Photosynthetic Linear Electron Flow Drives CO2 Assimilation in Maize Leaves.

Ginga Shimakawa,Chikahiro Miyake

doi:10.3390/ijms22094894

Abstract

Photosynthetic organisms commonly develop the strategy to keep the reaction center chlorophyll of photosystem I, P700, oxidized for preventing the generation of reactive oxygen species in excess light conditions. In photosynthesis of C4 plants, CO2 concentration is kept at higher levels around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) by the cooperation of the mesophyll and bundle sheath cells, which enables them to assimilate CO2 at higher rates to survive under drought stress. However, the regulatory mechanism of photosynthetic electron transport for P700 oxidation is still poorly understood in C4 plants. Here, we assessed gas exchange, chlorophyll fluorescence, electrochromic shift, and near infrared absorbance in intact leaves of maize (a NADP-malic enzyme C4 subtype species) in comparison with mustard, a C3 plant. Instead of the alternative electron sink due to photorespiration in the C3 plant, photosynthetic linear electron flow was strongly suppressed between photosystems I and II, dependent on the difference of proton concentration across the thylakoid membrane (ΔpH) in response to the suppression of CO2 assimilation in maize. Linear relationships among CO2 assimilation rate, linear electron flow, P700 oxidation, ΔpH, and the oxidation rate of ferredoxin suggested that the increase of ΔpH for P700 oxidation was caused by the regulation of proton conductance of chloroplast ATP synthase but not by promoting cyclic electron flow. At the scale of intact leaves, the ratio of PSI to PSII was estimated almost 1:1 in both C3 and C4 plants. Overall, the photosynthetic electron transport was regulated for P700 oxidation in maize through the same strategies as in C3 plants only except for the capacity of photorespiration despite the structural and metabolic differences in photosynthesis between C3 and C4 plants.

Highlights

In chloroplasts of plant leaves, photosynthetic CO2 assimilation is driven in the Calvin– Benson cycle utilizing NADPH and ATP produced by light energy [1]
Since cyclic electron flow around PSI (CEF) is mediated by the electron transport from Fd− to PQ, theoretically it is capable of pumping H+ from the stroma to the lumen of the thylakoid membrane in the Q-cycle; this results in an additional ATP production that is not linked to NADP+ reduction [22]
We analyzed in vivo photosynthetic parameters at a constant light intensity and different CO2 partial pressures to investigate the effects of limitation of electron sink on the photosynthetic electron transport

Summary

Introduction

In chloroplasts of plant leaves, photosynthetic CO2 assimilation is driven in the Calvin– Benson cycle utilizing NADPH and ATP produced by light energy [1]. In the Calvin–Benson cycle, ribulose 1,5bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the carboxylation of RuBP to produce two molecules of 3-phosphoglycerate (3-PGA), which are metabolized in the Calvin–Benson cycle with NADPH and ATP. In C3 plants, P700 oxidation is supported by the suppression of electron transport in the Cyt b6/f complex, dependent on the difference of proton concentration across thylakoid membranes (∆pH) [16,17,18]. In C4 plants, the ratio of PSI to PSII is much higher in isolated bundle sheath cells than in the mesophyll cells, which gives the hypothesis that

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