Effects of salicylic acid on protein kinase activity and chloroplast D1 protein degradation in wheat leaves subjected to heat and high light stress

In the north of China, wheat plants are often stressed by heat and high light during grain-filling stage, which leads to injury in photosynthetic apparatus and decline in photosynthetic rate. In order to develop a method to protect photosynthetic apparatus in wheat leaves subjected to heat and high light stress, the effects of SA (salicylic acid) and FSBA (5′-p-fluorosulfonylbenzoyl adenosine) on PK (protein kinase) activity, D1 protein degradation and the performance of PSII were investigated in present work. Our results showed that PK activity enhanced under heat and high light stress and declined when stress was removed. FSBA pretreatment resulted in marked decreases in PK activity and D1 protein level, suggesting a correlationship between degradation of D1 protein and phosphorylation. After 2 h of stress, D1 protein level in water-pretreated leaves decreased to 79% of control and then recovered to 81% after 3 h of recovery. This clearly indicated that the damage of D1 protein induced by heat and high light stress was reversible. Compared to the control, SA pretreatment could not only increase PK activity, retard the degradation of D1 protein during heat and high light stress, but also accelerate the recovery of D1 protein level when the stress was removed. Correspondingly, F_v/F_m (maximum photochemical efficiency of PSII), Φ_PSII (actual photochemical efficiency of PSII), ETR (electron transfer rate) and Pn (net photosynthetic rate) in SA-treated leaves were higher than that in leaves of control under both stress and non-stress conditions. Taken together, our results revealed that SA pretreatment could significantly alleviate damages of heat and high light stress on D1 protein and PSII of wheat leaves, and accelerate restoration of photosynthetic function.     

Exogenous progesterone alleviates heat and high light stress-induced inactivation of photosystem II in wheat by enhancing antioxidant defense and D1 protein stability

This experiment was conducted to test the effects of foliar application of progesterone on the photochemical efficiency of photosystem II (PSII) and photosynthetic rate in wheat flag leaves subjected to cross-stress of heat and high light during grain-filling stage. The results showed that progesterone pretreatment increased the activities of superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase, and the contents of ascorbic acid and glutathione under the cross-stress. Meanwhile, the rate of O₂ ^? production, hydrogen peroxide (H₂O₂) and malondialdehyde contents in progesterone pretreated leaves were significantly lower under heat and high light stress. In parallel with the alleviation of oxidative stress, higher content of D1 protein in PSII reactive center was observed in progesterone pretreated leaves, resulting in a significant increase in the potential (Fv/Fm) and actual (ΦPS II) photochemical efficiency of PSII, and the net photosynthetic rate. In summary, this study suggested that foliar application of progesterone might protect the PSII complex from heat and high light stress-induced damage through enhancing antioxidant defense system and further facilitating D1 protein stability in the wheat leaves.     

Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities

The nature of photosynthetic recovery was investigated in 10-d-old wheat (Triticum aestivum L., cv. Moskovskaya-35) seedlings exposed to temperatures of 40 and 42 °C for 20 min and to temperature 42 °C for 40 min in the dark. The aftereffect of heat treatment was monitored by growing the heat-treated plants in low/moderate/high light at 20 °C for 72 h. The net photosynthetic rates (P_N) and the fluorescence ratios F_v/F_m were evaluated in intact primary leaves and the rates of cyclic and non-cyclic photophosphorylation were measured in the isolated thylakoids. At least two temporally separated steps were identified in the path of recovery from heat stress at 40 and 42 °C in the plants growing in high and moderate/high light, respectively. Both photochemical activity of the photosystem II (PSII) and the activity of CO₂ assimilation system were lowered during the first step in comparison with the corresponding activities immediately after heat treatment. During the second step, the photosynthetic activities completely or partly recovered. Recovery from heat stress at 40 °C was accompanied by an appreciably higher rate of cyclic photophosphorylation in comparison with control non-heated seedlings. In pre-heated seedlings, the tolerance of the PSII to photoinhibition was higher than in non-treated ones. The mode of acclimation to different light intensities after heat exposures is analyzed.     

Effects of heat and high irradiance stress on energy dissipation of photosystem II in low irradiance-adapted peanut leaves

To increase crop yields and not to compete for land with food crops, intercropping agricultural cultivation approach was introduced into cultivation of peanut (Arachis hypogaca L.). This approach improves the total yield of the crop per unit area, but decreases the yield of a single crop compared with mono-cropped agricultural cultivation approach. In wheat-peanut relay intercropping system, peanut plants would suffer heat and high light (HI) stress after wheat harvest. In the present work, peanut seedlings were cultivated in low light to simulate wheat-peanut relay intercropping environments. Upon exposure to heat and HI stress, energy dissipation in PSII complexes was evaluated by comparing those cultivated in low irradiance conditions with the mono-cropped peanut. The maximal photochemical efficiency of PSII (F_v/F_m) and the net photosynthetic rate (P_n) decreased markedly in relay-cropped peanut (RP) after heat and HI stress, accompanied by higher degree of PSII reaction center closure (1–qP). After heat and HI stress, higher antioxidant enzyme activity and less ROS accumulation were observed in mono-cropped peanut (MP) seedlings. Meanwhile, higher content of D1 protein and higher ratio of (A + Z)/(V + A + Z) were also detected in MP plants under such stress. These results implied that heat and HI stress could induce photoinhibition of PSII reaction centers in peanut seedlings and both xanthophyll cycle-dependent thermal energy dissipation and the antioxidant system were down-regulated in RP compared to classical monocropping systems after heat and high irradiance stress.     

Effects of Different Treatments of Salicylic Acid on Heat Tolerance, Chlorophyll Fluorescence, and Antioxidant Enzyme Activity in Seedlings of Cucumis sativa L.

The effects of different treatments of salicylic acid (SA) on lipid peroxidation, chlorophyll fluorescence and antioxidant enzyme activity in seedlings of Cucumis sativa L. were studied before heat stress treatment, 36 h after heat stress and 24 h after recovery. Compared with the controls (foliar spray of distilled water), a foliar spray of 1 mM SA (SSA treatment) decreased electrolyte leakage and the concentration of H₂O₂ and thiobarbituric acid reactive substances (TBARS). SSA treatment also enhanced maximum yield of photosystem II photochemical reactions (Fv/Fm) and the quantum yield of the photosystem II electron transport (ΦPSII) after both heat stress and recovery; however, adding 1 mM SA to the nutrient solution (ASA treatment) or both adding 1 mM SA to the nutrient solution and foliar spray of 1 mM SA as well (SSA + ASA treatment) had the opposite effects. SOD activity was stimulated by all SA treatments. CAT activity was stimulated by SSA treatment and inhibited by ASA and SSA + ASA treatments after heat stress and recovery. This suggest that SSA treatment can efficiently remove H₂O₂ and decrease heat stress, and CAT plays a key role in removing H₂O₂ in cucumber seedlings under heat stress, while more H₂O₂ accumulates in ASA and SSA + ASA treatments and therefore induces serious oxidative stress. GPX, APX and GR showed higher activities in all SA treatments under heat stress, however, it appears that they were not key enzymes in removing H₂O₂ in cucumber subject to heat stress.     

Alleviation of photoinhibition in drought-stressed wheat (Triticum aestivum) by foliar-applied glycinebetaine

Effects of foliar application of 100 mmol/L glycinebetaine (GB) on PS II photochemistry in wheat (Triticum aestivum) flag leaves under drought stress combined with high irradiance were investigated. The results show that GB-treated plants maintained a higher net photosynthetic rate during drought stress than non-GB treated plants. Exogenous GB can preserve the photochemical activity of PSII, for GB-treated plants maintain higher maximal photochemistry efficiency of PSII (F(v)/F(m)) and recover more rapidly from photoinhibition. In addition, GB-treated plants can maintain higher anti-oxidative enzyme activities and suffer less oxidative stress. Our data suggest that GB may protect the PSII complex from damage through accelerating D1 protein turnover and maintaining anti-oxidative enzyme activities at higher level to alleviate photodamage. Diethyldithiocarbamate as well as streptomycin treatment can impair the protective effect of GB on PSII. In summary, GB can enhance the photoinhibition tolerance of PSII.     

The overaccumulation of glycinebetaine alleviated damages to PSII of wheat flag leaves under drought and high temperature stress combination

To analyze the physiological mechanisms underlying the increased tolerance to drought and high temperature stress combination by overproduction of glycinebetaine (GB) in wheat, a transgenic wheat line T6 and its wild-type (WT) Shi4185 were used. The transgenic line was generated by introducing a gene encoding betaine aldehyde dehydrogenase (BADH) into a wheat line Shi4185. The gene was cloned from Garden Orache (Atriplex hortensis L.). Wheat plants were exposed to drought (withholding irrigation), high temperature stress (40 °C), and their combination at the flowering stage. Analyses of oxygen-evolving activity and photosystem II (PSII) photochemistry, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients (OJIP) were used to evaluate PSII photochemistry in wheat plants. The results suggest that the PSII in transgenic plants showed higher resistance than that in wild-type plants under the stresses studied here, this increased tolerance was associated with an improvement in stability of the oxygen-evolving complex and the reaction center of PSII; streptomycin treatment can impair the protective effect of overaccumulated GB on PSII. The overaccumulated GB may protect the PSII complex from damage through accelerating D1 protein turnover to alleviate photodamage. The results also suggest that the PSII under combined high temperature and drought stress shows higher tolerance than under high temperature stress alone in both transgenic and wild-type plants.     

MPH1 is a thylakoid membrane protein involved in protecting photosystem II from photodamage in land plants

Photosystem II (PSII) is highly susceptible to photoinhibition caused by environmental stimuli such as high light; therefore plants have evolved multifaceted mechanisms to efficiently protect PSII from photodamage. We previously published data suggesting that Maintenance of PSII under High light 1 (MPH1, encoded by AT5G07020), a PSII-associated proline-rich protein found in land plants, participates in the maintenance of normal PSII activity under photoinhibitory stress. Here we provide additional evidence for the role of MPH1 in protecting PSII against photooxidative damage. Two Arabidopsis thaliana mutants lacking a functional MPH1 gene suffer from severe photoinhibition relative to the wild-type plants under high irradiance light. The mph1 mutants exhibit significantly decreased PSII quantum yield and electron transport rate after exposure to photoinhibitory light. The mutants also display drastically elevated photodamage to PSII reaction center proteins after high-light treatment. These data add further evidence that MPH1 is involved in PSII photoprotection in Arabidopsis. MPH1 homologs are found across phylogenetically diverse land plants but are not detected in algae or prokaryotes. Taken together, these results suggest that MPH1 protein began to play a role in protecting PSII against excess light following the transition from aquatic to terrestrial conditions.     

Interruption of the Calvin cycle inhibits the repair of Photosystem II from photodamage

In photosynthetic organisms, impairment of the activities of enzymes in the Calvin cycle enhances the extent of photoinactivation of Photosystem II (PSII). We investigated the molecular mechanism responsible for this phenomenon in the unicellular green alga Chlamydomonas reinhardtii. When the Calvin cycle was interrupted by glycolaldehyde, which is known to inhibit phosphoribulokinase, the extent of photoinactivation of PSII was enhanced. The effect of glycolaldehyde was very similar to that of chloramphenicol, which inhibits protein synthesis de novo in chloroplasts. The interruption of the Calvin cycle by the introduction of a missense mutation into the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) also enhanced the extent of photoinactivation of PSII. In such mutant 10-6C cells, neither glycolaldehyde nor chloramphenicol has any additional effect on photoinactivation. When wild-type cells were incubated under weak light after photodamage to PSII, the activity of PSII recovered gradually and reached a level close to the initial level. However, recovery was inhibited in wild-type cells by glycolaldehyde and was also inhibited in 10-6C cells. Radioactive labelling and Northern blotting demonstrated that the interruption of the Calvin cycle suppressed the synthesis de novo of chloroplast proteins, such as the D1 and D2 proteins, but did not affect the levels of psbA and psbD mRNAs. Our results suggest that the photoinactivation of PSII that is associated with the interruption of the Calvin cycle is attributable primarily to the inhibition of the protein synthesis-dependent repair of PSII at the level of translation in chloroplasts.     

The Functional Significance of Black-Pigmented Leaves: Photosynthesis,Photoprotection and Productivity in Ophiopogon planiscapus ‘Nigrescens’

Black pigmented leaves are common among horticultural cultivars, yet are extremely rare across natural plant populations. We hypothesised that black pigmentation would disadvantage a plant by reducing photosynthesis and therefore shoot productivity, but that this trait might also confer protective benefits by shielding chloroplasts against photo-oxidative stress. CO₂ assimilation, chlorophyll a fluorescence, shoot biomass, and pigment concentrations were compared for near isogenic green- and black-leafed Ophiopogon planiscapus ‘Nigrescens’. The black leaves had lower maximum CO₂ assimilation rates, higher light saturation points and higher quantum efficiencies of photosystem II (PSII) than green leaves. Under saturating light, PSII photochemistry was inactivated less and recovered more completely in the black leaves. In full sunlight, green plants branched more abundantly and accumulated shoot biomass quicker than the black plants; in the shade, productivities of the two morphs were comparable. The data indicate a light-screening, photoprotective role of foliar anthocyanins. However, limitations to photosynthetic carbon assimilation are relatively small, insufficient to explain the natural scarcity of black-leafed plants.     

Genetic engineering of the biosynthesis of glycinebetaine enhances thermotolerance of photosystem II in tobacco plants

Genetically engineered tobacco (Nicotiana tabacum L.) with the ability to accumulate glycinebetaine was established. The wild type and transgenic plants were exposed to heat treatment (25–50°C) for 4 h in the dark and under growth light intensity (300 μmol m⁻² s⁻¹). The analyses of oxygen-evolving activity and chlorophyll fluorescence demonstrated that photosystem II (PSII) in transgenic plants showed higher thermotolerance than in wild type plants in particular when heat stress was performed in the light, suggesting that the accumulation of glycinebetaine leads to increased tolerance to heat-enhanced photoinhibition. This increased tolerance was associated with an improvement on thermostability of the oxygen-evolving complex and the reaction center of PSII. The enhanced tolerance was caused by acceleration of the repair of PSII from heat-enhanced photoinhibition. Under heat stress, there was a significant accumulation of H₂O₂, O₂⁻ and catalytic Fe in wild type plants but this accumulation was much less in transgenic plants. Heat stress significantly decreased the activities of catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monodehydroascorbate reductase in wild type plants whereas the activities of these enzymes either decreased much less or maintained or even increased in transgenic plants. In addition, heat stress increased the activity of superoxide dismutase in wild type plants but this increase was much greater in transgenic plants. Furthermore, transgenic plants also showed higher content of ascorbate and reduced glutathione than that of wild type plants under heat stress. The results suggest that the increased thermotolerance induced by accumulation of glycinebetaine in vivo was associated with the enhancement of the repair of PSII from heat-enhanced photo inhibition, which might be due to less accumulation of reactive oxygen species in transgenic plants.     

Salicylic acid affects growth,essential oil and chemical compositions of thyme (Thymus daenensis Celak.) under reduced irrigation

Salicylic acid (SA) may reduce the negative impact of water deficit on growth and metabolite yield of Thymus daenensis Celak subsp. daenensis Celak. The effect of foliar application of SA and reduced irrigation on growth, oil yield, chemical components, and antibacterial and antioxidant activities of T. daenensis in field condition were investigated. Treatments comprised 0.0, 1.5 and 3.0 M SA applied to plants under normal irrigation and stressed conditions. Results indicated that irrigation regime had a significant effect on growing degree days (GDD) required to reach early and full flowering. Foliar application of SA influenced GDD from early growing stage to 50 % and full flowering, minimum radius and canopy diameter. The highest values of oil content (3.2 % v/w) and yield (14.9 g m^?2) were obtained from application of 3.0 M SA. Percentage of some chemical constituents in the essential oil extracted from the plants under stress was higher than non-stressed plants. Thymol content was significantly reduced under stressed conditions. Foliar application of SA significantly improved carvacrol, α-thujene, α-pinene and p-cymene contents in the oils, but reduced thymol and, β-caryophyllene amounts. Our results showed that foliar application of SA reduced the negative effect of water deficit on thymol content in the essential oil of T. daenensis. The essential oils of T. daenensis exhibited antioxidant and antibacterial activities when plants were sprayed with 1.5 and 3.0 M SA, respectively.     

Overexpression of GASA5 increases the sensitivity of Arabidopsis to heat stress

Basal thermotolerance is very important for plant growth and development when plants are subjected to heat stress. However, little is known about the functional mechanism of gibberellins (GAs) in the basal thermotolerance of plants. In the present work, we provide molecular evidence that a member of the gene family encoding the GA-stimulated Arabidopsis (GASA) peptides, namely GASA5, is involved in the regulation of seedling thermotolerance. The GASA5-overexpressing plants displayed a weak thermotolerance, with a faster cotyledon-yellowing rate, lower seedling-survival rate, and slower hypocotyl elongation, in comparison to the wild-type and GASA5 null-mutant (gasa5-1) plants, after heat-stress treatment. The short-hypocotyl phenotype of GASA5-overexpressing plants could be rescued by the exogenous application of salicylic acid (SA), the hormone found to protect plants from heat stress-induced damage. GASA5 expression was inhibited by heat stress but unaffected by the application of exogenous SA. However, expression of the gene encoding the noexpresser of PR genes 1 (NPR1), a key component of the SA-signaling pathway, was downregulated by GASA5 overexpression. Importantly, when different GASA5-genotype plants were treated with heat stress, several genes encoding heat-shock proteins, including HSP101, HSP70B, HSP90.1, HSP17.6-C1, and HSP60, were inhibited by GASA5 overexpression. Meanwhile, hydrogen peroxide was accumulated at high levels in heat stress-treated GASA5-overexpressing plants. These results suggest that the Arabidopsis GASA5 gene acts as a negative regulator in thermotolerance by regulating both SA signaling and heat shock-protein accumulation.     

Natural Variants of Photosystem II Subunit D1 Tune Photochemical Fitness to Solar Intensity

Photosystem II (PSII) is composed of six core polypeptides that make up the minimal unit capable of performing the primary photochemistry of light-driven charge separation and water oxidation in all oxygenic phototrophs. The D1 subunit of this complex contains most of the ligating amino acid residues for the Mn₄CaO₅ core of the water-oxidizing complex (WOC). Most cyanobacteria have 3–5 copies of the psbA gene coding for at least two isoforms of D1, whereas algae and plants have only one isoform. Synechococcus elongatus PCC 7942 contains two D1 isoforms; D1:1 is expressed under low light conditions, and D1:2 is up-regulated in high light or stress conditions. Using a heterologous psbA expression system in the green alga Chlamydomonas reinhardtii, we have measured growth rate, WOC cycle efficiency, and O₂ yield as a function of D1:1, D1:2, or the native algal D1 isoform. D1:1-PSII cells outcompete D1:2-PSII cells and accumulate more biomass in light-limiting conditions. However, D1:2-PSII cells easily outcompete D1:1-PSII cells at high light intensities. The native C. reinhardtii-PSII WOC cycles less efficiently at all light intensities and produces less O₂ than either cyanobacterial D1 isoform. D1:2-PSII makes more O₂ per saturating flash than D1:1-PSII, but it exhibits lower WOC cycling efficiency at low light intensities due to a 40% faster charge recombination rate in the S₃ state. These functional advantages of D1:1-PSII and D1:2-PSII at low and high light regimes, respectively, can be explained by differences in predicted redox potentials of PSII electron acceptors that control kinetic performance.     

Combined effect of salicylic acid and potassium mitigates drought stress through the modulation of physio-biochemical attributes and key antioxidants in wheat

Improving physio-biochemical traits in wheat under drought stress conditions has received more research attention in recent years for better adaptability and higher yield. In this study, we explored the potential bio-physiological mechanisms underlying improved plant growth and water use efficiency in wheat following soil application of potassium (0 and 100 kg ha^?1) and seed primed salicylic acid (SA) (150 mg per L) and SA foliar application (100 mg per L) under drought stresses (100%, 60% and 30% FC). Two years' average data revealed that inducing drought stress resulted in a decrease in plant pigments content, growth traits, and plant water status however, the influence was substantially reduced with the combined application of K and SA under drought stress conditions. The SA foliar spray in combination with K had increased chlorophyll a (174% and 83%), chl b (130% and 192%), chl a + b (156% and 120), carotenoid (22% and 11%), proline contents (24% and 29%) leaf relative water content (24% and 29%) while reduced leaf WSD (17% and 20%), WRC (6% and 7%), and WUC (23% and 28%) under mild and severe drought stresses, respectively. The increase in grain yield by 41% and 37% with enhanced water use efficiency was obtained with combined foliar SA and K under mild and severe drought stress, respectively indicating its vital role in overcoming the deleterious effects of drought via regulation of osmotic and metabolic processes and stabilizes cell components. RDA analysis revealed that the studied traits were completely discriminated under severe stress than mild or no drought stress. A positive and significant association was found between plant pigments with seed yield whereas a negative and significant correlation existed between water leaf traits and plant pigments. It was concluded that both foliar SA and seed primed SA with K fertilization combat the adverse effects of drought and improved plant water status as well as growth and bio-physiological traits of wheat under drought stress conditions.     

The occurrence of a thylakoid-localized small zinc finger protein in land plants

Previous studies showed that LOW QUANTUM YIELD OF PHOTOSYSTEM II 1 (LQY1), a small thylakoid zinc finger protein was involved in maintenance and repair of Photosystem II (PSII). Here the author provide additional evidence for the role of LQY1 in PSII maintenance and repair and further commentary on the occurrence of LQY1 protein among land plants. After exposure to high light, Arabidopsis thaliana mutants lacking functional LQY1 gene (At1g75690) are more photoinhibited than wild-type control plants; display higher total non-photochemical quenching and photoinhibitory quenching. These results are consistent with the initial observation that lqy1 mutants have lower PSII efficiency than wild-type plants after high-light treatment. The low-PSII-efficiency phenotype can be suppressed upon complementation of lqy1 mutants with the LQY1 gene from wild-type plants. This further demonstrates that LQY1 is important in maintaining the activity of photosystem II in Arabidopsis. LQY1 homologs are present in land plants but are absent from sequenced genomes of green algae and cyanobacteria, which may reflect plant adaptation to excess light stress during the transition to land.     

Salicylic Acid-Mediated Regulation of Morpho-Physiological and Yield Attributes of Wheat and Barley Plants in Deferring Salinity Stress

Salinity has been observed to be a global problem that impede the physiological characteristics of plants. Salicylic acid (SA) as a phytohormone play multifaceted role in plants in terms of development as well as stress management. The current study was conducted to evaluate the effect of salinity and salicylic acid on the performance of wheat and barley plants under field experimentation followed by on-farm study to validate the results. This research was firstly conducted in a 4-year research barley field (2012–2013 and 2013–2014) and wheat (2014–2015 and 2015–2016) and subsequently in an on-farm research in four places (2017–2018). Results depicted that salinity decreased plant yield components and altered ion concentrations (Na⁺/K⁺) causing reduced grain and biological yield. However, SA foliar application induced yield components, especially grain number of plants in both years in non-saline and saline conditions. Exogenously SA application not only led to higher grain yield of barley and wheat but also significantly improved their salt tolerance. Our findings revealed that optimum SA concentrations for achieving highest barley yield were 0.85 and 0.78 mM under saline and non-saline conditions, respectively, while on-farm scale studies observed that foliar application of SA increased grain and biological yield of wheat in Ardakan, Ashkzar (saline soil and water) and Mehrabad (non-saline field) regions. There was no significant effect in Tijerd, a completely non-saline field. The grain yields were higher in SA-treated Ardakan, Ashkzar, and Mehrabad plants in field by 19, 16, and 15%, respectively. Based on present detailed studies, it was concluded that SA improved salinity tolerance and increased crop yield. So, optimum concentration (1.0–1.5 mM) with proper time application (double ridges), SA increased wheat and barley yields up to 20%. Therefore, SA priming could be used as a potent strategy to cope up salinity stress from plants.