WUS has been shown to activate CLAVATA3 ( CLV3) expression ( 7). WUS synthesized in few cells of the RM has been shown to migrate into adjacent cells where it accumulates at a lower level than in the RM ( 6). In Arabidopsis SAMs, WUSCHEL (WUS), a homeodomain transcription factor that is expressed in the RM, has been shown to provide cues for stem cell maintenance ( 4, 5). Shoot apical meristems (SAMs) contain a set of pluripotent stem cells in the central zone (CZ) the stem cell descendants that are displaced into the adjacent peripheral zone (PZ) differentiate as lateral organs ( 3), and the stem cell descendants that are displaced into the rib meristem (RM), located beneath the CZ, differentiate and become part of the stem ( 3). Transcriptional mechanisms that underlie dose-dependent modulation of gene expression in plant development have not been discovered, but cell-fate specification is known to rely heavily on positional cues ( 2). Various mechanisms have been proposed to explain dose-dependent transcriptional regulation mediated by morphogen gradients in animal development ( 1). The concentration-dependent transcriptional discrimination provides a mechanistic framework to explain the regulation of CLV3 levels that is critical for stem cell homeostasis. By deleting cis elements, manipulating the WUS-binding affinity and the homodimerization threshold of cis elements, and manipulating WUS levels, we show that the same cis elements mediate both the activation and repression of CLV3 at lower and higher WUS levels, respectively. Here we show that WUS binds a group of cis-elements, cis- regulatory module, in the CLV3-regulatory region, with different affinities and conformations, consisting of monomers at lower concentration and as dimers at a higher level. How WUS regulates CLV3 levels has not been understood.
In Arabidopsis shoot apical meristems, WUSCHEL (WUS), a stem cell-promoting transcription factor, accumulates at a higher level in the rib meristem and at a lower level in the central zone where it activates its own negative regulator, CLAVATA3 ( CLV3). However, the mechanisms of dose-dependent transcriptional regulation in plant development have not been understood. Transcriptional mechanisms that underlie the dose-dependent regulation of gene expression in animal development have been studied extensively.
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