Too much light? How beta-carotene protects the photosystem II reaction centre.
ABSTRACT The photosystem II reaction centre of all oxygenic organisms is subject to photodamage by high light i.e. photoinhibition. In this review I discuss the reasons for the inevitable and unpreventable oxidative damage that occurs in photosystem II and the way in which beta-carotene bound to the reaction centre significantly mitigates this damage. Recent X-ray structures of the photosystem II core complex (reaction centre plus the inner antenna complexes) have revealed the binding sites of some of the carotenoids known to be bound to the complex. In the light of these X-ray structures and their known biophysical properties it is thus possible to identify the two beta-carotenes present in the photosystem II reaction centre. The two carotenes are both bound to the D2 protein and this positioning is discussed in relation to their ability to act as quenchers of singlet oxygen, generated via the triplet state of the primary electron donor. It is proposed that their location on the D2 polypeptide means there is more oxidative damage to the D1 protein and that this underlies the fact that this latter protein is continuously re-synthesised, at a far greater rate than any other protein involved in photosynthesis. The relevance of a cycle of electrons around photosystem II, via cytochrome b(559), in order to re-reduce the beta-carotenes when they are oxidised and hence restore their ability to quench singlet oxygen, is also discussed.
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ABSTRACT: Sunburn is a physiological disorder of apples and other fruit species caused by excess solar radiation. Damage occurs in practically all growing regions of the world, causing severe crop loss every year. Direct factors required for induction of the three currently-known types of sunburn (i.e., sunburn necrosis, sunburn browning, and photooxidative sunburn) include excess radiant heating and/or exposure to excess sunlight. Several other factors (e.g., relative humidity, wind velocity, acclimation of fruit, and cultural management practices), which alone cannot induce sunburn damage, indirectly influence the induction of sunburn by interacting with the direct factors to influence the appearance and severity of the symptoms. Sunburn affects apple fruit at many levels; it causes structural and morphological changes, alters pigment composition, influences adaptive mechanisms, impairs photosynthesis, and consequently decreases fruit quality. Fruits employ multiple physiological and biochemical mechanisms as complex defense systems to minimize damage. Photoprotective pigments, antioxidant enzymes and metabolites, heat-shock proteins, and the xanthophyll cycle help mitigate damage, but are often inadequate under field conditions to fully protect from sunburn. Quality loss significantly affects postharvest behavior, marketing and consumer acceptance of fruit. Internal fruit quality (e.g., firmness, soluble solids concentration, and titratable acidity) is affected by sunburn, and changes in these traits continue during cold storage. Sunburn-related disorders (e.g., sunburn scald in ‘Granny Smith’ and ‘Fuji’ stain) can appear in cold storage. There are several methods with various modes of action (e.g., climate ameliorating techniques, and sunburn suppressants) available to growers to decrease sunburn under field conditions. At the end of this review, the potential impact of a changing climate on sunburn incidence is considered, as both UV-B radiation and temperature are projected to change. Finally, several topics that need further research are discussed.Critical Reviews in Plant Sciences 11/2012; 31(6):455-504. DOI:10.1080/07352689.2012.696453 · 5.29 Impact Factor
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ABSTRACT: In the present study, we examined the photoprotective mechanisms of the homoiochlorophyllous desiccation-tolerant (DT) Haberlea rhodopensis Friv. (Gesneriaceae) during the desiccation-rehydration cycle in its natural understory shaded and sunny habitats within drought prone regions. The integration of classical analysis of chlorophyll fluorescence with a detailed analysis of energy partitioning showed a re-adjustment in the function and extent of the different components of energy management in photosystem II (PSII) depending on the degree of dehydration. At mild dehydration, the non-photochemical quenching by active centres (NPQ) played a relevant role in preventing photoinhibition, while under the photoinhibitory conditions occurring when dehydration was severe, the light-dependent quenching by inactive centres provided strong photoprotection. These analyses of the photosynthetic and PSII functionality together with measurements of carotenoid changes showed that the photoprotective mechanisms of this resurrection species were also affected by growth irradiance. Plants growing in understory shaded habitats were mainly protected against dehydration by the mechanism of thermal re-emission by inactive PSII reaction centres together with the relevant contribution of an operative xanthophyll-cycle involved in a quenching process triggered by dehydration itself. Conversely, plants growing in sunny open environments relied more on the quenching mechanism in the light-harvesting antennae. One additional role of carotenoids, in particular of β-carotene, in dehydration tolerance of H. rhodopensis may well be quenching of ROS, a mechanism that could occur in homoiochlorophyllous resurrection plants as a consequence of the retained chlorophylls and photosynthetic apparatus, especially when plants grow in open sunny habitats.Environmental and Experimental Botany 01/2015; 113. DOI:10.1016/j.envexpbot.2015.01.007 · 3.00 Impact Factor
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ABSTRACT: We assessed whether the cow1 mutant defects are associated with growth of Tos17 and T-DNA insertional rice in blue light (BL). Growth of oscow1 mutants which encoded a member of the YUCCA protein family was retarded in BL. Root to shoot ratios of the mutants were reduced about 2 times lower in the absence of NAA and about 2.5 times lower in the presence of NAA; the shoot growth was not significantly changed by NAA addition. Photosynthetic activity of the mutants was however inhibited in high light. Pigment analysis showed significant difference between wild-type (Chl a:b = 3.02) and mutants (3.84). Carotenoid contents of the mutants were also decreased considerably, implying the involvement of cow1 in pigment formation. These findings lead us to suggest that the growth retardation of oscow1 mutant plants by BL results from the difference of photosynthetic activity in part.12/2010; 37(4). DOI:10.5010/JPB.2010.37.4.465