Enhancement of thiamin in plants by metabolic engineering

Thiamin is an essential nutrient in the human diet. Severe thiamin deficiency leads to beriberi, a lethal disease which is common in developing countries. Thiamin biofortification of staple food crops is a possible strategy to alleviate thiamin deficiency-related diseases. In plants, thiamin plays a role in the response to abiotic and biotic stresses, and data from the literature suggest that boosting thiamin content could increase resistance to stresses. Here, we tested an engineering strategy to increase thiamin content in Arabidopsis. Thiamin is composed of a thiazole ring linked to a pyrimidine ring by a methylene bridge. THI1 and THIC are the first committed steps in the synthesis of the thiazole and pyrimidine moieties, respectively. Arabidopsis plants were transformed with a vector containing the THI1 coding sequence under the control of a constitutive promoter. Total thiamin leaf content in THI1 plants was up ~2-fold compared to the wild-type. THI1-overexpressing lines were then crossed with preexisting THIC-overexpressing lines ([Bocobza et al. (2013) Orchestration of Thiamin Biosynthesis and Central Metabolism by Combined Action of the Thiamin Pyrophosphate Riboswitch and the Circadian Clock in Arabidopsis. Plant Cell 25:288-307]). Resulting THI1 x THIC plants accumulated up to 3.4- and 2.6-fold more total thiamin than wild-type plants in leaf and seeds, respectively. After inocu-lation with Pseudomonas syringae, THI1 x THIC plants had lower populations than the wild-type control. However, THI1 x THIC plants subjected to various abiotic stresses did not show any visible or bio-chemical changes compared to the wild-type. We discuss the impact of engineering thiamin biosyn-thesis on plants nutritional value and resistance to biotic and abiotic stresses.