Overexpression of NIMA-related kinase suppresses cell proliferation and tip growth in a liverwort Marchantia polymorpha

NIMA-related kinases (NEK) regulate various mitotic events in fungi and animals, whereas plant NEKs regulate growth direction of cells and organs. The liverwort Marchantia polymorpha has a single functional Mp NEK1 gene, whose knockout leads to aberrant growth direction of rhizoids. Its gain-of-function phenotype, however, remains unknown. Here, we generated inducible transgenic lines of Mp NEK1 , using an estrogen receptor mediated system. Estradiol treatment efficiently increased accumulation of Mp NEK1 mRNA and overexpression at the protein level was confirmed in the inducible lines of MpNEK1-Citrine. Overexpression of MpNEK1 severely suppressed growth of rhizoids and thalli, eventually causing lethality of juvenile plants. The effect of estradiol was reversible until 3 days, whereas 7-days treatment resulted in irreversible suppression of growth. This severe growth suppression was observed even at the nanomolar level of estradiol. EdU staining clearly indicated elimination of cell proliferation by estradiol-induced MpNEK1. In conclusion, overexpression of MpNEK1 suppresses cell division and elongation, leading to severe growth defects and lethality. Our results imply that the expression of MpNEK1 is tightly regulated at the appropriate level for cell growth. Plant NEK might control cell division as in the case of fungi and animal NEKs. ### Competing Interest Statement The authors have declared no competing interest.