Dancing Together. Social Controls in Parasitic Plant Development

In contrast to animals, the basic body plan of plants develops largely postembryonically and is directed by two primary meristems located on opposite ends of a bipolar embryo (Jürgens, 2001). The basal root meristem serves to extend the primary root established in the embryo, whereas the apical shoot meristem both maintains growth and provides a source of cells for new organs such as leaves and flowers. In accordance, the shoot organ is more complex, with new primordia forming in an established pattern, whereas the root apex is streamlined and utilitarian, consisting of long continuous files of developing cells radiating from the quiescent center. In both organs, cell division is arrested and cell fate established within the space of a few cell layers; cellular destiny is precisely defined by geographical position. Cell-cell contacts, “social controls,” operating within the meristem appear to be most critical for establishing the cellular pattern. Cell ablation/regeneration experiments in the root have implicated direct cell-cell surface contacts as critical for cell fate (Van den Berg et al., 1995, 1997), and local auxin gradients appear necessary for the differentiation of new organs in the shoot (Reinhardt et al., 2000).