Integration of hormonal signaling networks and mobile microRNAs is required for vascular patterning in Arabidopsis roots

Abstract

As multicellular organisms grow, positional information is continually needed to regulate the pattern in which cells are arranged. Inthe Arabidopsis root, most cell types are organized in a radiallysymmetric pattern; however, a symmetry-breaking event generates bisymmetric auxin and cytokinin signaling domains in thestele. Bidirectional cross-talk between the stele and the surrounding tissues involving a mobile transcription factor, SHORT ROOT(SHR), and mobile microRNA species also determines vascular pattern, but it is currently unclear how these signals integrate. We usea multicellular model to determine a minimal set of componentsnecessary for maintaining a stable vascular pattern. Simulationsperturbing the signaling network show that, in addition to themutually inhibitory interaction between auxin and cytokinin, signaling through SHR, microRNA165/6, and PHABULOSA is requiredto maintain a stable bisymmetric pattern. We have verified thisprediction by observing loss of bisymmetry in shr mutants. Themodel reveals the importance of several features of the network,namely the mutual degradation of microRNA165/6 and PHABU-LOSA and the existence of an additional negative regulator ofcytokinin signaling. These components form a plausible mechanismcapable of patterning vascular tissues in the absence of positionalinputs provided by the transport of hormones from the shoot.