  
A maize plant that carries a mutation in the leafbladeless1 (lbl1)
gene shows threadlike leaves and stunted growth. These defects result from the
failure to distinguish the upper and lower portions of the leaf during its development.
lbl1 functions in the production or perception of a stem cell derived signal
that sets up dorsoventral polarity. |
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| Marja Timmermans
Associate Professor Ph.D., Rutgers University, 1996 Small RNA regulation, pattern formation, stem cell function, plant development email timmerma@cshl.edu, phone (516) 367-8835 , fax (516) 367-8369 We are using Arabidopsis and maize as model organisms to study the role of small regulatory RNAs as potential signals. We have shown that dorsoventral polarity in leaves is specified through a novel patterning mechanism involving a cascade of opposing small RNAs. The ventral/lower side of the leaf is specified by a 21-nucleotide microRNA, miR166. Its polarized expression is established by the transacting siRNA, tasiR-ARF, which in turn is regulated by miR390. Our current research efforts are aimed at characterizing genes involved in the regulation of this small RNA network and at understanding small RNAs as morphogen-type signals in development. A second objective of our research is to elucidate the mechanisms that distinguish indeterminate stem cells from differentiating cells in plants. We use laser capture microdissection and genomics approaches to dissect genetic pathways required for meristem function and early leaf development. We also used mutational analyses to gain insight into stem cell regulation. This allowed the identification of a highly conserved protein complex that suppresses stem cell fate during organ development. This complex includes the chromatin-remodeling factor HIRA, and we are currently analyzing the molecular mechanism by which this epigenetic regulator controls stem cell identity. Selected Publications 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. Phelps-Durr, T.L., Thomas, J., Vahab, P., and Timmermans, M.C.P. 2005. Maize rough sheath2 and its Arabidopsis orthologue ASYMMETRIC LEAVES1 interact with HIRA, a predicted histone chaperone, to maintain knox gene silencing and determinacy during organogenesis. Plant Cell 17: 2886–2898. Juarez, M.T., Kui, J.S., Thomas, J., Heller, B.A., and Timmermans, M.C.P. 2004. microRNA mediated repression of rolled leaf1 specifies maize leaf polarity. Nature 428: 84–88. Kim, M., McCormick, S., Timmermans, M.C.P., and Sinha, N. 2003. The expression domain of PHANTASTICA determines leaflet placement in compound leaves. Nature 424: 438–443. Timmermans, M.C.P., Hudson, A., Becraft, P.W., and Nelson, T. 1999. ROUGH SHEATH2: A Myb domain protein that represses knox homeobox genes in maize lateral organ primordia. Science 284: 151–153.
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Plants grow
and develop organs, such as leaves, throughout their lifetime. The growing
tips of plants, called meristems, contain a population of indeterminate
stem cells. These cells serve as a persistent source of daughter cells
from which new organs arise and, in addition, produce signals important
for the determination and patterning of lateral organs.