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Understanding the “romantic journey” of plant reproduction

July 8, 2022
By: Mathew Burciaga
thale cress plant (Arabidopsis thaliana)
A recent PMB study of the thale cress plant (Arabidopsis thaliana) has identified a previously unknown molecular process that serves as a method of communication during fertilization and reproduction. Photos courtesy of Chun Yan.

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.

Green calcium waves "pulse" in this recording provided by the Luan Lab.

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.”

Reinventing Molecular Messaging

According to Luan, understanding the intricate molecular processes of fertilization may help improve the commercial yields in flowering plants. Other researchers or plant geneticists might use the findings to break the interspecies barrier, potentially opening the door to the creation of new hybrid crop species through cross-pollination. But, in addition to the potential commercial application, these findings further highlight plants’ miraculous ability to communicate via molecular emissions. “From an evolutionary point of view, plants reinvented their own molecules specific to their unique communication process,” he added. The calcium channels identified in this study are unique to plants, suggesting they invented a way to produce signals that are different than those found in animals. Luan said researchers have studied calcium channels for more than 30 years, uncovering how they confer resistance to powdery mildew (a fungal disease that affects a wide variety of plants) or enable mechanical sensing in root systems. Their biochemical role remained unknown until this study uncovered the specific channel activity. “Reinventing new channels to communicate in their own way, consistent with different lifestyles of plants and animals, is of general importance to biology,” Luan said. Co-authors include Qifei Gao, Chao Wang, Yasheng Xi, and Qiaolin Shao, postdoctoral researchers in Luan’s Lab; and Legong Li, professor of biology at Capital Normal University in Beijing and a former postdoc in Luan lab.