Transcriptomic analysis of Pak Choi under acute ozone exposure revealed regulatory mechanism against ozone stress

Background: Ground-level ozone (O) is one of the major air pollutants, which cause oxidative injury to plants. The physiological and biochemical mechanisms underlying the responses of plants to O stress have been well investigated. However, there are limited reports about the molecular basis of plant responses to O. In this study, a comparative transcriptomic analysis of Pak Choi (Brassica campestris ssp. chinensis) exposed to different O concentrations was conducted for the first time. Results: Seedlings of Pak Choi with five leaves were exposed to non-filtered air (NF, 31 ppb) or elevated O (E-O, 252 ppb) for 2 days (8 h per day, from 9:00-17:00). Compared with plants in the NF, a total of 675 differentially expressed genes (DEGs) were identified in plants under E-O, including 219 DEGs with decreased expressions and 456 DEGs with increased expressions. Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that O stress invoked multiple cellular defense pathways to mitigate the impaired cellular integrity and metabolism, including 'glutathione metabolism', 'phenylpropanoid biosynthesis', 'sulfur metabolism', 'glucosinolate biosynthesis', 'cutin, suberine and wax biosynthesis' and others. Transcription factors potentially involved in this cellular regulation were also found, such as AP2-ERF, WRKY, JAZ, MYB etc. Based on the RNA-Seq data and previous studies, a working model was proposed integrating O caused reactive oxygen burst, oxidation-reduction regulation, jasmonic acid and downstream functional genes for the regulation of cellular homeostasis after acute O stress. Conclusion: The present results provide a valuable insight into the molecular responses of Pak Choi to acute O stress and the specific DEGs revealed in this study could be used for further functional identification of key allelic genes determining the O sensitivity of Pak Choi.