Movement Required

Abstract

Plant cell fate is primarily the result of cell position. Cellular proteins, including transcription factors, can be transported from one cell into another through regulated pores (plasmodesmata) that connect adjacent cells (see the Perspective and associated animations by Jorgensen ) and may be one mechanism by which information about cell location is transmitted. Nakajima et al. determined that restricted expression of and the movement of the transcription factor shoot-root (SHR) was essential for proper cell fate specification and division in Arabidopsis thaliana roots. SHR transcripts are normally only expressed in the stele cells of the root (see Hake). Expression of a green fluorescent protein (GFP)-SHR fusion protein demonstrated that SHR protein appeared in the cells of the adjacent layer (quiescent center, the cortex-endodermis precursors, and the endodermis), where it was localized to the nucleus, supporting a role for SHR that is not cell autonomous. If SHR gene expression was forced by fusion to the SCARECROW ( SCR ) promoter in the cells of the adjacent layer, then there was an expansion of the root layers with an increase in the number of endodermal cells and quiescent center cells. SHR stimulates the transcription of SCR , thus SHR expressed from the SCR promoter establishes a positive feedback system. The ability of ectopically expressed SHR to stimulate increased cell numbers and endodermal cell fates in the adjacent layers was lost in SCR mutant plants, consistent with SHR acting through activation of SCR. Thus, restricted expression and subsequent movement into adjacent cells appears to be essential for proper cell fate specification by SHR. R. A. Jorgensen, Directed cell-to-cell movement of functional proteins: Do transcription factors double as signal molecules in plants? Science's STKE (2000), http://stke.sciencemag.org/cgi/content/full/OC_sigtrans;2000/58/pe2 [Abstract] [Full Text] K. Nakajima, G. Sena, T. Nawy, P. N. Benfey, Intercellular movement of the putative transcription factor SHR in root patterning. Nature 413 , 307-311 (2001). [Online Journal] S. Hake, Mobile protein signals cell fate. Nature 413 , 261-264 (2001). [Online Journal]