Plant breeding is both the science and art of developing elite crop cultivars by creating and reassembling desirable inherited traits for human benefit. From the bulk selection of wild plants for cultivation during early civilization to Mendelian genetics and genomics-assisted breeding in modern society, breeding methodologies have evolved over the last thousand years. In the past few decades, the "Green Revolution" through breeding of semi-dwarf wheat and rice varieties, and the use of heterosis and transgenic crops have dramatically enhanced crop productivity and helped prevent widespread famine (Hickey et al., 2019). Integration of these technologies can significantly improve breeding efficiency in the development of super crop varieties (Li et al., 2018). For example, a hybrid cotton variety CCRI63 and six related hybrid varieties account for nearly 90% of cotton production in the Yangtze River Basin (Wan et al., 2017; Wang et al., 2018). These varieties have successfully combined high yield, good quality, and biotic stress tolerance through the integration of conventional breeding, hybrid and genetically modified organism (GMO) technologies (Lu et al., 2019; Ma et al., 2019; Song et al., 2019). Unfortunately, such technology integration is not practical for most staple food crops, including rice and wheat, because of social or technical restrictions. Furthermore, plant breeding is still labor-intensive and time-consuming, and conventional breeding remains the leading approach for the release of commercial crop varieties worldwide. This is especially true for breeding cultivars and hybrids with high yield, good quality, and resistance to biotic or abiotic stresses (Liu et al., 2015; Gu et al., 2016). New germplasm, knowledge, and breeding techniques are required to breed the next generation of crop varieties.

Salinity affects more than 6% of the world's total land area, causing massive losses in crop yield. Salinity inhibits plant growth and development through osmotic and ionic stresses; however, some plants exhibit adaptations through osmotic regulation, exclusion, and translocation of accumulated Na+ or Cl-. Currently, there are no practical, economically viable methods for managing salinity, so the best practice is to grow crops with improved tolerance. Germination is the stage in a plant's life cycle most adversely affected by salinity. Barley, the fourth most important cereal crop in the world, has outstanding salinity tolerance, relative to other cereal crops. Here, we review the genetics of salinity tolerance in barley during germination by summarizing reported quantitative trait loci (QTLs) and functional genes. The homologs of candidate genes for salinity tolerance in Arabidopsis, soybean, maize, wheat, and rice have been blasted and mapped on the barley reference genome. The genetic diversity of three reported functional gene families for salt tolerance during barley germination, namely dehydration-responsive element-binding (DREB) protein, somatic embryogenesis receptor-like kinase and aquaporin genes, is discussed. While all three gene families show great diversity in most plant species, the DREB gene family is more diverse in barley than in wheat and rice. Further to this review, a convenient method for screening for salinity tolerance at germination is needed, and the mechanisms of action of the genes involved in salt tolerance need to be identified, validated, and transferred to commercial cultivars for field production in saline soil.
Gene Expression Regulation, Plant ; Genetic Variation ; Germination/physiology* ; Hordeum/physiology* ; Salt Tolerance/genetics*

Abiotic stresses, predominately drought, heat, salinity, cold, and waterlogging, adversely affect cereal crops. They limit barley production worldwide and cause huge economic losses. In barley, functional genes under various stresses have been identified over the years and genetic improvement to stress tolerance has taken a new turn with the introduction of modern gene-editing platforms. In particular, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) is a robust and versatile tool for precise mutation creation and trait improvement. In this review, we highlight the stress-affected regions and the corresponding economic losses among the main barley producers. We collate about 150 key genes associated with stress tolerance and combine them into a single physical map for potential breeding practices. We also overview the applications of precise base editing, prime editing, and multiplexing technologies for targeted trait modification, and discuss current challenges including high-throughput mutant genotyping and genotype dependency in genetic transformation to promote commercial breeding. The listed genes counteract key stresses such as drought, salinity, and nutrient deficiency, and the potential application of the respective gene-editing technologies will provide insight into barley improvement for climate resilience.