Involvement of a NIMA-related kinase in cell division of the liverwort Marchantia polymorpha

Never-in-mitosis A (NIMA)-related kinases (NEKs) regulate a series of mitotic events in fungi and animals, whereas plant NEKs have been shown to control the growth direction of cells and organs. Plant NEKs are highly expressed in the meristem, but whether they regulate meristematic activity remains unknown. The liverwort Marchantia polymorpha has a single functional MpNEK1 gene, and its knockout results in twisted rhizoid growth. For a gain-of-function approach, we generated lines for the inducible expression of MpNEK1 using an estrogen receptor-mediated system. Estradiol treatment effectively induced the accumulation of MpNEK1 mRNA and MpNEK1-Citrine fusion protein throughout the plant. MpNEK1 overexpression severely suppressed rhizoid and thallus growth, ultimately leading to the lethality of juvenile plants. This severe effect was observed even at the nanomolar level of estradiol. EdU (5-ethynyl-2'-deoxyuridine) staining and microtubule imaging clearly indicated suppression of cell division by estradiol-induced MpNEK1. MpNEK1 induction also reduced cortical microtubule density and dynamics but did not severely affect cell growth and morphology in thalli. Overexpression of kinase-deficient MpNEK1 also suppressed thallus and rhizoid growth, although to a slightly lesser extent than wild-type MpNEK1, indicating a phosphorylation-independent mechanism of growth suppression. Furthermore, Mpnek1 mutants exhibited growth suppression in their reproductive organs, the gametangiophores. This supports the role of MpNEK1 in cell division, as observed in both fungi and animals.