Translocated signals regulating root meristem activity in lupins (Lupinus albus and L. angustifolius)

Pluripotent stem cells in flowering plants occur at the root and shoot apices, at the cambium of shoot organs and the root pericycle. These meristematic cells provide sites for cell division and postembryonic organ differentiation. Their activity responds to environmental and endogenous cues that determine rate and direction of growth, developmental pattern and change in organ function. Recent analysis of gene expression in the shoot apical meristem (SAM) of Arabidopsis has revealed close cell/cell interactions and an exchange of signals between differentiating cell types. However, it is clear that the long distance translocation channels of vascular plants, phloem and xylem, also provide regulatory signals that influence the course of events in the SAM, such as the transition from vegetative to floral development. These channels serve as pathways for translocation of assimilates providing the vascular link between 'sources' and 'sinks' on the plant. Similarly, the below ground meristems responsible for root growth, lateral root initiation and branching as well as the initiation of nodules on legumes receive translocated shoot-derived 'signals' as well as assimilates in phloem. Physiological studies have established that such signals are integral components of meristem activity but their nature has not been clearly established. (author)