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Intercellular protein movement plays a critical role in animal and plant development. SHORTROOT (SHR) is a moving transcription factor essential for endodermis specification in the Arabidopsis root. Unlike diffusible animal morphogens, which form a gradient across multiple cell layers, SHR movement is limited to essentially one cell layer. However, the molecular mechanism is unknown. We show that SCARECROW (SCR) blocks SHR movement by sequestering it into the nucleus through protein-protein interaction and a safeguard mechanism that relies on a SHR/SCR-dependent positive feedback loop for SCR transcription. Our studies with SHR and SCR homologs from rice suggest that this mechanism is evolutionarily conserved, providing a plausible explanation why nearly all plants have a single layer of endodermis. [ABSTRACT FROM AUTHOR]

The article reports that the phytohormone auxin plays a major role in plant growth. To determine the direction of auxin flow researchers studied the directional auxin translocation during root gravitropism in Arabidopsis thaliana. They designed hemagglutinin epitope-tagged PIN1 and PIN2 genes under the transcriptional control of the PIN2 promoter. It was observed that in cortex cells, both proteins were localized at the lower basal side. Researchers tested the relationship between polarity and the direction of auxin flow by examining auxin translocation during gravity response. It was found that the localization of PIN proteins at the upper side of epidermal cells was related to their ability to facilitate upward auxin movement for root gravitropic response. Finally it was concluded that PIN polarity is a primary direction determining factor in auxin transport in meristematic tissues. It also provides information on how auxin flows can be redirected through rapid changes in PIN polarity in response to developmental and environmental signals.