SalinityHORT 301 – Plant Physiology

November 18, 2009Taiz and Zeiger, Chapter 26 (p. 692-698), Web Topics 26.5 & 26.6

paul.m.hasegawa.1@purdue.edu

Epstein and Bloom 2005

Salt stress is caused by ion concentrations greater than that required for optimum growth

NaCl is the primary cause of soil and water salinity

Courtesy of Tim Flowers

Biogeochemical cycling, incursions and inadequate management practices cause salinity stress

Cultivated Land (dry land) 1500 million ha

Salt affected 32 million ha (2%)

Irrigated Land 230 million haSalt affected 45 million ha (20%)

Munns (2005) New Phytol

World wide salinity problem

Plants are classified as glycophytes (sweet plants) or halophytes (salt plants)Halophytes (salt plants) are natives of a saline environment, adapted

Glycophytes are all other plants including crops

Smith et al. (2010)

Relative salt tolerance of halophytes and glycophytes

26.14 The growth of different species subjected to salinity relative to that of unsalinized controls

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

PrimaryWater deficit

Ion disequilibrium caused by Na+ and Cl-, Na+ reduces K+

acquisition causing K+ deficiency, Na+ is a cytotoxin

SecondaryReduced cell expansion

Reduced assimilate production

Reduced membrane function

Decreased cytosolic metabolism

Production of reactive oxygen intermediates (ROSs)

Salt Stress Effects on Plants

NaCl is a cytotoxin

Metabolic enzymes of halophytes and glycophytes are equally sensitive to NaCl

(halophyte)

(glycophte)

(halophyte)

Bray et al. (2000)

Salinity causes rapid osmotic stress and then ion disequilibrium in leaves

Munns and Tester (2008) Annu Rev Plant Biol

Xylem loading of Na+ and Cl- in roots controls ion accumulation in the shootNa+ and Cl- move radially across roots with the soil solutionMovement to the shoot is driven by transpirational flux

4.4 Pathways for water uptake by the root

Na+ transport across the root is regulated in the cortex and at xylem loading

Tester & Munns (2008) Annu Rev Plant Biol

Osmotic adjustment and ion compartmentalization are necessary responses to water deficit and ion toxicity caused by salinity

K+

PlasmaMembrane

polyols proline betaine

trehalose ectoine DMSPNa+

Cl-

Ca2+

TonoplastOH-*-scavenging

perox

cp

mt

Na+/H+

K+ H+

H+

Na+

H2

O

H2O

pH 5.5

pH 7.5

pH 5.5

-120 to -200 mV

+20 to +50 mV

K+(Na+)

H+

H+

PPi

H+ATP

K+(Na+)

Cl-

Na+Inositol

H+Cl-

H+

Cl-

ATP

Na+

H+

Na+, Cl-

Ca2+

Ca2+

Ca2+ATP

Ca

2+H

+

Cl-

Ca2+

Ca2+ATP

NaCl↑

Smith et al. (2010) Plant Biology

Compatible osmotic solutes accumulate in the cytosol and organelles

Bray et al. (2000)

Ion transport proteins coordinate net Na+ and Cl- uptake across the plasma membrane and tonoplast

26.15 Membrane transport proteins mediating sodium, potassium, and calcium transport

-120 mV

Na+ transport in planta is regulated at xylem loading and by recirculation from the shoot

Tester & Munns (2008) Annu Rev Plant Biol

Apse & Blumwald (2007) FEBS Lett

[Na+]ext↑ → [Ca2+]cyt↑ → SOS3 → SOS2 → SOS1SOS3 - Ca2+ binding protein, SOS2 - kinase, SOS1 - H+ driven Na+ antiporter

[Ca2+]ext blocks Na+ uptake through NSCC

26.16 Regulation of ion homeostasis by the SOS signal transduct ion pathway

Salt stress signaling regulates Na+ ion homeostasis

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

Salt-affected wheat in Aragon, northeastern Spain

Courtesy of Tim Flowers