction in flowering plants.

Published July 6 in Nature, the findings document a previously unknown molecular process that serves as a method of communication during fertilization.

According to Professor Sheng Luan, chair of the PMB department and the paper’s senior author, the exact mechanism for signaling has previously eluded researchers.

“At the molecular level, this whole process is now more clear than ever before,” he said.

Sending Molecular "Love Notes"

Flowers reproduce sexually through pollination, a process that involves the transfer of pollen from a flower’s stamen (the male fertilizing organ) to the stigma on the pistil (the female reproductive organ). Once the pollen grain lodges on the stigma, a pollen tube grows from the pollen grain to an ovule to facilitate the transfer of sperm to the egg.

Luan said researchers have previously recorded the presence of calcium waves preceding the fertilization process and noted that “they knew the calcium signal is important but didn’t know exactly how it is produced.”

To analyze how the calcium wave was produced by the female cell, Luan and his co-authors introduced a biosensor to report calcium levels in the specific cell to look for signals from the male parts that trigger calcium waves.

They found that pollen tubes emit several small peptides—short chains of amino acids—that can be recognized by peptide receptors on the surface of the female cell. Once activated, these receptors recruit a calcium channel to produce a calcium wave that guides the pollen tube to the ovule and initiates fertilization.

“You could compare this to a delivery service,” Luan explained. “We know the small peptide molecule serves as a signal to the female part of the flower, almost like a knock on the door letting it know the pollen tube is here.”

The calcium waves ultimately cause the pollen tube to rupture and release the immobile sperm once it is inside the ovule, ensuring a successful fertilization process.

“In a way, they basically commit suicide to release the sperm,” Luan said. “Sometimes the female reproductive cell also dies in order to expose the egg so they can meet and produce new life. It’s kind of a romantic journey for plant reproduction.”