Tolerance To High Light Stress Research Articles

Mutations in hik26 and slr1916 lead to high-light stress tolerance in Synechocystis sp. PCC6803

Increased tolerance to light stress in cyanobacteria is a desirable feature for their applications. Here, we obtained a high light tolerant (Tol) strain of Synechocystis sp. PCC6803 through an adaptive laboratory evolution, in which the cells were repeatedly sub-cultured for 52 days under high light stress conditions (7000 to 9000 μmol m−2 s−1). Although the growth of the parental strain almost stopped when exposed to 9000 μmol m−2 s−1, no growth inhibition was observed in the Tol strain. Excitation-energy flow was affected because of photosystem II damage in the parental strain under high light conditions, whereas the damage was alleviated and normal energy flow was maintained in the Tol strain. The transcriptome data indicated an increase in isiA expression in the Tol strain under high light conditions. Whole genome sequence analysis and reverse engineering revealed two mutations in hik26 and slr1916 involved in high light stress tolerance in the Tol strain.

Tradescantia-based models: a powerful looking glass for investigation of photoacclimation and photoadaptation in plants.

Here, we summarize diverse evidence from species that belong to the genus Tradescantia, which we propose as handy and versatile models for studies of the ecology of photosynthesis and the mechanisms of photoacclimation in higher plants. A valuable feature of this genus is the amazingly broad range of ecological niches occupied by its species: from shady understory of tropical rainforest to deserts and semideserts. The former habitats demand shade tolerance (e.g. that featured by Tradescantia fluminensis), whereas the latter requires succulence and/or high light stress tolerance (evident in e.g. Tradescantia navicularis). At the same time, the acclimative traits of Tradescantia species seem quite moderate at first glance. Certainly, their basic principles of acclimation seem to differ in some aspects from the ones typical for most of other higher plants. This review presents a systematic analysis of irradiance responses of Tradescantia species studied on different timescales. The specifics of Tradescantia responses to irradiance make the plants of this genus a 'multitool' for studies in this field. Similarity of irradiance acclimation patterns is a characteristic feature in the ecologically contrasting Tradescantia species, which may inspire further insights into physiology and evolution of plants.

Melatonin induction and its role in high light stress tolerance in Arabidopsis thaliana

In plants, melatonin is a potent bioactive molecule involved in the response against various biotic and abiotic stresses. However, little is known of its defensive role against high light (HL) stress. In this study, we found that melatonin was transiently induced in response to HL stress in Arabidopsis thaliana with a simultaneous increase in the expression of melatonin biosynthetic genes, including serotonin N-acetyltransferase1 (SNAT1). Transient induction of melatonin was also observed in the flu mutant, a singlet oxygen (1 O2 )-producing mutant, upon light exposure, suggestive of melatonin induction by chloroplastidic 1 O2 against HL stress. An Arabidopsis snat1 mutant was devoid of melatonin induction upon HL stress, resulting in high susceptibility to HL stress. Exogenous melatonin treatment mitigated damage caused by HL stress in the snat1 mutant by reducing O2- production and increasing the expression of various ROS-responsive genes. In analogy, an Arabidopsis SNAT1-overexpressing line showed increased tolerance of HL stress concomitant with a reduction in malondialdehyde and ion leakage. A complementation line expressing an Arabidopsis SNAT1 genomic fragment in the snat1 mutant completely restored HL stress susceptibility in the snat1 mutant to levels comparable to that of wild-type Col-0 plants. The results of the analysis of several Arabidopsis genetic lines reveal for the first time at the genetic level that melatonin is involved in conferring HL stress tolerance in plants.

In situ optical properties of foliar flavonoids: Implication for non-destructive estimation of flavonoid content

Flavonoids are a ubiquitous multifunctional group of phenolics of paramount importance for the terrestrial plants involved in protection from biotic and abiotic stresses, color and chemical signaling and other functions. Deciphering of in situ absorption of foliar Flv is important but was thought to be impossible due to a strong overlap with other pigments, complex in situ chemistry of Flv and sophisticated leaf optics.We deduced in situ absorbance of foliar Flv and introduced a concept of specific absorbance spectrum indicative of each pigment group contribution to light absorption and provided a rationale for the choice of spectral bands for non-destructive assessment of Flv in leaves with variable content of other pigments including anthocyanins.Only a narrow band 400–430nm was suitable for Flv assessment, however the effect of other pigments remained substantial, so subtraction of their contribution was necessary. The devised leaf absorbance-based algorithm allowed estimating Flv with error below 21%.Absorption by Flv in plant tissues might extend into the blue and can be commensurate to that of chlorophylls and carotenoids. The potential capacity of Flv to shield the cell in situ from the visible light might be essential for assessments of high light stress tolerance of plants.

Chlorophyll a fluorescence changes in response to short and long term high light stress in rice seedlings

Investigations were carried out to study the high light stress effects on photosystem (PS) II efficiency in intact leaves of four rice (Oryza sativa L.) cultivars, viz., Jyothi, Aiswarya, Swarnaprabha and Kanchana. Rice seedlings were exposed to high light stress of 2000 µmol m−2 s−1, and the photochemistry of the four rice cultivars was studied by analyzing chlorophyll-a (Chla) fluorescence at 0 (control), 2 and 8 h. The results showed that Fv/Fo, performance index, area above the flourescence curve, Φo/(1 − Φo), ΦDo, ΦEo, Vj and DIo/CSo were found to be more reliable for exploring the effect of changes in PSII activity after high light stress application for short term (2 h), which can aid in distinguishing the tolerant and sensitive rice cultivars. Some Chla fluorescence parameters (Fm, SFI, Fv/Fm, Ψo/(1 − Ψo), ETo/CSo, ABS/CSo) were modified only after exposing the plants to high light stress for 8 h. The order of high light stress tolerance of these cultivars according to the analysis of Chla fluorescence parameters was Jyothi > Aiswarya > Swarnaprabha > Kanchana.

Redox regulation of ascorbate and glutathione by a chloroplastic dehydroascorbate reductase is required for high-light stress tolerance in Arabidopsis.

Chloroplasts are a significant site for reactive oxygen species production under illumination and, thus, possess a well-organized antioxidant system involving ascorbate. Ascorbate recycling occurs in different manners in this system, including a dehydroascorbate reductase (DHAR) reaction. We herein investigated the physiological significance of DHAR3 in photo-oxidative stress tolerance in Arabidopsis. GFP-fused DHAR3 protein was targeted to chloroplasts in Arabidopsis leaves. A DHAR3 knockout mutant exhibited sensitivity to high light (HL). Under HL, the ascorbate redox states were similar in mutant and wild-type plants, while total ascorbate content was significantly lower in the mutant, suggesting that DHAR3 contributes, at least to some extent, to ascorbate recycling. Activation of monodehydroascorbate reductase occurred in dhar3 mutant, which might compensate for the lack of DHAR3. Interestingly, glutathione oxidation was consistently inhibited in dhar3 mutant. These findings indicate that DHAR3 regulates both ascorbate and glutathione redox states to acclimate to HL.

Enhancement of Cytochrome b 559 Indicates Its Possible Involvement in Long-Term High Light Stress Tolerance in Intertidal Macroalgae

The effects of high light (HL) stress on the photosynthetic proteins of green tide-forming macroalga were studied. The naturally grown Ulva prolifera was collected from an intertidal habitat and subjected to 1000 mu E m(-2) s(-1) HL for 12 h. The thylakoid membrane proteins were subsequently extracted and qualitatively analyzed using LC/MS. Quantitative analysis of the thylakoid membrane proteins showed that both PSII reaction center proteins and PSII chlorophyll antenna proteins were decreased. Comparatively, the main PSI reaction center proteins were well preserved. The cytochrome f subunit was significantly decreased, while the other subunits of cytochrome b (6) /f complex were maintained. These results demonstrated that the HL-induced cytochrome b (559) might play a role in photoacclimation of U. prolifera by involving in cyclic photosynthetic electron transfer (PET) flow around PSII. The LhcSR and PsbS proteins were also identified in U. prolifera, and PsbS was noted to be significantly increased under HL stress. The long-term photoacclimation mechanism of U. prolifera under HL stress might not depend on cyclic PET flow around PSI.

Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis.

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in high light stress tolerance, the effects of AOX on chlorophyll synthesis are unclear. Previous studies indicated that during greening, chlorophyll accumulation was largely delayed in plants whose mitochondrial cyanide-resistant respiration was inhibited by knocking out nuclear encoded AOX gene. Here, we showed that this delay of chlorophyll accumulation was more significant under high light condition. Inhibition of cyanide-resistant respiration was also accompanied by the increase of plastid NADPH/NADP(+) ratio, especially under high light treatment which subsequently blocked the import of multiple plastidial proteins, such as some components of the photosynthetic electron transport chain, the Calvin-Benson cycle enzymes and malate/oxaloacetate shuttle components. Overexpression of AOX1a rescued the aox1a mutant phenotype, including the chlorophyll accumulation during greening and plastidial protein import. It thus suggests that light intensity affects chlorophyll synthesis during greening process by a metabolic signal, the AOX-derived plastidial NADPH/NADP(+) ratio change. Further, our results thus revealed a molecular mechanism of chloroplast-mitochondria interactions.

Production of Squalene in Synechocystis sp. PCC 6803

In recent years, there has been an increased interest in the research and development of sustainable alternatives to fossil fuels. Using photosynthetic microorganisms to produce such alternatives is advantageous, since they can achieve direct conversion of carbon dioxide from the atmosphere into the desired product, using sunlight as the energy source. Squalene is a naturally occurring 30-carbon isoprenoid, which has commercial use in cosmetics and in vaccines. If it could be produced sustainably on a large scale, it could also be used instead of petroleum as a raw material for fuels and as feedstock for the chemical industry. The unicellular cyanobacterium Synechocystis PCC 6803 possesses a gene, slr2089, predicted to encode squalene hopene cyclase (Shc), an enzyme converting squalene into hopene, the substrate for forming hopanoids. Through inactivation of slr2089 (shc), we explored the possibility to produce squalene using cyanobacteria. The inactivation led to accumulation of squalene, to a level over 70 times higher than in wild type cells, reaching 0.67 mg OD750 −1 L−1. We did not observe any significant growth deficiency in the Δshc strain compared to the wild type Synechocystis, even at high light conditions, suggesting that the observed squalene accumulation was not detrimental to growth, and that formation of hopene by Shc is not crucial for growth under normal conditions, nor for high-light stress tolerance. Effects of different light intensities and growth stages on squalene accumulation in the Δshc strain were investigated. We also identified a gene, sll0513, as a putative squalene synthase in Synechocystis, and verified its function by inactivation. In this work, we show that it is possible to use the cyanobacterium Synechocystis to generate squalene, a hydrocarbon of commercial interest and a potential biofuel. We also report the first identification of a squalene hopene cyclase, and the second identification of squalene synthase, in cyanobacteria.

Chloroplast movement behavior varies widely among species and does not correlate with high light stress tolerance

It is well known that chloroplasts move in response to changes in blue light intensity in order to optimize light interception, however, little is known about interspecific variation and the relative importance of this mechanism for the high light stress tolerance of plants. We characterized chloroplast movement behavior as changes in light transmission through a leaf in a variety of species ranging from ferns to monocots and eudicots and found a wide spectrum of responses. Most species exhibited a distinct accumulation response compared to the dark positioning, and all species showed a distinct avoidance response. The speed with which transmission values changed during the avoidance response was consistently faster than that during the accumulation response and speeds varied greatly between species. Plants thriving in higher growth light intensities showed greater degrees of accumulation responses and faster changes in transmission than those that prefer lower light intensities. In some species, the chloroplasts on both the adaxial and abaxial leaf surfaces changed their positioning in response to light, while in other species only the chloroplasts on one leaf side responded. No correlation was found between high light stress tolerance and the speed or degree of transmission changes, indicating that plants can compensate for slow and limited transmission changes using other photoprotective mechanisms.