No abstract

No abstract

Dynamic acclimation of the photosynthetic apparatus in response to environmental cues, particularly light quantity and quality, is a widely-observed and important phenomenon which contributes to the tolerance of plants against stress and helps to maintain, as far as possible, optimal photosynthetic efficiency and resource utilization. This mini-review represents a scrutiny of a number of possible photoreceptors (including the two photosystems acting as light sensors) and signal transducers that may be involved in producing acclimation responses. We suggest that regulation by signal transduction may be effected at each of several possible points, and that there are multiple regulatory mechanisms for photosynthetic acclimation.

lhe relationship between the susceptibility of photosystem II (PSII) to photoinhibition in vivo and the rate of degradation of the D1 protein of the PSll reaction center heterodimer was investigated in leaves from pea plants (Pisum safivum 1. cv Creenfeast) grown under widely contrasting irradiances. lhere was an inverse linear relationship between the extent of photoinhibition and chlorophyll (Chl) a/b ratios, with low-light leaves being more susceptible to high light. In the presence of the chloroplast-encoded protein synthesis inhibitor lincomycin, the differential sensitivity of the various light-acclimated pea leaves to photoinhibition was largely removed, demonstrating the importance of D1 protein turnover as the most crucial mechanism to protect against photoinhibition. In the differently light-acclimated pea leaves, the rate of D1 protein degradation (measured from [35S]methionine pulse-chase experiments) increased with increasing incident light intensities only if the light was not high enough to cause photoinhibition in vivo. Under moderate illumination, the rate constant for D1 protein degradation corresponded to the rate constant for photoinhibition in the presence of lincomycin, demonstrating a balance between photodamage to D1 protein and subsequent recovery, via D1 protein degradation, de novo synthesis of precursor D1 protein, and reassembly of functional PSII. In marked contrast, in light sufficiently high to cause photoinhibition in vivo, the rate of D1 protein degradation no longer increased concomitantly with increasing photoinhibition, suggesting that the rate of D1 protein degradation is playing a regulatory role. lhe extent of thylakoid stacking, indicated by the Chl a/b ratios of the differently lightacclimated pea leaves, was linearly related to the half-life of the D1 protein in strong light. We conclude that photoinhibition in vivo occurs under conditions in which the rate of D1 protein degradation can no longer be enhanced to rapidly remove irreversibly damaged D l protein. We suggest that low-light pea leaves, with more stacked membranes and less stroma-exposed thylakoids, ate more susceptible to photoinhibition in vivo mainly due to their slower rate of D1 protein degradation under sustained high light and their slower repair cycle of the photodamaged PSll centers.Photoinhibition of PSII is a phenomenon that occurs in a11 oxygenic photosynthetic organisms under high irradiance (Powles, 1984;Prasil et al., 1992). Although light is the driving force of PSII, it also inactivates electron transport and destroys structural components of the PSII reaction center. Susceptibility of plants to photoinhibition under any given '

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...

The photosynthetic apparatus of plants responds to changing light quantity and quality with coordinated changes in both the light-harvesting antennae of the photosystems and the amounts of electron transport components and ATP synthase. These compositional modulations are accompanied by changes in thylakoid membrane organisation and photosynthetic capacity. It is now clear that there is a dynamic continuum of organisation and function of the photosynthetic apparatus from the appressed granal and non-appressed stroma thylakoids within a chloroplast, to different chloroplasts within a leaf, to leaves within and between species. While it is very unlikely that there is a unique solution to photosynthesis in the sun or shade, substantial changes in composition, and hence thylakoid membrane organisation and function, are elicited as part of sun/shade responses.

Influx of solar UV-B radiation (280-320 nm) will probably increase in the future due to depletion of stratospheric ozone. In plants, there are several targets for the deleterious UV-B radiation, especially the chloroplast. This review summarizes the early effects and responses of low doses of UV-B at the molecular level. The DNA molecules of the plant cells are damaged by UV due to the formation of different photoproducts, such as pyrimidine dimers, which in turn can be combatted by specialized photoreactivating enzyme systems. In the chloroplast, the integrity of the thylakoid membrane seems to be much more sensitive than the activities of the photosynthetic components bound within. However, the decrease of mRNA transcripts for the photosynthetic complexes and other chloroplast proteins are among very early events of UV-B damage, as well as protein synthesis. Other genes, encoding defence-related enzymes, e.g., of the flavonoid biosynthetic pathway, are rapidly up-regulated after commencement of UV-B exposure. Some of the cis-acting nucleotide elements and trans-acting protein factors needed to regulate the UV-induced expression of the parsley chalcone synthase gene are known.

The obligate shade plant, Tradescantia albiflora Kunth grown at 50 μmol photons · m(-2) s(-1) and Pisum sativum L. acclimated to two photon fluence rates, 50 and 300 μmol · m(-2) · s(-1), were exposed to photoinhibitory light conditions of 1700 μmol · m(-2) · s(-1) for 4 h at 22° C. Photosynthesis was assayed by measurement of CO2-saturated O2 evolution, and photosystem II (PSII) was assayed using modulated chlorophyll fluorescence and flash-yield determinations of functional reaction centres. Tradescantia was most sensitive to photoinhibition, while pea grown at 300 μmol · m(-2) · s(-1) was most resistant, with pea grown at 50 μmol · m(-2) · s(-1) showing an intermediate sensitivity. A very good correlation was found between the decrease of functional PSII reaction centres and both the inhibition of photosynthesis and PSII photochemistry. Photoinhibition caused a decline in the maximum quantum yield for PSII electron transport as determined by the product of photochemical quenching (qp) and the yield of open PSII reaction centres as given by the steady-state fluorescence ratio, F'vF'm, according to Genty et al. (1989, Biochim. Biophys. Acta 990, 81-92). The decrease in the quantum yield for PSII electron transport was fully accounted for by a decrease in F'vF'm, since qp at a given photon fluence rate was similar for photoinhibited and noninhibited plants. Under lightsaturating conditions, the quantum yield of PSII electron transport was similar in photoinhibited and noninhibited plants. The data give support for the view that photoinhibition of the reaction centres of PSII represents a stable, long-term, down-regulation of photochemistry, which occurs in plants under sustained high-light conditions, and replaces part of the regulation usually exerted by the transthylakoid ΔpH gradient. Furthermore, by investigating the susceptibility of differently lightacclimated sun and shade species to photoinhibition in relation to qp, i.e. the fraction of open-to-closed PSII reaction centres, we also show that irrespective of light acclimation, plants become susceptible to photoinhibition when the majority of their PSII reaction centres are still open (i.e. primary quinone acceptor oxidized). Photoinhibition appears to be an unavoidable consequence of PSII function when light causes sustained closure of more than 40% of PSII reaction centres.