Simon Gilroy, Ph.D., is Associate Professor of Biology at Pennsylvania State University.Patrick Masson, Ph.D., is Professor of Genetics at the University of Wisconsin.

• Contributors. Preface.1. Mechanisms of Gravity Perception in Higher Plants: Aline H. Valster and Elison B. Blancaflor.1.1 Introduction.1.2 Identification and characterization of gravity perception sites in plant organs.1.2.1 Roots.1.2.2 Hypocotyls and inflorescence stems (dicotyledons).1.2.3 Cereal pulvini (monocotyledons).1.3 The Starch-statolith hypothesis.1.3.1 A variety of plant organs utilize sedimenting amyloplasts to sense gravity.1.3.2 Amyloplast sedimentation is influenced by the environment and developmental stage of the plant.1.4 The gravitational pressure model for gravity sensing.1.5 The cytoskeleton in gravity perception.1.6 Concluding remarks and future prospects.1.7 Acknowledgment.1.8 Literature Cited.2. Signal Transduction in Gravitropism: Benjamin R. Harrison, Miyo T. Morita, Patrick H. Masson and Masao Tasaka.2.1 Introduction.2.2 Gravity signal transduction in roots and above-ground organs.2.2.1 Do mechano-sensitive ion channels function as gravity receptors?.2.2.2 Inositol 1,4,5 trisphosphate seems to function in gravity signal transduction.2.2.3 Do pH changes contribute to gravity signal transduction?.2.2.4 Proteins implicated in gravity signal transduction.2.2.5 Global ‘-omic’ approaches to the study of root gravitropism.2.2.6 Re-localization of auxin transport facilitators or activity regulation?.2.2.7 Could cytokinin also contribute to the gravitropic signal?.2.3 Gravity signal transduction in organs that do not grow vertically.2.4 Acknowledgments.2.5 Cited Literature.3. Auxin Transport and the Integration of Gravitropic Growth: Gloria K. Muday and Abidur Rahman.3.1 Introduction to auxins.3.2 Auxin transport and its role in plant gravity response.3.3 Approaches to Identify Proteins that Mediate IAA Efflux.3.4 Proteins that Mediate IAA Efflux.3.5 IAA influx carriers and their role in gravitropism.3.6 Regulation of IAA efflux protein location and activity during gravity response.3.6.1 Mechanisms that may control localization of IAA efflux carriers.3.6.2 Regulation of IAA efflux by synthesis and degradation of efflux carriers.3.6.3 Regulation of auxin transport by reversible protein phosphorylation.3.6.4 Regulation of auxin transport by flavonoids.3.6.5 Regulation of auxin transport by other signaling pathways.3.6.6 Regulation of gravity response by ethylene.3.7 Overview of the mechanisms of auxin induced growth.3.8 Conclusions.3.9 Acknowledgements.3.10 Cited Literature.4. Phototropism and its Relationship to Gravitropism: Jack L. Mullen and John Z. Kiss.4.1 Phototropism: General Description and Distribution.4.2 Light Perception.4.3 Signal Transduction and Growth Response.4.4 Interactions with Gravitropism.4.5 Importance to Plant Form and Function.4.6 Conclusions and outlook.4.7 References.5. Touch Sensing and Thigmotropism: Gabriele B. Monshausen, Sarah J. Swanson and Simon Gilroy.5.1 Introduction.5.2 Plant mechanoresponses.5.2.1 Specialized touch responses.5.2.2 Thigmomorphogenesis and thigmotropism.5.3 General principles of touch perception.5.3.1 Gating through membrane tension: the mechanoreceptor for hypoosmotic stress in bacteria, MscL.5.3.2 Gating through tethers: the mechanoreceptor for gentle touch in Caenorhabditis elegans.5.3.3 Evidence for mechanically gated ion channels in plants.5.4 Signal transduction in Touch & Gravity Perception.5.4.1 Ionic signaling.5.4.2 Ca2+ signaling in the touch and gravity response.5.5 Insights from transcriptional profiling.5.6 Interaction of touch and gravity signaling/response.5.7 Conclusion and Perspectives.5.8 Acknowledgements.5.9 Cited Literature.6. Other Tropisms and their Relationship to Gravitropism: Gladys I. Cassab.6.1 Introduction.6.2 Hydrotropism.6.2.1 Early studies of hydrotoprism.6.2.2 Genetic analysis of hydrotropism.6.2.3 Perception of moisture gradients and gravity stimuli by the root cap and the curvature response.6.2.4 ABA and the hydrotropic response.6.2.5 Future experiments.6.3 Electrotropism.6.4 Chemotropism.6.5 Thermotropism and oxytropism.6.6 Traumatropism.6.7 Overview.6.8 Acknowledgments.6.9 Literature cited.7. Single-Cell Gravitropism and Gravitaxis: Markus Braun and Ruth Hemmersbach.Introduction.7.1 Definitions of responses to environmental stimuli that optimize the ecological fitness of single-cell organisms.7.2 Occurrence and significance of gravitaxis in single-cell systems.7.3 Significance of gravitropism in single-cell systems.7.4 What makes a cell a biological gravity sensor?.7.5 Gravity susception - the initial physical step of gravity sensing.7.6 Susception in the statolith-based systems of Chara.7.7 Susception in the statolith-based system Loxodes.7.8 Susception in the protoplast-based systems of Euglena and Paramecium.7.9 Graviperception in the statolith-based systems of Chara.7.10 Graviperception in the statolith-based system Loxodes.7.11 Graviperception in the protoplast-based systems Paramecium and Euglena.7.12 Signal transduction pathways and graviresponse mechanisms in the statolith-based systems of Chara.7.13 Signal transduction pathways and graviresponse mechanisms in Euglena and Paramecium.7.14 Conclusions.7.15 Acknowledgements.7.18 Cited Literature.8. Space-Based Research on Plant Tropisms: Melanie J. Correll and John Z. Kiss.8.1 Introduction - the variety of plant movements.8.2 The microgravity environment.8.3 Ground-based studies: mitigating the effects of gravity.8.4 Gravitropism.8.4.1 Gravitropism: gravity perception.8.4.2 Gravitropism: signal transduction.8.4.3 Gravitropism: the curving response.8.5 Phototropism.8.6 Hydrotropism, autotropism and oxytropism.8.7 Studies of other plant movements in microgravity.8.8 Spaceflight hardware used to study tropisms.8.9 Future outlook and prospects.8.10 Cited Literature..9. Plan(t)s for Space Exploration: Christopher S. Brown, Heike Winter Sederoff, Eric Davies, Robert J. Ferl, and Bratislav Stankovic.Introduction.9.1 Human missions to space.9.2 Life support.9.3 Genomics and space exploration.9.4 Nanotechnology.9.5 Sensors, biosensors and intelligent machines.9.6 Plan(t)s for space exploration.9.7 Imagine….9.8 Literature cited