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

AbstractNIMA-related kinases (NEKs) regulate various mitotic events in fungi and animals, whereas plant NEKs regulate growth direction of cells and organs. The liverwortMarchantia polymorphahas a single functional MpNEK1gene, whose knockout leads to aberrant growth direction of rhizoids. Its gain-of-function phenotype, however, remains unknown. Here, we generated inducible transgenic lines of MpNEK1, using an estrogen receptor mediated system. Estradiol treatment efficiently induced accumulation of MpNEK1mRNA 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 clearly indicated elimination of cell proliferation by estradiol-induced MpNEK1. 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.