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 (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.

Increased Ascorbate Biosynthesis Does Not Improve Nitrogen Fixation Nor Alleviate the Effect of Drought Stress in Nodulated Medicago truncatula Plants

“…As an example of this variability in experimental approaches, contrasting results regarding the effect of overexpressing VTC2 on plant growth and stress tolerance have been reported. For instance, Zhang et al ( 2015 ) described improved seedling biomass in salt-stressed rice plants overexpressing A. thaliana VTC2 , while subsequent studies in rice did not show significant variations in terms of shoot biomass when overexpressing the kiwifruit ortholog (Ali et al, 2019 ) or the OsVTC2 counterpart (Broad et al, 2020 ). Again, one important limitation of the conclusions reached by Zhang et al ( 2015 ) is that biomass results were expressed on a fresh-weight basis instead of on a dry-weight basis, as mandatory when working with water-deficit stresses.…”

The Multiple Roles of Ascorbate in the Abiotic Stress Response of Plants: Antioxidant, Cofactor, and Regulator

“…Salinity, drought, and temperature stresses can cause ROS accumulation, resulting in severe oxidative damage (Cruz de Carvalho, 2008;Bhattacharjee, 2013;Choudhury et al, 2017). ROS scavenging is critical for plants to cope with these stresses (Sato et al, 2011;Lisko et al, 2013;Shigeoka and Maruta, 2014;Akram et al, 2017;Laxa et al, 2019;Broad et al, 2020a). Increasing ASC content by promoting ASC synthesis can enhance ROS scavenging ability and significantly improve plant stress tolerance (Hemavathi et al, 2000;Wang et al, 2011bWang et al, , 2018Lisko et al, 2013;Ma et al, 2014;Ali et al, 2019;Broad et al, 2020a;Gaafar et al, 2020).…”

“…ROS scavenging is critical for plants to cope with these stresses (Sato et al, 2011;Lisko et al, 2013;Shigeoka and Maruta, 2014;Akram et al, 2017;Laxa et al, 2019;Broad et al, 2020a). Increasing ASC content by promoting ASC synthesis can enhance ROS scavenging ability and significantly improve plant stress tolerance (Hemavathi et al, 2000;Wang et al, 2011bWang et al, , 2018Lisko et al, 2013;Ma et al, 2014;Ali et al, 2019;Broad et al, 2020a;Gaafar et al, 2020). In contrast, disruption of ASC de novo synthesis significantly reduces plant stress tolerance (Conklin et al, 1996;Noctor and Foyer, 1998;Huang et al, 2005;Qin et al, 2016;Wang et al, 2018).…”

“…These stresses can cause ROS accumulation in plants, which severely damages cell composition and disturbs growth and development (Schieber and Chandel, 2014;You and Chan, 2015;Choudhury et al, 2017). As an antioxidant, ASC can effectively scavenge the accumulated ROS via direct or indirect pathways, and is thus critical for eliminating oxidative damage and enhancing abiotic stress tolerance in plants (Noctor and Foyer, 1998;Akram et al, 2017;Hasanuzzaman et al, 2019;Broad et al, 2020a;Elkelish et al, 2020).…”

Transcriptomics View over the Germination Landscape in Biofortified Rice

“…Although currently there are still only a handful of studies looking into this particular aspect, the information gathered from transcriptomics analyses can provide useful insights into the molecular players governing both traits, along with the identification of potential biomarkers to test seed quality. Several studies reported poor germination in different lines of biofortified rice [ 31 , 32 , 33 ], while in other cases, agricultural biofortification (e.g., soil or foliar application of zinc or selenium) proved to be beneficial for germination [ 34 , 35 ]. The current advances in omics can now provide tools and techniques for the efficient exploration of genetic resources along with a better understanding of the molecular mechanisms involved in trait development.…”