SALINITY STRESS • Affects 7% of the total surface • 5% of the cultivated land • 20% Irrigated land is under secondary salanization EXTENT OF SALT-AFFECTED SOILS World’s Total area 12.78 b FAO Database 397 x 106 (3.1%) – Saline soils 434 x 106 (3.4 %)– Sodic Soils Asia, Pacific and Australia (M ha) 195 249 Total : 444 M Source : FAO database “Salinity” represents all the problems of the soil accumulating excessive salts, which can be categorized into sodic (or alkaline) and saline soils Sodic soils : poor soil structure generally spread over arid and semi-arid regions; retain high concentrations of Na+ at the exchangeable site in the soil, which shows high pH (greater than 8.5) with a high exchangeable sodium percentage (ESP > 15) Saline soils : Generally found in arid regions and coastal fringes, which are dominated by Na + ions with electrical conductivity (EC) more than dS/m that corresponds to approximately 40  mM NaCl Moreover, saline soils exhibit ESP of  several hours or days) to accumulate enough solutes inside the cell to get a decrease in intracellular Ψ osm (osmotic adjustments) Signal transduction and changes of related-gene expression, in contrast, are a relatively quick response Ionic streee : The stress phase develops later when toxic ions such as Na+ accumulate in excess in plants exceed those of most macronutrients Salinity causes ion-specific stresses resulting in an altered (decreased) K/Na ratio The external Na can negatively impact intracellular K influx High concentrations of Na+ ( extreme ratios of Na+/Ca2+ or Na+/K+) Buildup of Na and Cl- concentrations in the cytosol, The Na can dissipate the membrane potential and therefore facilitates the uptake of Cl down the gradient Higher concentrations of sodium ions (above 100 mM) are toxic to cell metabolism and can inhibit the activity of many essential enzymes, cell division and expansion, membrane disorganization, and osmotic imbalance, which finally can lead to growth inhibition Hydration shells of proteins are essential to maintain 3-D structure of proteins Na and Cl ions penetrate the shells of proteins Interfere with non covalent interactions between A acids loss of function Increase in leaf mortality with chlorosis and necrosis, and a decrease in the Alterations in K ions can disturb the osmotic balance, the function of stomata, and the function of some enzymes Lead to a reduction in photosynthesis and the production of reactive oxygen species This salt-specific or ion-excess effect of salinity causes a toxic effect of salt inside the plant The salt can concentrate in the old leaves and the leaves die, which is crucial for the survival and growth of a plant High salinity affects cortical microtubule organization, helical growth in Arabidopsis For most species, Na+ appears to reach a toxic concentration before Cl− does, and so most studies have concentrated on Na+ exclusion and the control of Na+ transport within the plant However for some species, such as soybean, citrus, and grapevine, Cl− is considered to be the more toxic ion Scheme of the two-phase growth response to salinity Manifestation of Salt Stress Physiological & Biochemical v     High Na+ transport to shoot v     Preferential accumulation of Na in older leaves v     High Cl- uptake v     Lower K+ uptake v     Low P and Zn uptake The salinity stress signal is perceived by a receptor or salt sensor present at the plasma membrane of the cell This signal is responsible for activating various ion pumps present at plasma and vacuolar membranes the salt overly sensitive (SOS) signal pathway This signal also activates the SOS pathway, the components of which help in regulating some of these pumps The various pumps/channels are the K+ inward‐rectifying channel (KIRC), histidine kinase transporter (HKT), nonspecific cation channels (NSCC), K+ outward‐rectifying channel (KORC), Na+/H+ antiporters (SOS1), vacuolar Na+/H+ exchanger (NHX), and H+/Ca+ antiporter (CAX1) Na+ extrusion from plant cells is powered by the electrochemical gradient generated by H+‐ATPases, which permits the Na+/H+ antiporters to couple the passive movement of H+ inside along the electrochemical gradient and extrusion of Na+ out of the cytosol The stress signal sensed by SOS3 activates SOS2, which activates SOS1 Components for the Na+ re absorption from the xylem vessel Na+ ions that reach the xylem by passing through barrier mechanisms in roots under salinity stress are transported to shoots It has been shown that Na+ re absorption occurs from the xylem stream by surrounding tissues, and as a result, reduces the net Na+ flow into shoots The AtHKT1;1 cDNA that encodes a Na+ transporter has been isolated from Arabidopsis The disruption of the AtHKT1;1 gene was found to render the plants salt hypersensitive with the severe chlorosis in leaves as a result of more Na+ in leaves than the wild type plants AtHKT1;1 is a crucial factor in Na+ reabsorption from the xylem vessel Na+ reabsorption at the xylem occurs in exchange for K+? OsHKT1;5-mediated salt tolerant mechanism in rice was later found to be similar to the AtHKT1 In wheat, T aestivum, an essential salt tolerant locus, Kna1, which maintains a high K+/Na+ ratio in shoots during salinity stress, had been identified • QTL analyses in durum wheat revealed that the Nax2 locus that reduces Na+ transport from roots to leaf blades by restricting Na+ contents of xylem sap is homoeologous to the Kna1 locus, which were eventually suggested tobe the HKT1;5 gene in wheat Shabala et al (2010) recently demonstrated in barley demonstrated salt tolerance not necessarily associated with Na+ accumulations in xylem sap, but rather related to a significantly higher K+ loading into the xylem stream higher K+/Na+ ratio in xylem sap and thus in shoots Moreover, salt tolerant cultivars were found to exhibit more efficient Na+ sequestration in leaves than susceptible Higher xylem K+ loading and Na+/H+ antiport activity in leaves could be more predominant mechanisms for barley plants to resist salinity stress In fact, rice cultured cells overexpressing OsKAT1 cDNA, which encodes a Shaker-type K+ channel, had enhanced cell growth in the presence of 100 mM and 200 mM NaCl OsKAT1-expressing cells accumulated more K+ during salinity stress, which resulted in higher K+/Na+ ratios A more recent study demonstrated that tobacco cultured cells expressing the rice OsHAK5 transporter that exhibits relatively Na+ insensitive K+ uptake activity showed an enhancement of growth under salinity stress due to increases in K+ accumulations accompanied with decreases in Na+ accumulations as proved by the high K+/Na+ ratios of the cells These findings further supported a positive impact of a stable K+ acquisition on the cellular salt tolerance The mechanism of salinity tolerance is a very complex phenomenon Studies have shown that components of various pathways are involved in imparting the salinity tolerance to the plants An interesting study lnvolving cDNA microarray analysis of 7000 Arabidopsis genes has shown that 194 genes are upregulated under high salinity stress (Seki et al., 2002), suggesting that many transcriptional regulatory mechanisms function in stress signal transduction pathways • The key transport systems involved in ion homoeostasis in plants grown under saline conditions are regulated by the salt overly sensitive (SOS) signal pathway • The SOS2, a serine/threonine protein kinase, regulates SOS1 (plasma membrane Na+/H+ antiporter)-mediated Na+ efflux from the cytosol, as well as regulating vacuolar Na+/H+ antiportermediated Na+ sequestration into the vacuole • In addition to this mechanism of ion homoeostasis, the high-affinity K+ transporters (HKT), which mediate Na+-specific transport or Na+/K+ co-transport, play an important role in maintaining Na+ homoeostasis in plants under saline conditions • An important locus (Kna1) in hexaploid breadwheat (Triticum aestivum) has been reported to control the selectivity of Na+ and K+ transport from root to shoot, thereby maintaining a high K+/Na+ ratio in leaves However, Na+ exclusion mechanism in durum wheat (Triticum turgidum L ssp durum Desf.) was found to be linked to Nax1 (Na+ exclusion 1) and Nax2 loci, which most probably relate to the Na+ transporters HKT1;4 (HKT7) and HKT1;5 (HKT8), respectively • • • The Nax1 and Nax2 loci have been reported to effectively decrease Na+ transport from root to shoot, thereby maintaining reasonably low Na+ content as well as high level of K+ in the leaf blades of durum wheat plants by excluding Na+ from, and loading K+ into, the xylem • Many other QTLs for ST traits have been identified in rice, including Saltol on chromosome 1, which explains most of the variation for ion uptake under salt stress QNa for high Na+ uptake on chromosome QNa:K for Na+/K+ discrimination on chromosome SKC1/OsHKT8 on chromosome 1, which regulates K+/Na+ homoeostasis in salt-tolerant indica variety ‘Nona Bokra’ several QTLs on all but chromosome for Na+/K+ ratio in the root, three QTLs for ion exchange on chromosomes and 10 and one QTL each for Na+ and K+ uptake and four QTLs for tissue Na+/K+ ratio on different chromosomes Furthermore, Ahmadi an Fotokian (2011) identified 14 QTLs for root and shoot Na+, K+ and K+/Na+ ratio on different rice chromosomes Among them, a QTL (QKr1.2) for root K+ content identified on chromosome was found to be most promising as it explained approximately 30% of the variation observed for ST in rice preprotein translocase, Sec23/Sec2 trunk Ser Thr SecA Kc subunit WD4 chloroplas SAM t synthetas membrane e protein Receptor like kinase CBL-interacting protein kinase 19 secretory peroxidase cold shock protein S_Tkc; WD40 Peroxidase , putative SALtol Region ( Major QTL K+/Na+) 12.0Mb 0.27 Mb (~40 genes) 12.11Mb 11.9 Mb 12.27 Mb 12.27Mb 12.13 Mb 12.25Mb 11.10Mb 12.40Mb 12.7Mb OSJNBa0011P19 B1153f04 B1135C02 P0426D06 cM 60.6 60.9 62.5 64.9 65.4 65.8 Chromosome of Rice (Source: Ellen Tumimbang) 66.2 67.6 67.9 The position of the candidate genes in chromosome Saltol region ( Major QTL K+-Na+ratio ) cM 60.6 60.9 62.5 • Pectinesterase • Phospholipase D Active site motif putative 64.9 65.4 65.8 66.2 • Putative SecA-type chloroplast protein transport factor • Serine/threonine kinase • Peroxidase 67.6 67.9 Plant neutral/alkaline invertase (Source: Ellen Tumimbang) List of genes that are located in the region of QTL and upregulated by high salinity in rice Gene name Pectinesterase Insertion lines 1B-23740, 1B-23741 CG408589 Ser/thr kinase Clone ID full length cDNA Rice 60k chip data under high salinity (fold-induction) References 0.5 h 2h 6h 1.1 3.3 4.9 AK065231 2.3 2.7 Guo et al., 2001 3.5 2.6 Kacperska, 2004 Zhu, 2002 2.6 3.05 Pastori and Foyer, 2002 Sottosanto et al., 2004 Ak105998 Phospholipase D 1515 AK120868 SecA/protein transport factor CL520490 CL520492 AK070488 3.1 1.5 Peroxidase AK099187 Alkaline Invertase AK120720 4.0 2.2 4.2 Unknown cDNA AK099887 0.37 1.6 2.4 (Source: Ellen Tumimbang) The control of Na+ transport and its effective exclusion from the mesophyll cells of leaves is therefore an important requirement for salinity tolerance Na+ exclusion from leaves is associated with salt tolerance in cereal crops Genetic analysis showed that Line 149 (Na exclusion) contained two major genes for Na+ exclusion, named Nax1 and Nax2 Nax1 was located on chromosome 2A by quantitative trait locus (QTL) analysis and was identified by fine mapping as an Na+ transporter of the HKT gene family HKT7 (HKT1;4) Nax2 was located on chromosome 5A and identified as HKT8 (HKT1;5) The Nax genes are not present in modern wheat These genes were therefore named TmHKT7 (TmHKT1;4-A2) and TmHKT8 (TmHKT1;5A) to recognize their origin in T monococcum In durum wheat, these genes enhanced removal of Na+ Major genes for Na+ exclusion, Nax1 and Nax2 (wheat HKT1;4 and HKT1;5), decrease Na+ accumulation in bread wheat leaves under saline and waterlogged conditions Two major genes for Na+ exclusion in durum wheat, Nax1 and Nax2, that were previously identified as the Na+ transporters TmHKT1;4-A2 and TmHKT1;5-A, were transferred into bread wheat in order to increase its capacity to restrict the accumulation of Na+ in leaves Nax1 decreased the leaf blade Na+ concentration by 50%, Nax2 decreased it by 30%, and both genes together decreased it by 60% The signature phenotype of Nax1, the retention of Na+ in leaf sheaths resulting in a high Na+ sheath:blade ratio, was found in the Nax1 lines The effect of Nax2 on lowering the Na+ concentration in bread wheat was surprising as this gene is very similar to the TaHKT1;5-D Na+ transporter already present in bread wheat, putatively at the Kna1 locus The results indicate that both Nax genes have the potential to improve the salt tolerance of bread wheat ... Adverse Effect of Salinity Stress Plants have to cope with two major stresses under high salinity, osmotic stress (Early occuring) and ionic stress (accumulating) Osmotic stress: Increases in... Therefore, high salinity and drought stresses overlaps with each other affect mostly all aspects of plant physiology and metabolism and cause both hyper ionic and hyper osmotic stresses The water... Ionic streee : The stress phase develops later when toxic ions such as Na+ accumulate in excess in plants exceed those of most macronutrients Salinity causes ion-specific stresses resulting in