A leucine-rich repeat containing receptor-like kinase marks somatic plant cells competent to form embryos

TLDR

Competent cells are a small subpopulation of enlarged, vacuolated cells derived from provascular elements of the hypocotyl. The authors identified the first embryogenic single cells in carrot hypocotyl explants using semi‑automatic cell tracking in the presence of 2,4‑D. A transiently expressed leucine‑rich repeat receptor‑like kinase, SERK, marks the small subpopulation of enlarged cells that are competent to form embryos, with expression active until the globular stage and shared with zygotic embryos, indicating a specific signal transduction pathway.

Abstract

ABSTRACT The first somatic single cells of carrot hypocotyl explants having the competence to form embryos in the presence of 2,4-dichlorophenoxyacetic acid (2,4-D) were identified using semi-automatic cell tracking. These competent cells are present as a small subpopulation of enlarged and vacuolated cells derived from cytoplasm-rich and rapidly proliferating 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 suspension cell cultures resulted in the identification of a gene transiently expressed in a small subpopulation of the same enlarged single cells that are formed during the initiation of the embryogenic cultures from hypocotyl explants. The predicted amino acid sequence and in vitro kinase assays show that this gene encodes a leucine-rich repeat containing receptor-like kinase protein, designated Somatic Embryogenesis Receptor-like Kinase (SERK). Somatic embryos formed from cells expressing a SERK promoterluciferase reporter gene. During somatic embryogenesis, SERK expression ceased after the globular stage. In plants, SERK mRNA 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 form embryos and early globular somatic embryos share a highly specific signal transduction chain with the zygotic embryo from shortly after fertilization to the early globular embryo.