Soluble Signals from Cells Identified at the Cell Wall Establish a Developmental Pathway in Carrot.

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Abstract
Cells in a plant differentiate according to their positions and use cell-cell communication to assess these positions. Similarly, single cells in suspension cultures can develop into somatic embryos, and cell-cell communication is thought to control this process. The monoclonal antibody JIM8 labels an epitope on cells in specific positions in plants. JIM8 also labels certain cells in carrot embryogenic suspension cultures. We have used JIM8 and secondary antibodies coupled to paramagnetic beads to label and immunomagnetically sort single cells in a carrot embryogenic suspension culture into pure populations. Cells in the JIM8(+) population develop into somatic embryos, whereas cells in the JIM8(-) population do not form somatic embryos. However, certain cells in JIM8(+) cultures (state B cells) undergo asymmetric divisions, resulting in daughter cells (state C cells) that do not label with JIM8 and that sort to JIM8(-) cultures. State C cells are competent to form somatic 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 suspension 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.