Plants are remarkable for their amazing diversity of forms. Yet despite enormous differences in their appearance, all plants have a similar underlying structure. This structure is derived from a reservoir of stem cells located at the growing tip, called the Shoot Apical Meristem (SAM). The stem cells in the SAM continuously divide and replenish themselves, enabling the plant to grow throughout its life. They also produce daughter cells that form organs such as leaves, stems and flowers.
Precise regulation of stem cell activity is essential to balance tip growth with organ formation, and the ability to maintain a dynamic equilibrium of meristem cells is critical for plants to achieve their characteristic architecture. Disruption of plant stem cell maintenance can cause dramatic effects including altered development, biomass accumulation and yield.
The goal of research in the Fletcher Lab is to determine the molecular mechanisms that maintain Arabidopsis shoot and flower stem cell reservoirs, and to understand the early steps of organ formation. Our approach involves a combination of genetics, molecular biology, cell biology and biochemistry.
Stem Cell Biology
We are currently analyzing several plant stem cell maintenance pathways.
The Arabidopsis CLAVATA3 (CLV3) gene encodes a small-secreted polypeptide that is expressed in the shoot and floral stem cells and perceived by several receptor complexes at the surface of the underlying cells. Intercellular signaling through the CLV3 pathway restricts stem cell accumulation by limiting the expression of the WOX family transcription factor gene WUSCHEL (WUS), which in turn promotes stem cell fate and directly activates CLV3 transcription. This regulatory pathway functions as a negative feedback loop that maintains a functional balance between stem cell accumulation and organ formation throughout the plant life cycle.
In addition to the CLV3 pathway, we have identified the Arabidopsis ULTRAPETALA1 (ULT1) locus as an important negative regulator of shoot and floral stem cell activity. ULT1 encodes a SAND domain putative transcriptional regulator that restricts stem cell accumulation and operates as a critical timing component of a pathway that terminates stem cell fate during flower formation. We have demonstrated that ULT1 acts as a trithorax Group (trxG) factor that regulates the chromatin conformation of large numbers of target gene loci. Our present goals are to further characterize the biochemical properties and downstream targets of ULT1 and the related ULT2 protein, and to identify additional components of the pathway.
We also use functional genomics to characterize members a plant-specific family of CLV3-related signaling molecules called CLE proteins and determine their roles in plant development. Intercellular signaling pathways convey cell fate information, regulate cell division and differentiation processes, and propagate and amplify specific signaling states. Yet members of only a few families of plant small signaling molecules have been studied and very little is known about how they coordinate growth and development. We have determined that most Arabidopsis tissues express multiple CLE genes in highly specific patterns, indicating that CLE-mediated signaling pathways are likely to play roles in many biological processes. Our work has also demonstrated that, like CLV3, the CLE proteins function as secreted polypeptides that act in diverse intercellular signaling modules along with other WOX family members. We are currently studying the roles of several CLE polypeptides in Arabidopsis shoot apical meristem function and leaf formation.