Overexpression of CsHMGB Alleviates Phytotoxicity and Propamocarb Residues in Cucumber.

Cucumber (Cucumis sativus L.) is one of the most economically important fruits of the Cucurbitaceae family, therefore consideration of potential pesticide residues in the fruit in the context of cucumber breeding and production programs is important. Propamocarb (a pesticide commonly used to prevent downy mildew) is widely used in cucumber cultivation, but the molecular mechanism underlying the degradation and metabolism of propamocarb in cucumber is not well understood. We screened a candidate CsHMGB gene (CsaV3-5G28190) for response to propamocarb exposure using transcriptome data. The coding region of CsHMGB was 624 bp in length and encoded the conserved HMB-box region. CsHMGB expression differed significantly between the "D0351" genotype, which accumulated low levels of propamocarb, and the "D9320" genotype, which accumulated high levels of propamocarb. CsHMGB expression was positively correlated with propamocarb levels in the cucumber peel. CsHMGB expression was upregulated in the fruit peels of the "D0351" genotype following exposure to propamocarb stress for 3-120 h, but no difference was observed in expression between propamocarb treatment and control for the "D9320" genotype. For the "D0351" genotype, CsHMGB expression was higher in the fruit peels and leaves than that in female flowers; expression was moderate in the stems and fruit pulps, and weak in male flowers and roots. The CsHMGB protein was targeted to the nucleus in Arabidopsis protoplasts and in the epidermis of Nicotiana benthamiana leaves. We measured MDA, O-2, and H2O2 levels in cucumber plants and found that they were likely to accumulate reactive oxygen species (ROS) in response to propamocarb stress. Analysis of antioxidant enzyme activity (SOD, POD, CAT, APX, GPX, GST, and GR) and the ascorbate-glutathione (AsA-GSH) system showed that the resistance of the plants was reduced and the levels of propamocarb residue was increased in CsHMGBsilenced plants in response to propamocarb stress. Conversely, overexpression of CsHMGB promoted glutathione-dependent detoxification by AsA-GSH system and improved the antioxidant potential, reduced the accumulation of ROS. Ultimately, the metabolism of propamocarb in cucumber was increased via increase in the wax levels and the stomatal conductance.