ZMOCL1, AN HDGL2 FAMILY HOMEOBOX GENE, IS EXPRESSED IN THE OUTER CELL LAYER THROUGHOUT MAIZE DEVELOPMENT

The formation of a morphologically distinct outer cell layer or protoderm is one of the first and probably one of the most important steps in patterning of the plant embryo. Here we report the isolation of ZmOCL1 (OCL for outer cell layer), a member of the HDGL2 (also known as HD-ZIP IV) subclass of plant-specific HD-ZIP homeodomain proteins from maize. ZmOCL1 transcripts are detected very early in embryo development, before a morphologically distinct protoderm is visible, and expression then becomes localised to the protoderm of the embryo as it develops. Subsequently, expression is observed in the L1 cell layer of both the developing primary root and shoot meristems, and is maintained in developing leaves and floral organs. We propose that ZMOCL1 may play a role in the specification of protoderm identity within the embryo, the organisation of the primary root primordium or in the maintenance of the L1 cell layer in the shoot apical meristem. We also show that the expression of ZmOCL1 is different from that of another epidermal marker gene, LTP2 (lipid transfer protein) and, in meristems, is complementary to that of Kn1 (Knotted) which is transcribed only in underlying cell layers.