Pollen Tube Tropism and Cytoskeletal Control of Growth

Pollen tubes have the function to rapidly grow and deliver the sperm cells from the pollen grain to the ovule. In order to find its target, the pollen tube needs to be able to perceive external clues and to react by changing its growth direction. The direction of pollen tube growth is controlled by the configuration of the cytoskeleton as it directs secretory vesicles to the site of exocytosis.

Actin mediated vesicle transport

The actin cytoskeleton forms long arrays parallel to the longitudinal axis of the pollen tube (Geitmann et al. 2000, Geitmann and Emons 2000). Close to the apex fine actin filaments form a fringe.

Vesicles accumulate in an inverted cone-shaped region in the apical cytoplasm (Bove et al. 2008).

STICS (spatio-temporal image correlation spectroscopy) revealed that actin-myosin propelled transport of the vesicles towards the tip occcurs in the periphery of the cytoplasm. Vesicles in the central cone move rearward generating an inverse fountain like streaming (Bove et al. 2008).

High temporal resolution imaging in the confocal laser scanning microscope in combination with FRAP (fluorescence recovery after photobleaching) experiments revealed that vesicle flow in the apical region circulates to ensure effective vesicle delivery to the site of exocytosis (Bove et al. 2008). We were able to predict different motion patterns of these vesicles based on a mathematical model combining active, actin-mediated transport an diffusion (Kroeger et al. 2009)The position of exocytosis and endocytosis relative to the tube tip changes during a change in growth direction.

Role of actin in force exertion

In order to invade the pistil, the pollen tube needs to penetrate the apoplast of the transmitting tissue. We devised a pollen tube invasion assay to study the role of the actin and microtubule cytoskeleton in the pollen tube's capacity to invade a stiffened matrix (Gossot and Geitmann 2007).

Cytoskeletal control of growth dynamics

Pollen tube growth is rarely steady but displays oscillatory changes in the growth rate (Chebli and Geitmann 2008). Inhibition of actin polymerization revealed that a functional actin cytoskeleton is a prerequisite for oscillatory growth in pollen tubes (Geitmann et al. 1996)

References

Kroeger JH, Bou Daher F, Grant M, Geitmann A. 2009. Microfilament orientation contrains vesicle flow and spatial distribution in growing pollen tubes. Biophysical Journal 97: 1822-1831

Bove J, Vaillancourt B, Kroeger J, Hepler P, Wiseman PW, Geitmann A. 2008. Magnitude and direction of vesicle dynamics in growing pollen tubes using spatio-temporal image correlation spectroscopy (STICS). Plant Physiology 147: 1646-1658

Chebli Y, Geitmann A. 2007. Mechanical principles governing pollen tube growth. Functional Plant Science and Biotechnology 1: 232-245

Gossot O, Geitmann A. 2007. Pollen tube growth: coping with mechanical obstacles involves the cytoskeleton. Planta 226: 405-416

Geitmann A, Snowman B, Franklin-Tong VE, Emons AMC. 2000. Alterations in the actin cytoskeleton of the pollen tube are induced by the self-incompatibility reaction in Papaver rhoeas. Plant Cell 12: 1239-1251

Geitmann A, Emons AMC. 2000. The cytoskeleton in plant and fungal cell tip growth. Journal of Microscopy 198: 218-245

Geitmann A, Li YQ, Cresti M. 1996. The role of the cytoskeleton and dictyosome activity in the pulsatory growth of Nicotiana tabacum and Petunia hybrida pollen tubes. Botanica Acta, 109: 102-109

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