Cell-context signalling.
In plants, cells differentiate according to their position with relation to their cell neighbours. Monoclonal antibody (MAb) probes to polysaccharide epitopes, present at the surfaces of all plant cells, have defined a family of proteoglycan antigens which signify cellular position. These MAbs have been used to sort the single cells present in carrot somatic cell cultures on the basis of the presence or absence of specific polysaccharide epitopes. This sorting allows embryo initial cells to be cultured among different cell collectives (based on their polysaccharide epitope expression) and thus in altered contextual backgrounds. These experiments have shown that specific populations of embryo initial precursor cells induce and sustain the early development of the embryo initials, revealing that the populations of different cell collectives which are defined by different polysaccharide epitopes (cell-context) serves important regulatory function in early plant development. Somatic embryo initials deprived of the influence of the cell collective-defined by the presence of the polysaccharide epitope recognised by the MAb JIM8-establish unorganised first divisions and develop as callus. However, in the presence of the JIM8-reactive cell collective, or medium conditioned by the collective, the initials develop into somatic embryos. This demonstrates that the cells defined by the JIM8 polysaccharide epitope are necessary to sustain the meristematic activity which drives the renewed development. Transfer of a cell-wall signal from the JIM8-reactive cells to cellular situations in carrot seedlings in which they would not normally occur (out-of-context signals) stimulates lateral root production, thus demonstrating that the inductive signal operative in suspension cultures can be reinterpreted by specific cells later in development and reinitiate meristematic activity. The communication between the precursor cells defined by JIM8 and embryo initials defines an early cell-cell interaction in developing carrot plants. Labelling of flower sections suggests that the same interaction exists between embryo apical and basal cells early in normal development.