THE ROLE OF ANION CHANNELS IN POLLEN GERMINATION AND TUBE GROWTH pp. 235-253
Authors: (Maria Breygina, Department of Plant Physiology, Biological Faculty, Lomonosov Moscow State University, Moscow, Russia)
Abstract: Germinating pollen grain provides the formation and delivery of the sperm cells to the ovule, and therefore plays a central role in sexual plant reproduction. Moreover, it is relatively simply organized, and became a widely used model in studies of plant physiology, including those of morphogenesis regulation and polar growth. An important feature of male gametophyte is a capability for polar growth based on structural and functional cytoplasm compartmentalization. Numerous studies demonstrated common mechanisms controlling the apical growth of pollen tubes, root hairs, fern rhizoids, fungal hyphae and axons of neurons (Geitmann, Emons, 2000; Palanivelu, Preuss, 2000; Bushart, Roux, 2007). The pivotal role in polar growth organization is played by inorganic ions (Hepler et al., 2006). Heterogeneous distribution of ion fluxes, intracellular ion gradients and their connection with morphogenesis has been shown for different cells, including growing fungal hyphae, frog and fish oocytes, drosophila and brown algae eggs (Nuccitelli, 1988). As early as in 1975 it was shown using nonselective microelectrodes that lily pollen grain activation leads to the appearance of transmembrane ion currents (Weisenseel et al., 1975). The inward current reached maximal values in the place of the would-be germination, while the outward current – on the opposite pole. After germination the inward currents were distributed along the tube, while the outward currents were shifted towards the pollen grain. According to these authors (Weisenseel, Wenisch, 1980), membrane potential values of the vegetative cell of lily pollen grain varied from -90 to -130 mV. In the later studies similar results (from -110 to -150 mV) were obtained for lily pollen using improved microelectrode methods (Obermeyer, Blatt, 1995). For pollen grains of other plant species, more positive membrane potential values have been reported: -30 mV in Petunia hybrida and -37 mV in Narcissus (Feijo et al., 1995), while potential of isolated pollen protoplasts of Brassica chinensis was -79 mV (Fan et al., 2003). Pollen tube membrane potential also varied in different plant species: e.g., in Agapanthus umbellatus it amounted to -55 mV (Malho et al., 1995), whereas in Arabidopsis – around -100 mV (Mouline et al., 2001).