The conserved machinery of bipolar prospindle formation controls acentrosomal spindle orientation in land plants

Correct spindle orientation is essential for the development of multicellular organisms.^1,2 In animal cells, astral microtubules control spindle orientation by determining the position of centrosomes.¹ However, the mechanisms that determine spindle orientation in acentrosomal plant cells are poorly understood. Here, we show that the microtubule-associated protein CORD^3,4 controls spindle orientation via prospindle assembly in the liverwort Marchantia polymorpha (Marchantia) and the angiosperm Arabidopsis thaliana (Arabidopsis). Wild-type cells formed bipolar microtubule structures called prospindles prior to nuclear envelope breakdown and assembled spindles along the prospindle axis. By contrast, CORD-deficient cells failed to maintain the bipolar prospindle, leading to the formation of multipolar prospindles and spindle misorientation. Spindle misorientation also caused phragmoplast misorientation, leading to abnormal cell plate alignment, although phragmoplast orientation was partially corrected during telophase. These results suggest that CORD maintains prospindle bipolarity, thereby controlling proper spindle orientation in Marchantia and Arabidopsis. This study reveals a conserved mechanism for the control of acentrosomal spindle orientation in land plants.