Identification of a transitional cell state in the developmental pathway to carrot somatic embryogenesis

We have located a novel carbohydrate epitope in the cell walls of certain single cells in embryogenic, but not in non-embryogenic, suspension cultures of carrot. Expression of this epitope, recognized by the mAb JIM8, is regulated during initiation, proliferation, and prolonged growth of suspension cultures such that changes in the abundance of JIM8-reactive cells always precede equivalent changes in embryogenic potential. Therefore, a direct correlation exists between the presence of the JIM8-reactive cell wall epitope and somatic embryo formation. The JIM8-reactive cell wall epitope is expressed in the cell walls of three types of single cells and one type of cell cluster. One of the single cell types seems able to follow one of two phytohormone-controlled developmental pathways, either a cell elongation pathway that eventually leads to cell death, or a cell division pathway that gives rise to proembryogenic masses. We demonstrate that all JIM8-reactive cell types in embryogenic carrot suspension cultures are developmentally related, and that the switch by one of them to somatic embryogenesis is accompanied by the immediate dissipation of the JIM8-reactive cell wall epitope. The cell wall carbohydrate epitope recognized by JIM8 therefore represents a cell wall marker for a very early transitional cell state in the developmental pathway to carrot somatic embryogenesis.