SOLUBLE SIGNALS FROM CELLS IDENTIFIED AT THE CELL-WALL ESTABLISH A DEVELOPMENTAL PATHWAY IN CARROT

Cells in a plant differentiate according to their positions and use ce ll-cell communication to assess these positions. Similarly, single cel ls in suspension cultures can develop into somatic embryos, and cell-c ell communication is thought to control this process. The monoclonal a ntibody JIM8 labels an epitope on cells in specific positions in plant s. JIM8 also labels certain cells in carrot embryogenic suspension cul tures. We have used JIM8 and secondary antibodies coupled to paramagne tic beads to label and immunomagnetically sort single cells in a carro t embryogenic suspension culture into pure populations. Cells in the J IM8(+) population develop into somatic embryos, whereas cells in the J IM8(-) population do not form somatic embryos. However, certain cells in JIM8(+) cultures (state B cells) undergo asymmetric divisions, resu lting in daughter cells (state C cells) that do not label with JIM8 an d that sort to JIM8(-) cultures. State C cells are competent to form s omatic embryos, and we show here that a conditioned growth medium from a culture of JIM8(+) cells allows state C cells in a JIM8(-) culture to go on and develop into somatic embryos. JIM8 labels cells in suspen sion cultures at the cell wall. Therefore, a cell with a role in cell- cell communication and early cell fate selection can be identified by an epitope in its cell wall.