Control of Iodine Emissions from Rice Plants

Iodine deficiency, together with vitamin A and iron deficiency, is one of the most serious public health problems worldwide. To date, to increase the naturally low amount of iodine in plants and vegetables for human consumption, several iodine biofortification methods have been tried. They rely on the exogenous administration of iodine to plants by using different approaches and methodologies aiming at increasing its absorption and accumulation. However, none of these techniques seem to be universally applicable to all the different plant species. Iodine is not a stable element in plants. Higher plants are in fact able to emit considerable amounts of it in the form of methyl iodide. The primary responsible of methyl iodide emissions are the halide methyltransferase and the halide/thiol methyltransferase enzymes, encoded by the HARMLESS TO OZONE LAYER (HOL) genes. The production of this form of iodine and its release in the atmosphere represent a detrimental phenomenon contributing to the ozone layer destruction. Rice paddies are among the strongest producers of methyl iodide. As a consequence, the most common agronomic approach of iodine biofortification is not ideal for this crop, necessarily leading to further increase of the iodine emissions in the atmosphere. In this work, we investigated the function of the HOL genes of the Oryza sativa species, one of the most important staple food crops, whose seed iodine enrichment could represent a significant progress in the fight against iodine deficiency in many areas of the planet. We demonstrated that one of these genes, OsHOL1, strongly contributes to methyl iodide production from rice plants, since a significant reduction of the iodine volatilization process was observed in mutant plants where the gene was knocked-out by CRISPR/Cas9 technology. The other gene, OsHOL2, was mutagenized as well, without obtaining similar effects. In addition, OsHOL proteins were first described in this study in terms of subcellular localization and protein-protein interactions, in order to better highlight the physiological role of the halide/thiol methyltransferases of rice. Our experiments helped further elucidating the function of both the OsHOL genes of rice and the relative proteins, providing new tools to develop biofortification programs effective for this crop.