The delivery of sperm cells via tip‐growing pollen tubes is an innovation of seed plants and shows the importance of pollen tubes for reproduction and their specific growth kinetics. Fast‐growing pollen tubes demand an extensive and dynamic vesicular trafficking network to build new cell membrane and wall, to deliver proteins among endomembrane compartments, and also to respond to external stimuli for growth adjustment. In this review, we summarize current findings on endomembrane compartments and vesicular trafficking routes of pollen tubes, comparing and contrasting their features with those of most somatic cells. We discuss the importance of membrane homeostasis, either at the plasma membrane (PM) or between PM‐targeted trafficking and vacuolar trafficking, for pollen tube growth. We also provide perspectives to facilitate future studies of vesicular trafficking in pollen tubes.
Summary Pollen tubes have dynamic tubular vacuoles. Functional loss of AP‐3, a regulator of one vacuolar trafficking route, reduces pollen tube growth. However, the role of canonical Rab5 GTPases that are responsible for two other vacuolar trafficking routes in Arabidopsis pollen tubes is obscure. By using genomic editing, confocal microscopy, pollen tube growth assays, and transmission electron microscopy, we demonstrate that functional loss of canonical Rab5s in Arabidopsis, RHA1 and ARA7, causes the failure of pollen tubes to grow through style and thus impairs male transmission. Functional loss of canonical Rab5s compromises vacuolar trafficking of tonoplast proteins, vacuolar biogenesis, and turgor regulation. However, rha1;ara7 pollen tubes are comparable to those of wild‐type in growing through narrow passages by microfluidic assays. We demonstrate that functional loss of canonical Rab5s compromises endocytic and secretory trafficking at the plasma membrane (PM), whereas the targeting of PM‐associated ATPases is largely unaffected. Despite that, rha1;ara7 pollen tubes contain a reduced cytosolic pH and disrupted actin microfilaments, correlating with the mis‐targeting of vacuolar ATPases (VHA). These results imply a key role of vacuoles in maintaining cytoplasmic proton homeostasis and in pollen tube penetrative growth through style.
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