Dev Bio SuperGroup Meeting

Leslie Dunipace (Stathopoulos lab)

"Temporal gene expression at the brinker locus:  the switch between two
cis-regulatory modules is controlled by autoregulatory feedback"

Cis-regulatory modules (CRMs) act sequentially to regulate temporal expression of genes, but how
the switch from one to the next is accomplished is not well understood. To provide insight we
investigated the cis-regulatory system controlling brinker (brk) expression in the Drosophila
embryo. Two distally-located CRMs support expression at different times, while a promoter
proximal element (PPE) is required to support their action. In the absence of Brk protein itself or
upon deletion of a portion of the PPE, the late-acting CRM, specifically, is delayed. This block to
late-acting CRM function appears to be removed when the early-acting CRM is also deleted.
These results demonstrate that autoregulatory feedback is necessary for the early-acting CRM to
disengage from the promoter so that the late-acting CRM may act. Autoregulation may be a
commonly used mechanism to control sequential CRM action necessary for dynamic gene
expression throughout the course of development.

Zachary Nimchuk (Meyerowitz Lab)

"Control of plant stem cell proliferation by peptide and hormone signaling networks"

Above ground post-embryonic plant growth is derived from stem cell pools that are located in the
central apex of the shoot apical meristem (SAM), a structure akin to the animal stem cell niche.
The SAM functions to self-renew stem cells and also generate lateral organs such as leaves or
flowers. The synthesis, transport and perception of the hormone auxin are necessary for the
formation of lateral organs, which arise from the flanks of the SAM. The SAM must therefore
balance the continued proliferation of the stem cell pool with the allocation of cells for lateral organ
formation. This balance is maintained by a feedback loop consisting of the homeodomain
transcription factor WUSCHEL and the peptide ligand-receptor kinase pair of CLAVATA3 and
CLAVATA1. In the absence of CLAVATA signaling, stem cell pools hyper-proliferate in both shoot
and floral meristems. This hyper-proliferation is thought to occur via increased levels of
WUSCHEL. Despite this, the direct molecular mechanisms underlying this hyper-proliferation
remain unknown. Here I will present recent data demonstrating that the clavata-class of mutants
regulate the auxin signaling status of the SAM independent of pathways regulating auxin mediated
lateral organ formation. Stem cell proliferation in both wild type and clavata mutants is directly
impacted by auxin mediated synthesis and signaling and is independent or downstream of
WUSCHEL function. The function of auxin in the floral meristems of clavata mutants requires the
activity of an auxin responsive transcriptional cascade. These data point to a key role for auxin in
the execution of peptide signaling pathways in the SAM.