SIM and CCS52A1 control root hair cell endoreplication and expansion

Arabidopsis thaliana (Arabidopsis) root hair cells undergo rapid cell elongation that is correlated with an increased DNA ploidy level through endoreplication. However, the contribution of endoreplication to hair cell growth is currently unclear. Here, we determined ploidy levels within the root hair cell file from the proximal meristem to the elongation zone and correlated them with cellular parameters. Our data show that cells in the basal meristem reach a 4C level prior to rapid elongation, whereas in the elongation zone a 16C level is reached prior to the onset of tip growth, suggesting that endoreplication is a prerequisite for rapid volume increase. Consistently, we show that mutants in SIAMESE (SIM) and CELL CYCLE SWITCH52 A1 (CCS52A1), key endocycle regulators expressed in hair cells, display a decrease in cell volume and hair tip length. Interestingly, SIM and CCS52A1 affect cell width and length, respectively. In addition, CCS52A1 controls the growth rate of the root hair tip. Given the common association between cell expansion and cell wall modifications, we performed immunolabeling on wild-type and mutant roots, which revealed a reduced level of pectin methylesterification in the mutant backgrounds, suggesting that SIM and CCS52A1 may modulate root hair cell growth by affecting cell wall modifications. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (https://academic.oup.com/plphys/pages/General-Instructions) is Lieven De Veylder (lieven.deveylder@psb.vib-ugent.be). ONE-SENTENCE SUMMARYSIM and CCS52A1 control Arabidopsis root hair cell volume by controlling endoreplication.