The role of alanine and aspartate aminotransferases in C₄ photosynthesis

Abstract

Alanine and aspartate are essential transfer metabolites for C species of the NAD-malic enzyme and phosphoenolpyruvate carboxykinase subtype. To some degree both amino acids are also part of the metabolite shuttle in NADP-malic enzyme plants. In comparison with C species, the majority of C species are therefore characterised by enhanced expression and activity of alanine and aspartate aminotransferases (AT) in the photosynthetically active tissue. Both enzymes exist in multiple copies and have been found in different subcellular compartments. We tested whether different C species show preferential recruitment of enzymes from specific lineages and subcellular compartments. Phylogenetic analysis of alanine and aspartate ATs from a variety of monocot and eudicot C species and their C relatives was combined with subcellular prediction tools and analysis of the subsequent transcript amounts in mature leaves. Recruitment of aspartate AT from a specific subcellular compartment was strongly connected to the biochemical subtype. Deviation from the main model was however observed in Gynandropsis gynandra. The configuration of alanine AT generally differed in monocot and eudicot species. C monocots recruited an alanine AT from a specific cytosolic branch, but eudicots use alanine AT copies from a mitochondrial branch. Generally, plants display high plasticity in the setup of the C pathway. Beside the common models for the different C subtypes, individual solutions were found for plant groups or lineages.

Metabolic profiles in C3, C3–C4 intermediate, C4-like, and C4 species in the genus Flaveria

“…5 ), providing strong support for the operation of an expanded NADP-ME-type C 4 cycle with integration of aminotransferases ( Fig. 7 ; Furbank, 2011 ; Schlüter et al , 2018 ). Our analyses also point to increased exchange of C between the CBC and the C1–C3 positions of the carrier metabolites, presumably as a result of increased PEPC activity.…”

Reduction of bundle sheath size boosts cyclic electron flow in C4Setaria viridis acclimated to low light

“…Diffusion is non‐limited, except that of CO 2 , restrained by a finite conductance ( g BS ) that allows higher CO 2 concentrations in BS cells ( C BS ) based on classical underpinnings (Berry & Farquhar, 1978). Glutamate and αKG do not diffuse directly between M and BS cells, but exchange amino‐ groups with ALA and PYR (Mallmann et al., 2014; Pick et al., 2011; Schlüter et al., 2019). If reactions can proceed in the opposite direction, they are assumed to proceed only in the direction with positive balance.…”

Reduction of bundle sheath size boosts cyclic electron flow in C4Setaria viridisacclimated to low light

“…Diffusion is non-limited, except that of CO 2 , restrained by a finite conductance ( g BS ) that allows higher CO 2 concentration in BS cells ( C BS ) based on classical underpinnings (Berry and Farquhar, 1978). Glutamate and αKG do not diffuse directly between M and BS cells, but exchange amino-groups with ALA and PYR (Pick et al , 2011, Mallmann et al , 2014, Schlüter et al , 2019). If reactions can proceed in opposite direction, they are assumed to proceed only in the direction with positive balance.…”