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.
"Its role in photoprotection has been suggested to be to quench triplet chlorophyll (Jahns & Holzwarth, 2012). b-carotene was also strongly associated with PRI and has been noted to quench singlet oxygen (Telfer, 2005). Together these major carotenoids undergo seasonal changes in concentration in evergreen leaves, presumably to protect various photosynthetic components from photodamage. "
[Show abstract][Hide abstract] ABSTRACT: In evergreens, the seasonal down-regulation and reactivation of photosynthesis is largely invisible and difficult to assess with remote sensing. This invisible phenology may be changing as a result of climate change. To better understand the mechanism and timing of these hidden physiological transitions, we explored several assays and optical indicators of spring photosynthetic activation in conifers exposed to a boreal climate.The photochemical reflectance index (PRI), chlorophyll fluorescence, and leaf pigments for evergreen conifer seedlings were monitored over 1 yr of a boreal climate with the addition of gas exchange during the spring.PRI, electron transport rate, pigment levels, light-use efficiency and photosynthesis all exhibited striking seasonal changes, with varying kinetics and strengths of correlation, which were used to evaluate the mechanisms and timing of spring activation. PRI and pigment pools were closely timed with photosynthetic reactivation measured by gas exchange.The PRI provided a clear optical indicator of spring photosynthetic activation that was detectable at leaf and stand scales in conifers. We propose that PRI might provide a useful metric of effective growing season length amenable to remote sensing and could improve remote-sensing-driven models of carbon uptake in evergreen ecosystems.
New Phytologist 01/2015; 206(1). DOI:10.1111/nph.13251 · 7.67 Impact Factor
"The observation may be explained as the conformation changes of β-carotene (II) pool, which tends to more perpendicular orientation respective to the membrane plane. Multiple photoprotective hypotheses have been established including Mn-mediated UV photoinactivation (Hakala et al., 2005; Ohnishi et al., 2005; Wei et al., 2011; Hou et al., 2013), cytochrome b-559 cyclic electron flow or cytochrome b-559 reversible interconvertion between the two redox forms (Thompson and Brudvig, 1988; Barber and De Las Rivas, 1993; Shinopoulos and Brudvig, 2012), and a β-carotene photooxidation (Telfer et al., 2003; Alric, 2005; Shinopoulos et al., 2014). The unidirectional photodamage of pheophytin in photosynthesis is discovered. "
"The levels of chlorophyll and carotenoids are directly involved in the photosynthetic apparatus activity and can induce modifications in values of chlorophyll fluorescence parameters (Havaux et al. 2000). Carotenoids participate in the photoprotection of the photosynthetic apparatus by neutralizing triplet chlorophyll and reactive oxygen species (Telfer 2005). Another example of biochemically active substances is plant phenolics, which are effective photoprotectors and scavengers of reactive oxygen species that may not directly increase the activity of the photosynthetic apparatus and photosynthetic carbon metabolism (Blokhina et al. 2003). "
[Show abstract][Hide abstract] ABSTRACT: Stem canker (blackleg) caused by fungus Leptosphaeria maculans/L. biglobosa is one of the most damaging diseases of oilseed winter rape crops. Some winter oilseed rape varieties (Brassica napus L. var. oleifera ‘Bojan’, ‘Lisek’, ‘Liclassic’) that differ in blackleg resistance have been chosen for the experiment. In all tested cultivars during growth on a medium with a fungal elicitor, a distinct reduction in the length of the stems, the roots and the entire length of the seedlings was observed. However, only in the case of the ‘Liclassic’ cultivar, fresh and dry weight were reversibly affected during elicitation. The cultivar ‘Liclassic’, recognized as blackleg mildly resistant, was characterized by the most efficient photosynthetic apparatus under toxin elicitation. The efficient adaptation of photosynthetic apparatus in this cultivar was accompanied by an increase in the content of phenolics, chlorophyll and carotenoids. Only for ‘Liclassic’, did most of the measured parameters of chlorophyll fluorescence (F
m′, ΦPSII, q
P and q
N) exhibit a statistically significant correlation with regard to the level of carotenoids. Therefore, in‘Liclassic’, the observed increase in carotenoid content seems to be a significant biochemical factor which can raise the efficiency of the photosynthetic apparatus under elicitation by Phoma lingam toxins.
Acta Physiologiae Plantarum 10/2013; 36(2). DOI:10.1007/s11738-013-1410-y · 1.58 Impact Factor
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