Genetic and physiological basis of salinity tolerance in wheat

Soil salinity is one of the major abiotic stresses restricting plant growth and development and reducing crop yield. Approximately 1125 million ha are affected by salinity comprising about 6% of the total land area and 20% of the irrigated land worldwide. The detrimental effects of salinity in plants are multifaceted and are related to osmotic, ionic, and oxidative components of the stress. Osmotic stress leads to stomata closure, reducing plant‚ÄövÑv¥s ability to assimilate CO2, while ionic stress hampers the normal metabolic processes in plants. Oxidative stress is associated with the damage of plant‚ÄövÑv¥s cellular structures and macromolecules such as DNA, enzymes, and lipids. Wheat (Triticum aestivum) is one of the most important cereal crops but is very sensitive to salinity stress, with reported loss in grain yield in excess of 60% under saline conditions. Different approaches can be used to reduce the negative impact of salinity stress on plant growth and yield. The development of salt-resistant cultivars via breeding program has been regarded as the best solution. Some agronomic practices including inoculating seeds with halotolerant plant growth promoting rhizobacteria (PGPR) and the application of various plant growth regulators (PGRs) can also partially alleviate salinity damage. This project explored both these options and aimed to pave the foundation for improving wheat performance under saline conditions via (i) identifying tolerant wheat genotypes using screening based on integrative tool membership function value (MFV); (ii) identify the salinity tolerance QTL through genome wide association study (GWAS); (iii) detect salt tolerance QTL in the doubled haploid (DH) population; (iv) explore the possibility of applying exogenous PGR (cytokinin; CK) for alleviating salt stress in wheat. The multifaceted nature of salinity tolerance traits complicates plant screening and the identification of salt-tolerant germplasm to be used for the genetic advancement of corps. Many screening criteria have been suggested to distinguish between genotypes. Most of these were applied under controlled environmental conditions and limited to one developmental stage of plants. As a result, most of the reported tolerance could not be validated under field conditions. This study employed a MFV to assess NaCl tolerance of eight wheat genotypes measured at germination, vegetative and reproductive stages, as an integrative tool for the overall plant performance. Salt stresses had an adverse effect on plant physiological (residual transpiration, stomatal density, chlorophyll fluorescence characteristics; leaf N, Na+ and K+ content) and agronomical (plant height; biomass; root and tiller number; grain) characteristics. Based on this assessment, plants were divided into three contrasting groups: salt tolerant, moderately salt tolerant and salt sensitive. Although genotypes did not show the same degree of tolerance in germination, glasshouse and field experiments, the variety Yu-07 showed consistently better performance in all trials, whilst Aus-19720 was most sensitive in glasshouse and field experiments. These contrasting genotypes could be of a potential value for further studies to uncover the genetic mechanisms governing salt stress response in wheat. Identifying new salinity tolerance QTL or genes is crucial for breeders to pyramid different tolerance mechanisms to improve crop adaptability to salinity. In this study, glasshouse experiments were conducted to investigate the genotypic variation present in 328 wheat varieties in their salinity tolerance at the vegetative stage. A GWAS were carried out to identify QTL conferring salinity tolerance through a mixed linear model. Six, five and eight significant marker-trait associations (MTAs) were identified from pot experiments, tank experiments and average damage scores of pot and tank experiments, respectively. These markers are located on the wheat chromosomes 1B, 2B, 2D, 3A, 4B, and 5A. These tolerance alleles were additive in their effects and, when combined, increased tolerance to salinity. Candidate genes identified in these QTL regions encoded a diverse class of proteins involved in salinity tolerance in plants. Amongst discovered genes, plasma membrane Na+/H+ exchanger and a potassium transporter on chromosome 5A (IWB30519) may be of a potential value for improvement of salt tolerance of wheat cultivars using marker assisted selection programs. Some useful genotypes, which showed consistent tolerance in different trials, could be also recommended for the use in breeding programs. A total 194 double haploid population derived from cross between H-020 and H-132 were used to identify new QTL tolerant to salinity stress. H-020 is more salt tolerant than H-132. This population was genotyped using high-density Diversity Arrays Technology (DArT) and SNP markers. A total fourteen QTL were detected for different morpho-physiological traits such as salinity damage score, plant height, shoot fresh weight, shoot dry weight, relative water content; leaf Na+ and K+ content; and K+/Na+ ratio. These QTL were distributed on chromosome 2A, 2B, 2D, 3B, 4B, 5B, 5D, 6A and 7B. Three QTL for salinity damage score showed an additive effect. One major QTL for salinity damage score on chromosome 5D explained 35.7% phenotypic variance. QTL for leaf Na+ content and K+/Na+ ratio was also identified at similar positions with the nearest marker of D1095484 for this QTL. Some potentially active genes conferring salinity tolerance exist in this QTL region. This new QTL will be useful in breeding programs for pyramiding salinity tolerance genes. The influence of PGR on alleviating salinity damage was also investigated. CK are a class of plant hormone widely known for their beneficial role in plant growth and development as well as salinity tolerance. Based on our previous work, two varieties H-132 (tolerant) and Aus-19720 (sensitive) were used to investigate the role of cytokinin on growth and physiology of wheat growing under salinity stress. Two set of experiments, i.e., hydroponics and foliar spray, were conducted under laboratory and glasshouse conditions. Salt stresses had an adverse effect on plant agronomical (shoot length, root length, biomass, tiller number) and physiological (residual transpiration, Fv/Fm, SPAD; leaf Na+ and K+ content) characteristics. Effects of exogenous application of kinetin (one specific form of cytokinins) were mixed (both positive and negative) and depended on both the cultivar and application method. In hydroponic experiments, application of kinetin further decreased shoot length, root length and relative water content (RWC) in tolerant H-132 variety. In sensitive Aus-19720 genotype, seedling Na+ content increased in kinetin treated plants. When kinetin was given as a foliar spray, both varieties were able to maintain more efficient operation of PSII (higher Fv/Fm values) but also had lower residual transpiration (better water saving). The sensitive variety Aus-19720 showed better performance in terms of the tiller number, SPAD, leaf Na+ content and K+/Na+ ratio. It was concluded that the exogenous application of cytokinin is unlikely to be a viable tool in improving plant performance under saline conditions in the field, as its effects are strongly dependant on genotype, method of application, and growth environment. Therefore, practical application of cytokinin in field condition for coping with salinity damage requires further review. In conclusion, several new QTL were identified for salinity tolerance based on visual symptoms scoring and some physiological traits. The detected QTL were located at different positions to those for salinity tolerance. These QTL can be effectively used in breeding program. In addition, the identified contrasting genotypes from GWAS and MFV will be useful for breeders to uncover genetic and physiological mechanisms governing salt stress response in wheat.