Abstract
An improvement in photosynthetic rate promotes the growth of crop plants. The sink-regulation of photosynthesis is crucial in optimizing nitrogen fixation and integrating it with carbon balance. Studies on these processes are essential in understanding growth inhibition in plants with ammonium () syndrome. Hence, we sought to investigate the effects of using nitrogen sources with different states of reduction (during assimilation of versus ) on the photosynthetic performance of Arabidopsis thaliana. Our results demonstrated that photosynthetic functioning during long-term nutrition was not disturbed and that no indication of photoinhibition of PSII was detected, revealing the robustness of the photosynthetic apparatus during stressful conditions. Based on our findings, we propose multiple strategies to sustain photosynthetic activity during limited reductant utilization for assimilation. One mechanism to prevent chloroplast electron transport chain overreduction during nutrition is for cyclic electron flow together with plastid terminal oxidase activity. Moreover, redox state in chloroplasts was optimized by a dedicated type II NAD(P)H dehydrogenase. In order to reduce the amount of energy that reaches the photosynthetic reaction centers and to facilitate photosynthetic protection during nutrition, non-photochemical quenching (NPQ) and ample xanthophyll cycle pigments efficiently dissipate excess excitation. Additionally, high redox load may be dissipated in other metabolic reactions outside of chloroplasts due to the direct export of nucleotides through the malate/oxaloacetate valve. Mitochondrial alternative pathways can downstream support the overreduction of chloroplasts. This mechanism correlated with the improved growth of A. thaliana with the overexpression of the alternative oxidase 1a (AOX1a) during nutrition. Most remarkably, our findings demonstrated the capacity of chloroplasts to tolerate syndrome instead of providing redox poise to the cells.
Highlights
Photosynthesis enables several anabolic reactions and provides energy for the metabolism and growth of plants
The major reactive oxygen species (ROS) produced in the chlETC might be O2 ̇, and the activity of superoxide dismutase (SOD) was higher in the plants nourished with NH+4 (Figure 1A)
In our previous study, prolonged NH+4 nutrition increased the redox state of Arabidopsis leaf tissue (Podgórska et al, 2013) and increased the ROS level in leaves (Podgórska et al, 2013; Podgórska et al, 2015)
Summary
Photosynthesis enables several anabolic reactions and provides energy for the metabolism and growth of plants. Unfavorable changes in the environment are quickly perceived by plants to minimize disturbances in electron transport flows Different stress factors, such as changes in light intensity or nutrient availability, affect the intracellular metabolism of plants, and fluctuations in energy or redox state develop in cells. The bioenergetically expensive version involves the nourishment of plants with nitrate (NO−3 ), which initially undergoes a two-step reduction In this case, NO−3 reduction is expected to consume up to 20% of the total reductants produced by photosynthetic light reactions (Noctor and Foyer, 1998) because of the activity of nitrite reductase (NiR) accepting 6é and of 2-oxoglutarate aminotransferase (GOGAT) accepting 2é. Most crop plants show strong growth inhibition and other toxicity symptoms (Podgórska and Szal, 2015; Liu and von Wirén, 2017) in response to NH+4 nutrition, making this condition largely considered to be a severe one
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