1. HORT 301 – Plant Physiology December 8 and 10, 2008
Salinity
HORT 301 – Plant Physiology
December 8 and 10, 2008
Taiz and Zeiger, Chapter 26 (p ), Web Topics 26.5 & 26.6
Salt stress – soil and water salinity
Natural genetic variation for salt tolerance
Salinity causes hyperosmotic stress and ionic disequilibrium
NaCl uptake into roots and transport in plants
Osmotic adjustment and ion compartmentalization
Ion homeostasis transport proteins
Salt stress signaling

2. Sodic vs saline – synonyms for most plant biologists
Salt stress - caused by concentrations greater than that required for optimum growth of a typical crop plant, 1500 ppm or 25 mM Na+
Sodic vs saline – synonyms for most plant biologists
Oceans are the principal sources of salt - >99% of water on earth is in oceans, Na+ is 460 mM and Cl- is 540 mM
Sodic – 15% or more of the cation exchange capacity of soil is occupied by sodium
NaCl is the primary cause of soil and water salinity, there are soils where Na2SO4 and CaCl2 constitute soil salinity

3. Processes of soil and soil water saliniation
Biogeochemical cycling of salt – water droplets containing salt (ocean) are carried by wind over land where these evaporate and deposit salt onto the soil and increase the salt content
Flooding from oceans or estuaries - change in management of the neighboring estuary resulted in flooding of land used for rice production
Courtesy of Tim Flowers
Irrigation water quality and agricultural management practices – soils are irrigated with saline water and improperly leached resulting in salt accumulation, saline water infiltration into the ground water
Inadequate management practices have led to the destruction of some very fertile soils, e.g., Fertile Crescent in Mesopotamia (Middle East)

4. The Salinity Problem Worldwide
Cultivated Land (dryland) 1500 million ha
Salt affected million ha (2%)
Irrigated Land million ha
Salt affected million ha (20%)
Salinity decreases crop biomass production and limits development of new agricultural land
Munns (2005) New Phytol

5. Natural variation for salt tolerance - glycophytes (sweet plants) are salt intolerant
Halophytes (salt plants) - “native flora in a saline environment”
Halophytes are widely distributed amongst angiosperms indicating polyphyletic (multiple progenitors) or halophytes arose early in the evolution of higher plants
Halophytes exist in 1/3 of the angiosperm families, although the majority of species are confined to a few families
50% of the 500 halophytic species belong to 20 families
Monocots - 45 genera in the Poaceae family
Dicots - 44% of the halophytic genera are in the Chenopodiaceae (e.g. Atriplex, Salicornia and Suaeda)

6. Plant classifications based on salt stress tolerance
Halophytes – grow at 200 to 500 mM NaCl
Euhalophytes (IA) - or facultative halophytes, genotypes that require salt for optimum growth; e.g. Suaeda maritima, Atriplex nummularia

7. Miohalophytes (IB) - capable of growth at high salt concentrations but growth is inhibited by salt, e.g. Atriplex hastata, Spartina townsendii, sugar beet
Halophytes and some glycophytes (II) - substantial growth reduction at 200 mM NaCl, crop plants like cotton, barley, tomato, common bean and soybean
Very salt sensitive glycophytes (III) - e.g. fruit trees, avocado, stone fruits

8. Salinity causes hyperosmotic stress (water deficit) and ionic disequilibrium (ion toxicity)

9. Ion disequilibrium – Na+ rapidly enters the cell because the membrane potential is inside negative (~-120 to -200 mV), Na+ can accumulate to 102- to 103-fold greater concentration than in the apoplast, driven by the membrane potential
Na+ is a cytotoxin and K+ is an essential nutrient
High Na+ interferes with K+/Na+ selective uptake and K+ nutrition
K+/Na+ selectivity is controlled by Ca2+, high Na+ disturbs K+ and Ca2+ availability

10. Secondary effects of NaCl include:
Reduced cell expansion and photo-assimilate production – similar to water deficit, adaptation includes a reduction in cell expansion rate that affects photosynthate production, i.e., yield decrease
Carbon assimilation and photophosphorylation are salt sensitive – enzymes and membranes

11. Decreased cytosolic metabolism – metabolic poisoning, enzymes of halophytes and glycophytes are equally sensitive to NaCl
(halophyte)
(glycophte)
(halophyte)
100 mM Na in the cytosol is not toxic
Production of reactive oxygen species (ROS) – products of photosystem I, photorespiration and mitochondrial respiration, electron flow is too great for electron acceptor molecules resulting in production of reactive oxygen species (ROS) that cause cell death

12. NaCl uptake into roots and transport in plants - radial transport from the soil solution into roots is apoplastic/symplastic (epidermis and cortex), symplastic across the endodermis and then loaded into the xylem

13. Radial transport is regulated - Na+ and Cl- transport to the xylem is limited by apoplastic to symplastic movement into epidermal, cortical and xylem parenchyma cells, endodermis (Casparian strip) restricts radial Na+ transport
Loading of Na+ into xylem is also a regulated process
Salt movement through the xylem is determined by the transpirational flux – moves through the xylem to the shoot with water, greater transpiration results in greater salt load into the shoot
Plants minimize exposure of shoot meristematic cells to Na+ and Cl- - the lack of vasculature to the meristem reduces transport to cells in this tissue, fully expanded leaves are ion sinks and may abscise
Some halophytes deposit salt on the surface of leaves (sink) via glands or bladders

14. Osmotic adjustment and ion compartmentalization - cellular level response to water deficit is osmotic adjustment (water status adaptation)
Osmotic adjustment – mediated by accumulation and compartmentalization of Na+ and Cl- into the vacuole, compatible osmolytes are accumulated in the cytosol K+
Plasma
Membrane
polyols
proline
betaine
trehalose
ectoine
DMSP
Na+
Cl-
Ca2+
Tonoplast
OH-*-scavenging
perox cp mt
Na+/H+ H+
H2O
pH 5.5
pH 7.5
-120 to mV
+20 to +50 mV
K+(Na+)
PPi
ATP
Inositol
NaCl↑

15. Cell expansion during growth – volume increases 10- to 100-fold as cells develop to maturity, due mostly to an increase in the vacuole size, i.e., water uptake into the vacuole drives cell expansion
Vacuolar compartmentalization of Na+ and Cl- facilitates cell expansion and prevents metabolic poisoning of the cytosol and organelles
Energetically, ion accumulation is more efficient than organic osmotic solute biosynthesis and accumulation

16. presumably a Cl- - H+ symporter
Ion homeostasis transport proteins - coordinate net Na+ and Cl- uptake across the plasma membrane with the capacity to compartmentalize these ions into the vacuole, i.e., cytosolic Na+ and Cl- concentrations are maintained below toxic levels
Plasma membrane:
Na+ influx is passive (nonselective cation channel(s)) (NSCC), HKT1 transport system, and leak through K+ uptake systems because of the inside negative potential across the plasma membrane
Cl- uptake is active (because of the inside negative potential across the plasma membrane)
presumably a Cl- - H+ symporter
-120 mV

17. Tonoplast – active Na+ transport into the vacuole:
Plasma membrane Na+ efflux is active, H+ driven Na+ antiporter SOS1, H+ gradient is established by the plasma membrane (P-type) H+-ATPase, note ∆pH
-120 mV
Tonoplast – active Na+ transport into the vacuole:
H+ driven Na+ antiporter, NHX family
H+ gradient established by tonoplast ATPase and/or pyrophosphatase

18. Salt stress signaling – regulates Na+ ion homeostasis
Na+ (high concentrations) induces Ca2+ transients that regulate the SOS signaling pathway
[Na+]ext↑ → [Ca2+]cyt↑ → SOS3 → SOS2 → SOS1
SOS3 - Ca2+ binding protein, SOS2 - kinase, SOS1 - H+ driven Na+ antiporter
Regulation of ion homeostasis by the SOS signal transduct
ion pathway
[Ca2+]ext blocks Na+ uptake through NSCC

19. SOS3-SOS2 complex phosphorylates SOS1, Na+/H+ antiporter activity
SOS3-SOS2 complex induces the expression of SOS1 through some yet unknown transcription factor
SOS pathway regulates AtNHX family antiporters at the post- transcriptional level
The activated SOS pathway and outputs of the pathway are targets for bioengineering of salt tolerance by constitutive activation of the pathway
Ectopic expression of ion homeostasis determinants, NHX, facilitate vacuolar compartmentalization of Na+ and salt tolerance
HKT alleles increase salt tolerance of rice and wheat, genetic mapping

21. The effects of salinity stress on plants
Munns and Tester (2008) Annu Rev Plant Biol