Fig. 8. A putative role for ALE2 in the promotion of protoderm differentiation. The RLK ALE2 and another RLK, ACR4, might function in a single or closely overlapping pathway. ALE1, encoding a putative subtilisin-like serine protease, is predominantly expressed in the endosperm that surrounds the developing embryo (Tanaka et al., 2001) and functions to promote the formation of the protoderm in a manner independent of ALE2 and ACR4. The two pathways involving ALE2, ACR4 and ALE1 might act positively to regulate the specification of the protoderm and/or expression of protoderm-specific genes in an organized manner. The expression of protoderm-specific genes, including those for the ATML1 homeodomain protein and for the redundant factor PDF2, might promote the expression of these genes themselves and other protoderm-specific genes (Abe et al., 2001; Abe et al., 2003). Subsequently, expression of the FDH gene for a putative fatty-acid elongase (Yephremov et al., 1999; Pruitt et al., 2000) and of other protoderm-specific genes promotes the formation of the epidermis proper.