A LEUCINE-RICH REPEAT CONTAINING RECEPTOR-LIKE KINASE MARKS SOMATIC PLANT-CELLS COMPETENT TO FORM EMBRYOS

The first somatic single cells of carrot hypocotyl explants having the competence to form embryos in the presence of 2,4-dichlorophenoxyacet ic acid (2,4-D) were identified using semi-automatic cell tracking. Th ese competent cells are present as a small subpopulation of enlarged a nd vacuolated cells derived from cytoplasm-rich and rapidly proliferat ing non-embryogenic cells that originate from the provascular elements of the hypocotyl, A search for marker genes to monitor the transition of somatic into competent and embryogenic cells in established suspen sion cell cultures resulted in the identification of a gene transientl y expressed in a small subpopulation of the same enlarged single cells that are formed during the initiation of the embryogenic cultures fro m hypocotyl explants, The predicted amino acid sequence and in vitro k inase assays show that this gene encodes a leucine-rich repeat contain ing receptor-like kinase protein, designated Somatic Embryogenesis Rec eptor-like Kinase (SERK), Somatic embryos formed from cells expressing a SERK promoter-luciferase reporter gene, During somatic embryogenesi s, SERK expression ceased after the globular stage, In plants, SERK mR NA could only be detected transiently in the zygotic embryo up to the early globular stage but not in unpollinated flowers nor in any other plant tissue, These results suggest that somatic cells competent to fo rm embryos and early globular somatic embryos share a highly specific signal transduction chain with the zygotic embryo from shortly after f ertilization to the early globular embryo.