Apical pollen tube wall curvature correlates with growth and indicates localized changes in the yielding of the cell wall

Abstract

The shape of the apical region of lily pollen tube changes rhythmically as the growth rate of the tube oscillates becoming alternately more prolate then back to oblate. We quantified shape change by calculating the curvature of the cross-sectional edge of the pollen tube tip and cross-correlating curvature changes with growth rate. The apical region takes the form of a partial elliptical spheroid, with variation in the length and location of the minor axis. During oscillation curvature profiles show a sharp increase in curvature at the “shoulders” of the apex when oblate, 4–7 μm from the flatter central zone. As the tip becomes more prolate, the “shoulders” decrease rapidly in curvature and move towards the growth axis as curvature at the tip increases. We understand curvature changes to represent differential changes in local wall expansion rates, driven by uniform turgor pressure and mediated by changes in wall polysaccharides. To become more oblate, the tip region must become less extensible than the “shoulder” region. And, as the tip becomes more prolate, the increased curvature must be due to increased local expansion. We found that changes in the growth velocity of the “shoulders” of the cell measured as the progress of the cell edge along the growth axis are cyclically out of phase with growth velocity at the tip such that the shoulder regions lag for part of the oscillation cycle, then “catch up” as the growth rate at the tip reaches a maximum and begins to decline. In this way the cell becomes oblate. Cell shape and growth rate oscillate in concert and are functionally related. Spatial change in edge growth rate points to important cellular locations for further investigation of vesicle movement and exocytosis, calcium gradients, and actin dynamics in lily pollen tubes.