Ionic and osmotic disruptions of the lily pollen tube oscillator: testing proposed models

Abstract

Two mechanisms have been proposed as the primary control of oscillating tip growth in Lilium longiflorum Thunb. pollen tubes: changes in cell wall strength (Holdaway-Clarke et al. 1997) or alternatively, changes in turgor pressure (Messerli et al. 2000). Here we have modified the ionic and osmotic concentrations of the growth medium to test predictions derived from both models. Raising the [Ca²⁺]_o tenfold above normal reduced the amplitude of the [Ca²⁺]_i oscillations and growth oscillations while it raised the basal [Ca²⁺]_i and growth rate such that the average growth rate did not change. Raising the [H⁺] of the growth medium tenfold reversibly decreased and sometimes eliminated the [Ca²⁺]_i and growth oscillations without changing the average growth rate. Lowering the [H⁺] tenfold led to irregular frequency and amplitude [Ca²⁺]_i oscillations, reduced the average growth rate of tubes and led to cell bursting in 33% of tubes. Addition of 50 mM H⁺ buffer, MES, to prevent pH changes in the cell wall increased the period, amplitude and duration of both [Ca²⁺]_i and growth oscillations. Changing the [K⁺]_o did not markedly effect [Ca²⁺]_i oscillations. Reducing the osmolarity of the medium led to transient large-amplitude [Ca²⁺]_i and growth oscillations while reducing large-amplitude oscillations over long periods. In many different conditions under which growth still occurs, lily pollen tubes maintain growth oscillations, albeit with modified frequency, amplitude and duration. We conclude that modifications to both proposed models are necessary to explain oscillating growth in this system.