Actin cytoskeleton regulates pollen tube growth and tropism
Fertilization in plants depends on the delivery of the sperm cells in the pollen grain through the pollen tube to the ovule. The pollen tube is a highly anisotropic, fast growing cellular protuberance. Because the pollen tube is non autotrophic, it requires a steady supply of carbohydrates and minerals supplied by the pistil to sustain its growth. These elements serve for the synthesis of cell wall material, delivered to the site of cell wall assembly in secretory vesicles that are transported along the actin cytoskeleton and deposited at the growing apex of the tube. The tube has to resist external deformation forces in order to maintain its cylindrical shape and to respond to various directional signals in order to reach its target. My objectives were to identify the role of the cytoskeleton in the anisotropic growth of the pollen tube and to determine how the tube responds to directional cues. The cytoskeleton in the pollen tube consists of microfilaments and microtubules, both forming long filamentous elements. For in vitro growing pollen tubes, carbohydrates and growth minerals have to be added to the growth medium in specific amounts order to sustain pollen tube growth. I optimized the growth conditions of Arabidopsis thaliana and Camellia japonica pollen tubes which, in addition to pollen from Lilium longiflorum, were used as model species for my experiments. I developed a microwave based, fast and efficient fixation and labelling protocol for pollen tubes. I used pharmacological, mechanical, molecular and microscopical tools to study the role of the cytoskeleton in pollen tube growth and tropism. I found that the actin cytoskeleton, and more specifically the subapical actin fringe, plays an important role in the regulation of pollen tube growth and architecture through the controlled delivery of secretory vesicles to the growing apex. I constructed a galvanotropic chamber that can be mounted on an inverted microscope to induce controlled tropic triggers. I found that the actin cytoskeleton is also involved in the ability of the pollen tube to change its direction. This tropic behaviour was shown to be dependent on the concentration of calcium ions in the growth medium and calcium influx through calcium channels. The cytosolic calcium gradient in the pollen tube regulates the activity of various actin binding proteins that are responsible for remodelling the actin cytoskeleton. Among these proteins are two Arabidopsis gametophyte-specific actin depolymerizing factors (ADFs) that I tagged with two intrinsically fluorescent proteins. I found that ADF7 and ADF10 are differentially expressed during microsporogenesis and pollen tube germination and growth and that they likely divide important functions between them.
Actin Arabidopsis thaliana Arabidopsis thaliana Calcium Calcium Camellia japonica Camellia japonica Cytoskeleton Lilium longiflorum Lilium longiflorum Pollen Pollen Tropism Pollen tube ADF Actin depolymerizing factor