Top-down and inside-out: Directionality of signaling in vascular and embryo development

Although most aspects of vascular tissue patterning remain elusive, the alignment of vascular cells into continuous strands is becoming amenable to molecular genetic analysis. Recent data are consistent with an apical-basal signal-flow model underlying vascular strand formation, bur also body axis formation in embryos. Directional, top-down auxin flow could set up a basic axial coordinate system upon which a second tier of patterning cell interactions could elaborate the cell patterns in embryos and meristems. Most of these pattern-elaborating activities may still be unknown, but recent examples illustrate how inward-out signaling from the center could regulate the development in the overlying tissue layers, relative to the position of vascular tissue. A two-tiered control of cellular patterns, in which fine-tuning cell interactions generate a reproducible cell pattern on the basis of an underlying robust, feed-back stabilized axial architecture, could also account for the amazing pattern regeneration capacities of embryos and meristems.