Transcriptomic and physio-biochemical features in rice (Oryza sativa L.) in response to mercury stress

Mercury (Hg) is a toxic and nonessential element for organisms, and its contamination in the environment is a global concern. Previous research has shown that Hg stress may cause severe damage to rice roots; however, the transcriptomic changes in roots and physio-biochemical responses in leaves to different levels of Hg stress are not fully understood. In the present study, rice seedlings were exposed to 20, 80, and 160 μM HgCl₂ for three days in hydroponic experiments. The results showed that the majority of Hg was accumulated in rice roots after Hg exposure, and the 80- and 160-μM Hg stresses significantly increased the root-to-shoot translocation factors relative to 20-μM Hg stress, resulting in elevated Hg concentrations in rice shoots. Only the 160-μM Hg stress significantly inhibited root growth compared with the control, while photosynthesis capacity in leaves was significantly reduced under Hg stress. RNA transcriptome sequencing analyses of the roots showed that common responsive differentially expressed genes were strongly associated with glutathione metabolism, amino acid biosynthesis, and secondary metabolite metabolism, which may play significant roles in Hg accumulation by rice plants. Nine crucial genes identified by protein-protein interaction network analysis may be used as candidate target genes for further investigation of the detoxification mechanism, encoding proteins involved in jasmonic acid synthesis, sugar metabolism, allene oxide synthase, glutathione peroxidase, dismutase, and catalase. Furthermore, physio-biochemical analyses of the leaves indicated that higher production of reactive oxygen species was induced by Hg stress, while glutathione and antioxidant enzymes may play crucial roles in Hg detoxification. Our findings provide transcriptomic and physio-biochemical features of rice roots and shoots, which advance our understanding of the responsive and detoxification mechanisms in rice under different levels of Hg stress.