Transient expression in Nicotiana benthamiana for rapid functional analysis of genes involved in non-photochemical quenching and carotenoid biosynthesis

Implicating the red body of Nannochloropsis in forming the recalcitrant cell wall polymer algaenan

Stramenopile algae contribute significantly to global primary productivity, and one class, Eustigmatophyceae, is increasingly studied for applications in high-value lipid production. Yet much about their basic biology remains unknown, including the nature of an enigmatic, pigmented globule found in vegetative cells. Here, we present an in-depth examination of this "red body," focusing on Nannochloropsis oceanica. During the cell cycle, the red body forms adjacent to the plastid, but unexpectedly it is secreted and released with the autosporangial wall following cell division. Shed red bodies contain antioxidant ketocarotenoids, and overexpression of a beta-carotene ketolase results in enlarged red bodies. Infrared spectroscopy indicates long-chain, aliphatic lipids in shed red bodies and cell walls, and UHPLC-HRMS detects a C32 alkyl diol, a potential precursor of algaenan, a recalcitrant cell wall polymer. We propose that the red body transports algaenan precursors from plastid to apoplast to be incorporated into daughter cell walls.

FER-like iron deficiency-induced transcription factor (FIT) accumulates in nuclear condensates

The functional importance of nuclear protein condensation remains often unclear. The bHLH FER-like iron deficiency-induced transcription factor (FIT) controls iron acquisition and growth in plants. Previously described C-terminal serine residues allow FIT to interact and form active transcription factor complexes with subgroup Ib bHLH factors such as bHLH039. FIT has lower nuclear mobility than mutant FITmSS271AA. Here, we show that FIT undergoes a light-inducible subnuclear partitioning into FIT nuclear bodies (NBs). Using quantitative and qualitative microscopy-based approaches, we characterized FIT NBs as condensates that were reversible and likely formed by liquid-liquid phase separation. FIT accumulated preferentially in NBs versus nucleoplasm when engaged in protein complexes with itself and with bHLH039. FITmSS271AA, instead, localized to NBs with different dynamics. FIT colocalized with splicing and light signaling NB markers. The NB-inducing light conditions were linked with active FIT and elevated FIT target gene expression in roots. FIT condensation may affect nuclear mobility and be relevant for integrating environmental and Fe nutrition signals.

How to engineer the unknown: Advancing a quantitative and predictive understanding of plant and soil biology to address climate change

Our basic understanding of carbon cycling in the biosphere remains qualitative and incomplete, precluding our ability to effectively engineer novel solutions to climate change. How can we attempt to engineer the unknown? This challenge has been faced before in plant biology, providing a roadmap to guide future efforts. We use examples from over a century of photosynthesis research to illustrate the key principles that will set future plant engineering on a solid footing, namely, an effort to identify the key control variables, quantify the effects of systematically tuning these variables, and use theory to account for these observations. The main contributions of plant synthetic biology will stem not from delivering desired genotypes but from enabling the kind of predictive understanding necessary to rationally design these genotypes in the first place. Only then will synthetic plant biology be able to live up to its promise.

Zeaxanthin epoxidase is involved in the carotenoid biosynthesis and light-dependent growth of the marine alga Nannochloropsis oceanica

BACKGROUND

The marine alga Nannochloropsis oceanica, an emerging model belonging to Heterokont, is considered as a promising light-driven eukaryotic chassis for transforming carbon dioxide to various compounds including carotenoids. Nevertheless, the carotenogenic genes and their roles in the alga remain less understood and to be further explored.

RESULTS

Here, two phylogenetically distant zeaxanthin epoxidase (ZEP) genes from N. oceanica (NoZEP1 and NoZEP2) were functionally characterized. Subcellular localization experiment demonstrated that both NoZEP1 and NoZEP2 reside in the chloroplast yet with differential distribution patterns. Overexpression of NoZEP1 or NoZEP2 led to increases of violaxanthin and its downstream carotenoids at the expense of zeaxanthin in N. oceanica, with the extent of changes mediated by NoZEP1 overexpression being greater as compared to NoZEP2 overexpression. Suppression of NoZEP1 or NoZEP2, on the other hand, caused decreases of violaxanthin and its downstream carotenoids as well as increases of zeaxanthin; similarly, the extent of changes mediated by NoZEP1 suppression was larger than that by NoZEP2 suppression. Interestingly, chlorophyll a dropped following violaxanthin decrease in a well-correlated manner in response to NoZEP suppression. The thylakoid membrane lipids including monogalactosyldiacylglycerol also correlated with the violaxanthin decreases. Accordingly, NoZEP1 suppression resulted in more attenuated algal growth than NoZEP2 suppression did under either normal light or high light stage.

CONCLUSIONS

The results together support that both NoZEP1 and NoZEP2, localized in the chloroplast, have overlapping roles in epoxidating zeaxanthin to violaxanthin for the light-dependent growth, yet with NoZEP1 being more functional than NoZEP2 in N. oceanica. Our study provides implications into the understanding of carotenoid biosynthesis and future manipulation of N. oceanica for carotenoid production.

PeVDE, a violaxanthin de-epoxidase gene from moso bamboo, confers photoprotection ability in transgenic Arabidopsis under high light

Plants employ an array of photoprotection mechanisms to alleviate the harmful effects of high light intensity. The violaxanthin cycle, which is associated with non-photochemical quenching (NPQ), involves violaxanthin de-epoxidase (VDE), and zeaxanthin epoxidase (ZEP) and is one of the most rapid and efficient mechanisms protecting plants under high light intensity. Woody bamboo is a class of economically and ecologically important evergreen grass species widely distributed in tropical and subtropical areas. However, the function of VDE in bamboo has not yet been elucidated. In this study, we found that high light intensity increased NPQ and stimulated the de-epoxidation of violaxanthin cycle components in moso bamboo (Phyllostachys edulis), whereas, samples treated with the VDE inhibitor (dithiothreitol) exhibited lower NPQ capacity, suggesting that violaxanthin cycle plays an important role in the photoprotection of bamboo. Further analysis showed that not only high light intensity but also extreme temperatures (4 and 42°C) and drought stress upregulated the expression of PeVDE in bamboo leaves, indicating that PeVDE is induced by multiple abiotic stresses. Overexpression of PeVDE under the control of the CaMV 35S promoter in Arabidopsis mutant npq1 mutant could rescue its NPQ, indicating that PeVDE functions in dissipating the excess absorbed light energy as thermal energy in bamboo. Moreover, compared with wild-type (Col-0) plants, the transgenic plants overexpressing PeVDE displayed enhanced photoprotection ability, higher NPQ capacity, slower decline in the maximum quantum yield of photosystem II (F _v /F _m ) under high light intensity, and faster recovery under optimal conditions. These results suggest that PeVDE positively regulates the response to high light intensity in bamboo plants growing in the natural environment, which could improve their photoprotection ability through the violaxanthin cycle and NPQ.

Cloning and homologous characterization of geranylgeranyl pyrophosphate synthase (GGPPS) from Withania somnifera revealed alterations in metabolic flux towards gibberellic acid biosynthesis

Overexpression of a novel geranylgeranyl pyrophosphate synthase gene (WsGGPPS) in planta resulted in increased levels of gibberellic acid and decrease in withanolide content. Withania somnifera (L.) Dunal, the herb from family Solanaceae is one of the most treasured medicinal plant used in traditional medicinal systems owing to its unique stockpile of pharmaceutically active secondary metabolites. Phytochemical and pharmacological studies in this plant were well established, but the genes affecting the regulation of biosynthesis of major metabolites were not well elucidated. In this study cloning and functional characterization of a key enzyme in terpenoid biosynthetic pathway viz. geranylgeranyl pyrophosphate synthase (EC 2.5.1.29) gene from Withania somnifera was performed. The full length WsGGPPS gene contained 1,104 base pairs that encode a polypeptide of 365 amino acids. The quantitative expression analysis suggested that WsGGPPS transcripts were expressed maximally in flower tissues followed by berry tissues. The expression levels of WsGGPPS were found to be regulated by methyl jasmonate (MeJA) and salicylic acid (SA). Amino acid sequence alignment and phylogenetic studies suggested that WsGGPPS had close similarities with GGPPS of Solanum tuberosum and Solanum pennellii. The structural analysis provided basic information about three dimensional features and physicochemical parameters of WsGGPPS protein. Overexpression of WsGGPPS in planta for its functional characterization suggested that the WsGGPPS was involved in gibberellic acid biosynthesis.

CstMYB14 links ROS signaling, apocarotenoid metabolism, and stress response in Crocus sativus L

Reactive oxygen species (ROS) behave as signaling molecules and induce biosynthesis of many secondary metabolites, including apocarotenoids, which play critical roles in stress tolerance through radical scavenging. However, the mechanism that regulates ROS responsive apocarotenoid metabolism and subsequent stress response is unknown. In this study, an R2R3-MYB transcription factor (CstMYB14) was identified from Crocus sativus L., which acts as a regulator of apocarotenoid biosynthesis. CstMYB14 expression increases in response to H₂ O₂ in a concentration and time-dependent manner. CstMYB14 localizes to the nucleus and acts as a transcriptional activator. Over-expression of CstMYB14 in Crocus stigmas enhanced apocarotenoid biosynthesis. Yeast-one-hybrid demonstrated binding of CstMYB14 to promoters of two apocarotenoid pathway genes (phytoene synthase and carotenoid cleavage dioxygenase 2). Nicotiana benthamiana plants overexpressing CstMYB14 showed better growth and higher stress tolerance than wild type plants. Higher antioxidant activity in CstMYB14-Ox plants indicated that stress tolerance might be due to ROS scavenging. These results establish a molecular link between ROS signaling, apocarotenoid metabolism and stress tolerance. Further, CstMYB14 is shown to act as a key regulator which modulates ROS responsive biosynthesis of apocarotenoids which in turn impart stress tolerance through ROS scavenging.

Transient expression systems to rewire plant carotenoid metabolism

Enrichment of foodstuffs with health-promoting metabolites such as carotenoids is a powerful tool to fight against unhealthy eating habits. Dietary carotenoids are vitamin A precursors and reduce risk of several chronical diseases. Additionally, carotenoids and their cleavage products (apocarotenoids) are used as natural pigments and flavors by the agrofood industry. In the last few years, major advances have been made in our understanding of how plants make and store carotenoids in their natural compartments, the plastids. In part, this knowledge has been acquired by using transient expression systems, notably agroinfiltration and viral vectors. These techniques allow profound changes in the carotenoid profile of plant tissues at the desired developmental stage, hence preventing interference with normal plant growth and development. Here we review how transient expression approaches have contributed to learn about the structure and regulation of plant carotenoid biosynthesis and to rewire carotenoid metabolism and storage for efficient biofortification of plant tissues.

Toxicity of copper on marine diatoms, Chaetoceros calcitrans and Nitzchia closterium from Cochin estuary, India

The effects of copper (Cu) toxicity on the growth, pigments, protein, carbohydrate, lipid and antioxidant enzyme activities of two endemic microalgae, Chaetoceros calcitrans and Nitzchia closterium from Cochin estuary were studied and compared. The 96 h median inhibition concentration (IC₅₀) of Cu for C. calcitrans was 143.8 µg L^-1 and that for N. closterium was 204.5 µg L^-1. No observable effect concentration (NOEC), lowest observable effect concentration (LOEC) and chronic value of Cu on C. calcitrans were 17.93 µg L^-1, 31.91 µg L^-1and 24.92 µg L^-1 respectively, whereas that for N. closterium were 18.35 µg L^-1, 36.04 µg L^-1 and 27.2 µg L^-1 respectively. Chlorophyll a and c showed significant variation from the control at NOEC in both species. Carotenoid content showed significant increase at LOEC. The chlorophyll a/c ratio significantly decreased at NOEC and LOEC of N. closterium. In N. closterium catalase (CAT) activity showed significant increase at NOEC and LOEC, but in C. calcitrans it varied significantly above LOEC. Protein content showed a significant decrease at NOEC of C. calcitrans. No significant variation was observed for N. closterium. Carbohydrate showed significant variation between the species at NOEC. Lipid content varied significantly at NOEC of C. calcitrans. Chaetoceros calcitrans was observed to be more sensitive to copper toxicity than N. closterium. The metal stress tolerance mechanism of N. closterium and its bioremediation capacity can be established in further studies. This study also provides an insight on the biochemical changes that happened at NOEC.

Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.

Particle film technology modulates xanthophyll cycle and photochemical dynamics of grapevines grown in the Douro Valley

Field-grown grapevines are often exposed to multiple environmental stresses, which challenges wine-growers to develop sustainable measures to sustain vine growth, yield, and quality. Under field conditions this task is demanding, due to differences in the magnitudes of stresses and associated plant responses. In this study we explored the hypothesis that kaolin-particle film application improves grapevine photoprotection through the regulation of xanthophyll cycle genes, limiting the thermal dissipation of excess energy under harsh environmental conditions. Hence, we selected two grapevine varieties, Touriga-Nacional (TN) and Touriga-Franca (TF), grown in the Douro Demarcated Region, and evaluated changes in light dissipation mechanisms, xanthophyll cycle components, and the expression of xanthophyll cycle genes during the 2017 summer season. The results showed that, from veraison to ripening, kaolin triggered the up-regulation of violaxanthin de-epoxidase (VvVDE1) and zeaxanthin epoxidase (VvZEP1) genes, indicating optimised regulation of the xanthophyll cycle. Kaolin treatment also decreased chlorophyll (Chl_a, Chl_b, Chl_(a+b)) and carotenoid (Car) accumulation under increasing summer stress conditions in both varieties and lowered the non-photochemical quenching (NPQ) of grapevines on ripening, suggesting a long-term response to summer stress. In addition, kaolin-treated grapevines showed increased Chl_a/Chl_b and lower Chl_(a+b)/Car ratios, displaying some features of high light adapted leaves. Overall, this study suggests that kaolin application enabled grapevines to benefit from fluctuating periods of summer stress by managing chlorophyll and carotenoid content and limiting down-regulation of both photochemistry and photoinhibition processes. Under Mediterranean field conditions, kaolin application can be considered an efficient method of minimising summer stress impact on grapevines.

A multifocal approach towards understanding the complexities of carotenoid biosynthesis and accumulation in rice grains

Carotenoids are mostly C40 terpenoids that participate in several important functions in plants including photosynthesis, responses to various forms of stress, signal transduction and photoprotection. While the antioxidant potential of carotenoids is of particular importance for human health, equally important is the role of β-carotene as the precursor for vitamin A in the human diet. Rice, which contributes upto 40% of dietary energy for mankind, contains very low level of β-carotene, thereby making it an important crop for enhancing β-carotene accumulation in its grains and consequently targeting vitamin A deficiency. Biosynthesis of carotenoids in the endosperm of white rice is blocked at the first enzymatic step wherein geranylgeranyl diphosphate is converted to phytoene by the action of phytoene synthase (PSY). Strategies aimed at enhancing β-carotene levels in the endosperm of white rice identified Narcissus pseudonarcissus (npPSY) and bacterial CRT1 as the regulators of the carotenoid biosynthetic pathway in rice. Besides transcriptional regulation of PSY, posttranscriptional regulation of PSY expression by OR gene, molecular synergism between ε-LCY and β-LCY and epigenetic control of CRITSO through SET DOMAIN containing protein appear to be the other regulatory nodes which regulate carotenoid biosynthesis and accumulation in rice grains. In this review, we elucidate a comprehensive and deeper understanding of the regulatory mechanisms of carotenoid metabolism in crops that will enable us to identify an effective tool to alleviate carotenoid content in rice grains.

Maize GOLDEN2-LIKE genes enhance biomass and grain yields in rice by improving photosynthesis and reducing photoinhibition

Photosynthetic efficiency is a major target for improvement of crop yield potential under agricultural field conditions. Inefficiencies can occur in many steps of the photosynthetic process, from chloroplast biogenesis to functioning of the light harvesting and carbon fixation reactions. Nuclear-encoded GOLDEN2-LIKE (GLK) transcription factors regulate some of the earliest steps by activating target genes encoding chloroplast-localized and photosynthesis-related proteins. Here we show that constitutive expression of maize GLK genes in rice leads to enhanced levels of chlorophylls and pigment-protein antenna complexes, and that these increases lead to improved light harvesting efficiency via photosystem II in field-grown plants. Increased levels of xanthophylls further buffer the negative effects of photoinhibition under high or fluctuating light conditions by facilitating greater dissipation of excess absorbed energy as heat. Significantly, the enhanced photosynthetic capacity of field-grown transgenic plants resulted in increased carbohydrate levels and a 30–40% increase in both vegetative biomass and grain yield.

Li et al. improve photosynthetic efficiency in rice by constitutively expressing maize GOLDEN2-like (GLK) genes (ZmG2 and ZmGLK1). They are able to reduce photoinhibition and enhance the photosynthetic potential as well as increase the carbohydrate, biomass and grain yield.

An algal enzyme required for biosynthesis of the most abundant marine carotenoids

Fucoxanthin and its derivatives are the main light-harvesting pigments in the photosynthetic apparatus of many chromalveolate algae and represent the most abundant carotenoids in the world's oceans, thus being major facilitators of marine primary production. A central step in fucoxanthin biosynthesis that has been elusive so far is the conversion of violaxanthin to neoxanthin. Here, we show that in chromalveolates, this reaction is catalyzed by violaxanthin de-epoxidase-like (VDL) proteins and that VDL is also involved in the formation of other light-harvesting carotenoids such as peridinin or vaucheriaxanthin. VDL is closely related to the photoprotective enzyme violaxanthin de-epoxidase that operates in plants and most algae, revealing that in major phyla of marine algae, an ancient gene duplication triggered the evolution of carotenoid functions beyond photoprotection toward light harvesting.

Establishment and optimization of a hemp (Cannabis sativa L.) agroinfiltration system for gene expression and silencing studies

Industrial hemp (Cannabis sativa L.) is a high-yielding annual crop primarily grown for fiber, seeds, and oil. Due to the phytochemical composition of hemp, there has been an increased interest in the market for nutraceuticals and dietary supplements for human health. Recent omics analysis has led to the elucidation of hemp candidate genes involved in the syntheses of specialized metabolites. However, a detailed study of these genes has not been undertaken due to the lack of a stable transformation system. We report for the first time an agroinfiltration system in hemp utilizing vacuum infiltration, which is an alternative method to stable transformation. A combination of 0.015% Silwett L-77, 5 mM ascorbic acid, and thirty second sonication followed by a 10-minute vacuum treatment resulted in the highest β-glucuronidase expression in the leaf, male and female flowers, stem, and root tissues. The phytoene desaturase gene was silenced with a transient hairpin RNA expression, resulting in an albino phenotype in the leaves and the male and female flowers. This agroinfiltration system would be useful for overexpression and silencing studies of target genes to regulate the yield of specialized metabolites in hemp.

Photosynthesis in Arabidopsis Is Unaffected by the Function of the Vacuolar K+ Channel TPK3

Photosynthesis is limited by the slow relaxation of nonphotochemical quenching, which primarily dissipates excess absorbed light energy as heat. Because the heat dissipation process is proportional to light-driven thylakoid lumen acidification, manipulating thylakoid ion and proton flux via transport proteins could improve photosynthesis. However, an important aspect of the current understanding of the thylakoid ion transportome is inaccurate. Using fluorescent protein fusions, we show that the Arabidopsis (Arabidopsis thaliana) two-pore K+ channel TPK3, which had been reported to mediate thylakoid K+ flux, localizes to the tonoplast, not the thylakoid. The localization of TPK3 outside of the thylakoids is further supported by the absence of TPK3 in isolated thylakoids as well as the inability of isolated chloroplasts to import TPK3 protein. In line with the subcellular localization of TPK3 in the vacuole, we observed that photosynthesis in the Arabidopsis null mutant tpk3-1, which carries a transfer DNA insertion in the first exon, remains unaffected. To gain a comprehensive understanding of how thylakoid ion flux impacts photosynthetic efficiency under dynamic growth light regimes, we performed long-term photosynthesis imaging of established and newly isolated transthylakoid K+- and Cl--flux mutants. Our results underpin the importance of the thylakoid ion transport proteins potassium cation efflux antiporter KEA3 and voltage-dependent chloride channel VCCN1 and suggest that the activity of yet unknown K+ channel(s), but not TPK3, is critical for optimal photosynthesis in dynamic light environments.

Rust pathogen effectors: perspectives in resistance breeding

Identification and functional characterization of plant pathogen effectors promise to ameliorate future research and develop effective and sustainable strategies for controlling or containing crop diseases. Wheat is the second most important food crop of the world after rice. Rust pathogens, one of the major biotic stresses in wheat production, are capable of threatening the world food security. Understanding the molecular basis of plant-pathogen interactions is essential for devising novel strategies for resistance breeding and disease management. Now, it has been established that effectors, the proteins secreted by pathogens, play a key role in plant-pathogen interactions. Therefore, effector biology has emerged as one of the most important research fields in plant biology. Recent advances in genomics and bioinformatics have allowed identification of a large repertoire of candidate effectors, while the evolving high-throughput tools have continued to assist in their functional characterization. The repertoires of effectors have become an important resource for better understanding of effector biology of pathosystems and resistance breeding of crop plants. In recent years, a significant progress has been made in the field of rust effector biology. This review describes the recent advances in effector biology of obligate fungal pathogens, identification and functional analysis of wheat rust pathogens effectors and the potential applications of effectors in molecular plant biology and rust resistance breeding in wheat.

A basic Helix-Loop-Helix (SlARANCIO), identified from a Solanum pennellii introgression line, affects carotenoid accumulation in tomato fruits

Carotenoid accumulation in tomato (Solanum lycopersicum) fruits is influenced by environmental stimuli and hormonal signals. However, information on the relative regulatory mechanisms are scanty since many molecular players of the carotenoid biosynthetic pathway are still unknown. Here, we reported a basic Helix-Loop-Helix transcription factor, named SlARANCIO (SlAR), whose silencing influences carotenoid accumulation in tomato fruits. The SlAR gene was found in the S. pennellii introgression line (IL) 12-4SL that holds the carotenoid QTL lyc12.1. We observed that the presence of the wild region in a cultivated genetic background led to a decrease in total carotenoid content of IL12-4SL fruits. To get insights into the function of SlAR, a quick reverse genetic approach was carried out. Virus-induced gene silencing of SlAR in S. lycopersicum M82 and MicroTom fruits reproduced the same phenotype observed in IL12-4SL, i.e. decreased content of lycopene and total carotenoids. Vice versa, the overexpression of SlAR in Nicotiana benthamiana leaves increased the content of total carotenoids and chlorophylls. Our results, combined with public transcriptomic data, highly suggest that SlAR acts indirectly on the carotenoid pathway and advances current knowledge on the molecular regulators controlling lyc12.1 and, potentially, precursors of carotenoid biosynthesis.

Functional characteristics of phenolic compounds accumulated in young leaves of two subtropical forest tree species of different successional stages

The abundance of phenolic compounds (including anthocyanins) in leaves is associated with photosynthetic performance, but the regulatory mechanism is unclear. Schima superba Gardn. et Champ. and Cryptocarya concinna Hance., which exhibit distinct anthocyanin accumulation patterns, are dominant tree species in the early- and late-successional stages, respectively, of subtropical forests in China. RNA-seq and analyses of phenolic concentrations, antioxidant capacity and photosynthetic characteristics were performed on young and mature leaves of these two species under contrasting light conditions. The high-light-acclimated young leaves of S. superba and C. concinna and low-light-acclimated young leaves of C. concinna were red. These red leaves had higher ratios of electron transport rate to gross photosynthesis (ETR:Pgross) and total antioxidant capacity to chlorophyll (TAC:Chl) than did the green leaves, regardless of light conditions. In addition, the red leaves had a higher expression level of the UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT) gene than did the green leaves, irrespective of light conditions. Total antioxidant capacity was positively correlated with flavonoid content in C. concinna leaves and with total phenolic content in leaves of both species under both high and low light. Consistent with the measurements of photosynthetic performance and flavonoids:Chl ratio, photosynthesis-related genes were extensively downregulated and flavonoid-pathway-related genes were extensively upregulated in young leaves relative to mature leaves. Under high and low light, both non-photochemical quenching and TAC:Chl, which serve as different types of photoprotective tools, were enhanced in young leaves of S. superba, whereas only TAC:Chl was enhanced in young leaves of C. concinna. Our results indicate that the biosynthesis of phenolic compounds in young leaves is likely enhanced by an imbalance between photosynthetic electron supply and demand and that flavonoids play a larger role in meditating photoprotection in late-successional species than in early-successional ones.

A functional zeaxanthin epoxidase from red algae shedding light on the evolution of light-harvesting carotenoids and the xanthophyll cycle in photosynthetic eukaryotes

The epoxy-xanthophylls antheraxanthin and violaxanthin are key precursors of light-harvesting carotenoids and participate in the photoprotective xanthophyll cycle. Thus, the invention of zeaxanthin epoxidase (ZEP) catalyzing their formation from zeaxanthin has been a fundamental step in the evolution of photosynthetic eukaryotes. ZEP genes have only been found in Viridiplantae and chromalveolate algae with secondary plastids of red algal ancestry, suggesting that ZEP evolved in the Viridiplantae and spread to chromalveolates by lateral gene transfer. By searching publicly available sequence data from 11 red algae covering all currently recognized red algal classes we identified ZEP candidates in three species. Phylogenetic analyses showed that the red algal ZEP is most closely related to ZEP proteins from photosynthetic chromalveolates possessing secondary plastids of red algal origin. Its enzymatic activity was assessed by high performance liquid chromatography (HPLC) analyses of red algal pigment extracts and by cloning and functional expression of the ZEP gene from Madagascaria erythrocladioides in leaves of the ZEP-deficient aba2 mutant of Nicotiana plumbaginifolia. Unlike other ZEP enzymes examined so far, the red algal ZEP introduces only a single epoxy group into zeaxanthin, yielding antheraxanthin instead of violaxanthin. The results indicate that ZEP evolved before the split of Rhodophyta and Viridiplantae and that chromalveolates acquired ZEP from the red algal endosymbiont and not by lateral gene transfer. Moreover, the red algal ZEP enables engineering of transgenic plants incorporating antheraxanthin instead of violaxanthin in their photosynthetic machinery.

Engineering the lutein epoxide cycle into Arabidopsis thaliana

Although sunlight provides the energy necessary for plants to survive and grow, light can also damage reaction centers of photosystem II (PSII) and reduce photochemical efficiency. To prevent damage, plants possess photoprotective mechanisms that dissipate excess excitation. A subset of these mechanisms is collectively referred to as NPQ, or nonphotochemical quenching of chlorophyll a fluorescence. The regulation of NPQ is intrinsically linked to the cycling of xanthophylls that affects the kinetics and extent of the photoprotective response. The violaxanthin cycle (VAZ cycle) and the lutein epoxide cycle (LxL cycle) are two xanthophyll cycles found in vascular plants. The VAZ cycle has been studied extensively, owing in large part to its presence in model plant species where mutants are available to aid in its characterization. In contrast, the LxL cycle is not found in model plants, and its role in photosynthetic processes has been more difficult to define. To address this challenge, we introduced the LxL cycle into Arabidopsis thaliana and functionally isolated it from the VAZ cycle. Using these plant lines, we showed an increase in dark-acclimated PSII efficiency associated with Lx accumulation and demonstrated that violaxanthin deepoxidase is responsible for the light-driven deepoxidation of Lx. Conversion of Lx to L was reversible during periods of low light and occurred considerably faster than rates previously described in nonmodel species. Finally, we present clear evidence of the LxL cycle's role in modulating a rapid component of NPQ that is necessary to prevent photoinhibition in excess light.

Dissecting and modeling zeaxanthin- and lutein-dependent nonphotochemical quenching in Arabidopsis thaliana

Photosynthetic organisms use various photoprotective mechanisms to dissipate excess photoexcitation as heat in a process called nonphotochemical quenching (NPQ). Regulation of NPQ allows for a rapid response to changes in light intensity and in vascular plants, is primarily triggered by a pH gradient across the thylakoid membrane (∆pH). The response is mediated by the PsbS protein and various xanthophylls. Time-correlated single-photon counting (TCSPC) measurements were performed on Arabidopsis thaliana to quantify the dependence of the response of NPQ to changes in light intensity on the presence and accumulation of zeaxanthin and lutein. Measurements were performed on WT and mutant plants deficient in one or both of the xanthophylls as well as a transgenic line that accumulates lutein via an engineered lutein epoxide cycle. Changes in the response of NPQ to light acclimation in WT and mutant plants were observed between two successive light acclimation cycles, suggesting that the character of the rapid and reversible response of NPQ in fully dark-acclimated plants is substantially different from in conditions plants are likely to experience caused by changes in light intensity during daylight. Mathematical models of the response of zeaxanthin- and lutein-dependent reversible NPQ were constructed that accurately describe the observed differences between the light acclimation periods. Finally, the WT response of NPQ was reconstructed from isolated components present in mutant plants with a single common scaling factor, which enabled deconvolution of the relative contributions of zeaxanthin- and lutein-dependent NPQ.

Light-harvesting antenna complexes in the moss Physcomitrella patens: implications for the evolutionary transition from green algae to land plants

Plants have successfully adapted to a vast range of terrestrial environments during their evolution. To elucidate the evolutionary transition of light-harvesting antenna proteins from green algae to land plants, the moss Physcomitrella patens is ideally placed basally among land plants. Compared to the genomes of green algae and land plants, the P. patens genome codes for more diverse and redundant light-harvesting antenna proteins. It also encodes Lhcb9, which has characteristics not found in other light-harvesting antenna proteins. The unique complement of light-harvesting antenna proteins in P. patens appears to facilitate protein interactions that include those lost in both green algae and land plants with regard to stromal electron transport pathways and photoprotection mechanisms. This review will highlight unique characteristics of the P. patens light-harvesting antenna system and the resulting implications about the evolutionary transition during plant terrestrialization.

Engineering central metabolism - a grand challenge for plant biologists

The goal of increasing crop productivity and nutrient-use efficiency is being addressed by a number of ambitious research projects seeking to re-engineer photosynthetic biochemistry. Many of these projects will require the engineering of substantial changes in fluxes of central metabolism. However, as has been amply demonstrated in simpler systems such as microbes, central metabolism is extremely difficult to rationally engineer. This is because of multiple layers of regulation that operate to maintain metabolic steady state and because of the highly connected nature of central metabolism. In this review we discuss new approaches for metabolic engineering that have the potential to address these problems and dramatically improve the success with which we can rationally engineer central metabolism in plants. In particular, we advocate the adoption of an iterative 'design-build-test-learn' cycle using fast-to-transform model plants as test beds. This approach can be realised by coupling new molecular tools to incorporate multiple transgenes in nuclear and plastid genomes with computational modelling to design the engineering strategy and to understand the metabolic phenotype of the engineered organism. We also envisage that mutagenesis could be used to fine-tune the balance between the endogenous metabolic network and the introduced enzymes. Finally, we emphasise the importance of considering the plant as a whole system and not isolated organs: the greatest increase in crop productivity will be achieved if both source and sink metabolism are engineered.

Improving photosynthesis and crop productivity by accelerating recovery from photoprotection

Crop leaves in full sunlight dissipate damaging excess absorbed light energy as heat. When sunlit leaves are shaded by clouds or other leaves, this protective dissipation continues for many minutes and reduces photosynthesis. Calculations have shown that this could cost field crops up to 20% of their potential yield. Here, we describe the bioengineering of an accelerated response to natural shading events in Nicotiana (tobacco), resulting in increased leaf carbon dioxide uptake and plant dry matter productivity by about 15% in fluctuating light. Because the photoprotective mechanism that has been altered is common to all flowering plants and crops, the findings provide proof of concept for a route to obtaining a sustainable increase in productivity for food crops and a much-needed yield jump.

Evolution of an atypical de-epoxidase for photoprotection in the green lineage

Plants, algae and cyanobacteria need to regulate photosynthetic light harvesting in response to the constantly changing light environment. Rapid adjustments are required to maintain fitness because of a trade-off between efficient solar energy conversion and photoprotection. The xanthophyll cycle, in which the carotenoid pigment violaxanthin is reversibly converted into zeaxanthin, is ubiquitous among green algae and plants and is necessary for the regulation of light harvesting, protection from oxidative stress and adaptation to different light conditions(1,2). Violaxanthin de-epoxidase (VDE) is the key enzyme responsible for zeaxanthin synthesis from violaxanthin under excess light. Here we show that the Chlorophycean VDE (CVDE) gene from the model green alga Chlamydomonas reinhardtii encodes an atypical VDE. This protein is not homologous to the VDE found in plants and is instead related to a lycopene cyclase from photosynthetic bacteria(3). Unlike the plant-type VDE that is located in the thylakoid lumen, the Chlamydomonas CVDE protein is located on the stromal side of the thylakoid membrane. Phylogenetic analysis suggests that CVDE evolved from an ancient de-epoxidase that was present in the common ancestor of green algae and plants, providing evidence of unexpected diversity in photoprotection in the green lineage.