TY - JOUR

T1 - Overexpression of the chloroplastic 2-oxoglutarate/malate transporter disturbs carbon and nitrogen homeostasis in rice

AU - Zamani-Nour, Shirin

AU - Lin, Hsiang-Chun

AU - Walker, Berkley J

AU - Mettler-Altmann, Tabea

AU - Khoshravesh, Roxana

AU - Karki, Shanta

AU - Bagunu, Efren

AU - Sage, Tammy L

AU - Quick, W Paul

AU - Weber, Andreas P M

N1 - © The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology.

PY - 2021/1/20

Y1 - 2021/1/20

N2 - The chloroplastic 2-oxaloacetate (OAA)/malate transporter (OMT1 or DiT1) takes part in the malate valve that protects chloroplasts from excessive redox poise through export of malate and import of OAA. Together with the glutamate/malate transporter (DCT1 or DiT2), it connects carbon with nitrogen assimilation, by providing 2-oxoglutarate for the GS/GOGAT (glutamine synthetase/glutamate synthase) reaction and exporting glutamate to the cytoplasm. OMT1 further plays a prominent role in C4 photosynthesis: OAA resulting from phosphoenolpyruvate carboxylation is imported into the chloroplast, reduced to malate by plastidic NADP-malate dehydrogenase, and then exported for transport to bundle sheath cells. Both transport steps are catalyzed by OMT1, at the rate of net carbon assimilation. To engineer C4 photosynthesis into C3 crops, OMT1 must be expressed in high amounts on top of core C4 metabolic enzymes. We report here high-level expression of ZmOMT1 from maize in rice (Oryza sativa ssp. indica IR64). Increased activity of the transporter in transgenic rice was confirmed by reconstitution of transporter activity into proteoliposomes. Unexpectedly, overexpression of ZmOMT1 in rice negatively affected growth, CO2 assimilation rate, total free amino acid content, tricarboxylic acid cycle metabolites, as well as sucrose and starch contents. Accumulation of high amounts of aspartate and the impaired growth phenotype of OMT1 rice lines could be suppressed by simultaneous overexpression of ZmDiT2. Implications for engineering C4 rice are discussed.

AB - The chloroplastic 2-oxaloacetate (OAA)/malate transporter (OMT1 or DiT1) takes part in the malate valve that protects chloroplasts from excessive redox poise through export of malate and import of OAA. Together with the glutamate/malate transporter (DCT1 or DiT2), it connects carbon with nitrogen assimilation, by providing 2-oxoglutarate for the GS/GOGAT (glutamine synthetase/glutamate synthase) reaction and exporting glutamate to the cytoplasm. OMT1 further plays a prominent role in C4 photosynthesis: OAA resulting from phosphoenolpyruvate carboxylation is imported into the chloroplast, reduced to malate by plastidic NADP-malate dehydrogenase, and then exported for transport to bundle sheath cells. Both transport steps are catalyzed by OMT1, at the rate of net carbon assimilation. To engineer C4 photosynthesis into C3 crops, OMT1 must be expressed in high amounts on top of core C4 metabolic enzymes. We report here high-level expression of ZmOMT1 from maize in rice (Oryza sativa ssp. indica IR64). Increased activity of the transporter in transgenic rice was confirmed by reconstitution of transporter activity into proteoliposomes. Unexpectedly, overexpression of ZmOMT1 in rice negatively affected growth, CO2 assimilation rate, total free amino acid content, tricarboxylic acid cycle metabolites, as well as sucrose and starch contents. Accumulation of high amounts of aspartate and the impaired growth phenotype of OMT1 rice lines could be suppressed by simultaneous overexpression of ZmDiT2. Implications for engineering C4 rice are discussed.

KW - Carbon/metabolism

KW - Chloroplasts/metabolism

KW - Homeostasis

KW - Ketoglutaric Acids/metabolism

KW - Malates/metabolism

KW - Nitrogen/metabolism

KW - Oryza/genetics

KW - Photosynthesis

U2 - 10.1093/jxb/eraa343

DO - 10.1093/jxb/eraa343

M3 - Journal article

C2 - 32710115

VL - 72

SP - 137

EP - 152

JO - Journal of Experimental Botany

JF - Journal of Experimental Botany

SN - 0022-0957

IS - 1

ER -