Effect of rice GDP-L-Galactose phosphorylase constitutive overexpression on ascorbate concentrations, stress tolerance, and iron bioavailability in rice

Authors: BROAD, RONAN - University Of Melbourne; BONNEAU, JULIEN - University Of Melbourne; BEASLEY, JESSE - University Of Melbourne; RODEN, SALLY - Queensland University Of Technology; SADOWSKI, PAWEL - Queensland University Of Technology; BERGER, BETTINA - University Of Adelaide; TAKO, ELAD - Cornell University; Glahn, Raymond; HELLENS, ROGER - Queensland University Of Technology; JOHNSON, ALEXANDER - University Of Melbourne

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2020
Publication Date: 12/3/2020
Citation: Broad, R., Bonneau, J., Beasley, J., Roden, S., Sadowski, P., Berger, B., Tako, E., Glahn, R.P., Hellens, R., Johnson, A. 2020. Effect of rice GDP-L-Galactose phosphorylase constitutive overexpression on ascorbate concentrations, stress tolerance, and iron bioavailability in rice. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2020.595439.
DOI: https://doi.org/10.3389/fpls.2020.595439

Interpretive Summary: Ascorbate (vitamin C) is an essential multifunctional molecule for both plants and mammals. In plants, ascorbate is the most abundant water-soluble antioxidant that supports stress tolerance. In humans, ascorbate is an essential micronutrient and promotes iron (Fe) absorption in the gut. Engineering crops with increased ascorbate levels therefore has the potential to improve both crop stress tolerance and human health. In this study, rice plants were engineered to constitutively overexpress the rice enzymes that can produce more ascorbate. Subsequently, the rice grains produced were analyzed for delivery of bioavailable Fe via a cell culture assay. The results showed that ascorbate concentrations can be increased, and that such increase can improve Fe delivery from the rice grains. The results therefore suggest that ascorbate-enriched rice has potential to provide enhanced Fe nutrition.

Technical Abstract: Ascorbate (vitamin C) is an essential multifunctional molecule for both plants and mammals. In plants, ascorbate is the most abundant water-soluble antioxidant that supports stress tolerance. In humans, ascorbate is an essential micronutrient and promotes iron (Fe) absorption in the gut. Engineering crops with increased ascorbate levels therefore has the potential to improve both crop stress tolerance and human health. Here, rice (Oryza sativa L.) plants were engineered to constitutively overexpress the rice GDP-L-galactose phosphorylase coding sequence (35S-OsGGP) which encodes the rate-limiting enzymatic step of the L-galactose pathway. Ascorbate concentrations remained negligible in null segregant (NS) and 35S-OsGGP brown rice (BR) but were significantly increased in 35S-OsGGP germinated brown rice (GBR) relative to NS. Foliar ascorbate concentrations were significantly increased in the 35S-OsGGP plants in the vegetative growth phase relative to NS, but significantly reduced at the reproductive growth phase and were associated with reduced expression of the endogenous OsGGP gene. The 35S-OsGGP plants did not affect salt tolerance at the vegetative growth phase despite having elevated ascorbate concentrations. Caco-2 cell ferritin formation --an accurate predictor of human Fe absorption --was improved in 35S-OsGGP GBR relative to NS, suggesting that ascorbate-enriched crops may improve Fe bioavailability in human diets.