The Role of Actin Monomer Binding Proteins in Plant Cell Tip Growth

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Intellectual MeritOne very important aspect of the growth of plant cells is "tip growth." This project will address some fundamental aspects of the regulation of the actin filament network, a dynamic cytoskeletal structure which is the driving force behind plant cell tip growth. Dr. Bezanilla will study the role of an actin binding protein, ADF/cofilin (ADF), during tip growth in the moss model organism, Physcomitrella patens. Physcomitrella has the unique capacity among plant systems to undergo homologous recombination, making it an ideal model plant system for performing detailed in vivo complementation studies, which will be a cornerstone of the project. ADF is a monomeric actin binding protein that promotes actin filament disassembly. ADF is highly conserved across eukaryotic species of all taxa, and found in organisms ranging from yeast to animals to plants. Across such a diverse array of species, the kinds of actin-based motility phenomena are also quite diverse; yet the evolutionarily highly-conserved ADF protein has been implicated as a key regulator of actin dynamics in all of them. Thus, by studying the function of the ADF protein in plant cell tip growth, it may be possible to begin to address some of the fundamental differences among these actin-based processes. Dr. Bezanilla has already demonstrated that ADF is essential for tip growth in Physcomitrella. She will use a combination of conditional knockdowns employing RNAi, and gene replacements employing homologous recombination, to determine how ADF function is regulated in vivo. In addition she will investigate the potential molecular and genetic interactions between ADF and other evolutionarily conserved actin monomer binding proteins for controlling actin polymerization. Finally, she will begin to establish an in vivo map of the proteins required to regulate ADF and actin dynamics during tip growth in Physcomitrella. These studies will serve to lay groundwork for testing whether these highly conserved proteins function similarly in other plant systems. Understanding the mechanism of tip growth is important because tip growth in plant cells is a fundamental process required for the development of specific tissue types critical for the plant. As examples, rhizoids from algae, pollen tubes and root hairs from flowering plants, and protonemal filaments from mosses and ferns all exhibit tip growth. In all cases these tip-growing tissues represent an essential aspect of development for the plant. In flowering plants, proper development of root hairs is critical for the uptake of water and minerals required for growth and development. Root systems containing more abundant root hairs are more robust and are better able to withstand dryer environmental conditions. Without proper pollen tube growth to allow the sperm nucleus to reach the ovary, fertilization cannot occur and thus the very survival of the species is at risk. Similarly in mosses and ferns, protonemal tip growth is an essential developmental stage required for species viability. Thus, elucidating the mechanism that regulates tip growth will add to our understanding of plant vigor and species viability across a wide range of evolutionarily distinct plant species.Broader ImpactsThis project has potential agricultural benefits for society. For example, the knowledge and understanding that will result from this project may allow us to enhance the ability of crop plants to form root hairs and thus potentially increase drought tolerance and eventually lead to improving crop yields. The project also will integrate research with teaching and training and will broaden the participation of underrepresented groups in science. Dr. Bezanilla, herself a member of an underrepresented minority in science, is strongly interested in recruiting underrepresented minorities at all levels of scientific training (undergraduate, graduate and postdoctoral). She intends to integrate the techniques and findings from this research project into an undergraduate Plant Cell Biology course at the University of Massachusetts, Amherst. This course will serve to introduce undergraduate students to the research and encourage them to participate hands-on in the laboratory. In addition, Dr. Bezanilla intends to encourage high school students to explore science, by recruiting high school students to participate in the research during the summers.