Arabidopsis pollen tube integrity and sperm release are regulated by RALF-mediated signaling

Successful fertilization in plants requires sperm cells (SCs) to be carried along the growing pollen tube (PT) until reaching the female gametophyte where the PT then bursts to release the SCs. One challenge PTs must overcome in order to achieve fertilization is deciding to rupture or not to rupture. Ge et al. identified two receptor-like kinases (RLKs), BUPS1 and BUPS2, expressed at the PT surface. Loss of BUPS1 resulted in shorter siliques and a drastic reduction in seed set, observed to be male-specific. The bups1/2 mutant PTs also ruptured prematurely, most not travelling beyond the style. BUPS1/2 selectively bind to the pollen-expressed peptides RALF4 and RALF19 and ralf4/19 mutants were phenotypically similar to bups1/2 mutants. RALF4/19 also interact with ANX1 and ANX2, two other pollen expressed RLKs. The data suggest a potential heteromer between BUPS1/2 and ANX1/2, which bind RALF4/19 to maintain PT integrity. RALF34, an ovule-derived peptide, was found to induce PT rupture and competes against RALF4/19 for binding to BUPS1 and ANX1. The authors set forth a model in which male-derived ligands bind a PT receptor complex to maintain growth and, upon perception of a female-derived ligand, induces PT rupture (Summary by Alecia Biel) Science. 10.1126/science.aao3642.