ABA and Drought Adaptation HORT 301 – Plant Physiology November 28, 2007 Taiz and Zeiger, Chapter 25 (p ), Web Topic 26.1 Class Notes Lecture Outline: Water deficit stress – drought, water limitation Water deficit stress reduces plant growth and biomass production – reduced cell expansion and less photosynthetic production, less yield Cellular osmotic adjustment – causes a negative solute/osmotic potential through intracellular accumulation of solutes, facilitates water uptake into plants Water deficit-mediated leaf abscission – reduces leaf canopy area and plant transpiration

Water deficit stress-enhanced root elongation – facilitates acquisition of water Stomatal closure - water deficit stress-induced plant response that is regulated by ABA, reduces transpiration Leaf movement in response to water deficit – reduces heat absorption Gene expression induced by hyperosmotic stress (water deficit) – facilitates osmotic adjustment, ABA biosynthesis, etc.

Terminology: Abiotic stress – environmental factors that limit growth and production Stress tolerance – fitness of a plant to cope with adverse environments, i.e. relative to other plants Acclimation – exposure to a sub-lethal level of stress increases the capacity of the plant to tolerate extreme stress Adaptation – genetic capacity of a plant to tolerate a stress E.g., C 4 and CAM (crassulacean acid metabolism) plants are more water use efficient, g C fixed/g water C 4 plants – rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) has higher affinity for CO 2 than the enzyme in C 3 plants (i.e., less transpiration per CO 2 fixed) CAM plants – fix CO 2 at night

ABA and Drought Adaptation HORT 301 – Plant Physiology November 28, 2007 Taiz and Zeiger, Chapter 25 (p ), Web Topic 26.1 Class Notes Lecture Outline: Water deficit stress – drought, water limitation Water deficit stress reduces plant growth and biomass production – reduced cell expansion and less photosynthetic production, less yield Cellular osmotic adjustment – causes a negative solute/osmotic potential through intracellular accumulation of solutes, facilitates water uptake into plants Water deficit-mediated leaf abscission – reduces leaf canopy area and plant transpiration

Water deficit – water content of a cell is below that when the cell is fully hydrated, below 100% relative water content Caused primarily by reduced soil water content (more negative soil water potential (ψ w )) Drought - meteorological condition of insufficient water availability, manifested daily (mid-day turgor reduction due to transpirational demand), seasonally (periods without precipitation (e.g., Midwest summer) or over prolonged periods (desert)

Water status of plants is defined by the cellular water potential (ψ w ) and relative water content Water potential: ψ w = ψ s + ψ p, defines the chemical potential of water ∆ ψ w (water potential gradient) - drives water movement into or out of cells, water moves toward a more negative ψ w Drought - reduced soil moisture causes a more negative apoplastic water potential resulting in dehydration (cellular water loss) Dry soil

Relative water content (RWC) – water content of a cell relative to the water content at full turgidity RWC = [fresh wt – dry wt]/[fully turgid fresh wt – dry wt] x 100% When water uptake by roots = transpiration, then RWC is about 85 to 95% Wilting – cell turgor loss, no turgor Permanent wilting point – plants cannot regain turgor even if transpiration ceases because of very low soil water content

Water deficit stress is associated primarily with drought; however, other stresses cause water deficit Salinity - lowers solute (osmotic) potential (ψ s ) and water potential (ψ w ) of the soil solution reducing water absorption by roots Freezing (occurs first in the apoplast) – lowers the chemical potential of apoplastic water (ψ w - more negative water potential) causing a ψ w gradient (Δψ w ) between the symplast and apoplast, water leaves the cell NaCl or Freezing

Water deficit stress reduces plant growth and biomass production – drought stress reduces crop yields to ~30% of the genetic potential Plant water potential and effects on physiological processes Leaf cell expansion (growth/irreversible increase in cell volume) - the most sensitive physiological process to water deficit

Reduced leaf cell expansion caused by drought limits crop productivity – total photosynthetic production (grain fill) is linked directly to the leaf area, particularly at early stages of the life cycle However, reduced leaf area reduces plant transpiration, drought-induced reduced cell expansion is an adaptive response

Water deficit causes turgor (ψ p ) reduction, ψ p is necessary for cell (leaf) expansion At equilibrium, ψ w(ext) = ψ w(int) = ψ s(int) + ψ p(int) ψ w(ext) - external water potential ψ w(int) - internal water potential, ψ s(in) - internal solute/osmotic potential, ψ p - hydrostatic pressure/pressure potential/turgor Water deficit reduces the apoplastic (soil solution) water potential ψ w(ext) (more negative) Turgor( ψ p ) reduction is the initial cellular response to water deficit, re- establishes ψ w equilibrium with minimal water loss but reduces cell expansion

GR = m(ψ p - Y) (red line) sunflower leaves Cell expansion/growth is dependent on water uptake into cells Relationship between turgor and leaf cell expansion rate (growth) GR – leaf growth rate ψ p – turgor Y – yield threshold (minimum turgor for expansion that is irreversible) m – wall extensibility coefficient (turgor required to drive cell expansion rate), leaf growth

Decrease in turgor reduces the growth rate (GR), growth cessation occurs if turgor falls below the yield threshold (Y), (red line) m and Y – regulated by complicated physical, physiological and metabolic mechanisms that are not well defined as the turgor (biophysical) effect sunflower leaves

Cellular osmotic adjustment – facilitates turgor (ψ p ) re-establishment after water deficit stress Osmotic adjustment – net accumulation of solutes, ions and small organic molecules, more negative solute/osmotic potential (ψ s ) Common osmotic solutes are K +, sugars, organic acids, and amino acids Compatible solutes – organic compounds (species specific), not metabolically poisonous at high concentrations, highly water soluble, zwitterionic – no net charge, do not affect intracellular pH, “protect” enzyme and membrane functions Compatible solute molecules - proline, sugar alcohols and quaternary ammonium compounds, e.g., betaine (tri-methyl glycine)

After cellular adjustment has occurred, new m and Y values are established However, cell expansion is less than w/o stress, presumed to be an adaptive response (black line)

Osmotic adjustment increases water deficit stress tolerance but does not result in equivalent yield relative to without stress, i.e. enhances survival but has yield drag

Water deficit stress-mediated leaf abscission – cotton, ethylene- dependent abscission to reduce leaf area (i.e., transpirational loss), leaves re-develop (leaf canopy) if sufficient water is provided Adaptive response that reduce leaf canopy area minimizing transpiration, negative impact on biomass production

Water deficit stress-”enhanced” root elongation – root growth is relatively less inhibited than shoot growth Coordination of root and shoot growth ensures that transpiration does not exceed capacity of roots to supply water to the shoot Leaf canopy growth contiues until water demand is limited by root water uptake, root growth continues until sink demand is equivalent to photosynthate production Water deficit-”enhanced” root growth facilitates the capacity of roots to sense water (hydrotropism) and “mine” water in soils ABA regulates coordination of water-deficit stress responses of shoots and roots, inhibits leaf cell expansion and facilitates “enhanced” root elongation

Stomatal closure, water deficit-induced plant response that is regulated by ABA Soil water content decreases - water deficit → ABA → stomatal closure

Water deficit - more negative water potentials cause an increase in apoplastic pH (alkaline), greater proportion of dissociated ABA (ABA - ) ABA - is less readily transported across the plasma membrane of mesophyll cells than ABAH, more ABA is available for entry into the guard cells → stomatal closure Water-sufficient conditions – ABA is primarily in the undissociated form (ABAH) and accumulates in the mesophyll cells (major sink)

Also, ABA is synthesized in the chloroplasts of mesophyll cells as a response to water deficit ABA is released from mesophyll cells to the apoplast → guard cells → stomatal closure 1.ABA is synthesized in roots and transported to leaves 2.ABA is more available to guard cells, alkalization of apoplast in leaves 3. ABA is synthesized in mesophyll chloroplasts ABA facilitates water deficit-induced stomatal closure:

ABA-mediated stomatal closure mechanisms - regulate opening and closing K + is the principal osmotic solute for stomatal regulation – accumulation lowers the cellular solute/osmotic potential (ψ s ), increase in turgor (ψ p ), water uptake and an increase in cell volume that causes stomatal opening Stomatal opening - K + uptake → guard cell solute potential is lowered (more negative) → water uptake → turgor/cell volume change → stomatal opening Stomatal closure - K + efflux → turgor loss → stomatal closure

ABA → ROS → Ca 2+ ↑ → Cl - efflux, membrane potential depolarization → K + efflux → K + influx is blocked → turgor and water loss/volume reduction → stomatal closure

Leaf movement reduces water deficit-mediated heat stress – water deficit reduces transpiration, less circulation of water through the plant and less evaporative cooling (latent heat of vaporization), increased leaf temperatures Water sufficient (top) and drought stressed (bottom) soybean plants Change in leaf orientation reduces the absorbed light (heat energy) and water deficit-caused heat stress Maize – leaf rolling

Examples of genes that are regulated by hyperosmotic stress and whose products likely function in adaptation Osmotic adjustment – osmotic solute biosynthesis Δ 1 -Pyrolline-5-carboxylate synthase, key enzyme in proline biosynthesis Betaine aldehyde dehydrogenase, biosynthesis of betaine myo-Inositol 6-O-methyltransferase, rate-limiting enzyme in the biosynthesis of the compatible osmotic solute pinitol Hyperosmotic (water deficit) stress induces gene expression – drought stress induces a plant defensive response that results in induction or repression of gene expression Stress-regulated gene expression is presumably required for adaptation

Facultative CAM (crassulacean acid metabolism) transition – ice plant, Mesembryanthemum crystallinum CO 2 fixation occurs in the dark, requires phosphoenolpyruvate carboxylase activity Transition from C 3 to CAM is induced by severe NaCl stress (500 mM)/water deficit

Late embryogenesis abundant (LEA) proteins – function in membrane protection under stress conditions, conserved in all plants Abscisic acid biosynthesis NCED (9-cis-epoxycarotenoid dioxygenase) – gene encoding the enzyme is regulated by drought stress →

Water deficit-enhanced root growth facilitates the capacity of roots to sense water (hydrotropism) and “mine” water in soils

ABA inhibits shoot growth and facilitates root growth at more negative water potentials (water deficits) Shoot growth is inhibited by water deficit to a greater extent in wild type than in ABA-deficient plants (ABA deficient) High water potential – 0.03 MPa, low water potential – 0.3 MPa

Root growth is less inhibited by water deficit in wild type than in ABA deficient plants Root to shoot ratio is greater in wild type than in ABA deficient plants under water deficit stress B. High water potential – 0.03 MPa, low water potential – 1.6 MPa ABA coordinates shoot and root growth under water deficit stress

Photosynthesis is less affected by water deficit than leaf expansion As the water deficit becomes more severe, CO 2 uptake is affected first and then components of the photosynthetic apparatus Photosynthate is available for partitioning to the root for growth

Soil water content decreases - water deficit → ABA → stomatal closure ABA is synthesized in roots, synthesis increases as a response to water deficit ABA transported from roots to leaves in the tracheary elements (xylem), unloaded from xylem moved to guard cells to mediate stomatal closure

Gene expression is regulated by signal transduction pathways (signaling) but research to date has not defined these completely Abscisic acid (ABA) is an intermediate in some osmotic stress-regulated signal pathways