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 Ca²⁺ waves, a hallmark of tip growth, are coupled to zygote elongation through a bidirectional feedback loop, as in other tip-growing cells. However, Ca²⁺ 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 Ca²⁺ oscillations and cell elongation reinforce each other, but redirects the target cytoskeleton to the MT band, enabling zygote-specific tip growth for axis formation.