Reciprocal Crosses Between Astaxanthin and Capsanthin Rice Unravel Effects of Metabolic Gene Efficacy in Rice Endosperm

Carotenoids are not naturally accumulated in the endosperms of rice, which is a cereal crop. In our previous study, biofortified rice seeds accumulated two red color carotenoids, ketocarotenoids including astaxanthin and ketoxanthophylls including capsanthin, via four-step pathway engineering. Their biosynthetic pathways require zeaxanthin as a common precursor and then bifurcated. In this study, cross-fertilization between two astaxanthin (BP and sBP) and two capsanthin (C and CB) rice plants were reciprocally performed to elucidate the rice seed system’s metabolic preference. Filial seeds showed different colors in red hues between mutually interbred lines as well as from parents. The strongest driving force for metabolism was caused by the sBP line for astaxanthin regardless of the parent side and proved to be the most efficacious customized synthetic gene for rice crops. It further resulted in the highest astaxanthin levels when paternal sBP mated with maternal CB line, which has the biggest capacity of total carotenoids. Also, reciprocal crosses between CB and BP lines ascertained their metabolically balanced gene efficacy by showing the pathway preference toward either ketoxanthophylls or ketocarotenoids depending on the paternal parental side. Our study suggests that gene efficacy plays the most decisive role in leading the metabolic path and revealed the unexpectedly paternal parent-of-origin effects in the endosperm. Consequently, we simultaneously biofortified rice crops with two functional red carotenoids and finally generated new red-colored rice varieties.