Increased endocytosis of fluorescent phospholipid in tobacco pollen in microgravity and inhibition by verapamil.

Gravity sensing in plants occurs in specialised tissues, like in the columella in root tips or the endodermis in shoots. Generally, dense organelles, acting as statoliths, are thought to interact with the cytosekeleton and ion channels in gravitropism. We examined the possibility that tobacco pollen tubes (Nicotiana sylvestris) having an elaborate cytoskeleton could perceive gravity through interaction of the cytoskeleton and the endomembrane system and organelles. Using lipid endocytosis as a quantitative parameter, we show that endocytosis is increased transiently in microgravity within 3 min. This increase is inhibited by the calcium blocker verapamil, suggesting that calcium is lowered in the tip, which is known to increase endocytosis in the pollen tube.