Metabolic engineering of rice endosperm towards higher vitamin B1 accumulation

Abstract

Summary Rice is a major food crop to approximately half of the human population. Unfortunately, the starchy endosperm, which is the remaining portion of the seed after polishing, contains limited amounts of micronutrients. Here, it is shown that this is particularly the case for thiamin (vitamin B1). Therefore, a tissue‐specific metabolic engineering approach was conducted, aimed at enhancing the level of thiamin specifically in the endosperm. To achieve this, three major thiamin biosynthesis genes, THIC, THI1 and TH1, controlled by strong endosperm‐specific promoters, were employed to obtain engineered rice lines. The metabolic engineering approaches included ectopic expression of THIC alone, in combination with THI1 (bigenic) or combined with both THI1 and TH1 (trigenic). Determination of thiamin and thiamin biosynthesis intermediates reveals the impact of the engineering approaches on endosperm thiamin biosynthesis. The results show an increase of thiamin in polished rice up to threefold compared to WT, and stable upon cooking. These findings confirm the potential of metabolic engineering to enhance de novo thiamin biosynthesis in rice endosperm tissue and aid in steering future biofortification endeavours.

Metabolic engineering provides insight into the regulation of thiamin biosynthesis in plants

“…The observation that the intermediates predominantly occur in their phosphorylated form in the WT further highlights their metabolic role to be readily consumed in B1 biosynthesis. This B1 profile is indicative for an active ( de novo biosynthesis) metabolism and is in great contrast with what has been reported in seeds of food crops, in which B1 predominantly occurs as thiamin, serving as a stable storage form ( Shimizu et al., 1990 ; Golda et al., 2004 ; Strobbe et al., 2021 ).…”

“…Similarly, the limitation that inadequate salvage of (nonphosphorylated) intermediates poses to the flux in the pathway likely holds true in a range of different tissues. Indeed, inadequate salvage of intermediates was proposed to limit thiamin accumulation in rice endosperm ( Goyer, 2017 ; Strobbe et al., 2021 ), which is a target tissue in biofortification efforts of grain crops and is considered less metabolically active tissue. The observed sufficiency of endogenous TH2 to allow adequate thiamin accumulation was also reported in rice leaves ( Dong et al., 2016 ) and endosperm ( Strobbe et al., 2021 ).…”

“… Thiamin biosynthesis in plants. Comprehensive schematic representation of thiamin biosynthesis in planta (adapted from Strobbe and Van Der Straeten, 2018 ; Strobbe et al., 2021 ). Synthesis of pyrimidine and thiazole moieties as well as their condensation occurs in plastids.…”

Antioxidants in Potatoes: A Functional View on One of the Major Food Crops Worldwide

“…Some wild species of potato could contain up to two-fold the amount of thiamin found in potato varieties commonly grown in the United States [ 44 ], and could be used as gene sources for breeding thiamin-enriched varieties. Genetic engineering of the thiamin biosynthesis pathway is another biofortification alternative that has shown promise in rice endosperm [ 45 ] and should be tested in potato tubers. Environmental growth conditions can also affect thiamin content.…”

Riboflavin fortification of rice endosperm by metabolic engineering

“…Recently, biofortification of rice endosperm with micronutrients or phytonutrients via genetic engineering has been proposed as a potential strategy to promote human health and nutrition (Strobbe et al, 2021 ; Tian et al, 2020 ). Theoretically, riboflavin content in rice endosperm also can be enhanced via the metabolic engineering of enzymes involved in riboflavin biosynthetic pathways.…”