Linking hydrogen-mediated boron toxicity tolerance with improvement of root elongation, water status and reactive oxygen species balance: a case study for rice

Background and aims Boron is essential for plant growth but hazardous when present in excess. As the antioxidant properties of hydrogen gas (H-2) were recently described in plants, oxidative stress induced by excess boron was investigated along with other biological responses during rice (Oryza sativa) seed germination to study the beneficial role of H-2. Methods Rice seeds were pretreated with exogenous H-2. Using physiological, pharmacological and molecular approaches, the production of endogenous H-2, growth status, reactive oxygen species (ROS) balance and relative gene expression in rice were measured under boron stress to investigate mechanisms of H2-mediated boron toxicity tolerance. Key Results In our test, boron-inhibited seed germination and seedling growth, and endogenous H2 production, were obviously blocked by exogenously applying H-2. The re-establishment of ROS balance was confirmed by reduced lipid peroxidation and ROS accumulation. Meanwhile, activities of catalase (CAT) and peroxidase (POX) were increased. Suppression of pectin methylesterase (PME) activity and downregulation of PME transcripts by H2 were consistent with the alleviation of root growth inhibition caused by boron. Water status was improved as well. This result was confirmed by the upregulation of genes encoding specific aquaporins (AQPs), the maintenance of low osmotic potential and high content of soluble sugar. Increased transcription of representative AQP genes (PIP2; 7 in particular) and BOR2 along with decreased BOR1 mRNA may contribute to lowering boron accumulation. Conclusions Hydrogen provides boron toxicity tolerance mainly by improving root elongation, water status and ROS balance.