1. Drought Adaptation of Plants and Role of ABA in Water Deficit Tolerance HORT 301 – Plant Physiology December 3, 2008 Taiz and Zeiger, Chapter 26 (p. 671-682), Web Topic 26.1 Abiotic stress – environmental factors that limit growth and development; reduce yield and biomass production E.g. water deficit, temperature extremes, salinity, flooding (low or no O 2 ) Abiotic stress tolerance – capacity of a plant to cope with an abiotic environment, which may impose stresses on plants that are not adapted to the environment Stress adaptation – genetic capacity of a plant to tolerate an abiotic stress, fitness

2. For example, C 4 and CAM (crassulacean acid metabolism) plants are more water use efficient than C 3 plants, higher ratio of C fixed (g)/g water transpired C 3 plants - 1 g C fixed/400 to 500 g water transpired C 4 plants - 1 g C fixed/250 to 300 g water transpired CAM plants - 1 g C fixed/50 to 100 g water transpired C 4 plants – phosphoenolpyruvate carboxylase (PEPcase) has greater carbon fixation activity relative to rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) because it is 0 2 insensitive and CO 2 is not lost through photorespiration, consequently there is less transpiration per carbon fixed CAM plants – primary CO 2 fixation occurs at night Acclimation – increased capacity to tolerate a stress that results from prior exposure to a stress

3. Drought Adaptation of Plants and Role of ABA in Water Deficit Tolerance HORT 301 – Plant Physiology December 3, 2008 Taiz and Zeiger, Chapter 26 (p. 671-682), Web Topic 26.1 Abiotic stress – environmental factors that limit growth and development; reduce yield and biomass production E.g. water deficit, temperature extremes, salinity, flooding (low or no O 2 ) Abiotic stress tolerance – capacity of a plant to cope with an abiotic environment, which may impose stresses on plants that are not adapted to the environment Stress adaptation – genetic capacity of a plant to tolerate an abiotic stress, fitness Acclimation – increased capacity to tolerate a stress that results from prior exposure to a stress

4. Water deficit stress – insufficient water in a plant, water content is not optimal for physiological processes, drought Plant responses and adaptations to water deficit Reduced plant growth – decreased cell division and expansion, and less photosynthetic production Cellular osmotic adjustment – more negative solute/osmotic potential (ψ s ) through intracellular accumulation of solutes, facilitates water uptake into plants under drought Leaf abscission – reduces leaf canopy area and plant transpiration Enhanced root elongation relative to shoot elongation – regulated by ABA, facilitates acquisition of water with less transpirational demand

5. Leaf movements in response to water deficit – reduces light (heat) absorption Stomatal closure - regulated by ABA, reduces transpiration Induced gene expression – signaling cascades, osmotic adjustment, ABA biosynthesis, etc.

6. Water deficit stress – begins whenever the cell is not fully hydrated, below 100% relative water content (RHC) Caused primarily by reduced soil moisture content, more negative soil water potential (ψ w ) (below field capacity) that restricts water uptake into roots And, when transpiration is greater than water uptake or transport, e.g. mid-day turgor reduction or wilting due to transpirational demand Drought - meteorological condition of insufficient water availability, seasonal (dry-land agriculture) or prolonged (desert) periods with insufficient precipitation

7. Water status of plants is defined by the cellular ψ w and RWC Water potential: ψ w = ψ s + ψ p, 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

8. 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% Critical RWC – below which tissue death occurs, about 50% or less Wilting – cell turgor pressure (ψ p ) is 0, no turgidity Permanent wilting point – plants cannot regain turgor even if transpiration ceases because of very low soil water content

9. Water deficit stress is associated primarily with drought and transpirational demand; 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 (more negative ψ w ) causing a ψ w gradient (Δψ w ) between the symplast and apoplast, water leaves the cell NaCl or Freezing Salinity or Freezing

10. Water-deficit stress reduces plant growth – drought stress reduces yield of crops to about 20% of the genetic potential Water-deficit stress limits yield and biomass production

11. Plant water potential (ψ w ) effects on leaf canopy area and photosynthesis Photosynthetic production is less affected by lower ψ w ; however, whole plant phytosynthate production (yield or biomass/plant) is linked directly to the leaf area (canopy), particularly at early stages of the development Reduced leaf area limits transpiration, i.e. water-deficit-induced reduction in cell expansion is an adaptive response Leaf cell expansion (growth/irreversible increase in cell volume) - the most sensitive physiological process to water deficit

12. Water deficit causes turgor pressure (ψ p ) reduction, ψ p facilitates cell (leaf) expansion At equilibrium, ψ w(ext) = ψ w(int) = ψ s(int) + ψ p(int) ψ w(ext) - external (apoplast) water potential ψ w(int) - internal (intracellular/symplast ) water potential, ψ s(in) - internal solute/osmotic potential, ψ p - hydrostatic pressure/pressure potential/turgor pressure Water deficit reduces the apoplastic (soil solution) water potential ψ w(ext) (more negative) Turgor pressure ( ψ p ) reduction is the initial cellular response to water deficit, re-establishes ψ w equilibrium with minimal water loss but reduces cell expansion rate Plant cells have limited elasticity because of rigid walls, i.e. limited capacity for volume regulation in response to decreased ψ w(ext)

13. sunflower leaves Growth rate is dependent on ψ p and water uptake Decrease in ψ p reduces the growth rate (slope) and will result in growth cessation if ψ p falls below the yield threshold m and Y – regulated by complicated physiological processes that are not well defined Relationship between turgor pressure and leaf cell expansion rate (growth) GR = m(ψ p - Y) GR – leaf growth rate ψ p – turgor pressure Y – yield threshold (minimum turgor pressure for irreversible cell expansion) m – wall extensibility coefficient (turgor pressure required to drive cell expansion rate), leaf growth

14. Cellular osmotic adjustment – facilitates ψ p re-establishment in response to 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)

15. Growth rate is less than w/o stress (compare red and black lines) at equivalent ψ p, presumed to be an adaptive response Osmotic adjustment increases water deficit stress tolerance but causes a yield (biomass) drag, an adaptation that makes a plant more acclimated to water deficit After cellular osmotic adjustment that establishes ψ w equilibrium between the apoplast and symplast, new m (extensibility coefficient) and Y (yield threshold) values are established

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

17. Water deficit stress-enhanced relative root elongation – coordination of root and shoot growth, ensures that transpiration does not exceed the capacity of roots to “supply” water to the shoot Leaf canopy area increases until water demand exceeds water uptake capacity of roots Roots grow until sink demand is beyond the capacity of leaves to produce photosynthate Water deficit-enhanced relative root growth facilitates the capacity of roots to sense water (hydrotropism) and “mine” water in soils

18. ABA coordinates water-deficit stress responses of shoots and roots, promotes root growth relative to leaf cell expansion Shoot growth is inhibited by water deficit to a greater extent in wild type than in ABA-deficient plants Wild type - water deficit → ABA → shoot growth inhibition (ABA deficient) Wild type and vp maize, high water potential – 0.03 MPa, low water potential – 0.3 MPa

19. Root growth is less inhibited by water deficit in wild type than in ABA deficient plants Wild type → water deficit → ABA → enhanced root growth relative to shoot growth Root to shoot ratio is greater in wild type than in ABA deficient plants under water deficit stress ABA coordinates root and shoot growth of plants in response to water deficit, promotes root growth and inhibits shoot growth B. High water potential – 0.03 MPa, low water potential – 1.6 MPa, wild type and vp maize

20. Photosynthesis is less affected by water deficit than leaf expansion A reduction in leaf canopy reduces overall plant photosynthetic production resulting in yield/biomass reduction over a season 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, water acquisition

21. Leaf movement reduces water deficit-mediated heat stress – water deficit reduces transpiration resulting in 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

22. Stomatal closure, water deficit-induced plant response that is regulated by ABA Soil water content (ψ w ) decrease - water deficit → ABA → stomatal closure (reduced stomatal conductance)

23. ABA synthesis increases in roots as a response to water deficit ABA is transported in tracheary elements (xylem) from roots to shoots, unloaded from xylem moved to guard cells to mediate stomatal closure (increased stomatal resistance)

24. Water deficit - more negative water potentials cause an increase in apoplastic pH (alkaline) to above pH 7.0, 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 – apoplastic pH is about 6.0, ABA is primarily in the undissociated form (ABAH) and accumulates in the mesophyll cells (major sink)

25. 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 ABA facilitates water deficit-induced stomatal closure: ABA is synthesized in roots and transported to leaves ABA is more available to guard cells due to water-deficit-induced alkalization of apoplast of leaf cells ABA is synthesized in mesophyll chloroplasts and released to guard cells

26. Stomatal opening: K + is a principal osmotic solute for stomatal regulation – accumulation of K + in guard cells causes a more negative cellular solute/osmotic potential (ψ s ) Increase in turgor pressure (ψ p ), water uptake and cell volume that causes stomatal opening Stomatal opening - K + uptake → more negative guard cell ψ s → increased ψ p /water uptake → cell volume increase → stomatal opening

27. Drought Adaptation of Plants and Role of ABA in Water Deficit Tolerance HORT 301 – Plant Physiology December 3, 2008 Taiz and Zeiger, Chapter 26 (p. 671-682), Web Topic 26.1 Stomatal opening: K + is a principal osmotic solute for stomatal regulation – accumulation of K + in guard cells causes a more negative cellular solute/osmotic potential (ψ s ) Increase in turgor pressure (ψ p ), water uptake and cell volume that causes stomatal opening Stomatal opening - K + uptake → more negative guard cell ψ s → increased ψ p /water uptake → cell volume increase → stomatal opening

28. ABA mediated stomatal closure mechanism: Water deficit → ABA → stomatal closure ABA → ROS → Ca 2+ ↑ → Cl - efflux/membrane potential depolarization → K + efflux/K + influx is blocked → ψ p decrease/water loss → volume reduction → stomatal closure ABA → NO/S1P → cADP ribose/IP3 → Ca 2+ → pm H + -ATPase inhibited → H + gradient dissipation (pH) → K + efflux

29. Water deficit stress induces gene expression – plant defensive response that results in induction or repression of gene expression Gene expression induction – controls water deficit adaptation determinants Gene expression repression – down regulates determinants that are not necessary or counter productive for adaptation Negative regulators of stress-adaptation determinants under water sufficient conditions Function of most water-deficit-stress regulated genes in adaptation is not known, although recent reports provide evidence that some are involved in adaptation

30. Water-deficit induced gene expression is regulated by signal transduction pathways (signaling) Abscisic acid (ABA) is an intermediate in some osmotic stress-regulated signal pathways

31. B A DEBB2A over-expression can increase drought tolerance without a yield reduction in the absence of stress Sakuma et al. (2006) Plant Cell Water sufficient

32. Plant transcription factor ZmNF-YB2) increases drought tolerance and yield stability of maize Nelson et al. (2007) PNAS

33. Effector genes that are regulated by water-deficit stress and whose products likely function in adaptation Osmotic adjustment – compatible 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 pinitol

34. 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

35. 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 upregulated by drought stress NCED