Overexpression of caffeic acid O-methyltransferase from Lycoris aurea (LaCOMT) confers tolerance to mercury in Arabidopsis thaliana by modulating ROS homeostasis

Caffeic acid O-methyltransferase (COMT) catalyzes key steps in the biosynthesis of lignin. It can also act as an N-acetylserotonin O-methyltransferase (ASMT), which participates in the last step of melatonin biosynthesis. Melatonin has been demonstrated to play vital roles in the regulation of plant processes and stress responses. However, the ASMT activity of COMT has not yet been characterized in the non-model plant golden spider lily (Lycoris aurea [L’Hér.] Herb), which is an ornamental that is medicinally important. A previous transcriptome analysis identified the COMT gene (LaCOMT) in this plant. The recombinant LaCOMT protein from E. coli was highly active toward ASMT, and this activity was significantly inhibited by caffeic acid in a dose-dependent manner. LaCOMT-GFP was localized to the cytoplasm and nucleus. Considering that the bulbs of L. aurea can tolerate extreme environmental conditions, such as drought stress, waterlogging and poor soil conditions, the pattern of expression of LaCOMT in different tissues and after exposure to mercuric chloride (HgCl₂) was analyzed. The results revealed that LaCOMT is ubiquitously expressed in all the tissues studied and can be induced by HgCl₂. Moreover, the heterologous overexpression of LaCOMT led to mercury tolerance in transgenic Arabidopsis thaliana plants. This could be attributed to the accumulation of scavenged reactive oxygen species (ROS) by elevating antioxidant enzymes and augmenting antioxidants in the transgenic A. thaliana plants that overexpressed LaCOMT. Our results suggest that LaCOMT participates in the alleviation of Hg toxicity by modulating ROS homeostasis in plants.