Calcium signals are necessary to establish auxin transporter polarity in a plant stem cell niche.

In plants mechanical signals pattern morphogenesis through the polar transport of the hormone auxin and through regulation of interphase microtubule (MT) orientation. To date, the mechanisms by which such signals induce changes in cell polarity remain unknown. Through a combination of time-lapse imaging, and chemical and mechanical perturbations, we show that mechanical stimulation of the SAM causes transient changes in cytoplasmic calcium ion concentration (Ca) and that transient Ca response is required for downstream changes in PIN-FORMED 1 (PIN1) polarity. We also find that dynamic changes in Ca occur during development of the SAM and this Ca response is required for changes in PIN1 polarity, though not sufficient. In contrast, we find that Ca is not necessary for the response of MTs to mechanical perturbations revealing that Ca specifically acts downstream of mechanics to regulate PIN1 polarity response.