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

Significance The vascular tissues form a continuous network providing the long-distance transport of water and nutrients in all higher plants (tracheophytes). To incorporate separate organs into this network, it is essential that the position of different vascular cell types is tightly regulated. Several factors required for root vascular patterning (including hormones and gene products) have previously been identified in the model plant Arabidopsis. We have now established a mathematical model formulizing the interaction between these factors, allowing us to identify a minimal regulatory network capable of maintaining a stable vascular pattern in Arabidopsis roots. We envisage that this model will help future researchers understand how similar regulatory units can be applied to create alternative patterns in other species. As multicellular organisms grow, positional information is continually needed to regulate the pattern in which cells are arranged. In the Arabidopsis root, most cell types are organized in a radially symmetric pattern; however, a symmetry-breaking event generates bisymmetric auxin and cytokinin signaling domains in the stele. 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 use a multicellular model to determine a minimal set of components necessary for maintaining a stable vascular pattern. Simulations perturbing the signaling network show that, in addition to the mutually inhibitory interaction between auxin and cytokinin, signaling through SHR, microRNA165/6, and PHABULOSA is required to maintain a stable bisymmetric pattern. We have verified this prediction by observing loss of bisymmetry in shr mutants. The model reveals the importance of several features of the network, namely the mutual degradation of microRNA165/6 and PHABULOSA and the existence of an additional negative regulator of cytokinin signaling. These components form a plausible mechanism capable of patterning vascular tissues in the absence of positional inputs provided by the transport of hormones from the shoot.