Complexity of the heat stress response in plants.

Global warming has led to increased temperature of the earth which is a major abiotic stress posing a serious threat to the plants. Photosynthesis is amongst the plant cell functions that is highly sensitive to high temperature stress and is often inhibited before other cell functions are impaired. The primary sites of targets of high temperature stress are Photosystem II (PSII), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) while Cytochrome b559 (Cytb559) and plastoquinone (PQ) are also affected. As compared to PSII, PSI is stable at higher temperatures. ROS production, generation of heat shock proteins, production of secondary metabolites are some of the consequences of high temperature stress. In this review we have summarized the physiological, biochemical and molecular aspects of high temperature stress on the process of photosynthesis, as well as the tolerance and adaptive mechanisms involved.

Photosynthesis: Response to high temperature stress

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

/pdf/in-posidonia-oceanica-cadmium-induces-changes-in-dna-3cu7uj83mr.pdf

In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

Plants encounter various abiotic stresses due to their sessile nature which include heavy metals, salt, drought, nutrient deficiency, light intensity, pesticide contamination, as well as extreme temperatures. These stresses impose major constraints limiting crop production and food security worldwide. Abiotic stresses primarily reduce the photosynthetic efficiency of plants, due to their negative consequences on chlorophyll biosynthesis, performance of the photosystems, electron transport mechanisms, gas exchange parameters, and many others. A better understanding of the photochemistry of plants under these abiotic stresses can help in the development of pragmatic interventions for managing these stresses. Interestingly, in this review, we provide an overview of insight into different mechanisms affecting the photosynthetic ability of plants in relation to these abiotic factors. The present review describes how different abiotic stresses can pose deleterious impacts on plant photosynthetic machinery including cellular membranes, cell division and cell elongation, biosynthesis of photosynthetic pigments, as well as electron transport chain. It is important to understand the detrimental impacts of various abiotic stresses for better stress management because a comprehensive understanding of plant responses has pragmatic implication for remedies and management.

Photosynthetic Response of Plants Under Different Abiotic Stresses: A Review

The light reactions in photosynthesis convert light energy into chemical energy in the form of ATP and drive the production of NADPH from NADP+. The reactions involve two types of electron flow in the chloroplast. While linear electron transport generates both ATP and NADPH, photosystem I cyclic electron transport is exclusively involved in ATP synthesis. The physiological significance of photosystem I cyclic electron transport has been underestimated, and our knowledge of the machineries involved remains very limited. However, recent genetic approaches using Arabidopsis thaliana have clarified the essential functions of this electron flow in both photoprotection and photosynthesis. Based on several lines of evidence presented here, it is necessary to reconsider the fundamental mechanisms of chloroplast energetics.

Cyclic Electron Transport Around Photosystem I: Genetic Approaches

In this study, the function of the NAD(P)H dehydrogenase (NDH)-dependent pathway in suppressing the accumulation of reactive oxygen species in chloroplasts was investigated. Hydrogen peroxide accumulated in the leaves of tobacco (Nicotiana tabacum) defective in ndhC-ndhK-ndhJ (ΔndhCKJ) at 42°C and 4°C, and in that of wild-type leaves at 4°C. The maximum quantum efficiency of PSII decreased to a similar extent in both strains at 42°C, while it decreased more evidently in ΔndhCKJ at 4°C. The parameters linked to CO2 assimilation, such as the photochemical efficiency of PSII, the decrease of nonphotochemical quenching following the initial rise, and the photosynthetic O2 evolution, were inhibited more significantly in ΔndhCKJ than in wild type at 42°C and were seriously inhibited in both strains at 4°C. While cyclic electron flow around PSI mediated by NDH was remarkably enhanced at 42°C and suppressed at 4°C. The proton gradient across the thylakoid membranes and light-dependent ATP synthesis were higher in wild type than in ΔndhCKJ at either 25°C or 42°C, but were barely formed at 4°C. Based on these results, we suggest that cyclic photophosphorylation via the NDH pathway might play an important role in regulation of CO2 assimilation under heat-stressed condition but is less important under chilling-stressed condition, thus optimizing the photosynthetic electron transport and reducing the generation of reactive oxygen species.

/pdf/chloroplastic-nad-p-h-dehydrogenase-in-tobacco-leaves-1mjxx5b5ep.pdf

Chloroplastic NAD(P)H Dehydrogenase in Tobacco Leaves Functions in Alleviation of Oxidative Damage Caused by Temperature Stress

After incubation at 42°C for more than 48 h, brown damages occurred on the stems of tobacco (Nicotiana tabacum L.) ndhC-ndhK-ndhJ deletion mutant (ΔndhCKJ), followed by wilt of the leaves, while less the phenotype was found in its wild type (WT). Analysis of the kinetics of post-illumination rise in chlorophyll fluorescence indicated that the PSI cyclic electron flow and the chlororespiration mediated by NAD(P)H dehydrogenase (NDH) was significantly enhanced in WT under the high temperature. After leaf disks were treated with methyl viologen (MV), photosynthetic apparatus of ΔndhCKJ exhibited more severe photo-oxidative damage, even bleaching of chlorophyll. Analysis of P700 oxidation and reduction showed that the NDH mediated cyclic electron flow probably functioned as an electron competitor with Mehler reaction, to reduce the accumulation of reactive oxygen species (ROS). When leaf disks were heat stressed at 42°C for 6 h, the photochemical activity declined more markedly in ΔndhCKJ than in WT, accompanied with more evident decrease in the amount of soluble Rubisco activase. In addition, the slow phase of millisecond-delayed light emission (ms-DLE) of chlorophyll fluorescence indicated that NDH was involved in the building-up of transthylakoid proton gradient (ΔpH), while the consumption of ΔpH was highly inhibited in ΔndhCKJ after heat stress. Based on the results, we supposed that the cyclic electron flow mediated by NDH could be stimulated under the heat stressed conditions, to divert excess electrons via chlororespiration pathway, and sustain CO2 assimilation by providing extra ΔpH, thus reducing the photooxidative damage.

The role of chloroplast NAD(P)H dehydrogenase in protection of tobacco plant against heat stress.

Mechanism of temperature dependence of post-illumination transient increase in chlorophyll fluorescence in cyanobacterium synechocystis pcc 6803

One electron paramagnetic resonance (EPR) signal, named Signal IIslow, originates from the oxidized Tyrosine 160 (YD) of D2 polypeptide of photosystem II reaction center. After adding high concentration trichloroacetate (TCA) to theChlamydomonas reinhardtii thylakoid suspension, this signal was abolished in a minute. Treatment of TCA also removes a few of polypeptides, including three extrinsic polypeptides of oxygen-evolving complex, from the thylakoid membrane. Based upon the analysis of the microenvironment around YD with a three-dimensional model, it is indicated that relatively high hydrophobicity of this microenvironment may be the essential prerequisite for TCA to affect YD. It has been observed that TCA treatment also retards the decay of the Signal 1, produced by the oxidized reaction center chlorophyll dimer (P700+) of photosystem I.

/pdf/trichloroacetate-affects-the-epr-signal-ii-slow-and-signal-i-4fibq1xn7n.pdf

Trichloroacetate affects the EPR Signal II slow and Signal I in the thylakoid of Chlamydomonas reinhardtii

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Chloroplastic NAD(P)H Dehydrogenase in Tobacco Leaves Functions in Alleviation of Oxidative Damage Caused by Temperature Stress

The role of chloroplast NAD(P)H dehydrogenase in protection of tobacco plant against heat stress.

Mechanism of temperature dependence of post-illumination transient increase in chlorophyll fluorescence in cyanobacterium synechocystis pcc 6803

Trichloroacetate affects the EPR Signal II slow and Signal I in the thylakoid of Chlamydomonas reinhardtii