Salinity HORT 301 – Plant Physiology November 18, 2009 Taiz and Zeiger, Chapter 26 (p ), Web Topics 26.5 & 26.6 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 ha Salt 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

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

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 shoot Na + and Cl - move radially across roots with the soil solution Movement to the shoot is driven by transpirational flux

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+K+ Plasma Membrane polyols proline betaine trehalose ectoine DMSP Na + Cl - Ca 2+ Tonoplast OH-*-scavenging perox cp mt Na + /H + K+K+ H+H+ H+H+ Na + H2OH2O H2OH2O pH 5.5 pH 7.5 pH to -200 mV +20 to +50 mV K + (Na + ) H+H+ H+H+ PPi H+H+ ATP K + (Na + ) Cl - Na + Inositol H+H+ Cl - H+H+ ATP Na + H+H+ Na +, Cl - Ca 2+ ATP Ca 2+ H+H+ Cl - Ca 2+ 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 -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 ↑ → [Ca 2+ ] cyt ↑ → SOS3 → SOS2 → SOS1 SOS3 - Ca 2+ binding protein, SOS2 - kinase, SOS1 - H + driven Na + antiporter [Ca 2+ ] ext blocks Na + uptake through NSCC Regulation of ion homeostasis by the SOS signal transduction 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