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.
Schematic of genes that regulate stem cell activity in the Arabidopsis shoot apical meristem
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.
Promoter-driven GUS expression patterns of various CLE genes in Arabidopsis seedlings, leaves, stems, flowers and siliques
Monfared, M. and Fletcher, J.C. (2013). Genetic and phenotypic analysis of shoot apical and floral meristem development. In Flower Development: Methods and Protocols. Wellmer, F. and Reichmann, J.-L., Eds. Totowa, NJ : Humana Press, in press.
Pu, L., Liu, M.-S., Kim, S.Y., Chen, L.-F., Fletcher, J.C. and Sung, Z.R. (2013). EMBRYONIC FLOWER1 and ULTRAPETALA1 act antagonistically on Arabidopsis development and stress response. Plant Physiology 162: 812-830.
Monfared, M., Carles, C.C., Rossignol, P., Pires, H.R. and Fletcher, J.C. (2013). The ULT1 and ULT2 trxG genes play overlapping roles in Arabidopsis development and gene regulation. Molecular Plant in press.
Fiume, E. and Fletcher, J.C. (2012). Regulation of Arabidopsis embryo and endosperm development by the polypeptide signaling molecule CLE8. Plant Cell 24: 1000-1012.
Monfared, M., Simon, M., Meister, R., Roig-Villanova, I., Kooiker, M., Colombo, L., Fletcher, J.C., and Gasser, C. (2011) Overlapping and antagonistic activities of BASIC PENTACYSTEINE genes affect a range of developmental processes in Arabidopsis. Plant Journal 66: 1020-1031.
Fiume, E., Monfared, M., Jun, J.H. and Fletcher, J.C. (2011) CLE polypeptide signaling gene expression in Arabidopsis embryos. Plant Signaling & Behavior 6: 443-444.
Jun, J.H., Fiume, E., Roeder, A.H.K., Meng, L., Sharma, V.K., Osmont, K.S., Baker, C., Ha, C.M., Meyerowitz, E.M., Feldman, L.J. and Fletcher, J.C. (2010) Comprehensive analysis of CLE polypeptide signaling gene expression and over-expression activity in Arabidopsis. Plant Physiol. 154: 1721-1736.
Ha, C.M., Jun, J.H. and Fletcher, J.C. (2010) BLADE-ON-PETIOLE1 and 2 control Arabidopsis leaf morphogenesis by regulating YABBY and KNOX gene activity. Genetics 186: 197-206.
Ha, C.M., Jun, J.H. and Fletcher, J.C. (2010) Shoot apical meristem form and function. In Current Topics in Developmental Biology. Timmermans, M., Ed. New York, NY : Academic Press. vol. 91C, pp. 103-140.
Meng, L., Ruth, K.C., Fletcher, J.C. and Feldman, L.J. (2010) The roles of different CLE domains in Arabidopsis CLE polypeptide activity and functional specificity. Molecular Plant 3: 760-772.
Carles, C.C. and Fletcher, J.C. (2010) Missing links between histones and RNA Pol II arising from SAND? Epigenetics 381-385.
Ron, M., Alandete-Saez, M., Williams, L., Fletcher, J.C. and McCormick, S. (2010) Proper regulation of a sperm-specific cis-nat-siRNA is essential for double fertilization in Arabidopsis. Genes & Dev 24: 1010-1021.
Carles, C.C., Ha, C.M., Jun, J.H., Fiume, E. and Fletcher, J.C. (2010) Analyzing shoot apical meristem development. In Methods in Plant Developmental Biology. Hennig, L. and Kohler, C. Eds. Totowa, NJ : Humana Press. vol. 655, pp. 105-129.
Fiume, E., Pires, H.R., Kim, J.S. and Fletcher, J.C. (2010) Analyzing floral meristem development. In Methods in Plant Developmental Biology. Hennig, L. and Kohler, C. Eds. Totowa, NJ : Humana Press. vol. 655, pp. 131-142.
Jun, J.H., Ha, C.M. and Fletcher, J.C. (2010) BLADE-ON-PETIOLE1 coordinates organ determinacy and axial polarity in Arabidopsis by directly activating ASYMMETRIC LEAVES2. Plant Cell 22: 62-76.
Carles, C.C. and Fletcher, J.C. (2009) The SAND domain protein ULTRAPETALA1 acts as a trithorax group factor to regulate cell fate in plants. Genes & Dev 23: 2723-2728.
Leasure, C.D., Fiume, E. and Fletcher, J.C. (2009) The essential gene EMB1611 maintains shoot apical meristem function during Arabidopsis development. Plant J. 57: 579-592.