IDENTIFICATION OF A MERISTEM L1 LAYER-SPECIFIC GENE IN ARABIDOPSIS THAT IS EXPRESSED DURING EMBRYONIC PATTERN-FORMATION AND DEFINES A NEW CLASS OF HOMEOBOX GENES

Homeobox genes are master regulatory genes that specify the body plan and control development of many eukaryotic organisms, including plants , We isolated and characterized a cDNA designated ATML1 (for Arabidops is thaliana meristem L1 layer) that encodes a novel homeodomain protei n, The ATML1 protein shares high sequence homology inside and outside of the homeodomain with both the Phalaenopsis O39 and the Arabidopsis GLABRA2 (GL2) homeodomain proteins, which together define a new class of plant homeodomain-containing proteins, designated HD-GLS. The ATML1 gene was first expressed in the apical cell after the first asymmetri c division of the zygote and continued to be expressed in all proembry o cells until the eight-cell stage, In the 16-cell proembryo, the ATML 1 gene showed a distinct pattern of expression, with its mRNA becoming restricted to the protoderm, In the torpedo stage of embryo developme nt, ATML1 mRNA disappeared altogether but reappeared later only in the L1 layer of the shoot apical meristem in the mature embryo, After ger mination, this L1 layer-specific pattern of expression was maintained in the vegetative shoot apical meristem, inflorescence, and floral mer istems, as well as in the young floral organ primordia, Finally, ATML1 mRNA accumulated in the protoderm of the ovule primordia and integume nts and gradually became restricted in its expression to the endotheli um surrounding the embryo sac, We propose that ATML1 may be involved i n setting up morphogenetic boundaries of positional information necess ary for controlling cell specification and pattern formation, In addit ion, ATML1 provides an early molecular marker for the establishment of both apical-basal and radial patterns during plant embryogenesis.