Organ formation in animals and plants relies on precise control of cell state transitions to turn stem cell daughters into fully differentiated cells. In plants, cells cannot rearrange due to shared cell walls. Thus, differentiation progression and the accompanying cell expansion must be tightly coordinated across tissues. PLETHORA (PLT) transcription factor gradients are unique in their ability to guide the progression of cell differentiation at different positions in the growing Arabidopsis thaliana root, which contrasts with well-described transcription factor gradients in animals specifying distinct cell fates within an essentially static context. To understand the output of the PLT gradient, we studied the gene set transcriptionally controlled by PLTs. Our work reveals how the PLT gradient can regulate cell state by region-specific induction of cell proliferation genes and repression of differentiation. Moreover, PLT targets include major patterning genes and autoregulatory feedback components, enforcing their role as master regulators of organ development.

The mouse mdr2 P-glycoprotein (P-gp) and its human MDR3 homologue are present in high concentrations in the canalicular membrane of hepatocytes. Mice lacking this protein are unable to secrete phosphatidylcholine (PC) into bile, suggesting that this P-gp is a PC translocator. We have tested this in fibroblasts from transgenic mice expressing the MDR3 gene under a vimentin promoter. Transgenic and control fibroblasts were incubated with ['%]choline to label PC. When the labeled cells were incubated with a PC transfer protein and acceptor liposomes, transfer of radioactive PC was enhanced in transgenic cells relative to the wild type controls. We conclude that the MDR3 P-glycoprotein is able to promote the transfer of PC from the inner to the outer leaflet of the plasma membrane, supporting the idea that this protein functions as a PC flippase.

A phospholipid transfer protein was purified from chicken liver which, in addition to phosphatidylinositol (PI) and phosphatidylcholine (PC), carries sphingomyelin (SM) between membranes. For comparison, the PI-transfer protein from chicken liver only carries PI and PC. Specificity was established by use of phospholipids that carry a pyrene-labeled acyl chain. Based on the N-terminal sequence and Western blot analysis we conclude that this protein is an isoform of the PI-transfer protein. At increasing length of the pyrene-labeled acyl chain, the isoform expresses a high activity toward SM, a low activity toward PI, and virtually no activity toward PC.