Ca2+ oscillations promote microtubule-band turnover and support tip growth in Arabidopsis zygotes

The zygote is the origin of development, and in most angiosperms, it divides asymmetrically to establish the apical-basal axis. In Arabidopsis thaliana, the zygote undergoes tip growth-like polar elongation, using a subapical transverse microtubule band (MT band). Because canonical tip-growing cells rely on longitudinal actin filaments (F-actin), it remains unclear whether and how the zygote employs conserved tip-growth mechanisms. Here, using quantitative live-cell imaging, pharmacological perturbations, and mechanical simulations, we found that oscillatory Ca{superscript 2} waves, a hallmark of tip growth, are coupled to zygote elongation through a bidirectional feedback loop, as in other tip-growing cells. However, Ca{superscript 2} waves were dispensable for overall F-actin alignment but promoted MT band turnover. Our study provides a model showing that the zygote uses a conserved tip-growth module, in which Ca{superscript 2} oscillations and cell elongation reinforce each other, but redirects the target cytoskeleton to the MT band, enabling zygote-specific tip growth for axis formation.