Ion signaling oscillatory patterns as integrators of chemotropic responses in pollen tubes

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Periodic oscillations are present at all levels of biology, from ecology and circadian clocks, to neural function and molecular phenomena. Oscillatory phenomena in the minute/hour range are, however, particularly intriguing since they have important roles in cellular and organ physiology but are poorly understood in terms of mechanisms. Pollen tubes are the male gametophytes of plants, indispensable for the formation of seeds, hence food, and display conspicuous oscillations of small inorganic ions in the minute range when grown in vitro. Previous work has identified these oscillations as implied in intracellular coordination and/or cell-cell communication and this project builds on previous research supported by the National Science Foundation that resulted in the development of a suite of statistical tools to study such oscillations with quantitative precision. Besides pollen tubes, these tools are being applied to phenomena ranging from circadian clocks to astrophysics, revealing unsuspected oscillatory patterns and their underlying mechanisms. The prevalence of oscillations associated with such widespread phenomena and different areas of scientific knowledge, make them attractive topics for any STEM education program, bridging mathematics and physics with biology at various levels. This project will have extensive training opportunities for students at all levels in the quantitative analysis of these oscillations in cellular function.Pollen tubes, reach to the female gametophyte by chemotropism, which is the ability of some cells to grow towards or against an external stimuli. The present project will focus on the surprising finding that mutation of two genes coding for proteins that exchanges potassium against protons in the cell membrane of pollen tubes affect their chemotropic response to an extent that the mutant plant barely produces any seed. It thus unveils a mechanism which is essential for food production. Preliminary evidence showed that the mutant pollen tubes have distinct oscillatory "signatures", which were taken as representing information about specific aspects of cell physiology. The present project will dissect quantitatively the nature of these oscillations when challenged with various negative and positive chemotropic molecules in different genetic backgrounds. Researchers will utilize state-of-the-art methods of cell imaging, molecular genetics, biophysics and mathematical biology. The project is crafted around the hypothesis that the disturbances in proton or potassium concentration resulting from these mutations will affect cell physiology by perturbing some basal level or property of the cell membrane, namely its electric potential, which in turn will render the signaling underlining the chemotropic reaction null. This stringent theoretical framework will be tested, on one hand, through the elaboration of mathematical models that can predict the robustness of key features of cell growth and, on the other hand, by direct proof that the electric potential of the cell membrane is affected is different ways in the mutant and the wildtype pollen tubes. Maintenance of electric potential potentials through cell membrane is a vital property of all living cells and has been evolutionarily co-opted for many fundamental physiological processes, e.g. excitability in the nervous system. The major transducers of energy to create such electric potentials in nature are proton-ATPases, proteins that "pump" protons out of the cell, thus creating a pH gradient used by numerous other proteins to transport molecules into and out of cells. This project will genetically dissect the proton-ATPase family of proteins to determine the boundaries of electric potential in the cell membrane outside of which chemotropism signaling becomes mute.The widespread relevance of chemosensing in biology and the expected mechanistic advances, suggests the present project has a high potential to generate new fundamental models for understanding these phenomena by bridging cell biology, biophysics and mathematical biology.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.