PbrSYP71 regulates pollen tube growth by maintaining polar distribution of the endoplasmic reticulum in Pyrus

The polar distribution of endoplasmic reticulum (ER) underlies the rapid polar growth of pollen tubes. However, the mechanism governing ER distribution remains elusive. In this study, we have identified a pollen tube-specific syntaxin protein, PbrSYP71. Our findings reveal that both overexpression and reducing the transcription level of PbrSYP71 inhibit pollen tube growth. Subcellular localization analysis demonstrates that PbrSYP71 anchors to the ER via its transmembrane structure. Overexpression of PbrSYP71 leads to clustered ER distribution in the shank of the pollen tube, while reducing the transcription level of PbrSYP71 abolishes the ER polar distribution. Remarkably, transient overexpression of PbrSYP71ΔABD, lacking the actin-binding domain (ABD) of PbrSYP71, has no impact on ER distribution or pollen tube growth. Further investigation indicates that ABD is positioned on F-actin in the pollen tube and has a direct interaction with F-actin. PbrSYP71 assists the ER in moving toward the apex of the pollen tube, with ABD displaying autonomous mobility. Our study elucidates that PbrSYP71 maintains the polar distribution of the ER by tethering the ER to F-actin, facilitating ER movement towards the pollen tube apex for pear pollen tube elongation. These insights shed light on the mechanisms governing ER distribution in polarized cell growth.