Adjustments of photosystem stoichiometry in chloroplasts improve the quantum efficiency of photosynthesis.

Abstract

The efficiency of photosynthetic electron transport depends on the coordinated interaction of photosystem II (PSH) and photosystem I (PSI) in the electron-transport chain. Each photosystem contains distinct pigment-protein complexes that harvest light from different regions ofthe visible spectrum. The light energy is utilized in an endergonic electrontransport reaction at each photosystem. Recent evidence has shown a large variability in the PSI/PSI stoichiometry in plants grown under different environmental irradiance conditions. Results in this work are consistent with the notion of a dynamic, rather than static, thylakoid membrane in which the stoichiometry of the two photosystems is adjusted and optimized in response to different light quality conditions. Direct evidence is provided that photosystem stoichiometry adjustments in chloroplasts are a compensation strategy designed to correct unbalanced absorption of light by the two photosystems. Such adjustments allow the plant to maintain a high quantum efficiency of photosynthesis under diverse light quality conditions and constitute acclimation that confers to plants a significant evolutionary advantage over that of a fixed photosystem stoichiometry in thylakoid membranes.apparatus, given the contrasting light environments in different plant ecosystems (6-8) and the fact that substantially different pigments absorb light for PSI and for PSII in the thylakoid membrane of oxygenic photosynthesis.These findings suggested that higher plants and algae possess regulatory mechanisms that enable chloroplasts to adjust and optimize the function of the light reactions under diverse conditions. Recently, evidence in the literature suggested long-term adjustments in photosystem stoichiometry as a plant response to different light-quality conditions during growth (9, 10). Changes in photosystem stoichiometry, occurring in response to different light qualities, may be a compensation reaction in the thylakoid membrane, serving to correct uneven absorption of light by the two photosystems. However, the effect of these adjustments on the quantum yield of photosynthesis in higher plants has not been investigated before. This work provides direct evidence that adjustments of photosystem stoichiometry in chloroplasts permit the plant to retain a quantum efficiency of photosynthesis near the theoretical maximum.Energy transduction in photosynthesis depends on the coordinated electron turnover by two photosystems in a linear electron-transport process. Photosystem II (PSII) is involved in a light-dependent oxidation of water and reduction of plastoquinone. Electrons from plastohydroquinone reach photosystem I (PSI) via the cytochrome b6-f complex and plastocyanin. PSI is involved in a light-dependent electron transport to ferredoxin and to NADP'. Each photosystem is associated with distinct pigment-protein complexes, which absorb solar radiation and transfer excitation energy to the photochemical reaction center.In almost every photosynthetic organism, light-harvesting pi...

Overexpression of chlorophyllide a oxygenase (CAO) enlarges the antenna size of photosystem II in Arabidopsis thaliana

“…A reduction in the Chl a : b ratio is accomplished not only by enlargement of the antenna size but also by an increase in the PSII: PSI ratio (Chow et al, 1990), since PSII binds more Chl b than PSI. Thus, we examined the amounts of proteins in the core complex of PSII and in the peripheral antenna complex by the green-gel system and by immunoblotting.…”

“…Recent studies using mutants defective in non-photochemical quenching have partially elucidated the mechanism that revealed the involvement of PsbS protein (Li et al, 2000). In the other strategy, the size of the photosynthetic antenna, the number of Chls associated with a reaction center, is regulated in response to the light environment (Chow et al, 1990;Morrissey et al, 1989). This mechanism is more important in long-term acclimation (hours to days) to light than non-photochemical quenching, which functions within minutes.…”

Dynamic Acclimation of Photosynthesis Increases Plant Fitness in Changing Environments

“…They have more chlorophyll-containing light-harvesting proteins relative to light-using enzymes involved in electron transport and metabolism, meaning that photosynthesis saturates with light at a lower irradiance. Plants can also adjust the relative proportions of the different photosystems to suit the light quality they experience (Chow et al, 1990;Walters andHorton, 1995a, 1995b).…”

Chloroplast redox signals: how photosynthesis controls its own genes

“…By contrast, photosystem stoichiometry adjustment is a long-term response (occurring in hours or days) in which the relative number of photosystems is changed in favour of one that is rate limiting. In this way, the efficiency of photosynthesis is enhanced [18]. Both processes are regulated by the redox state of the Box 1.…”