For more than 140 years, pollen tube guidance in flowering plants has been thought to be mediated by chemoattractants derived from target ovules. However, there has been no convincing evidence of any particular molecule being the true attractant that actually controls the navigation of pollen tubes towards ovules. Emerging data indicate that two synergid cells on the side of the egg cell emit a diffusible, species-specific signal to attract the pollen tube at the last step of pollen tube guidance. Here we report that secreted, cysteine-rich polypeptides (CRPs) in a subgroup of defensin-like proteins are attractants derived from the synergid cells. We isolated synergid cells of Torenia fournieri, a unique plant with a protruding embryo sac, to identify transcripts encoding secreted proteins as candidate molecules for the chemoattractant(s). We found two CRPs, abundantly and predominantly expressed in the synergid cell, which are secreted to the surface of the egg apparatus. Moreover, they showed activity in vitro to attract competent pollen tubes of their own species and were named as LUREs. Injection of morpholino antisense oligomers against the LUREs impaired pollen tube attraction, supporting the finding that LUREs are the attractants derived from the synergid cells of T. fournieri.

Directional control of tip-growing cells is essential for proper tissue organization and cell-to-cell communication in animals and plants. In the sexual reproduction of flowering plants, the tip growth of the male gametophyte, the pollen tube, is precisely guided by female cues to achieve fertilization. Several female-secreted peptides have recently been identified as species-specific attractants that directly control the direction of pollen tube growth. However, the method by which pollen tubes precisely and promptly respond to the guidance signal from their own species is unknown. Here we show that tip-localized pollen-specific receptor-like kinase 6 (PRK6) with an extracellular leucine-rich repeat domain is an essential receptor for sensing of the LURE1 attractant peptide in Arabidopsis thaliana under semi-in-vivo conditions, and is important for ovule targeting in the pistil. PRK6 interacted with pollen-expressed ROPGEFs (Rho of plant guanine nucleotide-exchange factors), which are important for pollen tube growth through activation of the signalling switch Rho GTPase ROP1 (refs 7, 8). PRK6 conferred responsiveness to AtLURE1 in pollen tubes of the related species Capsella rubella. Furthermore, our genetic and physiological data suggest that PRK6 signalling through ROPGEFs and sensing of AtLURE1 are achieved in cooperation with the other PRK family receptors, PRK1, PRK3 and PRK8. Notably, the tip-focused PRK6 accumulated asymmetrically towards an external AtLURE1 source before reorientation of pollen tube tip growth. These results demonstrate that PRK6 acts as a key membrane receptor for external AtLURE1 attractants, and recruits the core tip-growth machinery, including ROP signalling proteins. This work provides insights into the orchestration of efficient pollen tube growth and species-specific pollen tube attraction by multiple receptors during male-female communication.

AtLURE1 defensin-like peptides, which show species-specific evolution, are essential in Arabidopsis for attracting pollen tubes and can function in the breakdown of reproductive isolation barriers.

During sexual reproduction of flowering plants, pollen tube guidance by pistil tissue is critical for the delivery of nonmotile sperm cells to female gametes. Multistep controls of pollen tube guidance can be divided into two phases: preovular guidance and ovular guidance. During preovular guidance, various female molecules, including stimulants for pollen germination and pollen tube growth, are provided to support tube growth toward the ovary, where the ovules are located. After entering the ovary, pollen tubes receive directional cues from their respective target ovules, including attractant peptides for precise, species-preferential attraction. Successful pollen tube guidance in the pistil requires not only nutritional and directional controls but also competency controls to make pollen tubes responsive to guidance cues, regulation to terminate growth once a pollen tube arrives at the target, and strategies to stop ovular attraction depending on the fertilization of female gametes.