Abstract
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.
Highlights
Postembryonic growth and iterative organ formation of higher plants rely on the activity of pluripotent stem cells in organogenic centers, the meristems
The shoot meristem that will give rise to the above ground organs has been extensively studied in the model plant Arabidopsis thaliana
We show by positional cloning that wen9 is an allele of the OBE3 gene, and characterize its function together with its closest homologue OBE4 in the shoot meristem
Summary
Postembryonic growth and iterative organ formation of higher plants rely on the activity of pluripotent stem cells in organogenic centers, the meristems. The homeodomain transcription factor WUS is expressed in the organizing center (OC) underneath the stem cells [1] where it directly represses cytokinin response inhibitors [2] and, after moving into the overlying stem cells [3, 4], represses cell differentiation and activates expression of the signal peptide CLV3 [4,5,6]. CLV3 in turn represses WUS transcription via CLV1/CLV2-CRN receptor-like kinases to delimit the size of the OC [6,7,8]. This negative feedback loop balances stem cell maintenance and differentiation [7]. The WUS/CLV3 loop functions to maintain stem cells of the floral meristems [6, 7]. We show by positional cloning that wen is an allele of the OBE3 gene, and characterize its function together with its closest homologue OBE4 in the shoot meristem
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