Plant Water Deficit Responses HORT 301 – Plant Physiology November 11, 2009 Taiz and Zeiger, Chapter 26 (p ), Web Topic 26.1 Abiotic stress – environmental factor deficiency or excess that limits growth and development Water deficit, temperature extremes, salinity, flooding (low or no O 2 ) Stress tolerance results from plant adaptation and responses to the environment (acclimation)

Water deficit stress – insufficient plant water content for optimal physiological processes Taiz and Zeiger 2006

Plant water status affects critical physiological processes

Water status of plants is defined by the cellular ψ w and RWC ∆ ψ w (water potential gradient) - drives water movement into or out of cells, water moves toward a more negative ψ w Low soil moisture causes more negative apoplastic water potential resulting in reduced turgor pressure and cellular water loss

Water deficit causes cell turgor pressure reduction/loss, water loss and volume reduction

Water-deficit stress reduces plant growth – drought stress reduces yield of crops to about 20% of the genetic potential

Turgor Pressure (MPa) ∆  w gradient results in turgor pressure and facilitates water uptake for cell volume increase/expansion (fresh weight gain) Growth rate is dependent on ψ p and water uptake Decrease in ψ p reduces the growth rate until ψ p falls below the yield threshold (Y) m (extensibility) and Y – regulated by complicated physiological processes that are not well defined Increased ψ p is due to osmotic adjustment

Cellular osmotic adjustment – ψ p re-establishment in response to water deficit stress PLANT BIOLOGY, Smith et al. Figure 3-66.

Water deficit acclimation establishes ψ w equilibrium or ∆ ψ w Reduced growth rate due to higher m and Y, ψ p is required for growth, yield drag

Water deficit stress-mediated leaf abscission – ethylene-dependent abscission to reduce leaf area (i.e., transpirational loss) MPa

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 Facilitates the capacity of roots to sense water (hydrotropism) and “mine” water in soils (A) Irrigated soil(B) Dry soil

ABA coordinates water deficit stress responses of shoots and roots ABA promotes root growth relative to leaf cell expansion under water deficit 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

Root growth under water deficit is promoted by ABA Wild type → water deficit → ABA → enhanced root growth B. High water potential – 0.03 MPa, low water potential – 1.6 MPa, wild type and vp maize

Photosynthesis is less affected by water deficit than leaf expansion Photosynthate is partitioned to the root for growth, water acquisition

Stomatal closure is a water deficit-induced plant response that is regulated by ABA

ABA accumulates in guard cells in response to water deficit and causes stomatal closure Soil water content (ψ w ) decrease - water deficit → ABA → stomatal closure (reduced stomatal conductance) ABA is synthesized in roots and transported to leaves ABA is available to guard cells due to water-deficit-induced alkalization of the leaf apoplast ABA is synthesized in mesophyll chloroplasts and released to guard cells

ABA mediated stomatal closure mechanisms: 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

Stomatal opening: K + 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 increase Stomatal opening - K + uptake → more negative guard cell ψ s → increased ψ p /water uptake → cell volume increase → stomatal opening

Water deficit stress induces gene expression – plant defensive response that results in induction or repression of gene expression ABA dependent and independent pathways

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

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

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

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