The function of TONNEAU1 in moss reveals ancient mechanisms of division plane specification and cell elongation in land plants.

The preprophase band (PPB) is a transient ring of microtubules that forms before mitosis in land plants, and delineates the cytokinetic division plane established at telophase. It is one of the few derived traits specific to embryophytes, in which it is involved in the spatial control of cell division. Here we show that loss of function of Physcomitrella patens PpTON1 strongly affects development of the moss gametophore, phenocopying the developmental syndrome observed in Arabidopsis ton1 mutants: mutant leafy shoots display random orientation of cell division and severe defects in cell elongation, which are correlated with absence of PPB formation and disorganization of the cortical microtubule array in interphase cells.