# Calcium-dependent depletion zones in the cortical microtubule array coincide with sites of, but do not regulate, wall ingrowth papillae deposition in epidermal transfer cells

## TL;DR

This study shows that calcium plumes create microtubule depletion zones where wall ingrowth papillae form in transfer cells, but the microtubules themselves do not control the papillae's formation.

## Contribution

The paper reveals that microtubule depletion zones coincide with, but do not regulate, wall ingrowth papillae formation in transfer cells.

## Key findings

- Depletion zones in microtubules align with the sites of wall ingrowth papillae formation.
- Microtubule depolymerization or stabilization does not affect papillae deposition.
- Cytosolic calcium plumes are essential for creating depletion zones and papillae formation.

## Abstract

In developing transfer cells, inward-directed deposition of wall ingrowth papillae into the cytoplasm occurs independently of, but through, depletion zones in the cortical microtubule array created by cytosolic Ca2+ plumes.

Trans-differentiation to a transfer-cell morphology is characterized by the localized deposition of wall ingrowth papillae that protrude into the cytosol. Whether the cortical microtubule array directs wall ingrowth papillae formation was investigated using a Vicia faba cotyledon culture system in which their adaxial epidermal cells were spontaneously induced to trans-differentiate to transfer cells. During deposition of wall ingrowth papillae, the aligned cortical microtubule arrays in precursor epidermal cells were reorganized into a randomized array characterized by circular depletion zones. Concurrence of the temporal appearance, spatial pattern, and size of depletion zones and wall ingrowth papillae was consistent with each papilla occupying a depletion zone. Surprisingly, microtubules appeared not to regulate construction of wall ingrowth papillae, as neither depolymerization nor stabilization of cortical microtubules changed their deposition pattern or morphology. Moreover, the size and spatial pattern of depletion zones was unaltered when the formation of wall ingrowth papillae was blocked by inhibiting cellulose biosynthesis. In contrast, the depletion zones were absent when the cytosolic calcium plumes, responsible for directing wall ingrowth papillae formation, were blocked or dissipated. Thus, we conclude that the depletion zones within the cortical microtubule array result from localized depolymerization of microtubules initiated by elevated cytosolic Ca2+ levels at loci where wall ingrowth papillae are deposited. The physiological significance of the depletion zones as a mechanism to accommodate the construction of wall ingrowth papillae without compromising maintenance of the plasma membrane–microtubule inter-relationship is discussed.