Nitric Oxide Alleviates Photochemical Damage Induced by Cadmium Stress in Pea Seedlings

Cadmium (Cd), a life threatening hazardous heavy metal is abundant in nature. Cd amounts are greater in leaves than other plant parts, and it shows considerable effects on photosynthesis. Nitric oxide (NO), a free radical present in living organisms, is now known as an important signaling molecule playing various physiological processes in plants. In this study, the possible ameliorative effect of NO on photosynthesis was examined on pea seedlings grown under Cd stress. Results showed that chlorophyll, net photosynthetic rate, transpiration rate, stomatal conductance, photochemical efficiency of Photosystem II and Photosystem I decreased, and Fo and non-photochemical parameters for PSII and PSI significantly increased due to Cd stress. This suggests that Cd affects the photochemistry efficiency at both the PSII and PSI levels. Nitric oxide supplementation through SNP ameliorated Cd stress by enhancing all the above mentioned parameters but causing a reduction in the Fo, and non-photochemical parameters of PSII and PSI in pea plants. These data indicate that the exogenous application of NO was useful in mitigating Cd-induced damage to photosynthesis in pea seedling.     

Photoinhibition of photosystem I

The photoinhibition of Photosystem I (PSI) drew less attention compared with that of Photosystem II (PSII). This could be ascribed to several reasons, e.g. limited combinations of plant species and environmental conditions that cause PSI photoinhibition, the non-regulatory aspect of PSI photoinhibition, and methodological difficulty to determine the accurate activity of PSI under stress conditions. However, the photoinhibition of PSI could be more dangerous than that of PSII because of the very slow recovery rate of PSI. This article is intended to introduce such characteristics of PSI photoinhibition with special emphasis on the relationship between two photosystems as well as the protective mechanism of PSI in vivo. Although the photoinhibition of PSI could be induced only in specific conditions and specific plant species in intact leaves, PSI itself is quite susceptible to photoinhibition in isolated thylakoid membranes. PSI seems to be well protected from photoinhibition in vivo in many plant species and many environmental conditions. This is quite understandable because photoinhibition of PSI is not only irreversible but also the potential cause of many secondary damages. This point would be different from the case of PSII photoinhibition, which could be regarded as one of the regulatory mechanisms under stressed as well as non-stressed conditions.     

Photosystem II photoinhibition-repair cycle protects Photosystem I from irreversible damage

Photodamage of Photosystem II (PSII) has been considered as an unavoidable and harmful reaction that decreases plant productivity. PSII, however, has an efficient and dynamically regulated repair machinery, and the PSII activity becomes inhibited only when the rate of damage exceeds the rate of repair. The speed of repair is strictly regulated according to the energetic state in the chloroplast. In contrast to PSII, Photosystem I (PSI) is very rarely damaged, but when occurring, the damage is practically irreversible. While PSII damage is linearly dependent on light intensity, PSI gets damaged only when electron flow from PSII exceeds the capacity of PSI electron acceptors to cope with the electrons. When electron flow to PSI is limited, for example in the presence of DCMU, PSI is extremely tolerant against light stress. Proton gradient (ΔpH)-dependent slow-down of electron transfer from PSII to PSI, involving the PGR5 protein and the Cyt b6f complex, protects PSI from excess electrons upon sudden increase in light intensity. Here we provide evidence that in addition to the ΔpH-dependent control of electron transfer, the controlled photoinhibition of PSII is also able to protect PSI from permanent photodamage. We propose that regulation of PSII photoinhibition is the ultimate regulator of the photosynthetic electron transfer chain and provides a photoprotection mechanism against formation of reactive oxygen species and photodamage in PSI.     

Heat stress results in loss of chloroplast Cu/Zn superoxide dismutase and increased damage to Photosystem II in combined drought‐heat stressed Lotus japonicus

Drought and heat stress have been studied extensively in plants, but most reports involve analysis of response to only one of these stresses. Studies in which both stresses were studied in combination have less commonly been reported. We report the combined effect of drought and heat stress on Photosystem II (PSII) of Lotus japonicus cv. Gifu plants. Photochemistry of PSII was not affected by drought or heat stress alone, but the two stresses together decreased PSII activity as determined by fluorescence emission. Heat stress alone resulted in degradation of D1 and CP47 proteins, and D2 protein was also degraded by combined drought–heat stress. None of these proteins were degraded by drought stress alone. Drought alone induced accumulation of hydrogen peroxide but the drought–heat combination led to an increase in superoxide levels and a decrease in hydrogen peroxide levels. Furthermore, combined drought–heat stress was correlated with an increase in oxidative damage as determined by increased levels of thiobarbituric acid reactive substances. Heat also induced degradation of chloroplast Cu/Zn superoxide dismutase (SOD: EC 1.15.1.1) as shown by reduced protein levels and isozyme‐specific SOD activity. Loss of Cu/Zn SOD and induction of catalase (CAT: EC 1.11.1.6) activity would explain the altered balance between hydrogen peroxide and superoxide in response to drought vs combined drought–heat stress. Degradation of PSII could thus be caused by the loss of components of chloroplast antioxidant defence systems and subsequent decreased function of PSII. A possible explanation for energy dissipation by L. japonicus under stress conditions is discussed.     

A retrieval algorithm to evaluate the Photosystem I and Photosystem II spectral contributions to leaf chlorophyll fluorescence at physiological temperatures

A new computational procedure to resolve the contribution of Photosystem I (PSI) and Photosystem II (PSII) to the leaf chlorophyll fluorescence emission spectra at room temperature has been developed. It is based on the Principal Component Analysis (PCA) of the leaf fluorescence emission spectra measured during the OI photochemical phase of fluorescence induction kinetics. During this phase, we can assume that only two spectral components are present, one of which is constant (PSI) and the other variable in intensity (PSII). Application of the PCA method to the measured fluorescence emission spectra of Ficus benjamina L. evidences that the temporal variation in the spectra can be ascribed to a single spectral component (the first principal component extracted by PCA), which can be considered to be a good approximation of the PSII fluorescence emission spectrum. The PSI fluorescence emission spectrum was deduced by difference between measured spectra and the first principal component. A single-band spectrum for the PSI fluorescence emission, peaked at about 735?nm, and a 2-band spectrum with maxima at 685 and 740?nm for the PSII were obtained. A linear combination of only these two spectral shapes produced a good fit for any measured emission spectrum of the leaf under investigation and can be used to obtain the fluorescence emission contributions of photosystems under different conditions. With the use of our approach, the dynamics of energy distribution between the two photosystems, such as state transition, can be monitored in vivo, directly at physiological temperatures. Separation of the PSI and PSII emission components can improve the understanding of the fluorescence signal changes induced by environmental factors or stress conditions on plants.     

Early and late plastid development in response to chill stress and heat stress in wheat seedlings

Five-day-old etiolated wheat (Triticum aestivum L.) seedlings were transferred to 7°C (chill stress), 25°C (control), and 42°C (heat stress) and were kept in the dark or light for different time periods. Plastids were isolated from the control and stressed seedlings, and their low-temperature (77 K) fluorescence emission spectra were monitored. Most of the Protochlorophyllide (Pchlide) present in heat-stressed etiolated seedlings were in nonphototransformable form. The phototransformable Pchlide (F657) rapidly decreased when 5-day-old etiolated seedlings were transferred to 42°C in the dark for 24 h. A flash illumination of 0.2 s given to etiolated heat-stressed seedlings resulted in substantial arrest of Shibata shift, while in chill-stress conditions, it was only partially affected. In high temperature, due to disaggregation of polymeric Pchlide–Pchlide oxidoreductase (POR)–nicotinamide adenine dinucleotide phosphate (NADPH) molecules, the conversion of nonphototransformable Pchlide to its phototransformable form is substantially delayed resulting in impaired Shibata shift and belated development of the core antenna CP47 Photosystem II (PSII). Chill stress, however, did not disaggregate the polymeric Pchlide–POR–NADPH molecule-suppressed Pchlide and Chl synthesis and impaired of the assembly of PSII core antenna CP47 that emits F695 and PSI that emits F735. The decreased gene/protein expression and reduced posttranslational import of plastidic proteins, importantly POR in temperature-stressed plants, may be responsible for the delay in conversion of nonphototransformable to phototransformable form of Pchlide and plastid biogenesis.     

GmHSFA1基因克隆及其过量表达提高转基因大豆的耐热性

热激转录因子在调节植物对逆境胁迫应答和热激蛋白基因表达方面起重要作用。采用生物信息学和比较基因组学方法结合RACE技术从大豆基因组中克隆到一个新的热激转录因子基因GmHsfA1, 其cDNA全长1 781 bp, 包含1个1 533 bp的开放阅读框, 编码含有510个氨基酸残基的蛋白质(GenBank登录号为AY458843)。与其他转录因子的分子结构相似,GmHSFA1也含有4个典型的结构功能域—DNA结合域、寡聚域、核定位信号和C端激活域。BLAST分析表明, GmHSFA1与其同源性最高的番茄热激转录因子LpHSFA1之间的氨基酸序列相似性为52.46%。RT-PCR、Northern和遗传转化结果显示: 1)GmHsfA1在大豆的不同组织中呈现组成型表达模式; 2)常温下转基因大豆植株的GmHsfA1表达水平明显高于非转基因对照; 3)GmHsfA1的过量表达激活了转基因大豆植株中热激蛋白基因GmHsp22在非诱导条件下的转录, 并加强了高温胁迫下另2个热激蛋白基因GmHsp23和GmHsp70的表达; 4)转GmHsfA1大豆植株的耐热温度(达52℃)明显高于非转基因植株。上述结果说明, GmHsfA1的过量表达激活或促进其下游3个热激蛋白基因的转录或表达, 明显提高了转基因大豆植株的耐热能力。     

Effect of the anthocyanic epidermal layer on Photosystem II and I energy dissipation processes in Tradescantia pallida (Rose) Hunt

The effect of anthocyanic cells of the epidermal layer was investigated on photosynthetic activity of the higher plant Tradescantia pallida. To determine the possible indirect role of anthocyanin in photosynthesis, analysis was done on intact leaves and leaves where anthocyanic epidermal layer was removed. Energy dissipation processes related to Photosystem II (PSII) and Photosystem I (PSI) activity was done using simultaneously Chlorophyll a (Chl a) fluorescence and P700 transmittance signals change. In anthocyanic epidermal-less leaves, PSII photochemical activity was more decreased in dependence to increasing light irradiance exposure. We found that photoinhibition of PSII decreased PSI activity by reducing the electron flow toward PSI, especially under high light intensities. Under those conditions, it resulted in the accumulation of oxidized PSI reaction centers, which was stronger in leaves where the anthocyanic epidermal layer was removed. In conclusion, our results showed that the anthocyanic epidermal layer had a photoprotective effect only on the PSII and not on the PSI of T. pallida leaves, supporting the role of anthocyanin pigments in the regulation of photosynthesis for excess absorbed light irradiance.     

Modulation of photosynthetic electron transport in the absence of terminal electron acceptors: characterization of the rbcL deletion mutant of tobacco

Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The Delta rbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between Q(A) and Q(B), whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of Delta rbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F0 'dark rise' in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O2 in Delta rbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the Delta rbcL mutant under growth conditions. This protective capacity was rapidly exceeded in Delta rbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes.     

Characterization of photosystem II in transgenic tobacco plants with decreased iron superoxide dismutase

Iron superoxide dismutases (FeSODs) play an important role in preventing the oxidative damage associated with photosynthesis. To investigate the mechanisms of FeSOD in protection against photooxidative stress, we obtained transgenic tobacco (Nicotiana tabacum) plants with severely decreased FeSOD by using a gene encoding tobacco chloroplastic FeSOD for the RNAi construct. Transgenic plants were highly sensitive to photooxidative stress and accumulated increased levels of O??? under normal light conditions. Spectroscopic analysis and electron transport measurements showed that PSII activity was significantly reduced in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed that there was a slow electron transfer between Q(A) and Q(B) and decreased redox potential of Q(B) in transgenic plants, whereas the donor side function of PSII was not affected. Immunoblot and blue native gel analyses showed that PSII protein accumulation was also decreased in transgenic plants. PSII photodamage and D1 protein degradation under high light treatment was increased in transgenic plants, whereas the PSII repair was not affected, indicating that the stability of the PSII complex was decreased in transgenic plants. The results in this study suggest that FeSOD plays an important role in maintaining PSII function by stabilizing PSII complexes in tobacco plants.     

Implications of alternative electron sinks in increased resistance of PSII and PSI photochemistry to high light stress in cold-acclimated Arabidopsis thaliana

Exposure of control (non-hardened) Arabidopsis leaves to high light stress at 5?°C resulted in a decrease of both photosystem II (PSII) (45?%) and Photosystem I (PSI) (35?%) photochemical efficiencies compared to non-treated plants. In contrast, cold-acclimated (CA) leaves exhibited only 35 and 22?% decrease of PSII and PSI photochemistry, respectively, under the same conditions. This was accompanied by an accelerated rate of P700(+) re-reduction, indicating an up-regulation of PSI-dependent cyclic electron transport (CET). Interestingly, the expression of the NDH-H gene and the relative abundance of the Ndh-H polypeptide, representing the NDH-complex, decreased as a result of exposure to low temperatures. This indicates that the NDH-dependent CET pathway cannot be involved and the overall stimulation of CET in CA plants is due to up-regulation of the ferredoxin-plastoquinone reductase, antimycin A-sensitive CET pathway. The lower abundance of NDH complex also implies lower activity of the chlororespiratory pathway in CA plants, although the expression level and overall abundance of the other well-characterized component involved in chlororespiration, the plastid terminal oxidase (PTOX), was up-regulated at low temperatures. This suggests increased PTOX-mediated alternative electron flow to oxygen in plants exposed to low temperatures. Indeed, the estimated proportion of O(2)-dependent linear electron transport not utilized in carbon assimilation and not directed to photorespiration was twofold higher in CA Arabidopsis. The possible involvement of alternative electron transport pathways in inducing greater resistance of both PSII and PSI to high light stress in CA plants is discussed.     

Senescing leaves possess potential for stress adaptation: the developing leaves acclimated to high light exhibit increased tolerance to osmotic stress during senescence

Plants may experience environmental stress factors operating in nature either simultaneously or in sequence. In the study, we have acclimated the developing primary leaves of wheat seedlings to high light stress and examined their photosynthetic response to polyethylene glycol (PEG) mediated osmotic stress during different developmental phases including senescence. The high light acclimated leaves show higher level of total carotenoids as compared to their non-acclimated counterparts experiencing osmotic stress during senescence. They also exhibit greater membrane stability as indicated by the measurements of fluorescence polarisation and energy transfer efficiency in photosystem I (PSI) and Photosystem II (PSII). From the data of DCPIP photoreduction and pulse amplitude modulated (PAM) fluorimetry, a similar trend is observed for PSII photochemistry of the leaves experiencing osmotic stress during senescence. Our results may suggest that the stress adaptive potential induced by one stress during development is retained by the leaves and helps to mitigate another stress effect operating in sequence during another developmental phase, namely senescence.     

CP43-like chlorophyll binding proteins: structural and evolutionary implications

CP43, encoded by the psbC gene, is a chlorophyll (Chl)-binding protein of Photosystem II (PSII), the water-splitting and oxygen-evolving enzyme of photosynthesis. CP47, encoded by psbB, a Chl-binding protein of PSII, is closely related to CP43. The Chl-binding six transmembrane helical unit typified by CP43, is also structurally related to the N-terminal domains of the PsaA and PsaB proteins of Photosystem I (PSI) as well as to the family of light-harvesting proteins encoded by cyanobacterial isiA genes and prochlorophyte pcb genes. Here we use recent structural information derived for PSII and PSI to review similarities and differences between the various members of the CP43-like class of light-harvesting proteins, exploring both functional and evolutionary implications.     

Probing connection of PBS with the photosystems in intact cells of Spirulina platensis by temperature-induced fluorescence fluctuation

Temperature-dependent fluorescence for intact cells of cyanobacterium Spirulina platensis was detected to search for the connection of the phycobilisome (PBS) with Photosystem I (PSI) and Photosystem II (PSII). Some interesting results were obtained from the deconvoluted fluorescence components of C-phycocyanin (C-PC), allophycocyanin (APC), PSI and PSII as well as the fluorescence spectra of the intact cells at room temperature (RT=25 degrees C) and 0 degrees C. It was observed that, compared to those at RT, both of the fluorescence components for PSI and APC increased, whereas those for PSII and C-PC decreased at 0 degrees C with excitation at 580 nm, that is, the fluorescence for C-PC is not synchronous with that for APC, and the fluorescence fluctuation for PSI is not synchronous with that for PSII. On the other hand, the decrease in C-PC fluorescence is synchronous with the increase in PSI fluorescence, and the increase in APC fluorescence is synchronous with the decrease in PSII fluorescence. Therefore, it can be readily deduced that PBS should be coupled not only with PSII through the terminal acceptors in the APC core but also with PSI through C-PC in PBS rods at physiological condition, while at 0 degrees C, a migration of a PBS makes the APC partially detached from PSII but the C-PC more efficiently coupled with PSI. The results provide good evidences for "mobile PBS" model and "parallel connection" model but not for the "spillover" model.     

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.     

Dissipation of excess energy triggered by blue light in cyanobacteria with CP43' (isiA)

The chlorophyll-protein CP43' (isiA gene) induced by stress conditions in cyanobacteria is shown to serve as an antenna for Photosystem II (PSII), in addition to its known role as an antenna for Photosystem I (PSI). At high light intensity, this antenna is converted to an efficient trap for chlorophyll excitations that protects system II from photo-inhibition. In contrast to the 'energy-dependent non-photochemical quenching' (NPQ) in chloroplasts, this photoprotective energy dissipation in cyanobacteria is triggered by blue light. The induction is proportional to light intensity. Induction and decay of the quenching exhibit the same large temperature-dependence.