Biology subjects

Sage, T. L.

Publications and source records attributed to Sage, T. L..

3 recordsLinked to original sources

bioRxiv2026

Acclimation to enhanced photorespiratory conditions increases the effectiveness of C2 photosynthesis in C3-C4 intermediates

- C2 photosynthesis reduces photorespiratory carbon loss by shuttling glycine from mesophyll (M) to bundle sheath (BS) tissues, where glycine decarboxylase (GDC) releases CO2 that accumulates around rubisco. C2 photosynthesis has been considered a constitutive character, but as a metabolic trait, it may acclimate to the rate of photorespiratory glycine production. - We grew eleven species from two eudicot and two grass lineages spanning C3 to C2 phenotypes at 32 {degrees}C days and 200 or 800 {micro}mol mol-1 CO2. Acclimation was assessed using leaf gas exchange, GDC P-protein immunolocalization, M and BS ultrastructure, photosynthetic enzyme activity, and photosynthesis modelling. - Low relative to high growth CO2 enhanced C2-related phenotypes in sub-C2 and C2 species but not C3 or proto-Kranz species. Sub-C2 species positioned chloroplasts closer to mitochondria and had lower modelled BS conductance. C2 species increased rubisco capacity and GDC allocation to the BS. Aspartate aminotransferase activity increased in both sub-C2 and C2 species. - Acclimation to greater photorespiratory conditions strengthens C2 photosynthesis, but only in species whose BS has crossed a threshold of organelle and GDC investment. We hypothesize that acclimation could expose C2-related traits to selection and facilitate the origin of C4 photosynthesis.

plant biology↗

bioRxiv2021

Respiratory and C4-photosynthetic NAD-malic enzyme coexist in bundle sheath cells mitochondria and evolved via association of differentially adapted subunits

In different lineages of Cleomaceae, NAD-malic enzyme (NAD-ME) was independently co-opted to participate in C4 photosynthesis. In the C4 Cleome species Gynandropsis gynandra and Cleome angustifolia, all NAD-ME genes (NAD-ME, NAD-ME{beta}1, and NAD-ME{beta}2) were affected by C4 evolution and are expressed at higher levels than their orthologs in the C3 Cleome species Tarenaya hassleriana. In the latter C3 species, the NAD-ME housekeeping function is performed by two heteromers, NAD-ME/{beta}1 and NAD-ME/{beta}2, with similar biochemical properties. In both C4 species analyzed, this role is restricted the NAD-ME/{beta}2 heteromer. In the C4 species, NAD-ME/{beta}1 is exclusively present in the leaves, where it accounts for most of the enzymatic activity. GgNAD-ME/{beta}1 exhibits high catalytic efficiency and is differentially activated by the C4 intermediate aspartate, confirming its role as the C4-decarboxylase. During C4 evolution, GgNAD-ME{beta}1and CaNAD-ME{beta}1 lost their catalytic activity; their contribution to enzymatic activity results from a stabilizing effect on the associated -subunit. We conclude that in bundle sheath cell mitochondria of C4 Cleome species, the functions of NAD-ME as C4 photosynthetic decarboxylase and as a tricarboxylic acid cycle-associated housekeeping enzyme coexist and are performed by isoforms that combine the same subunit with differentially adapted {beta} subunits.

plant biology↗

bioRxiv2020

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

The chloroplastic oxaloacetate/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 oxaloacetate (OAA). Together with the glutamate/malate transporter (DCT1 or DiT2), it connects carbon with nitrogen assimilation, by providing -ketoglutarate for the GS/GOGAT reaction and exporting glutamate to the cytoplasm. OMT1 further plays a prominent role in C4 photosynthesis. OAA resulting from PEP-carboxylation is imported into the chloroplast, reduced to malate by plastidic NADP-MDH, and then exported for transport to bundle sheath cells. Both transport steps are catalyzed by OMT1, at the rate of net carbon assimilation. Therefore, 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, over-expression of ZmOMT1 in rice negatively affected growth, CO2 assimilation rate, total free amino acid contents, TCA 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 over-expression of ZmDiT2. Implications for engineering C4-rice are discussed.

plant biology↗