Spindle motility skews division site determination during asymmetric cell division in Physcomitrella

Asymmetric cell division (ACD) underlies the development of multicellular organisms. In animal ACD, the cell division site is determined by active spindle-positioning mechanisms. In contrast, it is considered that the division site in plants is determined prior to mitosis by the microtubule-actin belt known as the preprophase band (PPB) and that the localization of the mitotic spindle is typically static and does not govern the division plane. However, in some plant species, ACD occurs in the absence of PPB. Here, we isolate a hypomorphic mutant of the conserved microtubule-associated protein TPX2 in the moss Physcomitrium patens (Physcomitrella) and observe spindle motility during PPB-independent cell division. This defect compromises the position of the division site and produces inverted daughter cell sizes in the first ACD of gametophore (leafy shoot) development. The phenotype is rescued by restoring endogenous TPX2 function and, unexpectedly, by depolymerizing actin filaments. Thus, we identify an active spindle-positioning mechanism that, reminiscent of acentrosomal ACD in animals, involves microtubules and actin filaments, and sets the division site in plants.