single cotyledon (sic) Mutants of Pea and Their Significance in Understanding Plant Embryo Development

ABSTRACT Angiosperms are divided into two distinct classes—the dicotyledons (dicots) and monocoty-ledons (monocots)—based in part on the number of cotyledons in mature embryos. In this paper, we describe single-cotyledon pea mutants, termed sic (single cotyle-don), all of which show a degree of fusion between the cotyledons. The fusion in sic1 is along the margin of one cotyledon and is less complete than in sic2 embryos, but the effects of the mutations are additive in the double mutant. Occasionally sic2 mutants will show fusion of the two cotyledons into one cylindrical embryo in which the shoot apex becomes surrounded by the cotyledons. Both sic1 and sic2 mutants produce fertile plants. In the sic3 embryo, a single cotyledon is generated under the shoot apex that breaks the vascular connection between root and shoot, causing embryo lethality. The pattern of cotyledon development in all these mutants is identified by in situ mRNA hybridization and antibody labeling, using the storage protein vicilin as a cotyledon-specific marker. These patterns indicate that the joining of the cotyledons was due to zonal growth. The results indicate that there are genes in pea that influence the positioning and the morphology of the cotyle-don. A model for cotyledon development in pea is proposed that is based on the regulation of the positioning of cell clusters by the sic genes. Dev. Genet. 25:11–22