Analysis of IIId, IIIe and IVa group basic-helix-loop-helix proteins expressed in Arabidopsis root epidermis

Differentiation of Arabidopsis epidermal cells into root hairs and trichomes is a functional model system for understanding plant cell development. Previous studies showed that one of the Arabidopsis basic-helix-loop-helix (AtbHLH) proteins, GLABRA3 (GL3), is involved in root-hair and trichome differentiation. We analyzed 11 additional AtbHLH genes with homology to GL3. Estimation of the phylogeny based on amino acid sequences of the bHLH region suggests that 11 AtbHLH genes used in this study evolved by duplications of a single common GL3 ancestor. Promoter-GUS analysis showed that AtbHLH006, AtbHLH013, AtbHLH017 and AtbHLH020 were expressed in roots. Among them, AtbHLH006 and AtbHLH020 were preferentially expressed in root epidermal non-hair cells. Consistent with the expression patterns from promoter-GUS analysis, GFP fluorescence was observed in the nuclei of root epidermal non-hair cells of AtbHLH006p::AtbHLH006:GFP and AtbHLH020p::AtbHLH020:GFP transgenic plants. However, AtbHLH006 and AtbHLH0020 proteins did not interact with epidermis-specific MYB proteins and TTG1. Taken together, AtbHLH006 and AtbHLH020 may function in root epidermal cells, but other GL3-like bHLH proteins may have evolved to regulate different processes.