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

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 (<i>Oryza sativa</i> L.) plants were engineered to constitutively overexpress the rice <i>GDP-L-galactose phosphorylase</i> coding sequence (35S-<i>OsGGP</i>), which encodes the rate-limiting enzymatic step of the L-galactose pathway. Ascorbate concentrations were negligible in both null segregant (NS) and 35S-<i>OsGGP</i> brown rice (BR, unpolished grain), but significantly increased in 35S-<i>OsGGP</i> germinated brown rice (GBR) relative to NS. Foliar ascorbate concentrations were significantly increased in 35S-<i>OsGGP</i> plants in the vegetative growth phase relative to NS, but significantly reduced at the reproductive growth phase and were associated with reduced <i>OsGGP</i> transcript levels. The 35S-<i>OsGGP</i> 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 <i>in vitro</i> digests of NS and 35S-<i>OsGGP</i> BR and GBR samples.