Abstract：The function of the CYP90A subfamily genes in monocots remains unclear, but they are likely involved in the biosynthesis of the plant hormone brassinosteroids (BRs). This study used Hordeum vulgare var. nudum, a characteristic crop of the plateau, as the experimental material. By utilizing its genomic database, the CYP90A subfamily gene of H. vulgare var. nudum was identified, its cDNA sequence was cloned, and bioinformatics and expression pattern analyses were conducted. Additionally, genetic transformation of Arabidopsis thaliana was performed to explore its functions of salt resistance. The results showed that only one CYP90A subfamily gene was found in H. vulgare var. nudum genome, named HvvCYP90A. The coding region of the HvvCYP90A gene was 1 542 bp long, encoding 513 amino acids. The coding region similarity between the CYP90A genes of H. vulgare var. nudum and H. vulgare was as high as 96.76%, but there are significant differences in the promoter sequences, with HvvCYP90A having unique response elements for drought and circadian rhythm. Evolutionary analysis revealed that the CYP90A gene was differentiated between monocots and dicots, but there was no obvious regulation in the number of genes among different species. As the seedlings of H. vulgare var. nudum grew, the expression of HvvCYP90A gradually decreased in the roots but increased in the leaves. Moreover, the expression of HvvCYP90A was inhibited by exogenous 28-homobrassinolide (HBR). Additionally, after transferring HvvCYP90A into A. thaliana, the root length of transgenic A. thaliana was significantly increased compared with the wild type (P<0.05) under normal and salt stress conditions. It was speculated that the HvvCYP90A gene regulated the salt tolerance of A. thaliana by participating in the biosynthesis of BRs. This study provides reference for the mechanism of brassinosteroid regulated salt stress adaptation of H. vulgare var. nudum.
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