Transverse cortical microtubule arrays form persistent unipolar domains in hypocotyl cells of <i>Arabidopsis thaliana</i>

Cortical microtubules influence plant cell shape by guiding cellulose deposition. Epidermal hypocotyl cells in <i>Arabidopsis thaliana</i> create distinct cortical microtubule array patterns to enable axial cell growth. How these array patterns are created and maintained during cell wall formation is a critical and unsolved problem in cell biology. Previous work showed that arrays aligned longitudinally with the cell's growth axis have a "split bipolar" organization, with microtubules treadmilling toward the apical or basal ends of the cell from a region of antiparallel overlap at the cell's midzone. The underlying order or architecture of these coaligned arrays prompted us to ask whether microtubules oriented transversely to the cell's axis are organized to a similar degree. Creating new fluorescently tagged End-Binding Protein 1b (EB1b) probes to circumvent gain-of-function effects observed for GFP-EB1b, we found that transverse arrays form persistent, nearly unipolar domains of microtubules treadmilling around the short axis of the cell, independent of the EB1b probe used. Our findings reveal an organizational strategy for transverse arrays distinct from that of longitudinal arrays, with implications for the mechanisms of array pattern creation and maintenance.