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

NIMA-related kinases (NEKs) regulate a series of 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 twisted growth of rhizoids. Mp NEK1 is also expressed in the meristem of vegetative flat organ, thallus, while its function remains unknown. Here, we generated transgenic lines for the inducible expression of Mp NEK1 using an estrogen receptor mediated system. Estradiol treatment efficiently induced the accumulation of Mp NEK1 mRNA and MpNEK1-Citrine fusion protein throughout plant body. Overexpression of MpNEK1 severely suppressed growth of rhizoids and thalli, eventually causing the 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 effect was observed even at the nanomolar level of estradiol. EdU staining and microtubule imaging clearly indicated the suppression of cell proliferation by estradiol-induced MpNEK1. Unexpectedly, the overexpression of kinase-deficient MpNEK1 also suppressed thallus growth and rhizoid formation, despite their slightly mild effect than the full length MpNEK1, indicating phosphorylation-independent mechanism of growth suppression. In conclusion, overexpression of MpNEK1 suppresses cell division and elongation, leading to growth cessation and lethality. Our results imply that the expression of MpNEK1 is tightly regulated and plant NEKs might control cell division as in fungi and animals.