For more than a century, researchers have marveled at how pollen creeps down into a plant, growing a tube that twists and turns to reach the target ovary. But the nature of the siren song guiding this descent remained a mystery until now. For the first time, researchers have pinpointed the exact molecules one plant uses to attract pollen tubes to its reproductive organs.
The proteins, which researchers in Japan and Germany dubbed LUREs, can signal to pollen tubes from as great a distance as 200 micrometers in Torenia fournieri, the team reports in the March 19 Nature. While the proteins’ role seems specific to this flower, the findings still represent a breakthrough in understanding plant reproduction, other researchers say.
Scientists had long debated whether pollen tubes respond to chemical signals within the plant, notes plant geneticist Mark Johnson at Brown University in Providence, R.I. By demonstrating the role of these proteins in this flower, “this paper really nails that point very clearly now,” he says.
Landing on a flower petal is just the first hurdle for a pollen grain. It then has to grow a tube down a long cylinder called the style and locate the flower’s ovaries so sperm can move through it and actually fertilize an egg. T. fournieri, also known as the wishbone flower, makes a good model system because its ovaries, or embryo sacs, protrude from the base of the flower, plant biologist Tetsuya Higashiyama of Nagoya University in Japan and his colleagues note in the new study.
The team’s previous research suggested synergid cells, which hug the embryo sac, direct the tubes once they approaches the ovary, a bit like an air traffic controller motioning a plane to the landing strip. In the new study, the researchers surveyed more than 2,000 proteins expressed on synergid cells to clinch the exact molecules luring pollen tubes to the embryo sac.
A group of molecules called cysteine-rich proteins, or CRPs, were most common on the synergid cells. The researchers focused on the three largest: TfCRP1, TfCRP2 and TfCRP3, and tested the proteins’ attraction potential by injecting TfCRP3 near pollen tubes on a growth plate. Sixty percent of the tubes edged toward the protein.
When incorporated into gelatin beads that were placed about 50 micrometers from the pollen tubes, TfCRP2 didn’t seem to attract pollen tubes. But TfCRP1 attracted up to 56 percent of the pollen tubes and TfCRP3 up to 73 percent. This prompted the researchers to rename these proteins LURE1 and LURE2. When the researchers blocked these proteins’ production in the embryo sacs, significantly fewer pollen tubes sidled up to the ovary. This observation is further evidence that LUREs act as attractants in T. fournieri, the researchers write.
“This molecule was the missing molecule for plant reproduction,” Higashiyama says. Though these specific LURE proteins did not attract pollen tubes in tests with other plants, Higashiyama and his colleagues believe other cysteine-rich proteins may act as attractants in those systems, too. The next steps include identifying which of these proteins function as LUREs in other plants and which proteins direct the tubes when they’re farther from the embryo sac, he notes.
Figuring out how pollen cells perceive signals from LURE proteins will also be key, Johnson adds. Teasing apart the mechanisms of targeted cell growth could eventually help researchers understand how flowering plants make seeds or serve as a model for how other types of plant and animal cells send and receive signals, he says.