Potassium (K) deficiency is one of the major factors affecting crop growth and productivity. Development of low-K tolerant crops is an effective approach to solve the nutritional deficiency in agricultural production. Tibetan annual wild barley is rich in genetic diversity and can grow normally under poor soils, including low-K supply. However, the molecular mechanism about low K tolerance is still poorly understood. In this study, Illumina RNA-Sequencing was performed using two Tibetan wild barley genotypes differing in low K tolerance (XZ153, tolerant and XZ141, sensitive), to determine the genotypic difference in transcriptome profiling. We identified a total of 692 differentially expressed genes (DEGs) in two genotypes at 6 h and 48 h after low-K treatment, including transcription factors, transporters and kinases, oxidative stress and hormone signaling related genes. Meanwhile, 294 low-K tolerant associated DEGs were assigned to transporter and antioxidant activities, stimulus response, and other gene ontology (GO), which were mainly involved in starch and sucrose metabolism, lipid metabolism and ethylene biosynthesis. Finally, a hypothetical model of low-K tolerance mechanism in XZ153 was presented. It may be concluded that wild barley accession XZ153 has a higher capability of K absorption and use efficiency than XZ141 under low K stress. A rapid response to low K stress in XZ153 is attributed to its more K uptake and accumulation in plants, resulting in higher low K tolerance. The ethylene response pathway may account for the genotypic difference in low-K tolerance.
The drought-stimulated gene expression of NCED, SUS, and KS - DHN and ABA signal cross-talk with other phytohormones maintains barley root growth under drought stress at pH 4.0 plus polyethylene glycol plus aluminum. Aluminum (Al) toxicity and drought are two major factors that limit barley production. In this work, the individual and combined effects of Al/acid and polyethylene glycol (PEG 6000) induced drought stress that suppressed root growth and caused oxidative damage as characterized by increased H2O2 and O2(.-) accumulation. The wild-barley genotypes, XZ5 and XZ29, exhibited a higher tolerance than the two cultivars Dayton (Al tolerant) and Tadmor (drought tolerant) under combined stress (pH 4.0 + PEG + Al). The oxidative damage induced by PEG was more severe at pH 4.0 than at pH 6.0. In XZ29, the highest root secretion of malate and citrate was recorded, and the least Al uptake in the four genotypes. In XZ5, a peak accumulation of ABA and minor synthesis of zeatin riboside and ethylene were found being essential in maintaining primary root elongation and root hair development. PEG-induced drought stress repressed Al uptake in root tips, with a lower increase in callose formation and HvMATE (Hordeum vulgare multidrug and toxic compound exudation) expression compared to Al-induced callose production. Stress by pH 4.0 + PEG + Al up-regulated 9-cis-epoxycarotenoid dioxygenase (NCED) which is involved in ABA biosynthesis. Such treatment stimulated the regulation of ABA-dependent genes sucrose synthase (SUS) and KS-type dehydrin (KS-DHN) in root tips. Our results suggest that the tolerance ranking to pH 4.0 + PEG + Al stress in Tibetan wild barley by gene expression is closely correlated to physiological indices. The results show that acclimatisation to pH 4.0 + PEG + Al stress involves specific responses in XZ5 and XZ29. The present study provides insights into the effects of Al/acid and drought combined stress on the abundance of physiological indices in the roots of barley varieties.
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