Introduction. What controls guard cell movement and stomatal aperture? Plants are diverse. We could not live without plants. What is photosynthesis?

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1 What controls guard cell movement and stomatal aperture? Plants and Biotechnology Howard Univ. Summer Workshop July 18, 2011 By Heven Sze & Dan Czerny University of Maryland, College Park, MD 7/18/11 1 Introduction Plants: The only eukaryotic organism that can make its own food. Plants are non-motile. Plants must sense, change and adapt to the changing environment. Today, we will focus on one of the most important cells in land plants, the guard cells. 7/18/11 2 Plants are diverse Club mosses Mosses Liverworts Vascular Plants Land Plants Ferns Grasses Flowering Plants Seed Plants Conebearing plants Green algae Broadleafed plants Plants have evolved the ability to thrive in diverse land habitats. Images courtesy tom donald 7/18/11 3 Guard cells first appeared in early land plants We could not live without plants Plants produce most of the oxygen we breathe. Plants produce most of the chemically stored energy we consume as food and burn for fuel. Plants produce an amazing assortment of useful chemicals. 7/18/11 4 Plants fix carbon dioxide into energyrich molecules we animals can use as food What is photosynthesis? CO2 Plants convert CO2 gas into sugars through the process of photosynthesis. 7/18/11 5 7/18/11 6 1

2 Overview of photosynthesis Seed germination produces seedlings 1. Light absorption water is split 2. NADPH Reducing energy is formed 3. ATP Chemical energy is formed 4. CO 2 -->--> sugar CO 2 is converted to sugars 7/18/11 7 7/18/ Development of the vegetative plant body. Plants develop appropriate structures to carry out special functions. Vegetative plant body is made of 3 organs: leaf Stem root 7/18/11 9 Leaf has different cell types with distinct functions. Dermal Ground Vascular Main function of leaf is? 7/18/11 10 Leaves are the photosynthetic organs of land plants All land plants have guard cells Guard cells are found on the epidermis of leaves. A pair of guard cells surround a slit that can open or close. CO 2 can diffuse inside the leaf through open pores. Water can escape out of the leaf through the open pores /18/11 11 Taiz It is critical that plants can control the opening and closing of the pores. Why? 7/18/

3 Questions When do stomatal pores open? How do stomatal pores open? What happens to guard cells? Questions When do stomatal pores open? Ans: in the day How do stomatal pores open? What happens to guard cells? Ans: when guard cells increase in turgor. Ions enter cells, water follows, cells swell. How do pores close? 7/18/ /18/ Stomata. Open and closed state What controls opening? Increase in turgor pressure What controls closing? A. Photosynthesis & Guard cells Stomatal aperture is dependent on light light 7/18/ /18/11 16 Blue light starts the process 1. Blue light is received by a photoreceptor 2. Photoreceptor then signals to activate a H + -pump 3. Cell become hyperpolarized mv 4. K+ enters via K+ channels 5. Cl - enters 6. Water follows 7. Guard cell swells causing pore to open. At dusk or end of day, pore closes 7/18/ /18/

4 Vacuole number & size changes as aperture widens B. Water movement and guard cells Fluorescent dye, Acridine Orange accumulates in vacuole. 7/18/11 19 From Gao XQ et al 2005 Plant Physiol. Ficia faba 7/18/11 20 How do plants move water from roots to leaves? What is the driving force for water transport from roots to the leaves? Pressure-driven bulk flow. Evidence for a negative pressure in xylem. Negative pressure is generated by transpiration Driving force for water flow: 3 Main driving forces i. Root/soil (pressure gradient) ii. Across root (water potential iii. Xylem (pressure potential) iv. Leaf/air (water vapor conc difference) 7/18/11 21 Water moves from a region of high water potential to a region with low water potential. 7/18/ Water vapor conc. difference causes water loss HOW DOES WATER MOVE? (into & within seed and seedling) In which direction? Water movement is passive. i.e. energetically downhill. Water movement is down conc. gradient and pressure gradient a. Three ways water move: 1) Diffusion- movement down a conc. Gradient [short distance] 2) Bulk Flow: movement down a pressure gradient [long distance] 3) Osmosis: movement across a membrane and down both conc. and pressure gradient 7/18/ /18/11 24 Loss of water from the leaves produces negative potential in the leaf 4

5 What happens when there is no rain? Soil begins to get dry. How can plants reduce its water lose? How can plants reduce water uptake? Figure 23.4 Changes in maize in response to water stress Plants can sense drought stress. Plants respond by making a stress hormone, ABA. ABA will induce changes to protect plant from drought stress. Let s see how? 7/18/ /18/11 26 How do plants sense drought stress and then produce ABA? Model: 1. Water stress is sensed first in the root 2. ABA is synthesized in root 3. ABA moves up via xylem to the leaf 4. ABA moves to guard cells in the leaf. 7/18/11 27 In vivo Imaging of ABA pools increase in water-stressed Arabidopsis: ABA-specific promoter::luc The pathb6::luc reporter plants cultivated for 4 weeks under sterile conditions were exposed to water stress (Ψ = 1.0 MPa) via their root system for 24 h prior to in vivo measurement of reporter activity shown in false colors (A) for a single plant and the 7/18/11 28 corresponding brightfield image (B). Figure 23.6 Redistribution of ABA in the leaf from alkalinization of xylem sap during water stress Stomata. Open and closed state What controls opening? Increase in turgor pressure ABA will induce closure. What controls closing? 7/18/ /18/

6 ABA inhibits opening of guard cells. Blue-light stimulated H+ pumping in guard cell protoplasts is reduced. Model of ABA & stomata closure acid Light H + ABA ABA is sensed by an ABA receptor [Ca] Causes Cl- loss K+ leaves Water leaves Loss in turgor Cells become flaccid. Pore closes 7/18/ /18/11 32 Question & Discuss Is this true? How can I test this? 1. Does daylight really cause guard cell movement? 2. Is K+ and Cl- the major ions to move in? 3. What would a stress hormone do to guard cells? 7/18/11 33 Goals of Experiment Test Light vs dark on stomatal opening. How would you test this? Test what ions are important KCl, choline-cl vs mannitol What is the effect of ABA, the stress hormone? 7/18/