| Summary: | Dehydration proteins (dehydrins, DHNs) confer tolerance to water-stress deficit in plants. We performed a comparative genomics and evolutionary study of DHN genes in four model <i>Brachypodium</i> grass species. Due to limited knowledge on dehydrin expression under water deprivation stress in <i>Brachypodium,</i> we also performed a drought-induced gene expression analysis in 32 ecotypes of the genus’ flagship species <i>B. distachyon</i> showing different hydric requirements. Genomic sequence analysis detected 10 types of dehydrin genes (<i>Bdhn</i>) across the <i>Brachypodium</i> species. Domain and conserved motif contents of peptides encoded by <i>Bdhn</i> genes revealed eight protein architectures. <i>Bdhn</i> genes were spread across several chromosomes. Selection analysis indicated that all the <i>Bdhn</i> genes were constrained by purifying selection. Three upstream <i>cis</i>-regulatory motifs (BES1, MYB124, ZAT) were detected in several <i>Bdhn</i> genes. Gene expression analysis demonstrated that only four <i>Bdhn</i>1-<i>Bdhn</i>2, <i>Bdhn</i>3, and <i>Bdhn</i>7 genes, orthologs of wheat, barley, rice, sorghum, and maize genes, were expressed in mature leaves of <i>B. distachyon</i> and that all of them were more highly expressed in plants under drought conditions. <i>Brachypodium</i> dehydrin expression was significantly correlated with drought-response phenotypic traits (plant biomass, leaf carbon and proline contents and water use efficiency increases, and leaf water and nitrogen content decreases) being more pronounced in drought-tolerant ecotypes. Our results indicate that dehydrin type and regulation could be a key factor determining the acquisition of water-stress tolerance in grasses.
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