CELL-CONTEXT SIGNALING

In plants, cells differentiate according to their position with relati on to their cell neighbours. Monoclonal antibody (MAb) probes to polys accharide epitopes, present at the surfaces of all plant cells, have d efined a family of proteoglycan antigens which signify cellular positi on. These MAbs have been used to sort the single cells present in carr ot somatic cell cultures on the basis of the presence or absence of sp ecific polysaccharide epitopes. This sorting allows embryo initial cel ls to be cultured among different cell collectives (based on their pol ysaccharide epitope expression) and thus in altered contextual backgro unds. These experiments have shown that specific populations of embryo initial precursor cells induce and sustain the early development of t he embryo initials, revealing that the populations of different cell c ollectives which are defined by different polysaccharide epitopes (cel l-context) serves important regulatory function in early plant develop ment. Somatic embryo initials deprived of the influence of the cell co llective - defined by the presence of the polysaccharide epitope recog nised by the MAb JIM8 establish unorganised first divisions and develo p as callus. However, in the presence of the JIM8-reactive cell collec tive, or medium conditioned by the collective, the initials develop in to somatic embryos. This demonstrates that the cells defined by the JI M8 polysaccharide epitope are necessary to sustain the meristematic ac tivity which drives the renewed development. Transfer of a cell-wall s ignal from the JIM8-reactive cells to cellular situations in carrot se edlings in which they would not normally occur (out-of-context signals ) stimulates lateral root production, thus demonstrating that the indu ctive signal operative in suspension cultures can be reinterpreted by specific cells later in development and reinitiate meristematic activi ty. The communication between the precursor cells defined by JIM8 and embryo initials defines an early cell-cell interaction in developing c arrot plants. Labelling of flower sections suggests that the same inte raction exists between embryo apical and basal cells early in normal d evelopment.