The preprophase band is dispensable for robust cell division orientation in wood-forming cambium stem cells

The control of cell division plane orientation is fundamental for organizing developmental processes and shaping bodies of multicellular organisms. In plants, radial organ growth is mediated by the cambium, a stem cell niche embedded in expanding organs and continuously producing xylem and phloem in a strictly bidirectional manner. Cambium stem cells (CSCs) are unique in comparison with other plant stem cells as they consistently divide along their longest axis, clearly overriding the commonly found 'short axis' rule. Here, we investigated how cell division plane orientation is controlled in Arabidopsis thaliana (Arabidopsis) CSCs. We characterized successive microtubule organization during CSC divisions using immunolabelling and histologically compared wild-type, preprophase band (PPB)-deficient, and cortical division zone (CDZ)-deficient mutants. We found that division plane orientation is established independently from spindle orientation and the PPB. Instead, the orientation of division planes depends on the CDZ and CDZ-related PHRAGMOPLAST ORIENTING KINESIN (POK) proteins. Robust CSC division orientation is independent of the PPB, challenging the view that the PPB is universally required to stabilize plant cell divisions. Our results highlight the importance of wood-forming CSCs and their subcellular characterization as an instructive example for the determination of cell division orientation in plants.