The majority of food crops are flowering plants that require successful fertilization to produce grains and fruits. Fertilization requires pollen to land on the female flower and form a pollen tube, a rapidly elongating single-celled projection which grows through the female tissue, in some cases for several centimeters. This pollen tube is able to reorient its growth in response to chemical guidance cues to deliver the male reproductive cells to the female. The pollen tube, like other plant cells is surrounded by a cell wall and cell expansion requires the proper secretion of new cell wall material that is specifically targeted to the specialized, flexible cell wall at the growing tip. The internal secretory machinery and the cell wall must be appropriately coordinated with each other and with the direction of elongation for successful sustained growth. Despite the importance of this process for agricultural production, a major gap exists in our understanding of how the external cell wall and the intracellular secretory apparatus coordinate to maintain a stable but responsive tip region over time. This project will elucidate the linkage between these two compartments using genetics, biochemistry and cell imaging approaches. The research will also support the training of graduate and undergraduate students. Research activities will be integrated into an introductory plant biology lab course giving students an authentic research experience. In addition, the project will provide interactive, hands-on activities to the general public in collaboration with the University of Michigan Museum of Natural History. This project was co-funded by the Plant, Fungal and Microbial Developmental Mechanisms Program in the Division of Integrative Organismal Systems and the Cellular Dynamics and Function Program in the Division of Molecular and Cellular Biosciences.
Flowering plant fertility depends on the sustained rapid elongation of pollen tubes and their ability to reorient growth to effectively target receptive ovules. Proper pollen tube elongation requires hydroxyproline (hyp) O-arabinosylation, the posttranslational addition of arabinose sugar chains to hydroxyprolines. Arabinosyltransferase mutants exhibit shorter, wider and sometimes branched pollen tubes, suggesting impaired tip-region stability. The secretion required for pollen tube elongation depends on the highly dynamic F-actin structures at the pollen tube tip. F-actin organization is controlled, in part, by the nucleation activity of class 1 formin homology proteins. The extracellular domains of these proteins contain predicted arabinosylation motifs which have been implicated in interaction with the cell wall. This project will test the hypothesis that the pollen tube cell wall and actin cytoskeleton are coordinated via hyp O-arabinosylation of the class 1 formin homology proteins, limiting their actin organizing activity to appropriate cortical domains though interaction with the overlying cell wall. Altered cell wall polarity and actin organization will be assayed in the mutants and wild type. The in vivo glycosylation status of the class 1 formins will be determined for three important members of this protein family, including two known regulators of pollen tube actin. Furthermore, the effect of the extracellular domains on protein mobility and localization in the pollen tube will be determined. This research will establish an essential point of connection between two major cell components required for pollen tube polarization and growth.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.