RALF34 is a Paracrine Signal to Trigger Pollen Tubes Burst and Sperm Release

During the delivery of sperm cells, pollen tubes maintain the cell integrity via RALF4/19-BUPS/ANX signaling complex when they grow in the maternal tissues. After arriving at the female gametophyte, pollen tubes should rupture timely to release the sperm cells for successful double fertilization, the mechanism of which remains unknown. Here, we show that RALF34 could induce pollen tubes burst in a paracrine manner. RALF34 shares high sequence identity with RALF4 and RALF19, but it shows distinct expression pattern from RALF4/19 in that RALF34 is mainly expressed in the ovule while RALF4/19 are pollen-expressed. RALF34 is distributed in the inner integument cells. Furthermore, RALF34 peptide could not only interact with BUPS1/2 but also be able to bind to ANX1/2, which is consistent with the pull-down results of RALF4/19 and BUPS1/2–ANX1/2, suggesting RALF34 is another ligand of BUPS–ANX receptor complex. In the competition assay, RALF34 peptide could compete with RALF4 for BUPS/ANX binding. Therefore, our results suggest that RALF34 peptide might act as a paracrine signal to bind BUPS1/2–ANX1/2 complex and participates in regulating pollen tubes rupture and sperm cell release in the female tissues.