Chrysanthemum morifolium β-carotene hydroxylase overexpression promotes Arabidopsis thaliana tolerance to high light stress

The β-carotene hydroxylase gene (BCH) regulates zeaxanthin production in response to high light levels ro protect Chrysanthemum morifolium plants against light-induced damage. In this study, the Chrysanthemum morifolium CmBCH1 and CmBCH2 genes were cloned and their functional importance was assessed by overexpressing them in Arabidopsis thaliana. These transgenic plants were evaluated for gene-related changes in phenotypic characteristics, photosynthetic activity, fluorescence properties, carotenoid biosynthesis, aboveground/belowground biomass, pigment content, and the expression of light-regulated genes under conditions of high light stress relative to wild-type (WT) plants. When exposed to high light stress, WT A. thaliana leaves turned yellow and the overall biomass was reduced compared to that of the transgenic plants. WT plants exposed to high light stress also exhibited significant reductions in the net photosynthetic rate, stomatal conductance, Fv/Fm, qP, and ETR, whereas these changes were not observed in the transgenic CmBCH1 and CmBCH2 plants. Lutein and zaxanthin levels were significantly increased in the transgenic CmBCH1 and CmBCH2 lines, with progressive induction with prolonged light exposure, whereas no significant changes were observed in light-exposed WT plants. The transgenic plants also expressed higher levels of most carotenoid biosynthesis pathway genes, including phytoene synthase (AtPSY), phytoene desaturase (AtPDS), lycopene-β-cyclase (AtLYCB), and ζ-carotene desaturase (AtZDS). The elongated hypocotyl 5 (HY5) and succinate dehydrogenase (SDH) genes were significantly induced following exposure to high light conditions for 12h, whereas phytochrome-interacting factor 7 (PIF7) was significantly downregulated in these plants.

Exploring Nordic microalgae as a potential novel source of antioxidant and bioactive compounds

Nordic microalgae are a group of photosynthetic organisms acclimated to growth at low temperature and in varying light conditions; the subarctic climate offers bright days with moderate temperatures during summer and cold and dark winter months. The robustness to these natural stress conditions makes the species interesting for large-scale cultivation in harsh environments and for the production of high-value compounds. The aim of this study was to explore the ability of nineteen species of Nordic microalgae to produce different bioactive compounds, such as carotenoids or polyphenols. The results showed that some of these strains are able to produce high amounts of carotenoids (over 12 mg·g^-1 dry weight) and phenolic compounds (over 20 mg GAE·g^-1 dry weight). Based on these profiles, six species were selected for cultivation under high light and cold stress (500 μmol·m^-2·s^-1 and 10 ˚C). The strains Chlorococcum sp. (MC1) and Scenedesmus sp. (B2-2) exhibited similar values of biomass productivity under standard or stress conditions, but produced higher concentrations of carotenoids (an increase of 40% and 25%, respectively), phenolic compounds (an increase of 40% and 30%, respectively), and showed higher antioxidant capacity (an increase of 15% and 20%, respectively) during stress. The results highlight the ability of these Nordic microalgae as outstanding producers of bioactive compounds, justifying their cultivation at large scale in Nordic environments.

Functional characterization and comparison of lycopene epsilon-cyclase genes in Nicotiana tabacum

BACKGROUND

Lycopene epsilon-cyclase (ε-LCY) is a key enzyme in the carotenoid biosynthetic pathway (CBP) of higher plants. In previous work, we cloned two Ntε-LCY genes from allotetraploid tobacco (Nicotiana tabacum), Ntε-LCY2 and Ntε-LCY1, and demonstrated the overall effect of Ntε-LCY genes on carotenoid biosynthesis and stress resistance. However, their genetic and functional characteristics require further research in polyploid plants.

RESULTS

Here, we used CRISPR/Cas9 to obtain Ntε-LCY2 and Ntε-LCY1 mutants in allotetraploid N.tabacum K326. Ntε-LCY2 and Ntε-LCY1 had similar promoter cis-acting elements, including light-responsive elements. The Ntε-LCY genes were expressed in roots, stems, leaves, flowers, and young fruit, and their highest expression levels were found in leaves. Ntε-LCY2 and Ntε-LCY1 genes responded differently to normal light and high light stress. Both the Ntε-LCY2 and the Ntε-LCY1 mutants had a more rapid leaf growth rate, especially ntε-lcy2-1. The expression levels of CBP genes were increased in the ntε-lcy mutants, and their total carotenoid content was higher. Under both normal light and high light stress, the ntε-lcy mutants had higher photosynthetic capacities and heat dissipation levels than the wild type, and this was especially true of ntε-lcy2-1. The reactive oxygen species content was lower in leaves of the ntε-lcy mutants.

CONCLUSION

In summary, the expression patterns and biological functions of the Ntε-LCY genes Ntε-LCY1 and Ntε-LCY2 differed in several respects. The mutation of Ntε-LCY2 was associated with a greater increase in the content of chlorophyll and various carotenoid components, and it enhanced the stress resistance of tobacco plants under high light.

The major photoprotective role of anthocyanins in leaves of Arabidopsis thaliana under long-term high light treatment: antioxidant or light attenuator?

Anthocyanins are water-soluble pigments in plants known for their photoprotective role against photoinhibitory and photooxidative damage under high light (HL). However, it remains unclear whether light-shielding or antioxidant activity plays a major role in the photoprotection exerted by anthocyanins under HL stress. To shed light on this question, we analyzed the physiological and biochemical responses to HL of three Arabidopsis thaliana lines (Col, chi, ans) with different light absorption and antioxidant characteristics. Under HL, ans had the highest antioxidant capacity, followed by Col, and finally chi; Col had the strongest light attenuation capacity, followed by chi, and finally ans. The line ans had weaker physiological activity of chloroplasts and more severe oxidative damage than chi after HL treatment. Col with highest photoprotection of light absorption capacity had highest resistance to HL among the three lines. The line ans with high antioxidant capacity could not compensate for its disadvantages in HL caused by the absence of the light-shielding function of anthocyanins. In addition, the expression level of the Anthocyanin Synthase (ANS) gene was most upregulated after HL treatment, suggesting that the conversion of colorless into colored anthocyanin precursors was necessary under HL. The contribution of anthocyanins to flavonoids, phenols, and antioxidant capacity increased in the late period of HL, suggesting that plants prefer to synthesize red anthocyanins (a group of colored antioxidants) over other colorless antioxidants to cope with HL. These experimental observations indicate that the light attenuation role of anthocyanins is more important than their antioxidant role in photoprotection.

Transcriptome-based analysis of the effects of salicylic acid and high light on lipid and astaxanthin accumulation in Haematococcus pluvialis

BACKGROUND

The unicellular alga Haematococcus pluvialis has achieved considerable interests for its capacity to accumulate large amounts of triacylglycerol and astaxanthin under various environmental stresses. To our knowledge, studies focusing on transcriptome research of H. pluvialis under exogenous hormones together with physical stresses are rare. In the present study, the change patterns at transcriptome level were analyzed to distinguish the multiple defensive systems of astaxanthin and fatty acid metabolism against exogenous salicylic acid and high light (SAHL) stresses.

RESULTS

Based on RNA-seq data, a total of 112,463 unigenes and 61,191 genes were annotated in six databases, including NR, KEGG, Swiss-Prot, PFAM, COG and GO. Analysis of differentially expressed genes (DEGs) in KEGG identified many transcripts that associated with the biosynthesis of primary and secondary metabolites, photosynthesis, and immune system responses. Furthermore, 705 unigenes predicted as putative transcription factors (TFs) were identified, and the most abundant TFs families were likely to be associated with the biosynthesis of astaxanthin and fatty acid in H. pluvialis upon exposure to SAHL stresses. Additionally, majority of the fifteen key genes involved in astaxanthin and fatty acid biosynthesis pathways presented the same expression pattern, resulting in increased accumulation of astaxanthin and fatty acids in single celled H. pluvialis, in which astaxanthin content increased from 0.56 ± 0.05 mg·L-1 at stage Control to 0.89 ± 0.12 mg·L-1 at stage SAHL_48. And positive correlations were observed among these studied genes by Pearson Correlation (PC) analysis, indicating the coordination between astaxanthin and fatty acid biosynthesis. In addition, protein-protein interaction (PPI) network analysis also demonstrated that this coordination might be at transcriptional level.

CONCLUSION

The results in this study provided valuable information to illustrate the molecular mechanisms of coordinate relations between astaxanthin and fatty acid biosynthesis. And salicylic acid might play a role in self-protection processes of cells, helping adaption of H. pluvialis to high light stress.

PCC 6803

High light (HL) exposure leads to photoinhibition and excess accumulation of toxic reactive oxygen species (ROS) in photosynthetic organisms, negatively impacting the global primary production. In this study, by screening a mutant library, a gene related with bicarbonate transport, slr1512, was found involved in HL acclimation in model cyanobacterium Synechocystis sp. PCC 6803. Comparative growth analysis showed that the slr1512 knockout mutant dramatically enhanced the tolerance of Synechocystis towards long-term HL stress (200 μmol photons m^-2 s^-1) than the wild type, achieving an enhanced growth by ~1.95-folds after 10 d. The phenotype differences between Δslr1512 and the wild type were analyzed via absorption spectrum and chlorophyll a content measurement. In addition, the accessible bicarbonate controlled by slr1512 and decreased PSII activity were demonstrated, and they were found to be the key factors affecting the tolerance of Synechocystis against HL stress. Further analysis confirmed that intracellular bicarbonate can significantly affect the activity of photosystem II, leading to the altered accumulation of toxic ROS under HL. Finally, a comparative transcriptomics was applied to determine the differential responses to HL between Δslr1512 and the wild type. This work provides useful insights to long-term acclimation mechanisms towards HL and valuable information to guide the future tolerance engineering of cyanobacteria against HL.

Non-photochemical quenching-dependent acclimation and thylakoid organization of Chlamydomonas reinhardtii to high light stress

Light is essential for all photosynthetic organisms while an excess of it can lead to damage mainly the photosystems of the thylakoid membrane. In this study, we have grown Chlamydomonas reinhardtii cells in different intensities of high light to understand the photosynthetic process with reference to thylakoid membrane organization during its acclimation process. We observed, the cells acclimatized to long-term response to high light intensities of 500 and 1000 µmol m^-2 s^-1 with faster growth and more biomass production when compared to cells at 50 µmol m^-2 s^-1 light intensity. The ratio of Chl a/b was marginally decreased from the mid-log phase of growth at the high light intensity. Increased level of zeaxanthin and LHCSR3 expression was also found which is known to play a key role in non-photochemical quenching (NPQ) mechanism for photoprotection. Changes in photosynthetic parameters were observed such as increased levels of NPQ, marginal change in electron transport rate, and many other changes which demonstrate that cells were acclimatized to high light which is an adaptive mechanism. Surprisingly, PSII core protein contents have marginally reduced when compared to peripherally arranged LHCII in high light-grown cells. Further, we also observed alterations in stromal subunits of PSI and low levels of PsaG, probably due to disruption of PSI assembly and also its association with LHCI. During the process of acclimation, changes in thylakoid organization occurred in high light intensities with reduction of PSII supercomplex formation. This change may be attributed to alteration of protein-pigment complexes which are in agreement with circular dichoism spectra of high light-acclimatized cells, where decrease in the magnitude of psi-type bands indicates changes in ordered arrays of PSII-LHCII supercomplexes. These results specify that acclimation to high light stress through NPQ mechanism by expression of LHCSR3 and also observed changes in thylakoid protein profile/supercomplex formation lead to low photochemical yield and more biomass production in high light condition.

Transcriptome analysis of Panax ginseng response to high light stress

Background

Ginseng (Panax ginseng Meyer) is an essential source of pharmaceuticals and functional foods. Ginseng productivity has been compromised by high light (HL) stress, which is one of the major abiotic stresses during the ginseng cultivation period. The genetic improvement for HL tolerance in ginseng could be facilitated by analyzing its genetic and molecular characteristics associated with HL stress.

Methods

Genome-wide analysis of gene expression was performed under HL and recovery conditions in 1-year-old Korean ginseng (P. ginseng cv. Chunpoong) using the Illumina HiSeq platform. After de novo assembly of transcripts, we performed expression profiling and identified differentially expressed genes (DEGs). Furthermore, putative functions of identified DEGs were explored using Gene Ontology terms and Kyoto Encyclopedia of Genes and Genome pathway enrichment analysis.

Results

A total of 438 highly expressed DEGs in response to HL stress were identified and selected from 29,184 representative transcripts. Among the DEGs, 326 and 114 transcripts were upregulated and downregulated, respectively. Based on the functional analysis, most upregulated and a significant number of downregulated transcripts were related to stress responses and cellular metabolic processes, respectively.

Conclusion

Transcriptome profiling could be a strategy to comprehensively elucidate the genetic and molecular mechanisms of HL tolerance and susceptibility. This study would provide a foundation for developing breeding and metabolic engineering strategies to improve the environmental stress tolerance of ginseng.

The hydroperoxide lyase branch of the oxylipin pathway protects against photoinhibition of photosynthesis

This study describes a new role for hydroperoxide lyase branch of oxylipin biosynthesis pathway in protecting photosynthetic apparatus under high light conditions. Lipid-derived signaling molecules, oxylipins, produced by a multi-branch pathway are central in regulation of a wide range of functions. The two most known branches, allene oxide synthase (AOS) and 13-hydroperoxide lyase (HPL) pathways, are best recognized as producers of defense compounds against biotic challenges. In the present work, we examine the role of these two oxylipin branches in plant tolerance to the abiotic stress, namely excessive light. Towards this goal, we have analyzed variable chlorophyll fluorescence parameters of intact leaves of Arabidopsis thaliana genotypes with altered oxylipin profile, followed by examining the impact of exogenous application of selected oxylipins on functional activity of photosynthetic apparatus in intact leaves and isolated thylakoid membranes. Our findings unequivocally bridge the function of oxylipins to photosynthetic processes. Specifically, HPL overexpressing lines display enhanced adaptability in response to high light treatment as evidenced by lower rate constant of photosystem 2 (PS2) photoinhibition and higher rate constant of PS2 recovery after photoinhibition. In addition, exogenous application of linolenic acid, 13-hydroperoxy linolenic acid, 12-oxophytodienoic acid, and methyl jasmonate individually, suppresses photochemical activity of PS2 in intact plants and isolated thylakoid membranes, while application of HPL-branch metabolites-does not. Collectively these data implicate function of HPL branch of oxylipin biosynthesis pathway in guarding PS2 under high light conditions, potentially exerted through tight regulation of free linolenic acid and 13-hydroperoxy linolenic acid levels, as well as competition with production of metabolites by AOS-branch of the oxylipin pathway.

Photoinhibition of photosystem I in a pea mutant with altered LHCII organization

Comparative analysis of in vivo chlorophyll fluorescence imaging revealed that photosystem II (PSII) photochemical efficiency (Fv/Fm) of leaves of the Costata 2/133 pea mutant with altered pigment composition and decreased level of oligomerization of the light harvesting chlorophyll a/b-protein complexes (LHCII) of PSII (Dobrikova et al., 2000; Ivanov et al., 2005) did not differ from that of WT. In contrast, photosystem I (PSI) activity of the Costata 2/133 mutant measured by the far-red (FR) light inducible P700 (P700(+)) signal exhibited 39% lower steady state level of P700(+), a 2.2-fold higher intersystem electron pool size (e(-)/P700) and higher rate of P700(+) re-reduction, which indicate an increased capacity for PSI cyclic electron transfer (CET) in the Costata 2/133 mutant than WT. The mutant also exhibited a limited capacity for state transitions. The lower level of oxidizable P700 (P700(+)) is consistent with a lower amount of PSI related chlorophyll protein complexes and lower abundance of the PsaA/PsaB heterodimer, PsaD and Lhca1 polypeptides in Costata 2/133 mutant. Exposure of WT and the Costata 2/133 mutant to high light stress resulted in a comparable photoinhibition of PSII measured in vivo, although the decrease of Fv/Fm was modestly higher in the mutant plants. However, under the same photoinhibitory conditions PSI photochemistry (P700(+)) measured as ΔA820-860 was inhibited to a greater extent (50%) in the Costata 2/133 mutant than in the WT (22%). This was accompanied by a 50% faster re-reduction rate of P700(+) in the dark indicating a higher capacity for CET around PSI in high light treated mutant leaves. The role of chloroplast thylakoid organization on the stability of the PSI complex and its susceptibility to high light stress is discussed.

Transcriptional regulation of the stress-responsive light harvesting complex genes in Chlamydomonas reinhardtii

Dissipating excess energy of light is critical for photosynthetic organisms to keep the photosynthetic apparatus functional and less harmful under stressful environmental conditions. In the green alga Chlamydomonas reinhardtii, efficient energy dissipation is achieved by a process called non-photochemical quenching (NPQ), in which a distinct member of light harvesting complex, LHCSR, is known to play a key role. Although it has been known that two very closely related genes (LHCSR3.1 and LHCSR3.2) encoding LHCSR3 protein and another paralogous gene LHCSR1 are present in the C. reinhardtii genome, it is unclear how these isoforms are differentiated in terms of transcriptional regulation and functionalization. Here, we show that transcripts of both of the isoforms, LHCSR3.1 and LHCSR3.2, are accumulated under high light stress. Reexamination of the genomic sequence and gene models along with survey of sequence motifs suggested that these two isoforms shared an almost identical but still distinct promoter sequence and a completely identical polypeptide sequence, with more divergent 3'-untranscribed regions. Transcriptional induction under high light condition of both isoforms was suppressed by treatment with a photosystem II inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and a calmodulin inhibitor W7. Despite a similar response to high light, the inhibitory effects of DCMU and W7 to the LHCSR1 transcript accumulation were limited compared to LHCSR3 genes. These results suggest that the transcription of LHCSR paralogs in C. reinhardtii are regulated by light signal and differentially modulated via photosynthetic electron transfer and calmodulin-mediated calcium signaling pathway(s).