Yoon-Ha Kim^1†, Abdul Latif Khan^2†, Duk-Hwan Kim¹, Seung-Yeol Lee¹, Kyung-Min Kim¹, Muhammad Waqas¹, Hee-Young Jung¹, Jae-Ho Shin¹, Jong-Guk Kim³ and In-Jung Lee¹

¹School of Applied Biosciences, College of Agriculture and Life Science, Kyungpook National University, Daegu 702-701, Republic of Korea.
²Department of Biological Science & Chemistry, University of Nizwa, Nizwa 616, Oman.
³Department of Life Sciences and Biotechnology, Kyungpook National University, Daegu 702-701, Republic of Korea.

^† Equal contributors

^* Correspondence

Background
Silicon (Si) application has been known to enhance the tolerance of plants against abiotic stresses. However, the protective mechanism of Si under heavy metals contamination is poorly understood. The aim of this study was to assess the role of Si in counteracting toxicity due to cadmium (Cd) and copper (Cu) in rice plants (Oryza sativa).

Results
Si significantly improved the growth and biomass of rice plants and reduced the toxic effects of Cd/Cu after different stress periods. Si treatment ameliorated root function and structure compared with non-treated rice plants, which suffered severe root damage. In the presence of Si, the Cd/Cu concentration was significantly lower in rice plants, and there was also a reduction in lipid peroxidation and fatty acid desaturation in plant tissues. The reduced uptake of metals in the roots modulated the signaling of phytohormones involved in responses to stress and host defense, such as abscisic acid, jasmonic acid, and salicylic acid. Furthermore, the low concentration of metals significantly down regulated the mRNA expression of enzymes encoding heavy metal transporters (OsHMA2 and OsHMA3) in Si-metal-treated rice plants. Genes responsible for Si transport (OsLSi1 and OsLSi2), showed a significant up-regulation of mRNA expression with Si treatment in rice plants.

Conclusion
The present study supports the active role of Si in the regulation of stresses from heavy metal exposure through changes in root morphology.