MUTATIONS IN THE HYDRA1 GENE OF ARABIDOPSIS PERTURB CELL-SHAPE AND DISRUPT EMBRYONIC AND SEEDLING MORPHOGENESIS

Mutations in the HYDRA1 (HYD1) gene of Arabidopsis thaliana can preven t normal morphological development of embryos and seedlings. Three all elic mutants (hydra 1-1, hydra1-2 and hydra1-3) have been identified, and in each the seedling is characterized by having a variable number of cotyledons, a short and wide hypocotyl and a much reduced root syst em, hydra1 embryos appear to develop normally to the octant stage, but fail to establish a distinct protoderm and lack bilateral symmetry, d eveloping multiple cotyledonary primordia of irregular size and shape, Cells of the embryo proper, but not the suspensor, exhibit abnormalit ies in size and shape. The hydra1 embryo fails to develop an embryonic root, but embryos and seedlings express molecular markers of apical-b asal polarity. Mutant seedlings produce leaves to form a small cabbage -like habit and may occasionally produce sterile flowers, though the m utation is commonly seedling-lethal. hydra1 seedlings exhibit abnormal radial patterning, but nevertheless express at least one molecular ma rker of vascular cell differentiation. A model is proposed in which th e HYDRA1 protein functions as an essential component of the cell expan sion system.