Indirect Export of Reducing Equivalents From the Chloroplast to Resupply NADP for C3 Photosynthesis—Growing Importance for Stromal NAD(H)?

Abstract

Plant productivity greatly relies on a flawless concerted function of the two photosystems (PS) in the chloroplast thylakoid membrane. While damage to PSII can be rapidly resolved, PSI repair is complex and time-consuming. A major threat to PSI integrity is acceptor side limitation e.g., through a lack of stromal NADP ready to accept electrons from PSI. This situation can occur when oscillations in growth light and temperature result in a drop of CO2 fixation and concomitant NADPH consumption. Plants have evolved a plethora of pathways at the thylakoid membrane but also in the chloroplast stroma to avoid acceptor side limitation. For instance, reduced ferredoxin can be recycled in cyclic electron flow or reducing equivalents can be indirectly exported from the organelle via the malate valve, a coordinated effort of stromal malate dehydrogenases and envelope membrane transporters. For a long time, the NADP(H) was assumed to be the only nicotinamide adenine dinucleotide coenzyme to participate in diurnal chloroplast metabolism and the export of reductants via this route. However, over the last years several independent studies have indicated an underappreciated role for NAD(H) in illuminated leaf plastids. In part, it explains the existence of the light-independent NAD-specific malate dehydrogenase in the stroma. We review the history of the malate valve and discuss the potential role of stromal NAD(H) for the plant survival under adverse growth conditions as well as the option to utilize the stromal NAD(H) pool to mitigate PSI damage.

The NAPstar family of NADP redox state sensors highlights glutathione as the primary mediator of anti-oxidative electron flux

“…To assess the in vivo responsiveness of the sensors, we illuminated leaf tissue, reasoning that activation of photosynthesis triggers the exchange of redox equivalents between the chloroplast stroma and the cytosol by different metabolite shuttles ( Figure 4 ) [35]. Consecutive illumination of discs from mature Arabidopsis leaves, for periods of 5, 15, 30 and 60 min, each of which was followed by fluorimetric time-lapse measurements, revealed a pronounced reduction of the cytosolic NAD pool as monitored by Peredox ( Figure 4D ) [23].…”

Inactivation of mitochondrial complex IV inPhyscomitrium patensreveals the essential role of respiration in coordinating plants metabolism

“…The transfer of electrons from complex I to molecular oxygen through the alternative oxidase is much less efficient than the cytochrome pathway in ATP production, but it nevertheless ensures the consumption of reducing power, which is likely a pivotal function in photosynthetically active cells (Dahal et al, 2017; Krämer & Kunz, 2021; Moreno-García et al, 2022). It is well known that the photosynthetic electron transport produces NADPH in excess of ATP for CO 2 fixation, and multiple mechanisms exist to balance this ratio, such as cyclic and pseudo cyclic electron transport (Shikanai & Yamamoto, 2017).…”

Visible-light-mediated green synthesis of tertiary alcohols from dicarbonyl compounds and arylamines in water

“…3 Moreover, the nicotinamide-dependent enzyme (NADPH) is an important electron and hydride donor in the CO 2 reduction reaction, concurrently delivering the NADP radical immediate. 4 Inspired by these natural systems, we recently questioned whether a water-phase electron donor–acceptor (WEDA) mechanism upon visible-light irradiation could be exploited to synthesize sterically hindered tertiary alcohols through a green chemistry approach. Indeed, merging water as a medium with visible-light catalysis has received little attention so far in the field of sustainable chemical synthesis.…”