Exogenous methyl jasmonate enhanced the antioxidant capacity of Malus baccata by stimulating jasmonate signalling under suboptimal low root-zone temperature

Methyl jasmonate (MeJA) has been documented to mitigate the oxidative stress caused by abiotic triggers. In this study, we investigated the mechanism through which exogenous MeJA alleviates oxidative damage in the roots of Malus baccata under a suboptimal low root-zone temperature. A suboptimal low root-zone temperature promoted increases in the activity and transcription of allene oxide synthase (AOS) and jasmonic acid carboxyl methyltransferase (JMT), elevated the levels of endogenous jasmonic acid (JA), MeJA and jasmonate isoleucine (JA-Ile), and induced the accumulation of malondialdehyde (MDA) and reactive oxygen species (ROS). The addition of exogenous MeJA further increased the JA (1 h, 12-24 h), MeJA (1-24 h), and JA-Ile (1-6 h) contents and enhanced the activities and transcription of lipoxygenase (LOX) and JMT. The upregulated expression of JASMONATE ZIM-DOMAIN (JAZ) and MdMYC2 indicated that suboptimal low root-zone temperature could activate JA signalling in roots, and exogenous MeJA could further strengthen JA signalling under lowtemperature stress. Exogenous MeJA alleviated oxidative damage to stressed roots by enhancing ascorbate-glutathione (AsA-GSH) cycle activity. In addition, pre-treatment with phenidone (an inhibitor of lipoxygenase) exerted the opposite effects to those induced by exogenous MeJA treatment. These findings indicated that exogenous MeJA enhanced antioxidant systems and detoxified ROS by regulating JA signalling and thereby protected M. baccata roots from damage caused by suboptimal low root-zone temperature stress.