Approximately 6.3 million hectares of land in Pakistan and 6% of world’s total land area is affected by salt genes stress. Majority of agricultural lands are becoming barren due to salt stress and their proportion increasing day by day due to land clearing and irrigation. To attain sustainability in food production, there is need to develop salt tolerant crops. Development of salt tolerance require identification of salt tolerant germplasm and manipulation of present cultivars to induce salt tolerance.
Salt stress inhibit plant growth either by effecting plant ability to take up water or it might enter into transpiration stream of plant and leads to cell damage in transpiring leaves. Salt stress affects plants in couple of ways i.e. In phase I, salt effects from outside of plant (neither Na+ nor Cl– is able to accumulate in the cell at concentration effecting plant growth rather plant accommodate these salts within vacuole of plant leaf). Response of plant specific to phase I depends upon extent of water stress induced in plant due to salt stress. There are certain signals coming from root which reduces leaf growth. Abscisic acid serve as candidate for such chemical signals because it is found in xylem sap and its concentration increase on the onset of salt stress. Cell differentiation and cell division is also effected by hormones. Leaves of salt treated plants are usually thick and leaf area is small, having high transpiration efficiency.
Second phase involves effects of salt inside the plant body. Salt continue to transfer from older leaves to younger leaves resulting in dying of leaves due to salt injury. Concentration of salts increases to an extent that plants are unable to compartmentalize it and salt accumulated in cytoplasm results in denaturation of enzymes and desiccation. Phase II response involves minimizing salt entry into plant and lowering salt concentration in cytoplasm. In roots concentration of Na+ must be in range of 10-30mM while in leaf it should not be more than 100mM. Plant roots exclude most of salts in soil. If a plant allows 1/50 of salts to enter in body, slat concentration in shoots of plants would never increases because water loss from transpiration is 50 times more as compared to amount of water retain in leaves of plant. Mostly plants excludes about 98% salt into soil and only 2% is transported through xylem but this ability vary in different crop plants. Wheat excludes more than 98% salt into soil solution and cytosolic concentration of Na+ never rises more than 50mM in leaves. Exclusion of salt in barley is less than 98% and Na+ cytosolic concentration can even rise up to 500mM. Similarly, exclusion of Cl– also vary between different species. There is strong correlation between exclusion of salt and salt tolerance in crop plants.
Induction of salt tolerance involves restricting sodium uptake. Genetic variation in Na+ uptake was observed in various ancient accessions and land races of different crop species. Numerous markers associated with different QTL labelled were identified in different crops. Some micro satellite markers associated with low sodium concentration were also identified and used to select progeny having low Na+ uptake. Some plants also partition salt in stem and in base of leaves other redirect salt from younger to older leaves. Most of the species which are not able to exclude slat, they compartmentalize salt into their vacuole; protecting cytoplasm from ion toxicity. Some physiological traits like osmotic adjustment, increasing water use efficiency and developmental and morphological patterns also aids in water conservation under salt stress.
Many genes play an important role in maintaining Na+ or K+ homeostasis. These have relevance for controlling uptake and transfer of Na+ and K+. These transporters and channels regulate transport of sodium ion either directly because they are partially selective for potassium or indirectly because they buffer cell against high sodium uptake maintaining potassium homeostasis. Following are list of genes responsible for regulating of sodium & potassium homeostasis in plants and maintenance of solute balance in cell.
AKT, KAT, KCO, KEA, HAK, KUP, HKT and CPA genes are all K+ channel expressed in roots and highly selective for K+ but under high salinities level, it could transport Na+. AKT and KAT are interrelated and show their expression in leaf phloem tissues and guard cells. SKOR are also K+ channel which maintain K+ homeostasis by influencing its loading in xylem. KCO are K+ channel expressed in leaves, possibly in tonoplast. KEA and CPA antiporters are important in homeostasis by loading K+ ions in vacuoles but are also carriers of Na+ ions. HAK and KUP have many variants in higher plants and important for K+ homeostasis. HKT has higher affinity for K+ uptake but mutation disrupts its function leading to transfer of Na+ form root towards shoot.
Some other transporters CNGC, GLR, CHX, NHX, SOS, AHA and AVP are all non-selective cation exporter genes. CNGC and GLR have permeability with Na+ and K+ and regulated by Ca+2. CHX are cation exchangers and their expression is down regulated during salt stress. NHX are Na+/K+ antiporter and selectively transport Na+ ions in vacuole along with K+ ion under saline condition. SOS are antiporter present on cell membrane and expressed in root cells and play important role in sodium extrusion form roots to external medium. AHA are proton pumps which mediates protons transport across cell membrane. AVP are also proton pumps but these mediates proton transport across vacuolar membrane.
Many genes codes for molecules which have protective functions, it includes small organic compounds that are called compatible solutes, osmolytes and osmoprotectants. These genes includes P5CS, OTS, Mt1, S6PDH and Imt1. P5CS which codes for Glycine betaine, proline, betaine and trigonelline solutes which plays an important role in maintenance of turgor pressure, regulation and buffering of cellular redox reaction and stress recovery. OTS genes codes for trehalose which altered growth through signaling pathways, mainly by hexose signaling. Mt1D genes produces Mannitol which regulates turgor pressure particularly in cytosol, also involved in signaling pathways and help in removal of hydroxyl free radicals. S6PDH controls expression of Sorbitol which is involved in turgor pressure maintenance and mopping of free radicals. Whereas Imt controls ononitol expression & maintains turgor potential in certain halophytic plants and also aids in removal of hydroxyl radical.
The purpose behind explaining these genes in detail is to create awareness among plant breeders, soil scientists, bio technologists and agriculturists to work with these genes either through markers assisted breeding if possible or through genetic engineering to incorporate these genes in locally grown major crops. Doing this will help in reclamation of salt affected soils of salt range and other slat affected areas which are lying barren. Further salt tolerance crops will produce more yield in salt affected areas and will boost the living standard of the farmers as well as it will strengthening of national economy.
This article is collectively authored by Rida Fatima, Rahil Shahzad, Shakra Jamil Agricultural Biotechnology Research Institute, AARI Faisalabad.