ABSTRACT
Precise signalling between pollen tubes and synergid cells in the ovule initiates fertilization in flowering plants1. Contact of the pollen tube with the ovule triggers calcium spiking in the synergids2,3 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 function4-6. 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 peptides7 that belong to the rapid alkalinization factor (RALF) family8 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.
Subject(s)
Arabidopsis Proteins , Arabidopsis , Calcium Signaling , Calcium , Calmodulin-Binding Proteins , Membrane Glycoproteins , Phosphotransferases , Pollen Tube , Pollen , Arabidopsis/anatomy & histology , Arabidopsis/metabolism , Arabidopsis Proteins/metabolism , Calcium/metabolism , Calcium Channels/metabolism , Calmodulin-Binding Proteins/metabolism , Cell Membrane/metabolism , Fertilization , Membrane Glycoproteins/metabolism , Ovule/metabolism , Peptide Hormones/metabolism , Phosphotransferases/metabolism , Pollen/metabolism , Pollen Tube/metabolismABSTRACT
We recently revealed that cyclic nucleotide-gated channel 18 (CNGC18) functioned as the main Ca2+ channel in pollen tube tips for pollen tube guidance to ovules by regulating external Ca2+ influx in Arabidopsis. In this study, we found that the reduction of external Ca2+ concentration ([Ca2+]ext) from 10 mM to 5 mM, and further to 2 mM, led to the decreases of pollen germination percentages, but led to the increases of the percentages of ruptured pollen grains and tubes, and branched pollen tubes in vitro in cngc18-17 compared with wild type. The second point mutant allele cngc18-22 showed similar phenotypes, including reduced pollen germination percentages, increased percentages of ruptured pollen tubes, but did not show obvious different percentages of ruptured pollen grains and branched pollen tubes compared with wild type. These data demonstrate that CNGC18 plays essential roles in pollen germination and tube growth as a Ca2+ channel in Arabidopsis.