Leaf epidermis transcriptome reveals drought-induced hormonal signaling for stomatal regulation in wild barley

Drought is one of the major abiotic stresses affecting crop yields. Understanding drought tolerance mechanism is pivotal for developing drought tolerant crop cultivars. Here, two Tibetan annual wild barley genotypes XZ5 (drought-tolerant) and XZ54 (drought-sensitive) were tested in this study. Gas exchange, stomatal parameters and yield analyses showed that XZ5 has superior drought tolerance than XZ54. Genome-wide transcriptome analysis with epidermal cell layer of XZ5 and XZ54 identified a total of 6,627 genes as drought-induced differentially expressed genes (DEGs) between the two genotypes. The key DEGs could be classified into abscisic acid, brassinosteroid, jasmonic acid, gibberellins, auxin indole-3-acetic acid pathways, reactive oxygen species signaling, Ca2+ signaling, nitric oxide signaling, stomatal development and membrane transport. Moreover, we discovered unique crosstalks among phytohormone pathways, cellular signaling and membrane transport, which are better regulated in the drought tolerant genotype XZ5. For instance, brassinosteroid may participate in co-regulation of stomatal movement with Abscisic acid through suppressing the expression of Brassinosteroid Insensitive 1-Associated Receptor Kinase (HvBAK) to release the interaction target Open Stomata 1 (HvOST1) in barley epidermal layers. This study provides some tissue-specific insights into the role of a crucial layer of cells—leaf epidermis for drought tolerance in the wild progenitors of cultivated barley.