The bHLH genes GL3 and EGL3 participate in an intercellular regulatory circuit that controls cell patterning in the Arabidopsis root epidermis

The specification of the hair and non-hair cells in the Arabidopsis root epidermis provides a useful model for the study of cell fate determination in plants. A network of putative transcriptional regulators, including the related bHLH proteins GLABRA3 (GL3) and ENHANCER OF GLABRA3 (EGL3), is known to influence the patterning of these cell types. Here, we analyze the expression and regulation of GL3 and EGL3 during root epidermis development. Although they are thought to act in both the hair and non-hair cell types, we surprisingly found that GL3 and EGL3 gene expression and RNA accumulation occurs preferentially in the developing hair cells. By analyzing the expression of GL3::GUS and EGL3::GUS reporter fusions in various mutant and overexpression lines, we discovered that the expression of both genes is negatively regulated by WER, GL3 and EGL3 in the developing non-hair cells, and positively regulated by the CPC and TRY proteins in the developing hair cells. Further, the analysis of a GL3-YFP translational fusion, expressed under the GL3 promoter, indicates that the GL3 protein moves from the hair cells to the non-hair cells. These results suggest that GL3/EGL3 accumulation in the N cells is dependent on specification of the hair cell fate, which itself is known to be influenced (via CPC-mediated lateral inhibition) by the non-hair cells. This bi-directional signaling mechanism defines a new regulatory circuit of intercellular communication to specify the epidermal cell types.