Mutant combinations of lycopene epsilon-cyclase and beta-carotene hydroxylase 2 homoeologs increased beta-carotene accumulation in endosperm of tetraploid wheat (Triticum turgidum L.) grains

Grains of tetraploid wheat (Triticum turgidum L.) mainly accumulate the non-provitamin A carotenoid lutein-with low natural variation in provitamin A beta-carotene in wheat accessions necessitating alternative strategies for provitamin A biofortification. Lycopene e-cyclase (LCYe) and beta-carotene hydroxylase (HYD) function in diverting carbons from beta-carotene to lutein biosynthesis and catalyzing the turnover of beta-carotene to xanthophylls, respectively. However, the contribution of LCYe and HYD gene homoeologs to carotenoid metabolism and how they can be manipulated to increase beta-carotene in tetraploid wheat endosperm (flour) is currently unclear. We isolated loss-of-function Targeting Induced Local Lesions in Genomes (TILLING) mutants of LCYe and HYD2 homoeologs and generated higher order mutant combinations of lcye-A, lcye-B, hyd-A2, and hyd-B2. Hyd-A2 hyd-B2, lcye-A hyd-A2 hyd-B2, lcye-B hyd-A2 hyd-B2, and lcye-A lcye-B hyd-A2 hyd-B2 achieved significantly increased beta-carotene in endosperm, with lcye-A hyd-A2 hyd-B2 exhibiting comparable photosynthetic performance and light response to control plants. Comparative analysis of carotenoid profiles suggests that eliminating HYD2 homoeologs is sufficient to prevent beta-carotene conversion to xanthophylls in the endosperm without compromising xanthophyll production in leaves, and that beta-carotene and its derived xanthophylls are likely subject to differential catalysis mechanisms in vegetative tissues and grains. Carotenoid and gene expression analyses also suggest that the very low LCYe-B expression in endosperm is adequate for lutein production in the absence of LCYe-A. These results demonstrate the success of provitamin A biofortification using TILLING mutants while also providing a roadmap for guiding a gene editing-based approach in hexaploid wheat.