(2) How Plants Manage Water Deficit and Why It Matters
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Plant-Water Relations (2) How plants manage water deficit and why it matters Photo credits: Mel Oliver; William M. Ciesla, Forest Health Management International; R.L. Croissant Bugwood.org © 2014 American Society of Plant Biologists Outline 1. Water scarcity is a growing problem 2. Desiccation-tolerant plants and xerophytes 3. Plant responses to water deficit Perception and signaling Transcriptional responses Effects on photosynthesis Effects on growth and development 4. Towards water-optimized and drought-tolerant agriculture Optimizing irrigation Monitoring water stress 5. Breeding for drought-tolerance Classical approaches Candidate gene approaches © 2014 American Society of Plant Biologists Key definitions and concepts Desiccation: Extreme water loss, equilibration to the water potential of the air which is generally low Desiccation tolerance: The ability to withstand and recover from desiccation Dehydration: The process of water loss from a cell or system Water deficit: Sub-optimal water status from the perspective of plant function Drought: Lack of rainfall or water supply leading to water deficit Drought tolerance: The ability to withstand suboptimal water availability through adaptations or acclimations (not the same as desiccation tolerance) Xerophyte: A plant adapted to live in a low-water environment Mesophyte: A plant without specialized adaptions to low-water or high- water environments Adaptation: Long-term evolutionary process by which an organism becomes suited to its environment Acclimation: Short-term physiological process by which an organism adjusts to changes in its environment © 2014 American Society of Plant Biologists Water scarcity is a growing problem In the past 40 years, Lake Chad has Severe droughts are increasing in shrunk by 95% because of irrigation frequency and forcing up food prices demands & climate change In 2013, a major drought affected large parts of the United States US National Drought Mitigation Center / NASA © 2014 American Society of Plant Biologists Pressure on freshwater resources threatens security and biodiversity Reprinted by permission from Macmillan Publishers Ltd. NATURE: Vorosmarty, C.J., McIntyre, P.B., Gessner, M.O., Dudgeon, D., Prusevich, A., Green, P., Glidden, S., Bunn, S.E., Sullivan, C.A., Liermann, C.R. and Davies, P.M. (2010). Global threats to human water security and river biodiversity. Nature. 467: 555-561, copyright 2010. © 2014 American Society of Plant Biologists The earth’s surface is 75% water, but most is not available to plants <1% of Earth’s water is 97% saltwater fresh and potentially available to plants 3% freshwater 68.7% snow and ice (including polar icecaps) 30.9% groundwater 0.4% lakes and rivers NASA: Earth Observatory © 2014 American Society of Plant Biologists Water is both a renewable and a nonrenewable resource Rainwater and snowmelt are renewable Oil is a Solar energy is a nonrenewable renewable resource resource, limited Aquifer-derived only by our ability water is not to capture and harness it © 2014 American Society of Plant Biologists The Ogallala (High Plains) aquifer is being depleted of its reserves Almost one third of the groundwater used for irrigation in the United Aquifers around the world States comes from the are similarly running dry Ogallala aquifer (shown in color), but this water resource is running out USGS © 2014 American Society of Plant Biologists Agricultural yields in most countries are likely to decrease by 2050 World Development Report 2010 © 2014 American Society of Plant Biologists The growing population demands a more efficient use of water Understanding how plants manage water deficit is essential for meeting the needs of the growing (and growing in affluence) human population Source: FAO Water at a Glance © 2014 American Society of Plant Biologists Some plants and plant tissues are desiccation-tolerant Seeds and pollen – usually desiccation tolerant Bryophytes (mosses, liverworts, hornworts) – many are desiccation tolerant Some vascular plants have acquired vegetative desiccation tolerance Vegetative tissues of vascular plants – usually not desiccation tolerant Photo credit: Anne Wangalachi/CIMMYT; Mel Oliver; Dartmouth Electron Microscope Facility © 2014 American Society of Plant Biologists Some plants are desiccation or drought tolerant, others not Desiccation tolerant Desiccation sensitive Xerophytes Mesophytes Within families and species, a wide range of drought sensitivities are present Drought tolerant Drought sensitive Many bryophytes A small number of Succulents and vascular plants non-succulents Photo credits: Mel Oliver ; Hoekstra, F.A., Golovina, E.A. and Buitink, J. (2001). Mechanisms of plant desiccation tolerance. Trends Plant Sci. 6: 431-438; Mwilliams; Halley; Lopes, M.S., Araus, J.L., van Heerden, P.D.R., and Foyer, C.H. (2011). Enhancing drought tolerance in C4 crops. J. Exp. Bot. 62: 3135 – 3153; Paxton Payton © 2014 American Society of Plant Biologists Plants fall in a continuum from desiccation tolerant to sensitive Desiccation-tolerant plants Desiccation-sensitive plants • Equilibrate their Cellular cellular water potential Use morphological, Adaptations protection and with the ambient water phenological and and acclimations repair potential biochemical strategies to to avoid mechanisms • Tolerate desiccation maintain constant through biochemical cellular water potential desiccation and structural and / or relative water Ψ fluctuates adjustments content Ψ fairly constant Death Cellular water water Cellular potential potential Cellular water water Cellular External water External water potential potential © 2014 American Society of Plant Biologists Desiccation-tolerant plants Desiccation-tolerant Most plants – bryophytes under -3 to -4 MPa lab lethal limit - conditions 250 MPa All desiccation- (-12 MPa Larrea) - tolerant plants MPa 40 0 Dehydration levels - - - 10 MPa 10 600 600 MPa 100 100 MPa All resurrection plants - tends to be the lowest end point in natural environments Most bryophytes for brief periods In a practical context: Plants and plant organs (e.g., seeds) equilibrated at water potentials Adapted from Oliver, M.J., Cushman, J.C. and Koster, K.L. (2010).Dehydration tolerance below -100 MPa are considered desiccated in plants. Methods in Molecular Biology (Clifton, N.J.) 639: 3-24. © 2014 American Society of Plant Biologists Vegetative desiccation tolerance is the ancestral strategy Note that most seeds and pollen retain desiccation tolerance Vegetative DT lost Desiccation tolerance (DT) A small number of is common in bryophytes tracheophytes (vascular plants) (Iiverworts, mosses) called “resurrection plants” have reacquired vegetative desiccation tolerance Oliver, M.J., Velten, J. and Mishler, B.D. (2005). Desiccation tolerance in bryophytes: A reflection of the primitive strategy for plant survival in dehydrating habitats? Integr. Comp. Biol. 45: 788-799 by permission of The Society for Integrative and Comparative Biology. © 2014 American Society of Plant Biologists Bryophytes, “non-vascular” plants, are mostly desiccation tolerant Liverworts (~7000 Mosses (~10,000 – Hornworts (~200 species) – 8500 species) 17,000 species) Vascular plants Green algae Land plants Adapted from Chang, Y. and Graham, S.W. (2011). Inferring the higher-order phylogeny of mosses (Bryophyta) and relatives using a large, multigene plastid data set. Am. J. Bot. 98: 839-849 and Ligrone, R., Duckett, J.G. and Renzaglia, K.S. (2012). Major transitions in the evolution of early land plants: a bryological perspective. Ann. Bot. 109: 851-871. Photo credits Tom Donald, Mary Williams and gjshepherd_br / Foter.com / CC BY-NC-SA © 2014 American Society of Plant Biologists Most seeds are mostly desiccation tolerant, some are not Wheat, corn, rice, beans and other human food staples produce “orthodox” seeds that desiccate and can be stored stably Rhizophora mangle Cocos nucifera Some mangroves, Stenocereus alamosensis red mangrove coconut cacti and large- seeded plants such as coconut have “recalcitrant” seeds that do not desiccate or become quiescent Viviparous seeds germinating on the tree or in the fruit Reprinted with permission of the Botanical Society of America from Cota-Sánchez, J.H., Reyes-Olivas, Á. and Sánchez-Soto, B. (2007). Vivipary in coastal cacti: a potential reproductive strategy in halophytic environments. Am. J. Bot. 94: 1577-1581. Jaro Nemcok Vencel © 2014 American Society of Plant Biologists Orthodox seeds go through a desiccated state Seed reserve accumulation Germination Acquisition of desiccation tolerance Embryogenesis seeds Orthodox Dry state, Water content Dispersal Embryogenesis Germination Seed reserve accumulation Recalcitrant seeds disperse without the stability associated with Dispersal desiccation Recalcitrant Recalcitrant seeds Water content Adapted from Franchi, G.G., Piotto, B., Nepi, M., Baskin, C.C., Baskin, J.M. and Pacini, E. (2011). Pollen and seed desiccation tolerance in relation to degree of developmental arrest, dispersal, and survival. J. Exp. Bot. 62: 5267-5281. © 2014 American Society of Plant Biologists Most pollen desiccates for stability during dispersal Some plants make recalcitrant pollen that Most plants produce does not desiccate and cannot tolerate Some grasses desiccated, orthodox pollen prolonged exposure to dry air live in dense that can tolerates exposure groups so pollen to dry air during dispersal Some orchids package disperses very pollen inside a protective short distances membrane, shown here attached to the head of a pollinator Squash (Cucrbita