KATANIN-mediated microtubule severing is required for MTOC formation and function in Marchantia polymorpha

Microtubule organising centres (MTOCs) are sites of localised microtubule nucleation in eukaryotic cells. Regulation of microtubule dynamics often involves KATANIN (KTN); a microtubule severing enzyme which cuts microtubules to generate new negative ends leading to catastrophic depolymerisation. In Arabidopsis thaliana, KTN is required for the organisation of microtubules in the cell cortex, preprophase band, mitotic spindle and phragmoplast. However, as angiosperms lack MTOCs, the role of KTN in MTOC formation has yet to be studied in plants. Two unique MTOCs - the polar organisers - form on opposing sides of the prophase nucleus in liverworts. Here we show that KTN-mediated microtubule depolymerisation is required for the de novo formation of polar organisers in the liverwort, Marchantia polymorpha. In Mpktn mutants that lack KTN function, the formation, shape, number, and function of polar organisers is defective. This is in addition to defective microtubule organisation in the cell cortex, preprophase band, mitotic spindle, and phragmoplast. These data demonstrate that KTN-mediated microtubule dynamics are required for the formation of liverwort-specific MTOCs.