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Pollen Tube Growth

When a pollen grain (the male gametophyte) lands on the stigma of a receptive flower, it rehydrates and germinates. A pollen tube emerges from one of the three apertures in the pollen coat and begins to grow rapidly down into the tissue of the flower.  The pollen tube is guided through the stigma until reaches an ovule and penetrates the embryo sac (the female gametophyte).  The two sperm cells migrate through the pollen tube while it is growing.  One sperm cell fertilizes the egg cell, giving rise to the embryo, while the other sperm cell fertilizes the central cell, giving rise to the endosperm.  This process is known as double fertilization, and is a hallmark of angiosperms.  The vegetative nucleus also migrates down the pollen tube, but more slowly, and it does not participate in double fertilization.

image of pollen grain
Pollen grain just prior to emergence of the pollen tube from an aperture (lower right corner). 

Pollen can also be induced to germinate in vitro, and this has greatly aided the study of pollen tube growth.  The pollen tube grows by "tip-directed growth", a mode of cell expansion used in highly polarized tissues such as root hairs where new membrane and cell wall is targeted specifically to the growing tip.

Pollen tubes are often used as a model for studying tip-directed growth and vesicular trafficking in general.  Much of this work has been performed in Lily, but Arabidopsis pollen is also seeing increasing use.  Arabidopsis pollen and pollen tubes are pictured to the right and in the movies below.

image of pollen grain with growing pollen tube
Pollen grain with growing pollen tube.
Red = Pollen Coat
Green = DRP1C-GFP

Movie 1: Growing pollen tube expressing DRP1C-GFP, an endocytic marker that localizes just distal to the growing tip.  Pollen coat autofluoresence is shown in red.  Movie is 120x real time.  From Konopka et al. 2008 Plant Cell 20, 1363.

Movie 2: Growing pollen tube expressing CDC48-YFP, which labels the ER and nuclear envelope.  The large, brightly fluorescent vegetative nucleus can be seen migrating down the growing pollen tube, well behind the growing tip.  From Park et al. 2008 Plant Phys. 148, 246.

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