Effect of Rice GDP-L-Galactose Phosphorylase Constitutive Overexpression on Ascorbate Concentration, Stress Tolerance, and Iron Bioavailability in Rice.

Ronan C Broad,Bettina Berger,Alexander A T Johnson,Julien P Bonneau,Pawel Sadowski,Sally Roden,Chris Brien,Raymond P Glahn,Jesse T Beasley,Nathaniel Jewell,Roger P Hellens,Elad Tako

doi:10.3389/fpls.2020.595439

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 have 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 were negligible in both null segregant (NS) and 35S-OsGGP brown rice (BR, unpolished grain), but significantly increased in 35S-OsGGP germinated brown rice (GBR) relative to NS. Foliar ascorbate concentrations were significantly increased in 35S-OsGGP plants in the vegetative growth phase relative to NS, but significantly reduced at the reproductive growth phase and were associated with reduced OsGGP transcript levels. The 35S-OsGGP plants did not display altered salt tolerance at the vegetative growth phase despite having elevated ascorbate concentrations. Ascorbate concentrations were positively correlated with ferritin concentrations in Caco-2 cells – an accurate predictor of Fe bioavailability in human digestion – exposed to in vitro digests of NS and 35S-OsGGP BR and GBR samples.

Highlights

Producing sufficient and nutritious food to feed the growing world population represents one of the great challenges of the 21st century
We found that constitutive overexpression of the OsGGP coding sequence in rice did not affect ascorbate concentrations in brown rice (BR, unpolished grain), but significantly increased ascorbate concentrations in germinated brown rice (GBR) and tissues at the vegetative growth phase
A total of 27 independent, hemizygous 35S-OsGGP transformation events were regenerated from tissue culture and ascorbate concentrations were measured in the T1 BR

Summary

Introduction

Producing sufficient and nutritious food to feed the growing world population represents one of the great challenges of the 21st century. Abiotic stresses, such as drought, salinity, and extreme temperatures are major limiting factors of global crop productivity, and are predicted to be exacerbated and co-occur more frequently due to the effects of climate change (Zhang et al, 2000; Pandey et al, 2017). The development of crops with an enhanced capacity to mitigate the damaging effects caused by the excess production of ROS represents one solution to help offset the forecasted yield losses associated with climate change (Koyro et al, 2012). The production of crops containing increased micronutrient density and/or bioavailability – a process known as biofortification – offers a cost-effective and sustainable intervention to reduce hidden hunger (Bouis et al, 2011)

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