Characterization of SHORT-ROOT function in the Arabidopsis root vascular system.

Development of the vascular tissues is a dynamic process that integrates extrinsic and intrinsic factors to control vascular tissue formation throughout the plant life cycle. During vascular tissue formation in Arabidopsis roots, radial and longitudinal signals, including nuclear factors and plant hormones, control the developmental processes involved in the specification, differentiation, and maintenance of the correct cell types. SHR, a GRAS transcription factor, has been known to regulate the specification of the stem cell niche and ground tissue identity in the root meristem in a non-cell-autonomous manner. However, the role of SHR in the root vasculature is relatively overlooked, despite localization of its mRNA and protein in the stele. Here, we investigated the role of SHR in the vascular system of the primary root using a reverse genetic approach and detailed phenotypic analysis. A novel, loss-of-function null mutant, shr-6, was isolated in the Columbia background, and vascular patterning was characterized in detail. Our results reveal that shr mutants have developmental defects in both protophloem and protoxylem elements. Our study also suggests that SHR plays a central role in the root vascular system to control patterning processes, possibly regulated by longitudinal and radial signals.