In flowering plants, fertilization occurs after the male gametophyte (pollen) germinates and produces a pollen tube, which grows through the pistil to reach the female gametophyte, where it ruptures to deliver sperm cells to the ovule. The receptor-like tyrosine kinase (RLK) FER is required in the female gametophyte of Arabidopsis thaliana for termination of pollen tube growth and the subsequent fertilization process, but little is known about the male gametophyte–specific factors required for fertilization. Boisson-Dernier and colleagues have identified two RLKs related to FER, ANX1 and ANX2, which are required in the male gametophyte for pollen tube growth. Microarray and reverse transcription polymerase chain reaction (RT-PCR) analyses showed that anx1 and anx2 were expressed in pollen, and transgenically encoded fluorescent ANX1 and ANX2 fusion proteins were enriched in the plasma membrane at the tip of the pollen tube. Although mutation of either ANX1 or ANX2 had no effect on fertilization, anx1;anx2 double mutants exhibited male sterility, and, although anx1;anx2 pollen germinated on wild-type pistils, it failed to grow or fertilize the female gametophyte. In an in vitro pollen tube assay, anx1;anx2 pollen germinated and initiated tube growth, but the mutant pollen tubes grew slowly and burst prematurely compared with wild-type or single-mutant pollen. Pollen tube bursting in anx1;anx2 mutant pollen was morphologically similar to wild type, suggesting that only the timing of pollen tube rupture was affected in the mutants. The authors propose that signaling through the RLKs ANX1 and ANX2 in the male gametophyte sustains pollen tube growth and that signaling through this pathway is deactivated when the pollen tube reaches the female gametophyte, thus inducing the pollen tube to rupture and deliver its cargo of sperm.
A. Boisson-Dernier, S. Roy, K. Kritsas, M. A. Grobei, M. Jaciubek, J. I. Schroeder, U. Grossniklaus, Disruption of the pollen-expressed FERONIA homologs ANXUR1 and ANXUR2 triggers pollen tube discharge. Development 136, 3279–3288 (2009). [Abstract] [Full Text]