Ion dynamics and hydrodynamics in the regulation of pollen tube growth
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A coherent picture of pollen tube growth is beginning to emerge that couples ion dynamics with biochemical, biophysical and cytological processes in ordered and controlled feedback circuits that define the nature of polarized apical growth. It is a paradox, however, that complete understanding of the mechanical forces that drive cell elongation in this system still remains to be fully achieved. The results of our recent studies to characterize Cl– ion dynamics during apical growth in tobacco pollen tubes led us to re-examine this question in the light of a possible force-generating role provided by hydrodynamic flow. Previously we found that oscillatory Cl– efflux from the apex is closely coupled to oscillatory growth and the cell volume of the apical domain. Cl– influx occurs in a region of the tube that is distal to the clear zone; hence, a vectorial flow of anion traverses the apical domain and fluxes out of the tip with oscillatory dynamics. Because of the effects that this could induce on charge and osmotic potentials, water could potentially flow through the apical domain, linked to the flux of Cl–. This conjecture is consistent with studies in other plant cells that demonstrate a pivotal role for flux through anion channels in the control or normalization of osmotic status. In the current report, the relationship between Cl– efflux oscillations and the physical characteristics of the apical dome during oscillatory growth is examined in closer detail. Evidence is presented that shows a cyclic deformation of the extreme apex occurs during the growth pulse and is correlated with cyclic Cl– efflux. In addition, there is a dramatic increase in the number and density of clear thread-like zones traversing the apical plasma membrane during the process of tip elongation. Possible functional roles of Cl– flux and hydrodynamics are discussed in the context of what drives tip elongation during cycles of pollen tube growth.
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