Progress and perspective on drought and salt stress tolerance in cotton

Drought stress, caused by lack of precipitation or irrigation, is one of the most challenging problems in crop production in the US and worldwide. Drought alone affects 45% of the world's agricultural land, further, 19.5% of irrigated agricultural lands are considered saline. A combination of two or more abiotic stresses, such as drought and salinity results in more yield loss than a single stress. Drought along with salinization is expected to cause up to 50% of arable land loss worldwide. Development of drought and/or salt stress tolerant cultivars represents one of the most practical solutions. Genetic variation in abiotic stress tolerance exists within Upland cotton (Gossypium hirsutwn L.); however, most if not all Upland cultivars have been developed under normal well-watered and non-saline conditions. Pima, Sea-Island or Egyptian cotton (G. barbadense L), carries some level of tolerance to abioitc stresses due to their origin near sea-coasts, and this tolerance can be transferred to Upland cotton by interspecific introgression. Although drought and salt stress tolerances are presumed to be interconnected, the genetic basis is not fully understood due to complexity of the stress resistance and difficulties in phenotyping. The objective of this review was to summarize the progress in screening methodology, resistance germplasm sources, inheritance, biochemical and molecular aspects, transgenic approaches, and quantitative trait loci (QTL) for drought and salt stress tolerance in cotton. In the last 10-15 years, significant progress has been made in understanding the genetic basis of drought and salt tolerance through QTL mapping using molecular markers on biparental and multi-parental populations and natural populations. Numerous drought or salt responsive genes have been identified, some of which include those commonly associated with drought or salt tolerance in other plants and are used in transgenic approaches for enhancement of abiotic stress tolerance. However, none of these genes have been utilized in commercial cotton breeding programs, and no abiotic stress tolerance QTL has been used in cotton breeding through marker-assisted selection (MAS). More and larger permanent intra-specific and interspecific mapping populations using diverse and multiple parents should be developed. These populations are necessary for repeated phenotyping for abiotic stress tolerance and for high resolution mapping of QTL using genome-wide SNP markers and for MAS to transfer tolerance genes to high-yielding cultivars. Further, quick, reliable and high throughput screening methods applicable for large scale populations need to be developed to improve the reliability and scale of phenotyping of the cotton germplasm in these populations for drought and salt stress tolerance.