Assessment of molecular markers and marker-assisted selection for drought tolerance in barley (Hordeum vulgare L.)

This review updates the present status of the field of molecular markers and marker-assisted selection (MAS), using the example of drought tolerance in barley. The accuracy of selected quantitative trait loci (QTLs), candidate genes and suggested markers was assessed in the barley genome cv. Morex. Six common strategies are described for molecular marker development, candidate gene identification and verification, and their possible applications in MAS to improve the grain yield and yield components in barley under drought stress. These strategies are based on the following five principles: (1) Molecular markers are designated as genomic 'tags', and their 'prediction' is strongly dependent on their distance from a candidate gene on genetic or physical maps; (2) plants react differently under favourable and stressful conditions or depending on their stage of development; (3) each candidate gene must be verified by confirming its expression in the relevant conditions, e.g., drought; (4) the molecular marker identified must be validated for MAS for tolerance to drought stress and improved grain yield; and (5) the small number of molecular markers realized for MAS in breeding, from among the many studies targeting candidate genes, can be explained by the complex nature of drought stress, and multiple stress-responsive genes in each barley genotype that are expressed differentially depending on many other factors.