Pollen tubes grow inside the pistil and deliver their sperm cell to egg cells, which are located deep inside the pistil, to bring about fertilization. Higashiyama's group has previously discovered a pollen tube attractant peptide, called LURE, which is produced by the ovule to guide the pollen tube towards the egg cell. Studies have shown that the structure of LURE differs for each plant species and is specific for each plant's pollen tube, i.e. each LURE peptide preferentially attracts the pollen tube of the same plant species. However, the exact mechanism on how pollen tubes detect LURE has been unknown up to now.
In their recent report published in Nature, a pair of plant biologists at Nagoya University has now revealed for the first time, a key kinase receptor in the pollen tubes (male) of flowering plants responsible for allowing the pollen tubes to precisely detect LURE and reach the egg cell (female) to enable successful fertilization, without losing its way.
They also found that this receptor works with multiple receptors that have a similar structure, in order to precisely detect the signals transmitted from the pistil. By accepting the various signals sent from the pistil, the kinase receptors enable the pollen tubes to grow to a position inside the pistil where they can detect LURE. Subsequently, the pollen tubes are guided to reach the egg cell and pass on their sperm cells for fertilization.
"We found that the structure of LURE differs according to the plant species, and that LURE of a specific plant attracts pollen tubes of the same species, which preserves fertilization between the same species," describes the author. "Therefore, LURE has been identified as the key factor produced by the female organ to attract the male organ in plants."
Nevertheless, the mechanism on how pollen tubes can detect LURE, how the pollen tubes grow to a position inside the pistil where they can detect LURE, and the factors behind growth and responses of the pollen tubes have been unknown. The team decided to look into these questions by trying to unveil the key factors in pollen tubes that enable it to detect LURE.
"By using Arabidopsis thaliana as a model, we hypothesized that the 23 kinase receptors specifically localized on the membrane surface of pollen tubes could be candidates that are necessary to detect LURE," says the author. "I conducted bioassays of pollen tubes by deactivating the function of each kinase receptor and found that the PRK6 receptor was essential to detect LURE."
For PRK6, there are actually multiple families of receptors that have a similar amino acid sequence. Upon deactivating the function of other PRK receptors, researchers found that the loss of various combinations of PRK receptors led to reductions in responses of the pollen tubes to LURE or hindered pollen tube growth. This coincides with previous reports that the growth of pollen tubes is induced by the PRK receptor responding to the signals sent from the pistil. Hence, the team found that PRK6 and its other PRK receptors work together to detect LURE as well as enable pollen tubes to grow to a position inside a pistil where it can detect LURE.
Researchers next studied how PRK6 sends signals within the cells of the pollen tube to understand how it responds to LURE. "When the pollen tube is growing in a straight direction, PRK6 is distributed equally across the cell membrane," explains the author. "I used fluorescently labeled PRK6 and upon addition of LURE to the pollen tube, I observed that PRK6 moves towards the area of cell membrane on the tip of the pollen tube that faces LURE. The pollen tube then changes its direction and starts to grow towards LURE." From these results, the team showed that PRK6 collects the factors necessary for pollen tube growth in the direction of LURE.
"Although the attraction of pollen tubes is considered to occur preferentially between the same species, we wanted to see whether if we can make it occur between different species," says the author. Upon treatment of LURE from Arabidopsis thaliana to a pollen tube of aCapsella rubella (pink shepherd's-purse) plant, which is in the same Brassicaceae (Cruciferae) family as Arabidopsis thaliana, no response to LURE was observed.
Unraveling the unknown receptors and mechanism for fertilization in plants
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