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.

The neuron-restrictive silencer factor NRSF (also known as REST and XBR) can silence transcription from neuronal promoters in non-neuronal cell lines, but its function during normal development is unknown. In mice, a targeted mutation of Rest, the gene encoding NRSF, caused derepression of neuron-specific tubulin in a subset of non-neural tissues and embryonic lethality. Mosaic inhibition of NRSF in chicken embryos, using a dominant-negative form of NRSF, also caused derepression of neuronal tubulin, as well as of several other neuronal target genes, in both non-neural tissues and central nervous system neuronal progenitors. These results indicate that NRSF is required to repress neuronal gene expression in vivo, in both extra-neural and undifferentiated neural tissue.

The neuron-restrictive silencer factor (NRSF) represses transcription of several neuronal genes in nonneuronal cells by binding to a 21-bp element called the neuron-restrictive silencer element (NRSE). We have performed data base searches with a composite NRSE to identify additional candidate NRSF target genes. Twenty-two more genes, 17 of which are expressed mainly in neurons, were found to contain NRSE-like sequences. Many of these putative NRSEs bound NRSF in vitro and repressed transcription in vivo. Most of the neuronal genes identified contribute to the basic structural or functional properties of neurons. However, two neuronal transcription factor genes contain NRSEs, suggesting that NRSF may repress neuronal differentiation both directly and indirectly. Functional NRSEs were also found in several nonneuronal genes, implying that NRSF may play a broader role than originally anticipated.

Signaling centers within developing organs regulate morphogenesis in both plants and animals. The putative transcription factor SHORT-ROOT (SHR) is an organizing signal regulating the division of specific stem cells in the Arabidopsis root. Comparison of gene transcription with protein localization indicates that SHR moves in a highly specific manner from the cells of the stele in which it is synthesized outward. Here, we provide evidence that SHR intercellular trafficking is both regulated and targeted. First, we show that subcellular localization of SHR in the stele is intrinsic to the SHR protein. Next, we show that SHR must be present in the cytoplasm to move, providing evidence that SHR movement is regulated. Finally, we describe an informative new shr allele, in which the protein is present in the cytoplasm yet does not move. Thus, in contrast to proteins that move by a process resembling diffusion, a cytoplasmic pool of SHR is not sufficient for movement.