The role of GL2 in root epidermis development in Arabidopsis thaliana

A position-dependent pattern of epidermal cell types is produced during root development in Arabidopsis thaliana. This pattern is reflected in the expression pattern of GLABRA2 ( GL2), a homeobox gene that regulates cell differentiation in the root epidermis. To understand the origin and regulation of this patterning mechanism, both the seedling and the embryonic GL2 expression was examined in wild-type and different mutant backgrounds using RNA in situ hybridization and reporter gene fusion constructs. The results reveal that a common position-dependent expression pattern exist in the hypocotyl and the root of the Arabidopsis seedling. This position-dependent expression pattern of the GL2 gene is initiated during embryogenesis of Arabidopsis. It is established by the heart stage and is maintained throughout the remainder of embryogenesis. A 500-bp GL2 promoter region is required for the position-dependent expression of GL2 gene. Furthermore, the WER and the TTG gene are both required for the seedling and embryonic GL2 expression. However, TTG is necessary for maximal GL2 activity but is not required for the pattern of GL2 expression, while the WER gene controls both aspects of GL2 expression. The gl2 mutant does not have any effect on either the level or the pattern of GL2 expression. The CPC gene and the TRY gene are redundant in regulating of GL2 expression, however, the CPC gene appears to have a major effect on GL2 expression pattern in the mature region of the seedling root epidermis. In addition to expression studies, the morphology of the root hairs in gl2 mutants were analyzed to further understand the role of GL2 in root epidermis development. The results indicate that GL2 affects only a subset of the hairless cell development. Attempts to ectopically express the GL2 gene were not successful, likely due to the properties of the regulatory elements used. The results suggest that the meristematic and embryonic expression of GL2 gene are important for its function.