Localized endocytosis in tobacco pollen tubes: visualisation and dynamics of membrane retrieval by a fluorescent phospholipid.

Two modes of endocytosis are known to occur in eucaryotic cells: fluid phase and receptor-mediated endocytosis. Fluid-phase endocytosis in plant cells resembles the retrieval of excess plasma membrane material previously incorporated by exocytosis. Pollen tubes need to carry out strong membrane retrieval due to their fast polar tip growth. Plasma membrane labelling of pollen tubes, grown in suspension, was achieved by the incorporation of a fluorescently modified phospholipid, 1,2-bis-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-undecanoyl)-sn-glycero-3-phosphocholine (20 microM) and measured with a confocal laser-scanning microscope. Time course experiments revealed a highly localised and relatively fast plasma membrane retrieval below the tip within the first 5 min after phospholipid application. The retrieved fluorescent plasma membrane was quickly re-integrated into parts of the endomembrane pool and then redistributed to the pollen tube base and very tip of the apex, with the exception of the cortical endoplasmic reticulum (ER) and the mitochondria even after 1-h incubation period. Low temperature (10 degrees C) and the actin filament depolymerizing cytochalasin D (2 microM) completely abolished plasma membrane retrieval, whereas the microtubule destabilizing herbicide oryzalin (1 microM) had no effect. Our results provide strong support for a highly localised endocytotic pathway in tobacco pollen tubes. Passive uptake of bis-Bodipy FL C(11)-phosphocholine by mere penetration can be excluded. It is a valuable alternative to the styryl dyes often used in endocytotic studies, and may also be used to follow lipid turnover because membrane flow of labelled membranes occurs apparently not in a default manner as ascertained by its fast distribution.