Expression studies of the zeaxanthin epoxidase gene in nicotiana plumbaginifolia

Effects of Drought Stress on Abscisic Acid Content and Its Related Transcripts in Allium fistulosum-A. cepa Monosomic Addition Lines

Climate change has resulted in an increased demand for Japanese bunching onions (Allium fistulosum L., genomes FF) with drought resistance. A complete set of alien monosomic addition lines of A. fistulosum with extra chromosomes from shallot (A. cepa L. Aggregatum group, AA), represented as FF + 1A-FF + 8A, displays a variety of phenotypes that significantly differ from those of the recipient species. In this study, we investigated the impact of drought stress on abscisic acid (ABA) and its precursor, β-carotene, utilizing this complete set. In addition, we analyzed the expression levels of genes related to ABA biosynthesis, catabolism, and drought stress signal transduction in FF + 1A and FF + 6A, which show characteristic variations in ABA accumulation. A number of unigenes related to ABA were selected through a database using Allium TDB. Under drought conditions, FF + 1A exhibited significantly higher ABA and β-carotene content compared with FF. Additionally, the expression levels of all ABA-related genes in FF + 1A were higher than those in FF. These results indicate that the addition of chromosome 1A from shallot caused the high expression of ABA biosynthesis genes, leading to increased levels of ABA accumulation. Therefore, it is expected that the introduction of alien genes from the shallot will upwardly modify ABA content, which is directly related to stomatal closure, leading to drought stress tolerance in FF.

Expression pattern of candidate genes and their correlation with various metabolites of abscisic acid biosynthetic pathway under drought stress in rice

Drought hampers global rice production. Abscisic acid (ABA) plays versatile roles under different environmental stresses. While the link between drought and ABA is known, its effect on ABA biosynthesis genes and metabolites is unclear. This study explored the impact of drought on various metabolites, namely beta-carotene, zeaxanthin, antheraxanthin, violaxanthin, neoxanthin, and candidate genes viz. zeaxanthin epoxidase (ZEP) and 9-cis epoxycarotenoid dioxygenase (NCED) of ABA biosynthesis pathway in rice cultivars (N22 and IR64) at anthesis {65 DAT (Days after transplanting)} with different stress levels. In stressed plants, zeaxanthin significantly increased (92%), while the concentration of beta-carotene, antheraxanthin, violaxanthin and neoxanthin decreased as drought stress progressed. The concentration of metabolites in roots was notably lower than in leaves in both genotypes. The ZEP expression was upregulated in roots (8.24-fold) under drought stress. Among five NCED isoforms, NCED3 showed significant upregulation (7.29-fold) in leaf and root tissue. NCED1 was significantly downregulated as stress progressed and was negatively correlated with ABA accumulation. NCED2, NCED4 and NCED5 showed no significant change in their expression. Drying and rolling of rice leaves was observed after imparting drought stress. The findings revealed that drought stress significantly influenced the expression of candidate genes and the concentration of metabolites of the ABA biosynthesis pathway. There was a significantly higher accumulation of ABA in N22 leaves (47%) and roots (30%) compared to IR64. The N22, a drought-tolerant genotype, exhibited significantly higher concentrations of intermediates and demonstrated increased expression of ZEP and NCED3, potentially contributing to its resilience against drought.

Flashing lights affect the photophysiology and expression of carotenoid and lipid synthesis genes in Nannochloropsis gaditana

Nannochloropsis gaditana is a promising microalga for biotechnology. One of the strategies to stimulate its full potential in metabolite production is exposure to flashing lights. Here, we report how N. gaditana adapts to different flashing light regimes (5, 50, and 500 Hz) by changing its cellular physiology and the relative expression of genes related to critical cellular functions. We analyzed the differential mRNA abundance of genes related to photosynthesis, nitrogen assimilation and biosynthesis of chlorophyll, carotenoids, lipids, fatty acids and starch. Analysis of photosynthetic efficiency and high mRNA abundance of photoprotection genes supported the inference that excess excitation energy provided by light absorbance during photosynthesis was produced under low frequency flashing lights and was dissipated by photopigments via the xanthophyll-cycle. Increased relative expression levels of genes related to the synthesis of carotenoids and chlorophyll confirmed the accumulation of photopigments previously observed at low frequency flashing lights. Higher differential mRNA abundance of genes related to the triacylglycerol biosynthesis were observed at lower frequency flashing lights, possibly triggered by a poor nitrogen assimilation caused by low mRNA abundance of a nitrate reductase gene. This study advances a new understanding of algal physiology and metabolism leading to improved cellular performance and metabolite production.

High non-photochemical quenching of VPZ transgenic potato plants limits CO2 assimilation under high light conditions and reduces tuber yield under fluctuating light

Under natural conditions, photosynthesis has to be adjusted to fluctuating light intensities. Leaves exposed to high light dissipate excess light energy in form of heat at photosystem II (PSII) by a process called non-photochemical quenching (NPQ). Upon fast transition from light to shade, plants lose light energy by a relatively slow relaxation from photoprotection. Combined overexpression of violaxanthin de-epoxidase (VDE), PSII subunit S (PsbS) and zeaxanthin epoxidase (ZEP) in tobacco accelerates relaxation from photoprotection, and increases photosynthetic productivity. In Arabidopsis, expression of the same three genes (VPZ) resulted in a more rapid photoprotection but growth of the transgenic plants was impaired. Here we report on VPZ expressing potato plants grown under various light regimes. Similar to tobacco and Arabidopsis, induction and relaxation of NPQ was accelerated under all growth conditions tested, but did not cause an overall increased photosynthetic rate or growth of transgenic plants. Tuber yield of VPZ expressing plants was unaltered as compared to control plants under constant light conditions and even decreased under fluctuating light conditions. Under control conditions, levels of the phytohormone abscisic acid (ABA) were found to be elevated, indicating an increased violaxanthin availability in VPZ plants. However, the increased basal ABA levels did not improve drought tolerance of VPZ transgenic potato plants under greenhouse conditions. The failure to benefit from improved photoprotection is most likely caused by a reduced radiation use efficiency under high light conditions resulting from a too strong NPQ induction. Mitigating this negative effect in the future might help to improve photosynthetic performance in VPZ expressing potato plants.

A C2-Domain Abscisic Acid-Related Gene, IbCAR1, Positively Enhances Salt Tolerance in Sweet Potato (Ipomoea batatas (L.) Lam.)

Plant C2-domain abscisic acid-related (CAR) protein family plays an important role in plant growth, abiotic stress responses, and defense regulation. In this study, we cloned the IbCAR1 by homologous cloning method from the transcriptomic data of Xuzishu8, which is a sweet potato cultivar with dark-purple flesh. This gene was expressed in all tissues of sweet potato, with the highest expression level in leaf tissue, and it could be induced by NaCl and ABA. Subcellular localization analyses indicated that IbCAR1 was localized in the nucleus and plasma membrane. The PI staining experiment revealed the distinctive root cell membrane integrity of overexpressed transgenic lines upon salt stress. Salt stress significantly increased the contents of proline, ABA, and the activity of superoxide dismutase (SOD), whereas the content of malondialdehyde (MDA) was decreased in overexpressed lines. On the contrary, RNA interference plants showed sensitivity to salt stress. Overexpression of IbCAR1 in sweet potatoes could improve the salt tolerance of plants, while the RNAi of IbCAR1 significantly increased sensitivity to salt stress in sweet potatoes. Meanwhile, the genes involved in ABA biosynthesis, stress response, and reactive oxygen species (ROS)-scavenging system were upregulated in overexpressed lines under salt stress. Taken together, these results demonstrated that IbCAR1 plays a positive role in salt tolerance by relying on the ABA signal transduction pathway, activating the ROS-scavenging system in sweet potatoes.

Research progress of aldehyde oxidases in plants

Plant aldehyde oxidases (AOs) are multi-functional enzymes, and they could oxidize abscisic aldehyde into ABA (abscisic acid) or indole acetaldehyde into IAA (indoleacetic acid) as the last step, respectively. AOs can be divided into four groups based on their biochemical and physiological functions. In this review, we summarized the recent studies about AOs in plants including the motif information, biochemical, and physiological functions. Besides their role in phytohormones biosynthesis and stress response, AOs could also involve in reactive oxygen species homeostasis, aldehyde detoxification and stress tolerance.

Characterization and functional validation of β-carotene hydroxylase AcBCH genes in Actinidia chinensis

Carotenoids are the pigment substances of yellow-fleshed kiwifruit, and among them β-cryptoxanthin has only been detected in the brighter yellow-fleshed variety 'Jinshi 1'. β-Carotene hydroxylase (BCH) catalyzes the formation of β-cryptoxanthin and zeaxanthin, but its molecular characteristics and functions have not been fully explained. Here we isolated two β-carotene hydroxylase genes, AcBCH1 and AcBCH2 from kiwifruit (Actinidia chinensis), and their relative expression levels exhibited a close correlation with the content of β-cryptoxanthin. AcBCH1 catalyzed the formation of β-cryptoxanthin when transformed into β-carotene-accumulating yeast cells. Moreover, silenced expression of AcBCH1 in kiwifruit caused decreases in the contents of zeaxanthin, lutein, and β-cryptoxanthin, and an increase in β-carotene content. The content of β-carotene decreased significantly after the AcBCH1/2 genes were overexpressed in tomato. The content of zeaxanthin increased and β-carotene decreased in transgenic kiwifruit seedlings. The results will enrich our knowledge of the molecular mechanisms of carotenoid biosynthesis in kiwifruit.

Map-Based Cloning and Characterization of Br-dyp1, a Gene Conferring Dark Yellow Petal Color Trait in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Flower color is an important trait in Brassica species. However, genes responsible for the dark yellow flower trait in Chinese cabbage have not been reported. In this study, we identified a dark-yellow-flowered Chinese cabbage line SD369. Genetic analysis indicated that the dark yellow flower trait in SD369 was controlled by a single recessive locus, Br-dyp1 (dark yellow petal color 1 in Brassica rapa). Using bulked segregant RNA sequencing and kompetitive allele-specific PCR assays, Br-dyp1 was fine-mapped to an interval of 53.6 kb on chromosome A09. Functional annotation analysis, expression analysis, and sequence variation analysis revealed that Bra037130 (BraA09.ZEP), which encodes a zeaxanthin epoxidase, was the most likely candidate gene for Br-dyp1. Carotenoid profile analysis suggested that Bra037130 (BraA09.ZEP) might participate in the epoxidation from zeaxanthin to violaxanthin. The 679 bp insertion in dark yellow petal caused premature stop codon, thus caused the loss-of-function of the enzyme zeaxanthin epoxidase (ZEP), which disturbed the carotenoid metabolism, and caused the increased accumulation of total carotenoid, and finally converted the flower color from yellow to dark yellow. Comparative transcriptome analysis also showed that the "carotenoid biosynthesis" pathway was significantly enriched, and genes involved in carotenoid degradation and abscisic acid biosynthesis and metabolism were significantly downregulated. Furthermore, we developed and validated the functional marker Br-dyp1-InDel for Br-dyp1. Overall, these results provide insight into the molecular basis of carotenoid-based flower coloration in B. rapa and reveal valuable information for marker-assisted selection breeding in Chinese cabbage.

A Cys2His2 Zinc Finger Transcription Factor BpSZA1 Positively Modulates Salt Stress in Betula platyphylla

Zinc finger proteins (ZFPs) are widely involved in plant growth and abiotic stress responses, however, few of these proteins have been functionally characterized in tree species. In this study, we cloned and characterized the BpSZA1 gene encoding a C2H2-type ZFP from Betula platyphylla. BpSZA1 is a transcription factor localized in the nucleus, with a transcription activation domain located at the N-terminus. BpSZA1 was predominantly expressed in stems and was induced by salt. We generated transgenic birch lines displaying overexpression (OE) or RNAi silencing (Ri) of BpSZA1 and exposed these along with wild-type birch seedlings to salinity. Phenotypic and physiological parameters such as superoxide dismutase, peroxisome, H₂O₂ content, proline content, water loss rate, and malondialdehyde content were examined. Overexpression of BpSZA1 in birch conferred increased salt tolerance. Chromatin immunoprecipitation-qPCR and RNA-seq showed that BpSZA1 binds to the GAGA-motif in the promoter of downstream target genes including BpAPX1, BpAPX2, BpCAT, and Bp6PGDH to activate their transcription. BpSZA1 also participates in abscisic acid (ABA) biosynthesis, proline biosynthesis, and the ABA/jasmonic acid pathway to enhance the salt stress of B. platyphylla.

Synergistic interaction of fungal endophytes, Paecilomyces formosus LHL10 and Penicillium funiculosum LHL06, in alleviating multi-metal toxicity stress in Glycine max L

Heavy metal accumulation in crop grains due to hazardous metal contamination is considered a great concern. However, phytobeneficial fungi are reported to have important abilities for the biosafety of crops grown in contaminated soil. Therefore, the current study was undertaken to explore the mutualistic association of plant growth-promoting endophytic fungi in reducing heavy metal concentration in the seeds of soybean plants subsequently grown in contaminated soil, without comprising seed quality and biochemical profile. The results revealed that endophytic Paecilomyces formosus LHL10 and Penicillium funiculosum LHL06 synergistically produced higher amounts of GAs and IAA in a co-cultured medium. Moreover, the co-inoculation of LHL06 and LHL10 to soybean plants grown under multi-metal toxic conditions significantly mitigated the adverse effects of heavy metal toxicity and increased the seed production (number of pods per plants, number of seeds per pod, and 100 seed weight) of soybean plants grown under control and multi-metal toxic conditions. Moreover, the levels of carbohydrates (glucose, sucrose, and fructose), minerals (iron, calcium, magnesium, and potassium), amino acids (serine, glutamic acids, glycine, methionine, lysine, arginine, and proline), and antioxidants (superoxide dismutase, catalase, and peroxidase) were significantly enhanced in sole and co-inoculated plants under control and stress conditions. Whereas organic acids (citric acid, tartaric acid, malic acid, and succinic acid), lipid peroxidation (MDA) products, multi-metal accumulation (nickel, cadmium, copper, lead, chromium, and aluminum), and stress-responsive endogenous abscisic acid levels were significantly decreased in seeds of soybean plants grown under control and multi-metal toxic conditions upon LHL06 and LHL10 sole and co-inoculation. The current results suggested the positive biochemical regulation in seeds for improving the nutritional status and making it safe for human consumption.

Metabolism of abscisic acid in two contrasting rice genotypes submitted to recurrent water deficit

Drought is the main constrain for crops worldwide, however, the effects of recurrent water deficit remain still hidden. We analysed two rice genotypes, 'BRS-Querência' (lowlands) and 'AN-Cambará' (uplands), after 7 days of recurrent drought followed by 24 h of rehydration, hypothesising that genotypes grown in regions with different water availabilities respond differently to water deficits, and that a previous exposure to stress could alter abscisic acid (ABA) metabolism. The results showed that both genotypes reduced stomatal conductance and increased ABA concentration. After rehydration, the ABA levels decreased, mainly in the plants of BRS-Querência subjected to recurrent stress. However, the levels of ABA were higher in plants in recurrent water deficit compared to non-recurrent stress plants in both genotypes. Remarkably in the lowland genotype, the ABA glucosyl-ester (ABA-GE) concentration increased after recovery in the plants under recurrent stress. Regarding of gene expression, the genes associated in ABA biosynthesis with the highest expression levels were NCED2, NCED3, NCED4 and AAO2. However, 'AN-Cambará' showed less transcriptional activation. Taking into account the genes involved in ABA catabolism, ABAH1 appears to play an important role related to the recurrent stress in upland plants. These results indicate that one of the factors that can promote greater tolerance for the upland genotype is the tradeoff between ABA and ABA-GE when plants are subjected to water deficits. In addition, they indicate that abscisic acid metabolism is altered due to the genotype (upland or lowland) and pre-exposure to stress can also modify adaptive responses in rice varieties (recurrent stress).

Penicillium chrysogenum polypeptide extract protects tobacco plants from tobacco mosaic virus infection through modulation of ABA biosynthesis and callose priming

The polypeptide extract of the dry mycelium of Penicillium chrysogenum (PDMP) can protect tobacco plants from tobacco mosaic virus (TMV), although the mechanism underlying PDMP-mediated TMV resistance remains unknown. In our study, we analysed a potential mechanism via RNA sequencing (RNA-seq) and found that the abscisic acid (ABA) biosynthetic pathway and β-1,3-glucanase, a callose-degrading enzyme, might play an important role in PDMP-induced priming of resistance to TMV. To test our hypothesis, we successfully generated a Nicotiana benthamiana ABA biosynthesis mutant and evaluated the role of the ABA pathway in PDMP-induced callose deposition during resistance to TMV infection. Our results suggested that PDMP can induce callose priming to defend against TMV movement. PDMP inhibited TMV movement by increasing callose deposition around plasmodesmata, but this phenomenon did not occur in the ABA biosynthesis mutant; moreover, these effects of PDMP on callose deposition could be rescued by treatment with exogenous ABA. Our results suggested that callose deposition around plasmodesmata in wild-type plants is mainly responsible for the restriction of TMV movement during the PDMP-induced defensive response to TMV infection, and that ABA biosynthesis apparently plays a crucial role in PDMP-induced callose priming for enhancing defence against TMV.

Comparative proteomic analysis of Nicotiana benthamiana plants under Chinese wheat mosaic virus infection

BACKGROUND

Chinese wheat mosaic virus (CWMV) is a severe threat to winter wheat and is transmitted by Polymyxa graminis. The mechanisms of interactions between CWMV and plants are poorly understood. In this study, a comparative proteomics analysis based on nanoliquid chromatography mass spectrometry (MS)/MS was conducted to characterize proteomic changes in plants responding to CWMV infection.

RESULTS

In total, 2751 host proteins were identified, 1496 of which were quantified and 146 up-regulated and 244 down-regulated proteins were identified as differentially expressed proteins (DEPs). Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that DEPs were most strongly associated with photosynthesis antenna proteins, MAPK signaling plant and glyoxylate and dicarboxylate metabolism pathways. Subcellular localization analysis predicted that more than half of the DEPs were localized in the chloroplast, an organelle indispensable for abscisic acid (ABA) synthesis. Our results suggest that CWMV infection interrupts normal chloroplast functions and decreases ABA concentrations in Nicotiana benthamiana. Further analysis showed that the ABA pathway was suppressed during CWMV infection and that ABA treatment induced plant hosts defenses against CWMV.

CONCLUSIONS

We identified several candidate proteins expressed during CWMV infection, and the ABA pathway was strongly associated with responses to CWMV infection in N. benthamiana.

Comparative transcriptome analysis implied a ZEP paralog was a key gene involved in carotenoid accumulation in yellow-fleshed sweetpotato

The mechanisms of carotenoid accumulation in yellow-fleshed sweetpotato cultivars are unclear. In this study, we compared the transcriptome profiles of a yellow-fleshed cultivar, Beniharuka (BH) and two of its spontaneous white-fleshed mutants (WH2 and WH3) to reveal the genes involved in yellow flesh. As a result of RNA sequencing, a total of 185 differentially expressed genes (DEGs) were commonly detected in WH2 and WH3 compared to BH. Of these genes, 85 DEGs and 100 DEGs were commonly upregulated and downregulated in WH2 and WH3 compared to BH, respectively. g1103.t1, a paralog of zeaxanthin epoxidase (ZEP), was only DEG common to WH2 and WH3 among 38 genes considered to be involved in carotenoid biosynthesis in storage roots. The expression level of g1103.t1 was also considerably lower in five white-fleshed cultivars than in five yellow-fleshed cultivars. Analysis of carotenoid composition in the storage roots showed that the epoxidised carotenoids were drastically reduced in both WH2 and WH3. Therefore, we propose that the ZEP paralog, g1103.t1, may be involved in carotenoid accumulation through the epoxidation of β-carotene and β-cryptoxanthin in sweetpotato.

Gene silencing of BnaA09.ZEP and BnaC09.ZEP confers orange color in Brassica napus flowers

Brassica napus is currently cultivated as an important ornamental crop in China. Flower color has attracted much attention in rapeseed genetics and breeding. Here, we characterize an orange-flowered mutant of B. napus that exhibits an altered carotenoid profile in its petals. As revealed by map-based cloning, the change in color from yellow to orange is attributed to the loss of BnaC09.ZEP (zeaxanthin epoxidase) and a 1695-bp deletion in BnaA09.ZEP. HPLC analysis, genetic complementation and CRISPR/Cas9 experiments demonstrated that BnaA09.ZEP and BnaC09.ZEP have similar functions, and the abolishment of both genes led to a substantial increase in lutein content and a sharp decline in violaxanthin content in petals but not leaves. BnaA09.ZEP and BnaC09.ZEP are predominantly expressed in floral tissues, whereas their homologs, BnaA07.ZEP and BnaC07.ZEP, mainly function in leaves, indicating redundancy and tissue-specific diversification of BnaZEP function. Transcriptome analysis in petals revealed differences in the expression of carotenoid and flavonoid biosynthesis-related genes between the mutant and its complementary lines. Flavonoid profiles in the petals of complementary lines were greatly altered compared to the mutant, indicating potential cross-talk between the regulatory networks underlying the carotenoid and flavonoid pathways. Additionally, our results indicate that there is functional compensation by BnaA07.ZEP and BnaC07.ZEP in the absence of BnaA09.ZEP and BnaC09.ZEP. Cloning and characterization of BnaZEPs provide insights into the molecular mechanisms underlying flower pigmentation in B. napus and would facilitate breeding of B. napus varieties with higher ornamental value.

How Do Novel M-Rootstock (Vitis Spp.) Genotypes Cope with Drought?

Most of the vineyards around the world are in areas characterized by seasonal drought, where water deficits and high temperatures represent severe constraints on the regular grapevine growth cycle. Although grapevines are well adapted to arid and semi-arid environments, water stress can cause physiological changes, from mild to irreversible. Screening of available Vitis spp. genetic diversity for new rootstock breeding programs has been proposed as a way for which new viticulture challenges may be faced. In 2014, novel genotypes (M-rootstocks) were released from the University of Milan. In this work, the behavior of M1, M3 and M4 in response to decreasing water availabilities (80%, 50% and 20% soil water content, SWC) was investigated at the physiological and gene expression levels, evaluating gas exchange, stem water potential and transcript abundances of key genes related to ABA (abscisic acid) biosynthesis (VvZEP, VvNCED1 and VvNCED2) and signaling (VvPP2C4, VvSnRK2.6 and VvABF2), and comparing them to those of cuttings of nine commercial rootstocks widely used in viticulture. M-rootstocks showed a change at physiological levels in severe water-stressed conditions (20% soil water content, SWC), reducing the stomatal conductance and stem water potential, but maintaining high photosynthetic activity. Water use efficiency was high in water-limiting conditions. The transcriptional changes were observed at 50% SWC, with an increment of transcripts of VvNCED1 and VvNCED2 genes. M-rootstocks showed similar behavior to 1103P and 110R rootstocks, two highly tolerant commercial genotypes. These rootstocks adopted a tolerant strategy to face water-stressed conditions.

Overexpression of plant ferredoxin-like protein promotes salinity tolerance in rice (Oryza sativa)

High-salinity stress is one of the major limiting factors on crop productivity. Physiological strategies against high-salinity stress include generation of reactive oxygen species (ROS), induction of stress-related genes expression, accumulation of abscisic acid (ABA) and up-regulation of antiporters. ROS are metabolism by-products and involved in signal transduction pathway. Constitutive expression of plant ferrodoxin-like protein (PFLP) gene enhances pathogen-resistance activities and root-hair growth through promoting ROS generation. However, the function of PFLP in abiotic stress responses is unclear. In this study, PFLP-1 and PFLP-2-transgenic rice plants were generated to elucidate the role of PFLP under salinity stress. PFLP overexpression significantly increased salt tolerance in PFLP-transgenic rice plants compared with non-transgenic plants (Oryza sativa japonica cv. Tainung 67, designated as TNG67). In high-salinity conditions, PFLP-transgenic plants exhibited earlier ROS production, higher antioxidant enzyme activities, higher ABA accumulation, up-regulated expression of stress-related genes (OsRBOHa, Cu/Zn SOD, OsAPX, OsNCED2, OsSOS1, OsCIPK24, OsCBL4, and OsNHX2), and leaf sodium ion content was lower compared with TNG67 plant. In addition, transgenic lines maintained electron transport rates and contained lower malondialdhyde (MDA) content than TNG67 plant did under salt-stress conditions. Overall results indicated salinity tolerance was improved by PFLP overexpression in transgenic rice plant. The PFLP gene is a potential candidate for improving salinity tolerance for valuable agricultural crops.

The fungal sesquiterpenoid pyrenophoric acid B uses the plant ABA biosynthetic pathway to inhibit seed germination

Pyrenophoric acid (P-Acid), P-Acid B, and P-Acid C are three phytotoxic sesquiterpenoids produced by the ascomycete seed pathogen Pyrenophora semeniperda, a fungus proposed as a mycoherbicide for biocontrol of cheatgrass, an extremely invasive weed. When tested in cheatgrass bioassays, these metabolites were able to delay seed germination, with P-Acid B being the most active compound. Here, we have investigated the cross-kingdom activity of P-Acid B and its mode of action, and found that it activates the abscisic acid (ABA) signaling pathway in order to inhibit seedling establishment. P-Acid B inhibits seedling establishment in wild-type Arabidopsis thaliana, while several mutants affected in the early perception as well as in downstream ABA signaling components were insensitive to the fungal compound. However, in spite of structural similarities between ABA and P-Acid B, the latter is not able to activate the PYR/PYL family of ABA receptors. Instead, we have found that P-Acid B uses the ABA biosynthesis pathway at the level of alcohol dehydrogenase ABA2 to reduce seedling establishment. We propose that the fungus P. semeniperda manipulates plant ABA biosynthesis as a strategy to reduce seed germination, increasing its ability to cause seed mortality and thereby increase its fitness through higher reproductive success.

Transcriptional Regulation of Carotenoid Biosynthesis in Plants: So Many Regulators, So Little Consensus

In plants, the carotenoid biosynthesis pathway (CBP) is essential for the production of photosynthetic and protective pigments, plant hormones, and visual/olfactory attractants for animal pollinators and seed dispersers. The regulation of carotenoid biosynthesis at the transcriptional level is vitally important for all of these functions and has been the subject of intensive research. Many putative transcriptional regulators, both direct and indirect, have been identified through conventional mutant analysis, transcriptome profiling, yeast one-hybrid screening, and candidate gene approaches. Despite this progress, our understanding of the transcriptional regulation of carotenoid biosynthesis remains fragmented and incomplete. Frequently, a stimulus or regulator is known, but the mechanism by which it affects transcription has not been elucidated. In other cases, mechanisms have been proposed (such as direct binding of a CBP gene promoter by a transcription factor), but function was tested only in vitro or in heterologous systems, making it unclear whether these proteins actually play a role in carotenoid regulation in their endogenous environments. Even in cases where the mechanism is relatively well understood, regulators are often studied in isolation, either in a single plant species or outside the context of other known regulators. This presents a conundrum: why so many candidate regulators but so little consensus? Here we summarize current knowledge on transcriptional regulation of the CBP, lay out the challenges contributing to this conundrum, identify remaining knowledge gaps, and suggest future research directions to address these challenges and knowledge gaps.

Molecular and Hormonal Aspects of Drought-Triggered Flower Shedding in Yellow Lupine

The drought is a crucial environmental factor that determines yielding of many crop species, e.g., Fabaceae, which are a source of valuable proteins for food and feed. Herein, we focused on the events accompanying drought-induced activation of flower abscission zone (AZ)-the structure responsible for flower detachment and, consequently, determining seed production in Lupinus luteus. Therefore, detection of molecular markers regulating this process is an excellent tool in the development of improved drought-resistant cultivars to minimize yield loss. We applied physiological, molecular, biochemical, immunocytochemical, and chromatography methods for a comprehensive examination of changes evoked by drought in the AZ cells. This factor led to significant cellular changes and activated AZ, which consequently increased the flower abortion rate. Simultaneously, drought caused an accumulation of mRNA of genes inflorescence deficient in abscission-like (LlIDL), receptor-like protein kinase HSL (LlHSL), and mitogen-activated protein kinase6 (LlMPK6), encoding succeeding elements of AZ activation pathway. The content of hydrogen peroxide (H2O2), catalase activity, and localization significantly changed which confirmed the appearance of stressful conditions and indicated modifications in the redox balance. Loss of water enhanced transcriptional activity of the abscisic acid (ABA) and ethylene (ET) biosynthesis pathways, which was manifested by elevated expression of zeaxanthin epoxidase (LlZEP), aminocyclopropane-1-carboxylic acid synthase (LlACS), and aminocyclopropane-1-carboxylic acid oxidase (LlACO) genes. Accordingly, both ABA and ET precursors were highly abundant in AZ cells. Our study provides information about several new potential markers of early response on water loss, which can help to elucidate the mechanisms that control plant response to drought, and gives a useful basis for breeders and agronomists to enhance tolerance of crops against the stress.

Abiotic stress modulates root patterning via ABA-regulated microRNA expression in the endodermis initials

Patterning of the root xylem into protoxylem (PX) and metaxylem is regulated by auxin-cytokinin signaling and microRNA miR165a/166b-mediated suppression of genes encoding Class III HOMEODOMAIN LEU-ZIPPER (HD-ZIPIII) proteins. We found that in Arabidopsis osmotic stress via core abscisic acid (ABA) signaling in meristematic endodermal cells induces differentiation of PX in radial and longitudinal axes in association with increased VND7 expression. Similarly, in tomato ABA enhanced PX differentiation in the longitudinally and radially, indicating an evolutionarily conserved mechanism. ABA increased expression of miR165a/166b and reduced expression of miR165a/166b repressor ARGONAOUTE10/ZWILLE, resulting in reduced levels of all five HD-ZIPIII RNAs. ABA treatments failed to induce additional PX files in a miR165a/166b-resistant PHB mutant, phb1-d, and in scr and shr mutants, in which miR165a/166b expression are strongly reduced. Thus, ABA regulates xylem patterning and maturation via miR165a/166b-regulated expression of HD-ZIPIII mRNAs and associated VND7 levels. In lateral root initials, ABA induced increase in miR165a levels in endodermal precursors and inhibited their reduction in the future quiescent center specifically at pre-emergence stage. Hence, ABA-induced inhibition of lateral root is associated with reduced HD-ZIPIII levels.

Abscisic acid negatively modulates plant defence against rice black-streaked dwarf virus infection by suppressing the jasmonate pathway and regulating reactive oxygen species levels in rice

Abscisic acid (ABA) plays a multifaceted role in plant immunity and can either increase resistance or increase susceptibility to some bacterial and fungal pathogens depending on the pathosystem. ABA is also known to mediate plant defence to some viruses. In this study, the relationship between the ABA pathway and rice black-streaked dwarf virus (RBSDV) was investigated in rice. The expression of ABA pathway genes was significantly reduced upon RBSDV infection. Application of exogenous hormones and various ABA pathway mutants revealed that the ABA pathway plays a negative role in rice defence against RBSDV. Exogenous hormone treatment and virus inoculation showed that ABA inhibits the jasmonate-mediated resistance to RBSDV. ABA treatment also suppressed accumulation of reactive oxygen species by inducing the expression of superoxidase dismutases and catalases. Thus, ABA modulates the rice-RBSDV interaction by suppressing the jasmonate pathway and regulating reactive oxygen species levels. This is the first example of ABA increasing susceptibility to a plant virus.

Unraveling the induction of phytoene synthase 2 expression by salt stress and abscisic acid in Daucus carota

Phytoene synthase (PSY) is the first committed enzyme of the carotenoid biosynthesis pathway and the most important point of regulation. Carotenoids are precursors of abscisic acid (ABA), which mediates abiotic stress tolerance responses in plants. ABA activates the synthesis of its own precursors through induction of PSY expression. Carrot, a species that accumulates very high amounts of carotenoids in its reserve root, has two PSY paralog genes that are expressed differentially in the root. Here, we determined that DcPSY2 expression is induced by salt stress and ABA. A DcPSY2 promoter fragment was obtained and characterized. Bioinformatic analysis showed the presence of three ABA responsive elements (ABREs). Through overexpressing pPSY2:GFP in Nicotiana tabacum we determined that all three ABREs are necessary for the ABA response. In the carrot transcriptome, we identified three ABRE binding protein (DcAREB) transcription factor candidates that localized in the nucleus, but only one, DcAREB3, was induced under ABA treatment in carrot roots. We found that AREB transcription factors bind to the carrot DcPSY2 promoter and transactivate the expression of reporter genes. We conclude that DcPSY2 is involved in ABA-mediated salt stress tolerance in carrot through the binding of AREB transcription factors to its promoter.

ABA Transport and Plant Water Stress Responses

To understand the integrative networks of signaling molecules, the sites of their biosynthesis and action must be clarified, particularly for phytohormones such as abscisic acid (ABA). The relationship between the sites of ABA biosynthesis and transport has been discussed extensively in the context of guard cells and stomatal regulation. However, guard cells are not the only site of ABA action. Recent studies have reported multiple sites of ABA biosynthesis and multiple ABA transporters, indicating that ABA transport regulation is not unidirectional but rather forms complex networks. Therefore, it is important to determine how multiple ABA sources coordinately contribute to individual biological processes under various physiological conditions.

Characterization and Primary Functional Analysis of a Bamboo ZEP Gene from Phyllostachys edulis

Zeaxanthin epoxidase (ZEP) plays important roles in plant response to various environmental stresses by involving in abscisic acid (ABA) biosynthesis and xanthophyll cycle. A full-length cDNA of PeZEP was isolated from moso bamboo (Phyllostachys edulis), which comprised of a 138-bp 5'-untranslated region (UTR), a 381-bp 3'-UTR, and a 2013-bp open reading frame (ORF) encoding a putative protein of 670 amino acids. PeZEP was mainly expressed in leaf blades and leaf sheaths, and less in roots and culms. The transcript level of PeZEP in bamboo leaf was elevated with the increasing light intensity. PeZEP was significantly upregulated in response to high light (HL: 1200 μmol·m-2·s-1) and reached to a higher level after 1 h treatment, and kept higher levels in the following hours. Besides, PeZEP was upregulated under high temperature (42°C), and downregulated under low temperature (4°C) and exogenous ABA treatment. The expression vector of PeZEP driven by CaMV 35S was constructed and transformed into Arabidopsis thaliana. The transgenic plants overexpressing PeZEP were generated and subjected to drought stress for morphological and physiological assays. Compared with Col-0, the transgenic plants demonstrated enhanced tolerance to drought stress, which appeared later wilting and higher survival rate. Moreover, higher value of Fv/Fm, higher activities of superoxide dismutase, peroxidase, and catalase, and lower concentration of malondialdehyde were also observed in transgenic plants. Transcript levels of AtP5CS and AtRD29b related to drought stress were enhanced in transgenic plants. These results indicated that PeZEP might play an important function in response to drought stress in bamboo.

Endogenous circadian rhythms in pigment composition induce changes in photochemical efficiency in plant canopies

There is increasing evidence that the circadian clock is a significant driver of photosynthesis that becomes apparent when environmental cues are experimentally held constant. We studied whether the composition of photosynthetic pigments is under circadian regulation, and whether pigment oscillations lead to rhythmic changes in photochemical efficiency. To address these questions, we maintained canopies of bean and cotton, after an entrainment phase, under constant (light or darkness) conditions for 30-48 h. Photosynthesis and quantum yield peaked at subjective noon, and non-photochemical quenching peaked at night. These oscillations were not associated with parallel changes in carbohydrate content or xanthophyll cycle activity. We observed robust oscillations of Chl a/b during constant light in both species, and also under constant darkness in bean, peaking when it would have been night during the entrainment (subjective nights). These oscillations could be attributed to the synthesis and/or degradation of trimeric light-harvesting complex II (reflected by the rhythmic changes in Chl a/b), with the antenna size minimal at night and maximal around subjective noon. Considering together the oscillations of pigments and photochemistry, the observed pattern of changes is counterintuitive if we assume that the plant strategy is to avoid photodamage, but consistent with a strategy where non-stressed plants maximize photosynthesis.

De Novo Sequencing and Comparative Analysis of Schima superba Seedlings to Explore the Response to Drought Stress

Schima superba is an important dominant species in subtropical evergreen broadleaved forests of China, and plays a vital role in community structure and dynamics. However, the survival rate of its seedlings in the field is low, and water shortage could be a factor that limits its regeneration. In order to better understand the response of its seedlings to drought stress on a functional genomics scale, RNA-seq technology was utilized in this study to perform a large-scale transcriptome sequencing of the S. superba seedlings under drought stress. More than 320 million clean reads were generated and 72218 unique transcripts were obtained through de novo assembly. These unigenes were further annotated by blasting with different public databases and a total of 53300 unique transcripts were annotated. A total of 31586 simple sequence repeat (SSR) loci were presented. Through gene expression profiling analysis between drought treatment and control, 11038 genes were found to be significantly enriched in drought-stressed seedlings. Based on these differentially expressed genes (DEGs), Gene Ontology (GO) terms enrichment and Kyoto Encyclopedia of Genes and Genomes pathway (KEGG) enrichment analysis indicated that drought stress caused a number of changes in the types of sugars, enzymes, secondary mechanisms, and light responses, and induced some potential physical protection mechanisms. In addition, the expression patterns of 18 transcripts induced by drought, as determined by quantitative real-time PCR, were consistent with their transcript abundance changes, as identified by RNA-seq. This transcriptome study provides a rapid method for understanding the response of S. superba seedlings to drought stress and provides a number of gene sequences available for further functional genomics studies.

Abscisic acid content and the expression of genes related to its metabolism during maturation of triticale grains of cultivars differing in pre-harvest sprouting susceptibility

Abscisic acid (ABA) is a plant hormone that plays a predominant role in the onset and maintenance of primary dormancy. Peak ABA accumulation in embryos of triticale grains was observed before any significant loss of water and was higher in Fredro, a cultivar less susceptible to pre-harvest sprouting (PHS), than in Leontino, a cultivar more sensitive to PHS. At full maturity, embryonic ABA content in Fredro was twice as high as in Leontino. Two full-length cDNAs of 9-cis-epoxycarotenoid dioxygenase (TsNCED1, TsNCED2), an enzyme involved in ABA biosynthesis, and two full-length cDNAs of ABA 8'-hydroxylase (TsABA8'OH1 and TsABA8'OH2), an enzyme involved in ABA catabolism, were identified in triticale grains and characterized. The maximum transcript level of both TsNCED1 and TsNCED2 preceded the peak of ABA accumulation, suggesting that both TsNCEDs contribute to reach this peak, although the expression of TsNCED1 was significantly higher in Fredro than in Leontino. High expression of TsABA8'OH2 and TsABA8'OH1 was observed long before and at the end of the ABA accumulation peak, respectively, but no differences were observed between cultivars. The obtained results suggest that mainly TsNCED1 might be related to the higher ABA content and higher resistance of Fredro to PHS. However, Fredro embryos not only have higher ABA content, but also exhibit greater sensitivity to ABA, which may also have a significant effect on grain dormancy and lower susceptibility to PHS for grains of this cultivar.

Genetic Diversity and Molecular Evolution of a Violaxanthin De-epoxidase Gene in Maize

Violaxanthin de-epoxidase (VDE) has a critical role in the carotenoid biosynthesis pathway, which is involved in protecting the photosynthesis apparatus from damage caused by excessive light. Here, a VDE gene in maize, ZmVDE1, was cloned and shown to have functional domains in common with the gramineous VDE protein. Candidate gene association analysis indicated that no polymorphic sites in ZmVDE1 were significant association with any of the examined carotenoid-related traits at P = 0.05 in an association panel containing 155 maize inbred lines. Nucleotide diversity analysis of VDE1 in maize and teosinte indicated that its exon had less genetic variation, consistent with the conserved function of VDE1 in plants. In addition, dramatically reduced nucleotide diversity, fewer haplotypes and a significantly negative parameter deviation for Tajima’s D test of ZmVDE1 in maize and teosinte suggested that a potential selective force had acted across the ZmVDE1 locus. We further identified a 4.2 Mb selective sweep with low recombination surrounding the ZmVDE1 locus that resulted in severely reduced nucleotide diversity on chromosome 2. Collectively, natural selection and the conserved domains of ZmVDE1 might show an important role in the xanthophyll cycle of the carotenoid biosynthesis pathway.

ZEAXANTHIN EPOXIDASE Activity Potentiates Carotenoid Degradation in Maturing Seed

Elucidation of the carotenoid biosynthetic pathway has enabled altering the composition and content of carotenoids in various plants, but to achieve desired nutritional impacts, the genetic components regulating carotenoid homeostasis in seed, the plant organ consumed in greatest abundance, must be elucidated. We used a combination of linkage mapping, genome-wide association studies (GWAS), and pathway-level analysis to identify nine loci that impact the natural variation of seed carotenoids in Arabidopsis (Arabidopsis thaliana). ZEAXANTHIN EPOXIDASE (ZEP) was the major contributor to carotenoid composition, with mutants lacking ZEP activity showing a remarkable 6-fold increase in total seed carotenoids relative to the wild type. Natural variation in ZEP gene expression during seed development was identified as the underlying mechanism for fine-tuning carotenoid composition, stability, and ultimately content in Arabidopsis seed. We previously showed that two CAROTENOID CLEAVAGE DIOXYGENASE enzymes, CCD1 and CCD4, are the primary mediators of seed carotenoid degradation, and here we demonstrate that ZEP acts as an upstream control point of carotenoid homeostasis, with ZEP-mediated epoxidation targeting carotenoids for degradation by CCD enzymes. Finally, four of the nine loci/enzymatic activities identified as underlying natural variation in Arabidopsis seed carotenoids also were identified in a recent GWAS of maize (Zea mays) kernel carotenoid variation. This first comparison of the natural variation in seed carotenoids in monocots and dicots suggests a surprising overlap in the genetic architecture of these traits between the two lineages and provides a list of likely candidates to target for selecting seed carotenoid variation in other species.

Soybean Seed Development: Fatty Acid and Phytohormone Metabolism and Their Interactions

Vegetable oil utilization is determined by its fatty acid composition. In soybean and other grain crops, during the seed development oil accumulation is important trait for value in food or industrial applications. Seed development is relatively short and sensitive to unfavorable abiotic conditions. These stresses can lead to a numerous undesirable qualitative as well as quantitative changes in fatty acid production. Fatty acid manipulation which targets a higher content of a specific single fatty acid for food or industrial application has gained more attention. Despite several successes in modifying the ratio of endogenous fatty acids in most domesticated oilseed crops, numerous obstacles in FA manipulation of seed maturation are yet to be overcome. Remarkably, connections with plant hormones have not been well studied despite their critical roles in the regulation and promotion of a plethora of processes in plant growth and development. While activities of phytohormones during the reproductive phase have been partially clarified in seed physiology, the biological role of plant hormones in oil accumulation during seed development has not been investigated. In this review seed development and numerous effects of abiotic stresses are discussed. After describing fatty acid and phytohormone metabolism and their interactions, we postulate that the endogenous plant hormones play important roles in fatty acid production in soybean seeds.

MsZEP, a novel zeaxanthin epoxidase gene from alfalfa (Medicago sativa), confers drought and salt tolerance in transgenic tobacco

KEY MESSAGE

The zeaxanthin epoxidase gene ( MsZEP ) was cloned and characterized from alfalfa and validated for its function of tolerance toward drought and salt stresses by heterologous expression in Nicotiana tabacum. Zeaxanthin epoxidase (ZEP) plays important roles in plant response to various environment stresses due to its functions in ABA biosynthetic and the xanthophyll cycle. To understand the expression characteristics and the biological functions of ZEP in alfalfa (Medicago sativa), a novel gene, designated as MsZEP (KM044311), was cloned, characterized and overexpressed in Nicotiana tabacum. The open reading frame of MsZEP contains 1992 bp nucleotides and encodes a 663-amino acid polypeptide. Amino acid sequence alignment indicated that deduced MsZEP protein was highly homologous to other plant ZEP sequences. Phylogenetic analysis showed that MsZEP was grouped into a branch with other legume plants. Real-time quantitative PCR revealed that MsZEP gene expression was clearly tissue-specific, and the expression levels were higher in green tissues (leaves and stems) than in roots. MsZEP expression decreased in shoots under drought, cold, heat and ABA treatment, while the expression levels in roots showed different trends. Besides, the results showed that nodules could up-regulate the MsZEP expression under non-stressful conditions and in the earlier stage of different abiotic stress. Heterologous expression of the MsZEP gene in N. tabacum could confer tolerance to drought and salt stress by affecting various physiological pathways, ABA levels and stress-responsive genes expression. Taken together, these results suggested that the MsZEP gene may be involved in alfalfa responses to different abiotic stresses and nodules, and could enhance drought and salt tolerance of transgenic tobacco by heterologous expression.

Positive feedback regulation of a Lycium chinense-derived VDE gene by drought-induced endogenous ABA, and over-expression of this VDE gene improve drought-induced photo-damage in Arabidopsis

Violaxanthin de-epoxidase (VDE) plays an important role in protecting the photosynthetic apparatus from photo-damage by dissipating excessively absorbed light energy as heat, via the conversion of violaxanthin (V) to intermediate product antheraxanthin (A) and final product zeaxanthin (Z) under light stress. We have cloned a VDE gene (LcVDE) from Lycium chinense, a deciduous woody perennial halophyte, which can grow in a large variety of soil types. The amino acid sequence of LcVDE has high homology with VDEs in other plants. Under drought stress, relative expression of LcVDE and the de-epoxidation ratio (Z+0.5A)/(V+A+Z) increased rapidly, and non-photochemical quenching (NPQ) also rose. Interestingly, these elevations induced by drought stress were reduced by the topical administration of abamine SG, a potent ABA inhibitor via inhibition of NCED in the ABA synthesis pathway. Until now, little has been done to explore the relationship between endogenous ABA and the expression of VDE genes. Since V serves as a common precursor for ABA, these data support the possible involvement of endogenous ABA in the positive feedback regulation of LcVDE gene expression in L. chinense under drought stress. Moreover, the LcVDE may be involved in modulating the level of photosynthesis damage caused by drought stress. Furthermore, the ratio of (Z+0.5A)/(V+A+Z) and NPQ increased more in transgenic Arabidopsis over-expressing LcVDE gene than the wild types under drought stress. The maximum quantum yield of primary photochemistry of PSII (Fv/Fm) in transgenic Arabidopsis decreased more slowly during the stressed period than that in wild types under the same conditions. Furthermore, transgenic Arabidopsis over-expressing LcVDE showed increased tolerance to drought stress.

Tissue-specific accumulation and regulation of zeaxanthin epoxidase in Arabidopsis reflect the multiple functions of the enzyme in plastids

The enzyme zeaxanthin epoxidase (ZEP) catalyzes the conversion of zeaxanthin to violaxanthin, a key reaction for ABA biosynthesis and the xanthophyll cycle. Both processes are important for acclimation to environmental stress conditions, in particular drought (ABA biosynthesis) and light (xanthophyll cycle) stress. Hence, both ZEP functions may require differential regulation to optimize plant fitness. The key to understanding the function of ZEP in both stress responses might lie in its spatial and temporal distribution in plant tissues. Therefore, we analyzed the distribution of ZEP in plant tissues and plastids under drought and light stress by use of a ZEP-specific antibody. In addition, we determined the pigment composition of the plant tissues and chloroplast membrane subcompartments in response to these stresses. The ZEP protein was detected in all plant tissues (except flowers) concomitant with xanthophylls. The highest levels of ZEP were present in leaf chloroplasts and root plastids. Within chloroplasts, ZEP was localized predominantly in the thylakoid membrane and stroma, while only a small fraction was bound by the envelope membrane. Light stress affected neither the accumulation nor the relative distribution of ZEP in chloroplasts, while drought stress led to an increase of ZEP in roots and to a degradation of ZEP in leaves. However, drought stress-induced increases in ABA were similar in both tissues. These data support a tissue- and stress-specific accumulation of the ZEP protein in accordance with its different functions in ABA biosynthesis and the xanthophyll cycle.

The abscisic acid pathway has multifaceted effects on the accumulation of Bamboo mosaic virus

Accepted 29 October 2013. Abscisic acid (ABA) plays a key role in modulating plant responses to different biotic and abiotic stresses. However, the effect of ABA on virus infection is not fully understood. Here, we describe the effects of the ABA pathway on the accumulation of Bamboo mosaic virus (BaMV) and Cucumber mosaic virus (CMV) in two different hosts: Arabidopsis thaliana and Nicotiana benthamiana. We report that ABA2 plays a critical role in the accumulation of BaMV and CMV. Mutants downstream of ABA2 (aao3, abi1-1, abi3-1, and abi4-1) were susceptible to BaMV, indicating that the ABA pathway downstream of ABA2 is essential for BaMV resistance. The aba2-1 mutant decreased the accumulation of BaMV (+)RNA, (-)RNA, and coat protein, with the most dramatic effect being observed for (-)RNA. These findings were further validated by the use of virus-induced gene silencing and enzyme-linked immunosorbent assay in N. benthamiana. In addition, infecting N. benthamiana with BaMV or CMV increased ABA contents and activated the SA and ABA pathways, thereby disrupting the antagonism between these two cascades. Our findings uncover a novel role for ABA2 in supporting BaMV and CMV accumulation, distinct from the opposing role of its downstream genes.

Changes in the abscisic acid levels and related gene expression during fruit development and ripening in bilberry (Vaccinium myrtillus L.)

Abscisic acid (ABA) is a natural plant hormone playing an important role in many physiological processes including fruit ripening and is also recently found to be potential for biomedical applications. This study was aimed to measure ABA levels and its biosynthesis in bilberry (Vaccinium myrtillus L.), which is one of the best sources of anthocyanins. Five ABA biosynthetic genes were isolated from bilberry and their expression profiles were studied in bilberry tissues, particularly during berry development. The level of ABA highly increased at the onset of bilberry fruit ripening, at the stage when expression of anthocyanin biosynthetic genes, chalcone synthase (VmCHS) and anthocyanidin synthase (VmANS), also increased. In fully ripe berries and leaves, ABA levels were lower but none was detected in bilberry stem or rhizome. The expression of 9-cis-epoxycarotenoid dioxygenase (VmNCED1) and putative neoxanthin synthase (VmNSY) was high in berry tissues and their expression increased markedly at the onset of berry ripening along with the accumulation of ABA. In contrast, the expression of zeaxanthin epoxidase (VmZEP), short-chain dehydrogenase/reductase (VmSDR/ABA2) and aldehyde oxidase (VmAO) were most highly associated with leaf tissues with no obvious relation to ABA content during berry development. The obtained results indicate that the ABA biosynthesis may play an important role in the regulation of ripening of non-climacteric bilberry fruits through transcriptional regulation of key ABA biosynthetic genes.

The role of FaBG3 in fruit ripening and B. cinerea fungal infection of strawberry

In plants, β-glucosidases (BG) have been implicated in developmental and pathogen defense, and are thought to take part in abscisic acid (ABA) synthesis via hydrolysis of ABA glucose ester to release active ABA; however, there is no genetic evidence for the role of BG genes in ripening and biotic/abiotic stress in fruits. To clarify the role of BG genes in fruit, eight Fa/FvBG genes encoding β-glucosidase were isolated using information from the GenBank strawberry nucleotide database. Of the Fa/FvBG genes examined, expression of FaBG3 was the highest, showing peaks at the mature stage, coincident with the changes observed in ABA content. To verify the role of this gene, we suppressed the expression of FaBG3 via inoculation with Agrobacterium tumefaciens containing tobacco rattle virus carrying a FaBG3 fragment (RNAi). The expression of FaBG3 in FaBG3-RNAi-treated fruit was markedly reduced, and the ABA content was lower than that of the control. FaBG3-RNAi-treated fruit did not exhibit full ripening, and were firmer, had lower sugar content, and were pale compared with the control due to down-regulation of ripening-related genes. FaBG3-RNAi-treated fruit with reduced ABA levels were much more resistant to Botrytis cinerea fungus but were more sensitive to dehydration stress than control fruit. These results indicate that FaBG3 may play key roles in fruit ripening, dehydration stress and B. cinerea fungal infection in strawberries via modulation of ABA homeostasis and transcriptional regulation of ripening-related genes.

Comparative transcriptome profiling of developing caryopses from two rice cultivars with differential dormancy

Pre-harvest sprouting (PHS) in rice causes poor grain quality and results in significant reductions in yield, leading to significant economic losses. In contrast, deep dormancy can lead to equally unwanted non-uniform germination. Therefore, a suitable level of dormancy is a critically important agronomic trait. In this study, an analysis of PHS in developing seeds of two Korean rice cultivars (vivipary), Gopum and Samgwang, revealed differences in dormancy in caryopses at 25 d after heading (DAH). To assess the transcriptomic characteristics associated with vivipary, we compared RNA profiles at early (3-6 DAH), middle (25 DAH), and late (40 DAH) developmental stages. Transcriptomic differentiation was most pronounced in caryopses at 25 DAH, the developmental stage at which differential dormancy was also the most prominent. A k-means clustering analysis of the two cultivars revealed groups of genes with similar or dissimilar expression profiles. Many of the genes that showed distinct differential expression profiles were those involved in seed maturation. Intriguingly, differential gene expression levels between the two cultivars were positively correlated with fold-changes in their expression during the early half of caryopsis development. This implies that the establishment of seed dormancy is strongly correlated with the altered transcriptomic patterns related to the progression of maturation. Our global RNA profiling suggests that caryopsis development in Gopum proceeds at a greater speed than in the Samgwang cultivar. Thus, a high degree of maturity and early dormancy release may be present in 25 DAH caryopses of Gopum, although we cannot exclude the possibility of genetic defects modifying dormancy. The comparative transcriptomic analysis of the two cultivars did not reveal noticeable differences in RNA profiles with respect to differences in abscisic acid (ABA) content or ABA sensitivity. Therefore, it is unlikely that ABA is directly involved in the differences in dormancy observed between the two cultivars.

Abscisic Acid synthesis and response

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

Efficient heterologous transformation of Chlamydomonas reinhardtii npq2 mutant with the zeaxanthin epoxidase gene isolated and characterized from Chlorella zofingiensis

In the violaxanthin cycle, the violaxanthin de-epoxidase and zeaxanthin epoxidase catalyze the inter-conversion between violaxanthin and zeaxanthin in both plants and green algae. The zeaxanthin epoxidase gene from the green microalga Chlorella zofingiensis (Czzep) has been isolated. This gene encodes a polypeptide of 596 amino acids. A single copy of Czzep has been found in the C. zofingiensis genome by Southern blot analysis. qPCR analysis has shown that transcript levels of Czzep were increased after zeaxanthin formation under high light conditions. The functionality of Czzep gene by heterologous genetic complementation in the Chlamydomonas mutant npq2, which lacks zeaxanthin epoxidase (ZEP) activity and accumulates zeaxanthin in all conditions, was analyzed. The Czzep gene was adequately inserted in the pSI105 vector and expressed in npq2. The positive transformants were able to efficiently convert zeaxanthin into violaxanthin, as well as to restore their maximum quantum efficiency of the PSII (Fv/Fm). These results show that Chlamydomonas can be an efficient tool for heterologous expression and metabolic engineering for biotechnological applications.

A vacuolar β-glucosidase homolog that possesses glucose-conjugated abscisic acid hydrolyzing activity plays an important role in osmotic stress responses in Arabidopsis

The phytohormone abscisic acid (ABA) plays a critical role in various physiological processes, including adaptation to abiotic stresses. In Arabidopsis thaliana, ABA levels are increased both through de novo biosynthesis and via β-glucosidase homolog1 (BG1)-mediated hydrolysis of Glc-conjugated ABA (ABA-GE). However, it is not known how many different β-glucosidase proteins produce ABA from ABA-GE and how the multiple ABA production pathways are coordinated to increase ABA levels. Here, we report that a previously undiscovered β-glucosidase homolog, BG2, produced ABA by hydrolyzing ABA-GE and plays a role in osmotic stress response. BG2 localized to the vacuole as a high molecular weight complex and accumulated to high levels under dehydration stress. BG2 hydrolyzed ABA-GE to ABA in vitro. In addition, BG2 increased ABA levels in protoplasts upon application of exogenous ABA-GE. Overexpression of BG2 rescued the bg1 mutant phenotype, as observed for the overexpression of NCED3 in bg1 mutants. Multiple Arabidopsis bg2 alleles with a T-DNA insertion in BG2 were more sensitive to dehydration and NaCl stress, whereas BG2 overexpression resulted in enhanced resistance to dehydration and NaCl stress. Based on these observations, we propose that, in addition to the de novo biosynthesis, ABA is produced in multiple organelles by organelle-specific β-glucosidases in response to abiotic stresses.

[Molecular mechanism for dynamic regulation of endogenous ABA signal level]

The process from stress signal perception and the trigger of ABA biosynthesis to dynamic regulation of ABA level is an important stress signaling pathway in cells. Compared to the downstream events in ABA signal transduction, the researches in this field are relatively lagged. Expression of synthase genes, such as ZEP in roots and rate-limiting enzyme genes NCED, AtRGS1 and ABA2, can be activated in response to stresses. However, the expression of genes encoding degradative enzymes, including 7'-, 8'-, 9'-hydroxylase and glucosyltransferase, negatively regulates ABA accumulation. Meanwhile, the expressions of the synthases, such as ZEP and NCED3, are induced by increasing endogenous ABA contents. Additionally, the analyses of gene expression and source-sink dynamics indicates that sustained supply from root-sourced ABA is required for the maintenance of leaf ABA dynamic pool. It is notable that miRNAs should be involved in ABA signal origin and ABA level dynamic adjustment. Further dynamic analysis of ABA metabolism revealed that endogenous ABA signal levels are synergistically controlled by the expressions of synthases and degradative enzymes.

Xanthophyll cycle--a mechanism protecting plants against oxidative stress

Six different xanthophyll cycles have been described in photosynthetic organisms. All of them protect the photosynthetic apparatus from photodamage caused by light-induced oxidative stress. Overexcitation conditions lead, in the chloroplast, to the over-reduction of the NADP pool and production of superoxide, which can subsequently be metabolized to hydrogen peroxide or a hydroxyl radical, other reactive oxygen species (ROS). On the other hand, overexcitation of photosystems leads to an increased lifetime of the chlorophyll excited state, increasing the probability of chlorophyll triplet formation which reacts with triplet oxygen forming single oxygen, another ROS. The products of the light-dependent phase of xanthophyll cycles play an important role in the protection against oxidative stress generated not only by an excess of light but also by other ROS-generating factors such as drought, chilling, heat, senescence, or salinity stress. Four, mainly hypothetical, mechanisms explaining the protective role of xanthophyll cycles in oxidative stress are presented. One of them is the direct quenching of overexcitation by products of the light phase of xanthophyll cycles and three others are based on the indirect participation of xanthophyll cycle carotenoids in the process of photoprotection. They include: (1) indirect quenching of overexcitation by aggregation-dependent light-harvesting complexes (LHCII) quenching; (2) light-driven mechanisms in LHCII; and (3) a model based on charge transfer quenching between Chl a and Zx. Moreover, results of the studies on the antioxidant properties of xanthophyll cycle pigments in model systems are also presented.

The activity pattern and gene expression profile of aldehyde oxidase during the development of Pisum sativum seeds

Aldehyde oxidase (AO, EC 1.2.3.1) is a molybdenohydroxylase that is considered to catalyze the final step in the synthesis of abscisic acid (ABA) and possibly of indole-3-acetic acid (IAA). Five AO activity bands were detected after native PAGE with indole-3-aldehyde (PsAO-α, -β, -γ, -δ, -κ) and three with abscisic aldehyde (PsAO-γ, -δ, -κ) in developing seeds of Pisum sativum. At early and mid-development, PsAO-α, -β, -γ and only PsAO-γ were observed, respectively, and their localization as well as the expression of PsAOs genes was almost exclusively restricted to the maternal fruit tissues, the seed coat and pericarp. Towards the end of rapid reserve synthesis, two additional isoforms (PsAO-δ, -κ) appeared in cotyledons, coinciding with a high transcript level of PsAO2. At this developmental stage, the activity level of PsAO-γ, was still considerable in the testa, and was higher than at earlier stages in the embryonic axis, which correlated with the PsAO3 transcript level. In mature dry seeds, AO activity and the expression of PsAOs became restricted to the embryonic tissues. The possible involvement of AO isoforms in ABA or IAA synthesis during pea seed development as well as the contribution of particular PsAO genes to the formation of the dimeric pea AO isoforms is discussed.

The APETALA-2-like transcription factor OsAP2-39 controls key interactions between abscisic acid and gibberellin in rice

The interaction between phytohormones is an important mechanism which controls growth and developmental processes in plants. Deciphering these interactions is a crucial step in helping to develop crops with enhanced yield and resistance to environmental stresses. Controlling the expression level of OsAP2-39 which includes an APETALA 2 (AP2) domain leads to phenotypic changes in rice. Overexpression of OsAP2-39 leads to a reduction in yield by decreasing the biomass and the number of seeds in the transgenic rice lines. Global transcriptome analysis of the OsAP2-39 overexpression transgenic rice revealed the upregulation of a key abscisic acid (ABA) biosynthetic gene OsNCED-I which codes for 9-cis-epoxycarotenoid dioxygenase and leads to an increase in the endogenous ABA level. In addition to OsNCED-1, the gene expression analysis revealed the upregulation of a gene that codes for the Elongation of Upper most Internode (EUI) protein, an enzyme that catalyzes 16α, 17-epoxidation of non-13-hydroxylated GAs, which has been shown to deactivate gibberellins (GAs) in rice. The exogenous application of GA restores the wild-type phenotype in the transgenic line and ABA application induces the expression of EUI and suppresses the expression of OsAP2-39 in the wild-type line. These observations clarify the antagonistic relationship between ABA and GA and illustrate a mechanism that leads to homeostasis of these hormones. In vivo and in vitro analysis showed that the expression of both OsNCED-1 and EUI are directly controlled by OsAP2-39. Together, these results reveal a novel mechanism for the control of the ABA/GA balance in rice which is regulated by OsAP2-39 that in turn regulates plant growth and seed production.

F-box protein DOR functions as a novel inhibitory factor for abscisic acid-induced stomatal closure under drought stress in Arabidopsis,

Guard cells, which form stoma in leaf epidermis, sense and integrate environmental signals to modulate stomatal aperture in response to diverse conditions. Under drought stress, plants synthesize abscisic acid (ABA), which in turn induces a rapid closing of stoma, to prevent water loss by transpiration. However, many aspects of the molecular mechanism for ABA-mediated stomatal closure are still not understood. Here, we report a novel negative regulator of guard cell ABA signaling, DOR, in Arabidopsis (Arabidopsis thaliana). The DOR gene encodes a putative F-box protein, a member of the S-locus F-box-like family related to AhSLF-S(2) and specifically interacting with ASK14 and CUL1. A null mutation in DOR resulted in a hypersensitive ABA response of stomatal closing and a substantial increase of drought tolerance; in contrast, the transgenic plants overexpressing DOR were more susceptible to the drought stress. DOR is strongly expressed in guard cells and suppressed by ABA treatment, suggesting a negative feedback loop of DOR in ABA responses. Double-mutant analyses of dor with ABA-insensitive mutant abi1-1 showed that abi1-1 is epistatic to dor, but no apparent change of phospholipase Dalpha1 was detected between the wild type and dor. Affymetrix GeneChip analysis showed that DOR likely regulates ABA biosynthesis under drought stress. Taken together, our results demonstrate that DOR acts independent of phospholipase Dalpha1 in an ABA signaling pathway to inhibit the ABA-induced stomatal closure under drought stress.

Mechanism and regulation of the violaxanthin cycle: the role of antenna proteins and membrane lipids

The violaxanthin cycle describes the reversible conversion of violaxanthin to zeaxanthin via the intermediate antheraxanthin. This light-dependent xanthophyll conversion is essential for the adaptation of plants and algae to different light conditions and allows a reversible switch of photosynthetic light-harvesting complexes between a light-harvesting state under low light and a dissipative state under high light. The photoprotective functions of zeaxanthin have been intensively studied during the last decade, but much less attention has been directed to the mechanism and regulation of xanthophyll conversion. In this review, an overview is given on recent progress in the understanding of the role of (i) xanthophyll binding by antenna proteins and of (ii) the lipid properties of the thylakoid membrane in the regulation of xanthophyll conversion. The consequences of these findings for the mechanism and regulation of xanthophyll conversion in the thylakoid membrane will be discussed.