How does Malus crabapple resist ozone? Transcriptomics and metabolomics analyses.

Ozone (O3), an oxidizing toxic air pollutant, is ubiquitous in industrialized and developing countries. To understand the effects of O3 exposure on apple (Malus) and to explore its defense mechanisms, we exposed 'Hongjiu' crabapple to O3 and monitored its responses using physiological, transcriptomics, and metabolomics analyses. Exposure to 300 nL L-1 O3 for 3 h caused obvious damage to the leaves of Malus crabapple, affected chlorophyll and anthocyanin contents, and activated antioxidant enzymes. The gene encoding phospholipase A was highly responsive to O3 in Malus crabapple. McWRKY75 is a key transcription factor in the response to O3 stress, and its transcript levels were positively correlated with those of flavonoid-related structural genes (McC4H, McDFR, and McANR). The ethylene response factors McERF019 and McERF109-like were also up-regulated by O3. Exogenous methyl jasmonate (MeJA) decreased the damaging effects of O3 on crabapple and was most effective at 200 μmol L -1. Treatments with MeJA altered the metabolic pathways of crabapple under O3 stress. In particular, MeJA activated the flavonoid metabolic pathway in Malus, which improved its resistance to O3 stress.