NAC1 directs CEP1-CEP3 peptidase expression and modulates root hair growth in Arabidopsis

Abstract

AbstractPlant genomes encode a unique group of papain-type Cysteine EndoPeptidases (CysEPs) containing a KDEL endoplasmic reticulum (ER) retention signal (KDEL-CysEPs or CEPs). CEPs process the cell-wall scaffolding EXTENSIN proteins (EXTs), which regulatede novocell wall formation and cell expansion. Since CEPs are able to cleave EXTs and EXT-related proteins, acting as cell wall-weakening agents, they may play a role in cell elongation.Arabidopsis thalianagenome encodes three CEPs (AtCPE1-AtCEP3). Here we report that the three Arabidopsis CEPs,AtCEP1-AtCEP3,are highly expressed in root-hair cell files. Single mutants have no evident abnormal root-hair phenotype, butatcep1-3 atcep3-2andatcep1-3 atcep2-2double mutants have longer root hairs (RHs) than wild type (Wt) plants, suggesting that expression ofAtCEPsin root trichoblasts restrains polar elongation of the RH. We provide evidence that the transcription factorNAC1activates AtCEPs expression in roots to limit RH growth. Chromatin immunoprecipitation indicates that NAC1 binds the promoter ofAtCEP1, AtCEP2,and to a lower extent toAtCEP3and may directly regulate their expression. Indeed, inducibleNAC1overexpression increasesAtCEP1 and AtCEP2transcript levels in roots and leads to reduced RH growth while the loss of functionnac1-2mutation reducesAtCEP1-AtCEP3gene expression and enhances RH growth. Likewise, expression of a dominant chimeric NAC1-SRDX repressor construct leads to increased RH length. Finally, we show that RH cell walls in theatcep1-1 atcep3-2double mutant have reduced levels of EXT deposition, suggesting that the defects in RH elongation are linked to alterations in EXT processing and accumulation. Taken together, our results support the involvement of AtCEPs in controlling RH polar growth through EXT-processing and insolubilization at the cell wall.