Identification of microRNAs in response to aluminum stress in the roots of Tibetan wild barley and cultivated barley

Background: Barley is relatively sensitive to Aluminum (Al) toxicity among cereal crops, but shows a wide genotypic difference in Al tolerance. The well-known Al-tolerant mechanism in barley is related to Al exclusion mediated by a citrate transporter HvAACT1 (Al-activated citrate transporter 1). A 1-kb insertion in the promoter region of HvAACT1 gene results in a dramatic increase of its expression level, which only occurs in some Al-tolerant cultivars. However, Al-tolerant Tibetan wild barley accession XZ29 did not have the 1-kb insertion. Results: We confirmed that the expression of HvAACT1 and secretion of citrate and other organic acids did not explain the difference in Al-tolerant wild barley XZ29 and Al-sensitive cultivated barley Golden Promise. To identify microRNAs (miRNAs) and their target genes responsive to Al stress in barley roots, eight small RNA libraries with two biological replicates from these two genotypes exposed to control and Al-treated conditions were constructed and submitted to deep sequencing. A total of 342 miRNAs were identified in Golden Promise and XZ29, with 296 miRNAs being commonly shared in the two genotypes. Target genes of these miRNAs were obtained through bioinformatics prediction or degradome identification. Comparative analysis detected 50 miRNAs responsive to Al stress, and some of them were found to be exclusively expressed in XZ29 and associated with Al tolerance. Conclusions: miRNAs exclusively expressing in the wild barley were identified and found to be associated with Al stress tolerance. The current results provide a model of describing the roles of some special miRNAs associated with Al tolerance in the Tibetan wild barley.