During the development of multicellular organisms, cell differentiation is tightly coordinated with cell division. In animals, loss of control of cell division leads to cancer. In some cell types, a modified cell cycle occurs during differentiation in which the DNA is replicated without concomitant cell division, resulting in an increase in nuclear DNA content. This process is called endoreplication; in plants, important examples of endoreplicated cell types include cereal endosperm, cotton fibers, and nitrogen-fixing symbiotic nodules in legumes. The coordination of this modified cell cycle with cell differentiation remains poorly understood. Previous work using Arabidopsis leaf hairs (trichomes) as a model for cell differentiation demonstrated that the SIAMESE (SIM) gene encodes a cell cycle regulator necessary for the suppression of mitosis during establishment of endoreplication in trichomes. The goal of the proposed work is to test a proposed mechanism for the establishment of endoreplication. The proposed work is expected to shed light on the establishment of the endocycle, and on the integration of the cell cycle with development. The proposed role for SIM focuses on a stage of the cell cycle that is not well-understood in any organism; thus the proposed work may also shed light on the mitotic cycle in both plants and animals. Additionally, the proposed work will play a role in the education of graduate students, undergraduates, and high school students from a wide diversity of backgrounds.

Project Report

Intellectual Merit of the work: Cell differentiation must be closely coordinated with cell division for proper development in both plants and animals. For example, in animals, loss of control of cell division leads to cancer. In some cell types of both plants and animals, a modified cell cycle occurs during differentiation in which the DNA is replicated without concomitant cell division, resulting in an increased nuclear DNA content over the normal diploid amount of DNA. This process is called endoreplication; in plants, important examples of endoreplicated cell types include cereal endosperm, cotton fibers, and nitrogen-fixing root nodules in legumes. In mammals, cells in the trophoblast layer of the placenta endoreplicate their DNA to contain approximately 1000 copies of the genome. Endoreplication is essential for nitrogen fixation in legume root nodules, and plays an important role in implantation of mammalian embryos, but the detailed mechanism and function of this modified cell cycle remains poorly understood. In our previous work using the genetic model plant Arabidopsis, we identified the SIAMESE (SIM) gene as a key plant cell cycle regulator that is necessary for the establishment of endoreplication. All land plant genomes contain multiple genes related to SIM. This gene family and its role in controlling cell division was discovered in our laboratory. During the completed project, we identified protein partners that either interact with the SIM protein directly, or act in the same mitosis-suppressing pathway as SIM. Most of these proteins that interact with SIM are known cell cycle regulators, and the results support our main hypothesis that SIM functions by inhibiting the activity of proteins required for cell division. In collaboration with another lab, we also showed that one likely role for endoreplication is to stabilize the differentiated state of cells. This is a previously unrecognized function of endoreplication. These key results of our work were published in the journals Development, Genetics, Molecular Systems Biology and PloS Genetics. We also published a review of recent work on endoreplication in plants in Trends in Plant Biology, and three more minor papers, for a total of eight published papers. Our future work is aimed at understanding the biochemical mechanisms by which SIM inhibits division, understanding the mechanisms by which SIM function is itself regulated, and understanding the roles of the many SIM-Related proteins in controlling plant growth and development. By illuminating the mechanisms underlying endoreplication and the coordination of cell division and cell differentiation, this research is anticipated to have implications for plant growth and development relevant to both traditional crops and for the production of biomass for biofuels. Additionally, comparison of plant and animal cell cycle control mechanisms may give useful insights relevant to human diseases such as cancer. Broader Impacts of the Funded Work: This funded project contributed to the training of two graduate Ph.D. students, Remmy Kasili and Narender Kumar. One of these students, Dr. Remmy Kasili, did postdoctoral work in the laboratory of Dr. Mary Lilly at the NIH, and is now a professor at Jomo Kenyata University in Kenya, while Narender is still in our lab completing his degree. I am pleased that all four previously graduated Ph.D. students from my lab are still in science, with career paths including work as a staff scientist at St. Jude’s Cancer Institute, postdoctoral work and teaching. This project contributed to the training of at least 13 undergraduates, including six women, four African-Americans, and two Asian-Americans. One undergraduate student was a co-author on two papers for her crucial role in mapping and positional cloning of two genes that act in the same pathway as SIM. She is now a medical student. Another undergraduate was a coauthor on a collaborative paper with a group in France for her work measuring the DNA contents of nuclei in wild-type and mutant plants. She is currently working as a lab technician and is considering graduate school in biological sciences. A high school teacher, Sara Wheeler, did research in the lab in the summer of 2010, supported by this project. The PI chaired two mentoring committees for untenured Biol. Sci. Dept. faculty, and served on the mentoring committees of two others, and served on the Ph.D. committees of multiple graduate students. Thus this project contributed to the development of human resources in the life sciences at all levels, ranging from high school teachers through undergraduate and graduate students to university faculty.

Agency
National Science Foundation (NSF)
Institute
Division of Integrative Organismal Systems (IOS)
Application #
0744566
Program Officer
Amy Litt
Project Start
Project End
Budget Start
2008-02-15
Budget End
2013-01-31
Support Year
Fiscal Year
2007
Total Cost
$401,819
Indirect Cost
Name
Louisiana State University & Agricultural and Mechanical College
Department
Type
DUNS #
City
Baton Rouge
State
LA
Country
United States
Zip Code
70803