Drought is a devastating environmental constraint affecting the agronomic production of barley. To facilitate the breeding process, abundant germplasm resources and reliable evaluation systems to identify the true drought-tolerant barley genotypes are needed. In this study, 237 cultivated and 190 wild barley genotypes, originating from 28 countries, were screened for drought tolerance under the conditions of both water deficit and polyethylene glycol (PEG)-simulated drought at seedling stage. Drought stress significantly reduced the plant growth of all barley genotypes, but no significant difference in drought-induced reduction in the performance of barley seedlings was observed under these two drought conditions. Both cultivated and wild barley subspecies displayed considerable genotypic variability in drought tolerance, which underpinned the identification of 18 genotypes contrasting in drought tolerance. A comparative analysis of drought effects on biomass, water relation, photosynthesis, and osmotic adjustment was undertaken using these contrasting barley genotypes, in order to verify the reliability of the screening and to obtain the credible traits as screening criteria of drought tolerance in barley. As expected, the selected drought-tolerant genotypes showed much less reduction in shoot biomass than drought-sensitive ones under water deficit, which was significantly positively correlated with the results of large-scale screening, confirming the reliability of the screening for drought tolerance under two drought conditions in this study. Likewise, the traits of water relation, photosynthetic activity, and osmotic adjustment differed greatly between the contrasting genotypes under water deficit stress, and they were highly correlated to the growth of barley seedlings, suggesting the potential of them to be the selection criteria for drought tolerance. The analysis of the variable importance of these traits in drought tolerance indicated that sap osmolality and relative water content in the youngest fully-expanded leaf are the suitable selection criteria of screening for drought tolerance in barley at seedling stage.

Grain weight and protein content will be reduced and increased respectively when barley is subjected to water stress after anthesis, consequently deteriorating the malt quality. However such adverse impact of water stress differs greatly among barley genotypes. In this study, two Tibetan wild barley accessions and two cultivated varieties differing in water stress tolerance, were used to investigate the genotypic difference in metabolic profiles during grain-filling stage under drought condition. Totally 71 differently-accumulated metabolites were identified, including organic acids, amino acids / amines, and sugars / sugar alcohols. Their relative contents were significantly affected by water stress for all genotypes and differed distinctly between the wild and cultivated barleys. The PCA analysis of metabolites indicated that the Tibetan wild barley XZ147 possessed a unique response to water stress. When subjected to water stress, the wild barley XZ147 showed the most increase of β-amylase activity among the four genotypes, as a result of its higher lysine content, less IAA biosynthesis, more stable H2O2 homeostasis and more up-regulation of BMY1 gene. On the other hand, XZ147 had the most reduction of β-glucan content under water stress than the other genotypes, which could be explained by the faster grain filling process and the less expression of β-glucan synthase gene GSL7. All these results indicated a great potential for XZ147 in barley breeding for improving water stress tolerance.

In this work, the effect of drought on K+ uptake in root and its translocation from root to shoot was investigated using six barley genotypes contrasting in drought tolerance. Results showed that drought conditions caused significant changes in K+ uptake and translocation in a time- and genotype-specific manner, which consequently resulted in a significant difference in tissue K+ contents and drought tolerance levels between the contrasting barley genotypes. The role of K+ transporters and channels and plasma membrane (PM) H+-ATPase in barley’s adaptive response to drought stress was further investigated at the transcript level. The expression of genes conferring K+ uptake (HvHAK1, HvHAK5, HvKUP1, HvKUP2 and HvAKT1) and xylem loading (HvSKOR) in roots were all affected by drought stress in a time- and genotype-specific manner, indicating that the regulation of these K+ transporters and channels is critical for root K+ uptake and root to shoot K+ translocation in barley under drought stress. Furthermore, the barley genotypes showed a strong correlation between H+ efflux and K+ influx under drought stress, which was further confirmed by the significant up-regulation of HvHA1 and HvHA2. These results suggested an important role of plasma membrane H+-ATPase activity and/or expression in regulating the activity of K+ transporters and channels under drought stress. Taken together, it may be concluded that the genotypic difference in drought stress tolerance in barley is conferred by the difference in the ability to regulate K+ transporters and channels in root epidermis and stele.

Plant AT-rich sequence and zinc-binding proteins (PLATZs) are a novel category of plant-specific transcription factors involved in growth, development, and abiotic stress responses. However, the PLATZ gene family has not been identified in barley. In this study, a total of 11 HvPLATZs were identified in barley, and they were unevenly distributed on five of the seven chromosomes. The phylogenetic tree, incorporating PLATZs from Arabidopsis, rice, maize, wheat, and barley, could be classified into six clusters, in which HvPLATZs are absent in Cluster VI. HvPLATZs exhibited conserved motif arrangements with a characteristic PLATZ domain. Two segmental duplication events were observed among HvPLATZs. All HvPLATZs were core genes present in 20 genotypes of the barley pan-genome. The HvPLATZ5 coding sequences were conserved among 20 barley genotypes, whereas HvPLATZ4/9/10 exhibited synonymous single nucleotide polymorphisms (SNPs); the remaining ones showed nonsynonymous variations. The expression of HvPLATZ2/3/8 was ubiquitous in various tissues, whereas HvPLATZ7 appeared transcriptionally silent; the remaining genes displayed tissue-specific expression. The expression of HvPLATZs was modulated by salt stress, potassium deficiency, and osmotic stress, with response patterns being time-, tissue-, and stress type-dependent. The heterologous expression of HvPLATZ3/5/6/8/9/10/11 in yeast enhanced tolerance to salt and osmotic stress, whereas the expression of HvPLATZ2 compromised tolerance. These results advance our comprehension and facilitate further functional characterization of HvPLATZs.

β-amylase activity is related to the polymorphisms of Bmy1 intron III; however, no attention has been given to such relationships under environmental stresses such as drought. In this study, 73 cultivated barley genotypes and 52 Tibetan wild barley accessions were used to test the association between Bmy1 gene intron III polymorphisms and β-amylase activity under drought stress. Our results showed that three alleles, Bmy1.a, Bmy1.b, and Bmy1.c, existed in the examined barley genotypes. Tibetan wild barley had a higher proportion of Bmy1.b, whereas cultivated barley showed a higher proportion of Bmy1.a. Impressively, barley genotypes with Bmy1.b showed a significant increase in β-amylase activity under drought stress, compared with those with Bmy1.a or Bmy1.c, indicating that the Bmy1.b allele might provide more chances for developing barley cultivars with higher β-amylase activity under water stress than both the Bmy1.a and Bmy1.c alleles. Furthermore, the Tibetan wild barley XZ147, belonging to the Bmy1.b allele type, showed significantly higher β-amylase activity than Triumph under drought stress. This might be the result of the unique amino acid substitution M527 or the amino acid composition of R115, D165, A233, S347, and M527 of XZ147.