An ion-based pacemaker at the pollen tube tip: mechanism and functions in apical growth, guidance and cell-cell communication

University Of Maryland, College Park, College Park MD

Investigators

Abstract

An ion-based pacemaker at the pollen tube tip: mechanism and functions in apical growth, guidance and cell-cell communication The project will study the role of ion and electric signals generated within the pollen tube, a highly specialized cell involved in the reproduction of flowering plants. These cells are representative of apical growing cells, a group that in addition to pollen tubes, includes fungal hyphae and developing neurons of the mammalian nervous system. Specifically these ion signals are needed to coordinate pollen tube growth and function required for fertilization. The results will not only contribute to predicting and manipulating cellular behavior leading to seed production, with potential agricultural impacts, but also to discovering new principles by which electric signals can define cell shape, guidance and communication extending beyond plant cells. This investigation will entail the development and integration of different STEM areas, including cell biology, genetics, computational methods and mathematical modeling. A data analysis pipeline and novel algorithms developed from the research will be made publicly available in R, an open-source statistical programming language, allowing analysis of complex oscillations and cell growth imaging. Educational and societal impacts include professional development of two postdoctoral researchers, undergraduate student training, and mentoring of local high school students. In addition, a number of outreach and educational activities are expected outcomes of the project, where the results and original approaches appealing to broader interests will be presented to a wide variety of audiences. Fertilization in higher plants requires the male gametophyte (pollen tubes) to grow into the female tissues and locate available ovules, finally bursting to deliver the sperm cells. Pollen tubes are highly polarized cells and in some species have the fastest cellular growth rates known in nature. This vigorous growth is sustained by specific spatial and temporal choreographies, often involving stable oscillations, of intracellular ion concentrations focused at the tip, which in turn are generated by extracellular ion fluxes. The main hypothesis to be tested in this project poses the existence of an ion-based central pacemaker responsible for minute-range oscillations in pollen tubes, which (1) are generated by a negative feedback loop between channel activity and membrane potential, (2) coordinate macroscopic processes at the cellular level (e.g. cell polarity, apical growth, guidance), (3) are involved in cell-cell competition/cooperation between growing pollen tubes and (4) impact cell-cell communication with the female gametophyte. The first goal is to characterize oscillations in all parameters using custom-made electrophysiology tools and live imaging with genetically encoded probes (Ca2+, H+, Cl- and membrane potential). These will allow comparison between signatures of different species/ ecotypes and single and multiple mutants of gene families coding for channels and transporters in Arabidopsis. The features of the oscillations will be determined with data-analysis tools developed for the project, allowing the formulation of mathematical models. Specific predictions will then be derived from differential equation models in varying levels of detail, whereas experimental validation will be achieved by testing the predictions with mutations, pharmacology and in vivo fertilization protocols, among others.