Size matters for single-cell C₄photosynthesis inBienertia

Abstract

A spatial separation of 10 µm between primary and secondary carboxylases is sufficient for a single-cell C4 pathway, even in the absence of any intracellular diffusion barriers.

Spatial photosynthesis modelling sets guidelines to constructing a viable single-cell cytoplasm-to-stroma C4 cycle

“…The assimilation, determined by the throughput of the photorespiratory and Calvin-Benson cycle pathways, is expressed on a cell-surface-area basis. The photon cost of carbon fixation (the number of photons needed per assimilated carbon atom to cover for the costs of the Calvin-Benson cycle, the photorespiration, and the C 4 cycle) is quantified assuming optimal usage of the linear and cyclic electron transfer chains (Zhu et al, 2008(Zhu et al, , 2010Jurić et al, 2017). The total energy consumption is expressed in terms of (photosynthetically active) photons absorbed per stroma volume in unit time, as in Xiao et al (2016).…”

“…Spatial separation between PEPC and RubisCO aids the C 4 pump by providing increased diffusive resistance and essentially underpins single-cell C 4 efficacy (Jurić et al, 2017). However, it is not clear if such cell-scale spatial separation is strictly necessary.…”

“…However, their analysis assumed a relatively high conductance of the chloroplast envelope, the cell wall, and the plasmalemma (see Discussion). Due to a small spatial separation (∼ 1 µm) between the carboxylase and decarboxylase of the proposed C 4 pump (which is well below the threshold separation (∼ 10 µm) for cost efficient single-cell C 4 photosynthesis (Jurić et al, 2017)) the viability of a single-cell based system would be strongly influenced by the permeability of the chloroplast envelope since this determines the CO 2 leakage current. CO 2 permeabilities of biological barriers are uncertain, with estimates of chloroplast envelope permeability ranging over three orders of magnitude, 10 1 − 10 4 µm/s (Evans et al, 2009;Kaldenhoff et al, 2014).…”

“…The model we use is based on one previously constructed to study single-cell C 4 photosynthesis in Bienertia (Jurić et al, 2017). It focuses on the effect of intracellular geometry on the diffusive transport of photosynthetically relevant gasses (O 2 , CO 2 , and its hydrated form HCO − 3 ).…”

Methods of analysis of Chloroplast Genomes of C3, Kranz type C4 and Single Cell C4 photosynthetic members of Chenopodiaceae

“…In C 4 and CAM photosynthesis, CO 2 is rst captured by phosphoenolpyruvate carboxylase (PEPC) with synthesis of 4-carbon organic acids which are sequestered in a spatial manner in C 4 plants and a temporal manner in CAM plants. Decarboxylation of the 4-carbon organic acid generates a CO 2 -rich environment around Rubisco [29]. This mechanism suppresses the oxygenation reaction by Rubisco and the subsequent energetically-wasteful photorespiratory pathway.…”