#35 - Salinity HORT 301 – Plant Physiology November 22, 2010 Taiz and Zeiger, Chapter 26, Web Topic 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) Glycophytes are all other plants including crops Smith et al. (2010) Halophytes (salt plants) are natives of a saline environment, adapted

Relative salt tolerance of halophytes and glycophytes

Primary Water deficit Ion disequilibrium (NaCl), Na + reduces K + acquisition K + deficiency Secondary Reduced cell expansion Protein denaturation Reduced membrane function Reduced assimilate production and metabolic activities Production of reactive oxygen intermediates (ROS) Salinity causes hyperosmotic stress (water deficit) and ionic disequilibrium (ion toxicity)

Salinity causes rapid osmotic stress and then ion disequilibrium in leaves Munns and Tester (2008) Annu Rev Plant Biol

NaCl is a cytotoxin Protein denaturation caused by Na + and Cl - (halophyte) (glycophyte) (halophyte) Bray et al. (2000) Metabolic enzymes of halophytes and glycophytes are equally sensitive to NaCl

Osmotic adjustment in NaCl environments – ion compartmentalization in the vacuole and compatible solute accumulation in the cytosol

Plett et al. (2010) Plant, Cell & Environ 33, Cellular Na + homeostasis

[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

Smith et al. (2010) Plant Biology Compatible osmotic solutes accumulate in the cytosol and organelles Bray et al. (2000) Compatible solutes maintain hydration shell around proteins

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

SOS1 and NHX1 Na + -H + antiporters SOS1 – Na + exclusion at the plasma membrane NHX – Na + compartmentalization into the vacuole

Pardo, 2010 HKT1 regulates Na + loading into the xylem Na,K Vacuole NHX

Transfer of Nax2 (TmHKT1;5) from Triticum monococcum (einkorn) into Triticum turgidum spp durum (durum wheat) increases yield in saline soil by 25% Rana Munns, Richard James and others (including Caitlin Byrt, Mark Tester, Ray Hare)

Relationship between EC and yield of durum wheat cultivar Tamaroi with Nax2 gene + Nax2 - Nax2

Plett et al. (2010) PLoS One 5, 1-8 Root cortical cell expression of HKT1 and reduced shoot Na + accumulation and salt tolerance Plett et al. (2010) PLoS One

J2731* J2731*UAS:HKT 100 mM NaCl 0 mM NaCl % Salinity tolerance Stelar-specific AtHKT1;1 activation in Arabidopsis increases salinity tolerance 100 mM NaCl 0 mM NaCl salt sensitivesalt tolerant Møller et al. (2009) Plant Cell 21: 2163–2178

Salt Tolerance of Canola by Expressing NHX1 Tonoplast Na + /H + Antiporter Zhang et al. (2001) PNAS 200 mM NaCl

Salt/Drought Stress Tolerance of Rice through Expression of Genes that Result in Trehalose Accumulation Garg et al. (2002) PNAS

Genetic Determinants of Salt Tolerance Genetic determinants that are regulators and effectors of salt tolerance are being identified Forward genetics – phenotype to gene Mutational genetics (model systems) Genetic variation in natural and domesticated populations (crops) Reverse genetics – candidate gene to phenotype Allelic variation – transgene technologies Allelic variation – QTL and association mapping of natural and domesticated populations for gene identification and genetic introgression – nonGMO approach