Fig. 6. A model for BR action in vascular development. Schematic representation of the vascular bundles in which BRs act as a vascular patterning signal that promotes xylem while repressing phloem differentiation. (B) In the case of defective BR-signaling (as shown in bri1 and brl1-1 mutants), increased phloem differentiation occurs at the expense of xylem. This model supports the idea that the specification of phloem and xylem in the vascular bundles is tightly linked. Whether BRs are perceived by receptors of the BRI1-family on the plasma membrane of cells that secrete BRs (autocrine) or cells that do not secrete BRs (paracrine) remains unknown.