| Morphogenesis in plants results from the combination of cell growth and cell proliferation. Although morphogenesis is clearly controlled genetically, the extent to which the programmed behavior of individual cells leads to the final shape of the plant is an open question. To learn more about the genetic control of morphogenesis, we studied the Arabidopsis thaliana genes ARABIDOPSIS G-PROTEIN BETA 1 (AGB1), ECTOPIC ROOT HAIRS 2 (ERH2), and the three members of the FIZZY-RELATED (FZR) gene family.Chapter 1 gives an historical description of two prevailing theories of multicellular organism morphogenesis: the Cell Theory and the Organismal Theory. In brief, the Cell Theory puts control of morphogenesis within genetically programmed cell divisions and cell expansion. Conversely, the Organismal Theory postulates that another genetic mechanism exists that controls the shape of the organism, independently of the behavior of individual cells. Each theory has merits in describing plant morphogenesis, and we incorporate the research described in the following two chapters into this historical debate.In Chapter 2, we study the role of the three Arabidopsis members of the FZR family, each of which is expressed throughout the plant, in cell growth and cell proliferation. Focusing on FZR2, we discovered that mutations that reduce or prevent expression of this locus result in a decrease in endoreduplication in leaves and trichomes, concurrent with a general reduction in cell sizes in those tissues. We show that misexpression of FZR2 leads to ectopic endoreduplication in non-endocycling cells such as petal cone cells, and induces extra endoreduplication in tissues that normally endocycle. Together, these data support our conclusion that FZR2 expression is necessary and sufficient for endoreduplication in Arabidopsis. Furthermore, loss of fzr2 function results in leaves of the same size and shape as wild-type leaves, yet these leaves are composed of smaller cells. We close the chapter offering a new model in which a reduction in cell volume can trigger a compensatory increase in cell proliferation, and suggest methods to test this hypothesis.In Chapter 3, we discuss our research performed on the regulation of cell growth and proliferation in trichomes and their attendant socket cells. We have discovered that a recessive mutation in either of two genes, ARABIDOPSIS G-PROTEIN BETA1 (AGB1) and ECTOPIC ROOT HAIR2 (ERH2), causes a statistically significant over-proliferation of socket cells and an increase in trichome base diameter. In addition, plants homozygous for mutations in both genes show an additive effect in the trichomes as the base diameter increases beyond either single mutant. The socket cell number, however, is not significantly different from the single erh2-1 mutant, indicating an epistatic relationship. We place AGB1 and ERH2 in a common pathway that limits socket cell proliferation, and in parallel pathways that control trichome base morphology. We go on to describe other interesting phenotypes in these mutants, such as the development of putative guard cells adjacent to the mutants' trichomes, an event which, to our knowledge, has not been previously reported. Besides the trichome effects, the agb1-2 and erh2-1 double mutants showed an interesting mixture of phenotypes from each of the single mutants, such as large round Leaves like agh1-2 and short petioles like erh2-1. We close this chapter by suggesting next steps to investigate these phenotypes.Chapter 4 is a description of a novel project to design and implement an undergraduate introductory biology course. We joined a group of graduate students and post-doctoral fellows, first to receive training in active learning and assessment techniques, then to formulate teachable units, and finally to administer and evaluate the effectiveness of the course composed of the teachable units. The goal of the class was to implement and assess active learning strategies to introduce students to fundamental biology concepts and scientific thinking. Using qualitative assessment tools, we measured the students' knowledge and skill gains, through employing initial and terminal examinations. The project was a success, and we discuss its strengths as well as its weaknesses. |
