Pectate lyase-like lubricates the male gametophyte's path toward its mating partner

Abstract

The pollen tube is an extension of the male gametophyte in plants and mediates sexual reproduction by delivering the sperm cells to the female gametophyte. To accomplish this task, the elongating pollen tube must break through the thick wall of the pollen grain and penetrate multiple pistillar tissues. Both processes require the loosening of cell wall material-that of the pollen intine and that of the apoplast of the transmitting tract. The enzymatic toolbox for these cell wall modifying processes employed by the invading male gametophyte is elusive. We investigated the role of the pectin-digesting pectate lyase-like (PLL) by combining mutant analysis with microscopy observations, fluorescence recovery after photo-bleaching experiments, and immuno-detection. We show that in Arabidopsis (Arabidopsis thaliana), PLLs are required for intine loosening during the first steps of pollen tube germination. We provide evidence that during pollen tube elongation, PLLs are released by the pollen tube into the extracellular space, suggesting that they may be employed to soften the apoplast of the transmitting tissue. The synergistic enzymatic action of PLLs in the pollen grain, the pollen tube, and the transmitting track contribute to an effective fertilization process.