Inactivation Of PSII Reaction Centers Research Articles

Action of the fungal compound citrinin, a bioherbicide candidate, on photosystem II.

Bioherbicides are becoming more attractive as safe weed control tools towards sustainable agriculture. Natural products constitute an important source chemicals and chemical leads for discovery and development of novel pesticide target sites. Citrinin is a bioactive compound produced by fungi of the genera Penicillium and Aspergillus. However, its physiological-biochemical mechanism as a phytotoxin remains unclear. Citrinin causes visible leaf lesions on Ageratina adenophora similar to those produced by the commercial herbicide bromoxynil. Phytotoxicity bioassay tests using 24 plant species confirmed that citrinin has a broad activity spectrum and therefore has potential as a bioherbicide. Based on chlorophyll fluorescence studies, citrinin mainly blocks PSII electron flow beyond plastoquinone QA at the acceptor side, resulting in the inactivation of PSII reaction centers. Furthermore, molecular modeling of citrinin docking to the A. adenophora D1 protein suggests that it binds to the plastoquinone QB site by a hydrogen bond between the O1 hydroxy oxygen atom of citrinin and the histidine 215 of the D1 protein, the same way as classical phenolic PSII herbicides do. Finally, 32 new citrinin derivatives were designed and sorted according to free energies on the basis of the molecular model of an interaction between the citrinin molecule and the D1 protein. Five of the modeled compounds had much higher ligand binding affinity within the D1 protein compared with lead compound citrinin. Citrinin is a novel natural PSII inhibitor that has the potential to be developed into a bioherbicide or utilized as a lead compound for discovery of new derivatives with high herbicidal potency. © 2023 Society of Chemical Industry.

Polyphasic OKJIP Chlorophyll a Fluorescence Transient in a Landrace and a Commercial Cultivar of Sweet Pepper (Capsicum annuum, L.) under Long-Term Salt Stress.

In a soilless long-term salt-stress experiment, we tested the differences between the commercial sweet pepper cultivar “Quadrato d’Asti” and the landrace “Cazzone Giallo” in the structure and function of PSII through the JIP test analysis of the fast chlorophyll fluorescence transients (OKJIP). Salt stress inactivated the oxygen-evolving complex. Performance index detected the stress earlier than the maximum quantum yield of PSII, which remarkably decreased in the long term. The detrimental effects of salinity on the oxygen evolving-complex, the trapping of light energy in PSII, and delivering in the electron transport chain occurred earlier and more in the landrace than the cultivar. Performance indexes decreased earlier than the maximum quantum yield of PSII. Stress-induced inactivation of PSII reaction centers reached 22% in the cultivar and 45% in the landrace. The resulted heat dissipation had the trade-off of a correspondent reduced energy flow per sample leaf area, thus an impaired potential carbon fixation. These results corroborate the reported higher tolerance to salt stress of the commercial cultivar than the landrace in terms of yield. PSII was more affected than PSI, which functionality recovered in the late of trial, especially in the cultivar, possibly due to heat dissipation mechanisms. This study gives valuable information for breeding programs aiming to improve tolerance in salt stress sensitive sweet pepper genotypes.

Classification and characteristics of heat tolerance in Ageratina adenophora populations using fast chlorophyll a fluorescence rise O-J-I-P

Croftonweed originated from Mexico is a worldwide notorious invasive weed. The objectives of this study were to screen heat tolerance in different croftonweed populations, determine the effect of heat stress on two photosystems and probe the mechanism of acquired heat tolerance. According to conventional tests of plant injury and fast chlorophyll fluorescence rise kinetics, four different croftonweed populations collected from South China were successfully classified into three categories by exposing whole plants to heat treatment at 40°C: sensitive, intermediate, tolerant. Evidence from the JIP-test indicated that inhibition of the oxygen evolution complexes (OEC) and inactivation of PSII reaction centers (RCs) were the primary cause of heat damage. In mild heat stress (<40°C), slightly damaged the OEC without creating a visible K-step in the fluorescence rise OJIP curve. In moderate heat stress or stronger (≥40°C), a pronounced K-step due to irreversible severe damage on the OEC occurred. Additionally, inactivation of PSII RCs, down-regulation of energetic connectivity of PSII units, destruction of PSII antenna architecture, losing overall photosynthetic activity of PSII, increase of PSI activity also took place. Furthermore, the tolerant population had lesser damage degree on photosynthetic capacity relative to intermediate and sensitive populations. Finally, a reliable model, based on the most sensitive parameter PIABS and VK as a characteristic parameter for heat stress, is presented for ranking and identifying heat tolerance in different croftonweed populations. The heat sensitivity index (HSI) is also introduced as an indicator of plant heat sensitivity. The smaller the HSI value is, the higher the level of tolerance to heat stress is. We also found that the tolerance degree of four croftonweed populations to heat stress is significantly correlated to the extreme high temperature. This indicates that acquired heat tolerance in croftonweed populations results from plant adaptation to ambient high temperatures. Acquirement of heat tolerance confers a possible risk for croftonweed to spread further to currently hotter areas.

Early stage toxicity of excess copper to photosystem II of Chlorella pyrenoidosa–OJIP chlorophyll a fluorescence analysis

Acute toxicity of excess Cu on the photosynthetic performance of Chlorella pyrenoidosa was examined by using chlorophyll a fluorescence transients and JIP-test after exposure to elevated Cu concentrations for a short time period. High Cu concentration resulted in a significant suppression in photosynthesis and respiration. The absorption flux (ABS/RC) per PSII reaction center increased with increasing Cu concentration, but the electron transport flux (ET0/RC) decreased. Excess Cu had an insignificant effect on the trapping flux (TR0/RC). The decline in the efficiency, with which a trapped exciton can move an electron into the electron transport chain further than Q(A)-(Psi0), the maximal quantum yield of primary photochemistry (Phi0), and the quantum yield of electron transport (Phi(E)0) were also observed. The amount of active PSII reaction centers per excited cross section (RC/CS) was also in consistency with the change of photosynthesis when cells were exposed to excess Cu concentration. JIP-test parameters had a good linear relationship with photosynthetic O2 evolution. These results suggested that the decrease of photosynthesis in exposure to excess Cu may be a result of the inactivation of PSII reaction centers and the inhibition of electron transport in the acceptor side.

Xanthophyll Cycle and Inactivation of Photosystem II Reaction Centers Alleviating Reducing Pressure to Photosystem I in Morning Glory Leaves under Short‐term High Irradiance

Abstract Under 30‐min high irradiance (1500 μmol m−2 s−1), the roles of the xanthophyll cycle and D1 protein turnover were investigated through chlorophyll fluorescence parameters in morning glory (Ipomoea setosa) leaves, which were dipped into water, dithiothreitol (DTT) and lincomycin (LM), respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non‐photochemical quenching (NPQ) was inhibited greatly and the oxidation level of P700 (P700+) was the lowest one. However, the maximal photochemical efficiency of PSII (Fv/Fm) in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700+ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross‐section (RC/CSo), maximal trapping rate in a PSII cross‐section (TRo/CSo), electron transport in a PSII cross‐section (ETo/CSo) and the highest of 1‐qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSI. However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1‐qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700+ under high irradiance.

Photosynthetic pigments, photosynthesis and plastid ultrastructure in RbcS antisense DNA mutants of tobacco (Nicotiana tabacum).

RbcS antisense DNA mutants of tobacco have reduced amounts of ribulose bisphosphate carboxylase oxygenase (Rubisco). We found that carotenoid and chlorophyll contents decrease in parallel as Rubisco is decreased, however, pigment levels are not significantly altered until Rubisco levels are reduced sharply. The mutants have normal Chl a/Chl b ratios and normal plastid ultrastructures, suggesting that reductions in Rubisco do not dramatically alter the composition of the thylakoid membranes. Nevertheless, chlorophyll fluorescence measurements, in which developmentally homogenous leaves were sampled, showed that there is reduced photosynthetic capacity of PSII and an enhanced photosensitivity in the mutants, especially in transgenics with severe reductions in Rubisco content. Support for this conclusion comes from several observations: 1) light saturation occurs at a lower light intensity in the mutants, resulting in an earlier closure of PS II (lower photochemical quenching); 2) the mutants have reduced photosynthetic efficiency (lower deltaF/Fm'); and 3) the mutants have a slower recovery of Fv/Fm. We found that acclimation to increasing light intensies in the mutants appears to involve an enhanced inactivation of PSII reaction centers as well as an increased activation of photoprotective mechanisms, notably an engagement of the xanthophyll cycle at lower than normal light intensities. We conclude that the photosensitivity of the antisense mutants is due, in part, to a limitation in Rubisco activation state.

Effects of high-temperature treatments on a thermophilic cyanobacterium Synechococcus vulcanus.

Effects of high-temperature treatments on a thermophilic cyanobacterium, Synechococcus vulcanus, were studied, and the following results were obtained. (1) Oxygen evolution and the PSII photochemical reaction were the most sensitive sites and started to be inactivated at temperatures slightly higher than the cultivating temperature. (2) The decrease in the fluorescence Fv value reflected the inactivation of the charge separation reaction of PSII as well as that of the oxygen evolution reaction. (3) The dark fluorescence level, Fo, showed an increase at around 70 degrees C, which was partially reversed by further incubation at 50 degrees C. This increase reflected the inactivation of PSII reaction centers and probably dissociation of phycobilisomes from the PSII reaction center complexes. (4) At higher temperatures, phycobiliproteins disassembled and denatured in a pH-dependent manner, causing a large Fo decrease. (5) Cell membranes became leaky to low-molecular-weight substances at around 72 degrees C. (6) Inhibition of growth of the cells was recognized when the cells were pretreated at temperatures higher than 72 degrees C. Reversibility of the high-temperature effects and relationship between viability of the cells and the degradation of the cell membranes are discussed.

Photoinhibition in outdoor Spirulina platensis cultures assessed by polyphasic chlorophyll fluorescence transients

Photoinhibition in outdoor cultures of Spirulina platensis was studied by measuring the polyphasic rise of chlorophyll fluorescence transients, which provide information on the primary photochemistry of PSII. The maximum efficiency of PSII photochemustry (Fv/Fm) declined in response to daily increasing irradiance and recovered as daily irradiance decreased. The greatest inhibition (15%) in Fv/Fm was observed at 12:00 hr which responded to the highest irradiance. The absorption flux, the trapping flux, and the electron transport flux per PSII reaction center increased in response to daily increasing irradiance and decreased as irradiance decreased. The daily change in the concentration of PSII reaction centers followed the same pattern as Fv/Fm. However, no significant changes in the probability of electron transport beyond QA (Ψo) were observed during the day. The results suggest that the decrease in Fv/Fm induced by photoinhibition in outdoor Spirulina cultures was a result of the inactivation of PSII reaction centers. The results also suggest that the measurement of polyphasic fluorescence transients is a powerful tool to study the mechanism of photoinhibition in outdoor Spirulina cultures and to screen strains for photoinhibition tolerance.

Development of photoautotrophy and photoinhibition of Gardenia jasminoides plantlets during micropropagation

This paper reports on the fast fluorescence responses of Gardenia jasminoides Ellis plantlets, at two successive stages (shoot multiplication and root induction) of culture in vitro. We test whether plantlets in vitro suffer photoinhibition during culture and whether the degree of photoautotrophy of these mixotrophic plantlets has any effect on the extent of photoinhibitory impairment. In this regard the effects of different sucrose levels in the medium and PPFD during growth on the development of photoautotrophy and the extent of photoinhibition were evaluated. Plantlets were grown under low, intermediate, and high (50, 100, and 300 μmol m-2 s-1) PPFD, and at 3 different sucrose concentrations (0.5, 1.5, and 3.0%, w/v) in the medium, during shoot multiplication. During root induction the same growth conditions were assayed except for the high PPFD. The development of photoautotrophy was assessed via the difference between the stable carbon isotope composition of sucrose used as heterotrophic carbon source and that of leaflets grown in vitro. Plantlets from root induction showed more developed photoautotrophy than those from shoot multiplication. For both stages the low-sucrose medium stimulated the photoautotrophy of plantlets in vitro. In addition, intermediate PPFD induced photoautotrophy during shoot multiplication. For plantlets of both culture stages at the lowest PPFD no photoinhibition occurred irrespective of the sucrose concentration in media. However, during the shoot multiplication stage chlorophyll fluorescence measurements showed a decrease in Fv/Fm and in t1/2 as growing PPFD increased, indicating photoinhibitory damage. The decline of Fv/Fm was caused mostly by an increase in Fo, indicating the inactivation of PSII reaction centers. However plantlets growing under low sucrose showed reduced susceptibility to photoinhibition. During root induction, only plantlets cultured with high sucrose showed a decrease in Fv/Fm as PPFD increased, although t1/2 remained unchanged. In this case, the decline of Fv/Fm was mostly due to a decrease in Fm, which indicates increased photoprotection rather than occurrence of photodamage. Therefore, growth in low-sucrose media had a protective effect on the resistance of PSII to light stress. In addition, plantlets were more resistant to photoinhibition during root induction than during shoot multiplication. Results suggest that increased photoautotrophy of plantlets reduces susceptibility to photoinhibition during gardenia culture in vitro.

Leaf structure and patterns of photoinhibition in two neotropical palms in clearings and forest understory during the dry season

We studied the leaf structural, water status, and fast fluorescence responses of two palms, Socratea exorrhiza and Scheelea zonensis, under natural dry season conditions in a clearing (high light [HL] palms) and the forest understory (low light [LL] palms) on Barro Colorado Island, Panama. HL‐Socratea leaves were more shade‐adapted, less xeromorphic, and more strongly affected by drought than HL‐Scheelea. Fv/Fm (the ratio of variable to maximum chlorophyll fluorescence) and t½ (the half‐rise time of Fm) was lower in HL‐leaves of both species, indicating photoinhibition. In HL‐Scheelea, the light‐induced reduction of Fv/Fm was much less than in HL‐Socratea, and Fv/Fm recovered completely overnight. Patterns of relative water content, specific leaf dry weight, stable carbon isotope composition, and leaf conductance suggest that increased drought resistance in Scheelea reduces susceptibility to photoinhibition. An increase in Fo indicated the inactivation of PSII reaction centers in HL‐Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL‐Socratea are consistent with photoinhibition. Under LL, the species showed no appreciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of Fv/Fm in HL‐plants relative to LL‐plants on both leaf surfaces, particularly on the lower surface, due to a decrease of Fm in both surfaces and an increase in F., of lower surface. For both species, Fo for the lower surfaces of HL‐plants was higher and t½ was markedly lower than for the upper surface, as is typical for shade‐adapted leaves. Xeromorphic leaf structure may reduce susceptibility to photoinhibition during the dry season. Drought‐enhanced photoinhibition could limit the ability of some species to exploit treefall gaps.

Leaf Structure and Patterns of Photoinhibition in Two Neotropical Palms in Clearings and Forest Understory During the Dry Season

We studied the leaf structural, water status, and fast fluorescence responses of two palms, Socratea exorrhiza and Scheelea zonensis, under natural dry season conditions in a clearing (high light [HL] palms) and the forest understory (low light [LL] palms) on Barro Colorado Island, Panama. HL-Socratea leaves were more shade-adapted, less xeromorphic, and more strongly affected by drought than HL-Scheelea. Fv/Fm (the ratio of variable to maximum chlorophyll fluorescence) and t½ (the half-rise time of Fm) was lower in HL-leaves of both species, indicating photoinhibition. In HL-Scheelea, the light-induced reduction of Fv/Fm was much less than in HL-Socratea, and Fv/Fm recovered completely overnight. Patterns of relative water content, specific leaf dry weight, stable carbon isotope composition, and leaf conductance suggest that increased drought resistance in Scheelea reduces susceptibility to photoinhibition. An increase in Fo indicated the inactivation of PSII reaction centers in HL-Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL-Socratea are consistent with photoinhibition. Under LL, the species showed no appreciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of Fv/Fm in HL-plants relative to LL-plants on both leaf surfaces, particularly on the lower surface, due to a decrease of Fm in both surfaces and an increase in F., of lower surface. For both species, Fo for the lower surfaces of HL-plants was higher and t½ was markedly lower than for the upper surface, as is typical for shade-adapted leaves. Xeromorphic leaf structure may reduce susceptibility to photoinhibition during the dry season. Drought-enhanced photoinhibition could limit the ability of some species to exploit treefall gaps.