Enforced growth-rate fluctuation causes pectin ring formation in the cell wall of Lilium longiflorum pollen tubes

Normally growing lily (Lilium longiflorum Thunb.) pollen tubes cultured in standard sucrose medium display a relatively steady tip-growth pattern and a rather even pectin sheath in the cell wall. In an attempt to better understand pulsatory growth, observed in some species, e.g., Petunia, and its possible role in causing the formation of thickened cell wall rings, we have imposed marked fluctuations in the growth-rate of lily pollen tubes. The appropriate growth-perturbing conditions were achieved by modulating the medium osmolarity or by applying caffeine, a non-turgor inhibitor, in a specially designed incubation chamber with a controlled medium flow. The relatively non-esterified pectin deposition in the wall of the growth-interrupted pollen tubes was detected by immunofluorescence microscopy using a monoclonal antibody, JIM 5. The observations show that the periods of slow or inhibited growth correspond to the times when the thickened walls are deposited. Since the growth fluctuations were induced by both turgor- and non-turgor-related means, the proposed endogenous regulatory role of turgor pressure is questioned. Other factors, such as the tip-focused Ca²⁺ gradient which was demonstrated by ratiometric ion imaging, and the alteration in the extensibility of the cell wall, which correlated with pectin esterification/de-esterification, emerge as candidates for the regulation of growth fluctuations.