A receptor-channel trio conducts Ca signalling for pollen tube reception.

Precise signalling between pollen tubes and synergid cells in the ovule initiates fertilization in flowering plants. Contact of the pollen tube with the ovule triggers calcium spiking in the synergids that induces pollen tube rupture and sperm release. This process, termed pollen tube reception, entails the action of three synergid-expressed proteins in Arabidopsis: FERONIA (FER), a receptor-like kinase; LORELEI (LRE), a glycosylphosphatidylinositol-anchored protein; and NORTIA (NTA), a transmembrane protein of unknown function. Genetic analyses have placed these three proteins in the same pathway; however, it remains unknown how they work together to enable synergid-pollen tube communication. Here we identify two pollen-tube-derived small peptides that belong to the rapid alkalinization factor (RALF) family as ligands for the FER-LRE co-receptor, which in turn recruits NTA to the plasma membrane. NTA functions as a calmodulin-gated calcium channel required for calcium spiking in the synergid. We also reconstitute the biochemical pathway in which FER-LRE perceives pollen-tube-derived peptides to activate the NTA calcium channel and initiate calcium spiking, a second messenger for pollen tube reception. The FER-LRE-NTA trio therefore forms a previously unanticipated receptor-channel complex in the female cell to recognize male signals and trigger the fertilization process.