OBE3 and WUS Interaction in Shoot Meristem Stem Cell Regulation

The stem cells in the shoot apical meristem (SAM) are the origin of all above ground tissues in plants. In Arabidopsis thaliana, shoot meristem stem cells are maintained by the homeobox transcription factor gene WUS (WUSCHEL) that is expressed in cells of the organizing center underneath the stem cells. In order to identify factors that operate together with WUS in stem cell maintenance, we performed an EMS mutant screen for modifiers of the hypomorphic wus-6 allele. We isolated the oberon3-2 (obe3-2) mutant that enhances stem cell defects in wus-6, but does not affect the putative null allele wus-1. The OBE3 gene encodes a PHD (Plant Homeo Domain) protein that is thought to function in chromatin regulation. Single mutants of OBE3 or its closest homolog OBE4 do not display any defects, whereas the obe3-2 obe4-2 double mutant displays broad growth defects and developmental arrest of seedlings. Transcript levels of WUS and its target gene in the stem cells, CLAVATA3, are reduced in obe3-2. On the other hand, OBE3 and OBE4 transcripts are both indirectly upregulated by ectopic WUS expression. Our results suggest a positive feedback regulation between WUS and OBE3 that contributes to shoot meristem homeostasis.