A key question in developmental biology is how cells exchange positional information for proper patterning during organ development. In plant roots the radial tissue organization is highly conserved with a central vascular cylinder in which two water conducting cell types, protoxylem and metaxylem, are patterned centripetally. We show that this patterning occurs through crosstalk between the vascular cylinder and the surrounding endodermis mediated by cell-to-cell movement of a transcription factor in one direction and microRNAs in the other. SHORT ROOT, produced in the vascular cylinder, moves into the endodermis to activate SCARECROW. Together these transcription factors activate MIR165a and MIR166b. Endodermally produced microRNA165/6 then acts to degrade its target mRNAs encoding class III homeodomain-leucine zipper transcription factors in the endodermis and stele periphery. The resulting differential distribution

Plants and animals produce modular developmental units in a periodic fashion. In plants, lateral roots form as repeating units along the root primary axis; however, the developmental mechanism regulating this process is unknown. We found that cyclic expression pulses of a reporter gene mark the position of future lateral roots by establishing prebranch sites and that prebranch site production and root bending are periodic. Microarray and promoter-luciferase studies revealed two sets of genes oscillating in opposite phases at the root tip. Genetic studies show that some oscillating transcriptional regulators are required for periodicity in one or both developmental processes. This molecular mechanism has characteristics that resemble molecular clock-driven activities in animal species.Formation of periodic modular structures is a common developmental feature in both animals and plants (1,2). The body axis of many metazoans-including species of arthropods, annelids, and vertebrates-is organized into segments. This organization is established during embryogenesis by successive addition of segments to an elongating posterior body region (3). In plants, branching of shoots and roots during postembryonic development produces repeating units (phytomers and lateral roots, respectively) along the growing longitudinal axis. Leaves are generated in a regular phyllotactic pattern, and lateral roots (LRs) are continuously produced from the primary root (Fig. 1A) as it grows, following the gravity vector (4).In vertebrates, segmentation is coordinated by a molecular clock that converts temporal information into a periodic spatial pattern, which precisely positions somites along the anterior-posterior axis (3,5). In the plant shoot apical meristem, there is evidence that

The development of multicellular organisms relies on the coordinated control of cell divisions leading to proper patterning and growth [1][2][3] . The molecular mechanisms underlying pattern formation, particularly the regulation of formative cell divisions, remain poorly understood. In Arabidopsis, formative divisions generating the root ground tissue are controlled by SHORTROOT (SHR) and SCARECROW (SCR) 4-6. Here we show, using cell-type-specific transcriptional effects of SHR and SCR combined with data from chromatin immunoprecipitation-based microarray experiments, that SHR regulates the spatiotemporal activation of specific genes involved in cell division. Coincident with the onset of a specific formative division, SHR and SCR directly activate a D-type cyclin; furthermore, altering the expression of this cyclin resulted in formative division defects. Our results indicate that proper pattern formation is achieved through transcriptional regulation of specific cellcycle genes in a cell-type-and developmental-stage-specific context. Taken together, we provide evidence for a direct link between developmental regulators, specific components of the cell-cycle machinery and organ patterning.Growth and patterning are key processes that govern the development of multicellular organisms. In some cases, like early Drosophila embryogenesis 7 , these are independent. However, in many animals and plants, proper development frequently relies on tight coordination of growth and patterning. Disruption of this coordination can lead to unchecked cell growth, resulting in tumorigenesis or misshapen organs 8 . Although the molecular mechanisms involved in pattern formation 9-11 and in cell-cycle control [12][13][14][15] 5,6,23,24,25 . To gain insight into the role of the SHR/SCR network in controlling formative cell divisions, we expressed an inducible version of either SHR or SCR in its respective mutant background and characterized the timing of formative divisions after induction.Before induction, SHR-and SCR-inducible plants had a single mutant ground tissue layer 4,5 (Supplementary Fig. 1). After SHR induction, SCR expression was observed within 3 h, indicating that SHR rapidly activates its targets ( Supplementary Fig. 1). The first periclinal (parallel to the direction of growth) division in the mutant ground tissue layer occurred 6 h after SHR induction (Fig. 1a , Supplementary Fig. 1 and Supplementary Movie 1) and earlier after SCR induction (Fig. 1b). Two layers of ground tissue with SCR expression in the quiescent centre and endodermis (Supplementary Fig. 1), along with a nearly complete Casparian band 5 , were detected 24 h after SHR induction ( Supplementary Fig. 2). This underlines the combinatorial role of SHR and SCR in regulating formative cell divisions and also indicates that the two inducible systems have slightly different kinetics.To understand the dynamics of the SHR/SCR regulatory network, we sorted ground tissue cells at several time points after SHR and SCR induction and performed microarray a...