Ph.D., Rutgers University, 1996
|At the growing tip of plants sits a reservoir for stem cells, called the meristem, from which new organs, such as leaves, arise. We study genetic networks that distinguish stem cells from their differentiating descendants as well as the role of mobile small RNAs as novel meristem-derived signals that pattern leaves|
The growing tips of plants, called meristems, contain a population of stem cells that serve as a persistent source of daughter cells from which new organs, such as leaves, arise. Marja Timmermans and colleagues are studying the genetic networks that regulate plant stem cell activity. Using genomic approaches, they have defined gene expression signatures that distinguish indeterminate stem cells from their differentiating derivatives. They have also worked out the mechanism that suppresses stem cell fate to allow cells to differentiate and have shown that this process requires a highly conserved epigenetic gene silencing mechanism. In particular, Timmermans’ group has shown that specific DNA-binding proteins mediate the recruitment of Polycomb repressive complexes to stem cell factors, an action that stably represses their expression in differentiating organs. This work addresses a major unresolved question in the field of epigenetics: how Polycomb proteins, which do not bind DNA themselves, recognize defined targets. Plant stem cells also produce signals important for the patterning of lateral organs. The lab has discovered that small RNAs can traffic from cell to cell and are among the stem-cell-derived signals. They have found that polarity in leaves is established via opposing gradients of mobile small RNAs that act as morphogen-like signals. Their most recent findings identified a third small RNA gradient involved in maintenance of organ polarity. These findings illustrate the complexity with which small RNAs generate developmental patterns. Currently, they are investigating parameters of small RNA mobility and the unique patterning properties of resulting small RNA gradients. Mathematical modeling predicts that such gradients might serve to generate robustness during development.
Lodha, M., Marco, C.F., and Timmermans, M.C.P. 2013. The ASYMMETRIC LEAVES complex maintains repression of KNOX homeobox genes via direct recruitment of Polycomb-repressive complex2. Genes Dev. 27: 596–601.
Chitwood, D.H. and Timmermans, M.C.P. 2010. Small RNAs are on the move. Nature 467: 415–419.
Chitwood, D.H., Nogueira F.T.S., Howell, M., Montgomery, T.A. Carrington, J.C. and Timmermans, M.C.P. 2009. Pattern formation via small RNA mobility. Genes Dev. 23: 549–554.
Guo, M., Thomas, J., Collins, G., and Timmermans, M.C.P. 2008. Direct repression of KNOX loci by the ASYMMETRIC LEAVES1 complex of Arabidopsis. Plant Cell 20: 48–58.
Nogueira, F.T.S., Madi, S., Chitwood, D.H., Juarez, M.T., and Timmermans, M.C.P. 2007. Two small regulatory RNAs establish opposing fates of a developmental axis. Genes Dev. 21: 750–755.