Overview

Research in my laboratory is focused on the molecular genetic analysis of the early events of pollination in the crucifer family (Brassicaceae) and on understanding the genetic basis of outcrossing and self-fertilizing mating systems. In the crucifer family, as in many families, mating system and breeding structure are regulated through the operation of cell-to-cell communication and recognition events between the pistil on the one hand and the pollen grain or pollen tube on the other hand. Our analysis of early pollination events in two model systems, Brassica and Arabidopsis, allows us to investigate at the molecular level how the pistil acts as a selective barrier to fertilization. The long-term goal of our research is to elucidate the molecular mechanisms underlying the two possible fates followed by pollen grains once they are delivered to the stigma: 1) germination and growth of the pollen tube leading to fertilization and seed production; and 2) inhibition of germination and pollen tube development because of the operation of a genetic barrier to self-fertilization, termed self-incompatibility (SI), in which cells of the stigma epidermis inhibit the development of self-related, but not genetically unrelated, pollen. We determined that specificity in the SI response is determined by two highly polymorphic and co-adapted proteins, a receptor kinase displayed on the stigma surface and its pollen coat-localized peptide ligand. We transferred the SI trait into the highly self-fertile Arabidopsis thaliana by transformation with a receptor-ligand pair fderived rom its self-incompatible sister species, A. lyrata, and uncovered previously unsuspected natural variation for expression of SI in A. thaliana. Our current efforts are focused on exploiting this transgenic A. thaliana SI system and the many molecular genetic resources of this model plant to analyse this crypotic natural variation and to perform saturating mutagenesis of the SI response. This analysis is expected to elucidate the mechanism of SI and identify components of pollen-stigma signaling. It is also expected to explain how and when A. thaliana lost SI and became self-fertile, and more generally to elucidate the genetic basis of evolutionary switches from outcrossing to selfing in the crucifer family.

Research Focus

The focus of research in my laboratory is the study of mechanisms of pollen-pistil interactions and receptor-ligand signaling in plant reproduction.

Instruction Focus

The courses I developed and teach focus on plant development and receptor signaling, with an emphasis on plant reproduction.

Additional Links

• The Nasrallah Lab
• Link to CALS Research portal
• Link to CALS Impact statements

Honors, Awards and Appointments

• American Society of Plant Biologists Martin Gibbs Medal - 2003
• Member of the US National Academy of Sciences - 2003

Selected Publications

• Sherman-Broyles, S., Boggs, N., Farkas, A., Liu, P., Vrebalov, J., Nasrallah, M.E. and Nasrallah, J.B. 2007. S locus genes and the evolution of self-fertility in Arabidopsis thaliana. Plant Cell 19, 94-106.
• Liu, P., Sherman-Broyles, S., Nasrallah, M.E., and Nasrallah J.B. 2007. A cryptic modifier causing transient self-incompatibility in Arabidopsis thaliana. Curr. Biol. 17: 734-740.
• Tang, C., Toomajian, C., Sherman-Broyles, S., Plagnol, V., Guo, Y.L., Hu, T.T., Clark, R.M., Nasrallah, J.B., Weigel, D., and Nordborg, M. (2007). Evolution of Selfing in Arabidopsis thaliana. Science Jul 26; [Epub ahead of print]
• Naithani, S., Chookajorn, T., Ripoll, D.R., and Nasrallah, J.B. (2007). Structural modules for receptor dimerization in the S-locus recepto kinase extracellular domain. Proc. Natl. Acad. Sci. USA 104: 12211-12216.
• Nasrallah, J.B. 2005. Recognition and rejection of self in plant self-incompatibility: comparisons to animal histocompatibility. Trends Immunol. 26: 412-418.
• Chookajorn, T., Kachroo, A., Ripoll, D.R., Clark, A.G., and Nasrallah, J.B. 2004. Specificity determinants and diversification of the Brassica self-incompatibility pollen ligand. Proc. Natl. Acad. Sci. USA 101: 911-917.
• Nasrallah, M.E., P. Liu, and Nasrallah, J.B. 2002. Generation of self-incompatible Arabidopsis thaliana by transfer of two S locus genes from A. lyrata. Science 297: 247-249.
• Kachroo, A., Schopfer, C.R., Nasrallah, M.E., and Nasrallah, J.B. 2001. Allele-specific receptor-ligand interactions in Brassica self-incompatibility. Science 293: 1824-1826.
• Schopfer, C.R., Nasrallah, M.E., and Nasrallah, J.B. 1999. The male determinant of self-incompatibility in Brassica. Science 286: 1697-1700.
• Stein, J.C., Howlett, B., Boyes, D.C., Nasrallah, M.E. and Nasrallah, J.B. 1991. Molecular cloning of a putative receptor protein kinase gene encoded at the self-incompatibility locus of Brassica oleracea. Proc. Natl. Acad. Sci. USA 88: 8816-8820.

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