A PPO Promoter from Betalain-Producing Red Swiss Chard, Directs Petiole- and Root-Preferential Expression of Foreign Gene in Anthocyanins-Producing Plants

Comparative Transcriptomic Analysis of the Metabolism of Betalains and Flavonoids in Red Amaranth Hypocotyl under Blue Light and Dark Conditions

Amaranth plants contain abundant betalains and flavonoids. Anthocyanins are important flavonoids; however, they cannot coexist in the same plant with betalains. Blue light influences metabolite synthesis and hypocotyl elongation; accordingly, analyses of its effects on betalain and flavonoid biosynthesis in Amaranthus tricolor may provide insight into the distribution of these plant pigments. We analyzed the betalain and flavonoid content and transcriptome profiles in amaranth hypocotyls under blue light and dark conditions. Furthermore, we analyzed the expression patterns of key genes related to betalains and flavonoids. Amaranth hypocotyls were shorter and redder and showed higher betalain and flavonoid content under blue light than in dark conditions. Key genes involved in the synthesis of betalains and flavonoids were upregulated under blue light. The gene encoding DELLA was also upregulated. These results suggest that blue light favors the synthesis of both betalains and flavonoids via the suppression of bioactive gibberellin and the promotion of DELLA protein accumulation, which also suppresses hypocotyl elongation. The metabolite profiles differed between plants under blue light and dark conditions. These findings improve our understanding of the environmental cues and molecular mechanisms underlying pigment variation in Amaranthus.

The R2R3 Transcription Factor CsMYB59 Regulates Polyphenol Oxidase Gene CsPPO1 in Tea Plants (Camellia sinensis)

Polyphenol oxidase (PPO) plays a role in stress response, secondary metabolism, and other physiological processes during plant growth and development, and is also a critical enzyme in black tea production. However, the regulatory mechanisms of PPO genes and their activity in tea plants are still unclear. In this study, we measured PPO activity in two different tea cultivars, Taoyuandaye (TYDY) and Bixiangzao (BXZ), which are commonly used to produce black tea and green tea, respectively. The expression pattern of CsPPO1 was assessed and validated via transcriptomics and quantitative polymerase chain reaction in both tea varieties. In addition, we isolated and identified an R2R3-MYB transcription factor CsMYB59 that may regulate CsPPO1 expression. CsMYB59 was found to be a nuclear protein, and its expression in tea leaves was positively correlated with CsPPO1 expression and PPO activity. Transcriptional activity analysis showed that CsMYB59 was a transcriptional activator, and the dual-luciferase assay indicated that CsMYB59 could activate the expression of CsPPO1 in tobacco leaves. In summary, our study demonstrates that CsMYB59 represents a transcriptional activator in tea plants and may mediate the regulation of PPO activity by activating CsPPO1 expression. These findings provide novel insights into the regulatory mechanism of PPO gene in Camellia sinensis, which might help to breed tea cultivars with high PPO activity.

Phenoloxidases in Plants-How Structural Diversity Enables Functional Specificity

The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism.

A 1232 bp upstream sequence of glutamine synthetase 1b from Eichhornia crassipes is a root-preferential promoter sequence

BACKGROUND

Glutamine synthetase (GS) acts as a key enzyme in plant nitrogen (N) metabolism. It is important to understand the regulation of GS expression in plant. Promoters can initiate the transcription of its downstream gene. Eichhornia crassipes is a most prominent aquatic invasive plant, which has negative effects on environment and economic development. It also can be used in the bioremediation of pollutants present in water and the production of feeding and energy fuel. So identification and characterization of GS promoter in E. crassipes can help to elucidate its regulation mechanism of GS expression and further to control its N metabolism.

RESULTS

A 1232 bp genomic fragment upstream of EcGS1b sequence from E. crassipes (EcGS1b-P) has been cloned, analyzed and functionally characterized. TSSP-TCM software and PlantCARE analysis showed a TATA-box core element, a CAAT-box, root specific expression element, light regulation elements including chs-CMA1a, Box I, and Sp1 and other cis-acting elements in the sequence. Three 5'-deletion fragments of EcGS1b upstream sequence with 400 bp, 600 bp and 900 bp length and the 1232 bp fragment were used to drive the expression of β-glucuronidase (GUS) in tobacco. The quantitative test revealed that GUS activity decreased with the decreasing of the promoter length, which indicated that there were no negative regulated elements in the EcGS1-P. The GUS expressions of EcGS1b-P in roots were significantly higher than those in leaves and stems, indicating EcGS1b-P to be a root-preferential promoter. Real-time Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) analysis of EcGS1b gene also showed higher expression in the roots of E.crassipes than in stems and leaves.

CONCLUSIONS

EcGS1b-P is a root-preferential promoter sequence. It can specifically drive the transcription of its downstream gene in root. This study will help to elucidate the regulatory mechanisms of EcGS1b tissue-specific expression and further study its other regulatory mechanisms in order to utilize E.crassipes in remediation of eutrophic water and control its overgrowth from the point of nutrient metabolism.

Recent advances in polyphenol oxidase-mediated plant stress responses

Plant polyphenol oxidases (PPOs) are ubiquitous copper metalloenzymes with a biochemistry that has been known for more than a century. By the 1990s, biologists began to recognize the importance of PPOs in plant response to the infestation of herbivores and pathogens; ideas concerning a defensive role for PPOs arose to address observed evidence, and several testable hypotheses were suggested. Two pivotal discoveries in tomato (Lycopersicon esculentum Miller) plants, an inverse correlation between PPO levels and insect growth and PPO induction by defence signals, have driven many studies of PPO defence functions in the context of abiotic and biotic stresses. During the past three decades, extensive molecular research in transgenic and non-transgenic systems has partly revealed the sophisticated mechanisms underlying PPO defence against herbivores and pathogens. These understandings, rather than theoretical predictions, have driven the development of new hypotheses and advanced PPO-related studies. Here, we review progress in PPO family features, expression regulation and the defensive role of PPOs in plants. We propose assumptions of an extended range of co- and post-transcriptional processes to the regulation of unexplored PPO expression. In addition, the identification of endogenous PPO substrates and downstream targets of PPO action will be useful for elucidating PPO defensive roles. The potential effects of PPO-mediated oxidative defences on herbivore performance ultimately needs to be further investigated. Therefore, expanding multidisciplinary approaches to unexplored dimensions of PPO defence function should be a future priority.

Choice of the Promoter for Tissue and Developmental Stage-Specific Gene Expression

Transgenic technologies belong to important tools of reverse genetics and biotechnology in plants. Targeted genetic modifications can reveal functions of genes of interest, change metabolic and regulatory pathways, or result in accumulation of valuable proteins or metabolites. However, to be efficient in targeted genetic modification, the chimeric gene construct should be designed properly. In particular, the promoters used to control transgene expression need to be carefully chosen. Most promoters in widely used vectors belong to strong and constitutively expressed variants. However, in many cases transgene expression has to be restricted to certain tissue, stage of development, or response to some internal or external stimuli. In turn, a large variety of tissue-specific promoters have been studied and information on their characteristics may be recovered from the literature. An appropriate promoter may be selected and used in genetic construct to optimize the transgene transcription pattern. We have previously designed the TGP database (TransGene Promoters, http://wwwmgs.bionet.nsc.ru/mgs/dbases/tgp/home.html ) collecting information from the publications in this field. Here we review the wide range of noncanonical tissue-specific and developmentally regulated promoters that might be used for transgene expression control.

RNA-sequencing analysis reveals betalains metabolism in the leaf of Amaranthus tricolor L

Amaranth plants contain large amounts of betalains, including betaxanthins and betacyanins. Amaranthin is a betacyanin, and its molecular structure and associated metabolic pathway differ from those of betanin in beet plants. The chlorophyll, carotenoid, betalain, and flavonoid contents in amaranth leaves were analyzed. The abundance of betalain, betacyanin, and betaxanthin was 2-5-fold higher in the red leaf sectors than in the green leaf sectors. Moreover, a transcriptome database was constructed for the red and green sectors of amaranth leaves harvested from 30-day-old seedlings. 22 unigenes were selected to analyze the expression profiles in the two leaf sectors. The RNA-sequencing data indicated that many unigenes are involved in betalain metabolic pathways. The potential relationships between diverse metabolic pathways and betalain metabolism were analyzed. The validation of the expression of 22 selected unigenes in a qRT-PCR assay revealed the genes that were differentially expressed in the two leaf sectors. Betalains were biosynthesized in specific tissues of the red sectors of amaranth leaves. Almost all of the genes related to betalain metabolism were identified in the transcriptome database, and the expression profiles were different between the red sectors and green sectors in the leaf. Amaranth plants consist of diverse metabolic pathways, and the betalain metabolic pathway is linked to a group of other metabolic pathways.

Proteomic Analysis of Hylocereus polyrhizus Reveals Metabolic Pathway Changes

Red dragon fruit or red pitaya (Hylocereus polyrhizus) is the only edible fruit that contains betalains. The color of betalains ranges from red and violet to yellow in plants. Betalains may also serve as an important component of health-promoting and disease-preventing functional food. Currently, the biosynthetic and regulatory pathways for betalain production remain to be fully deciphered. In this study, isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomic analyses were used to reveal the molecular mechanism of betalain biosynthesis in H. polyrhizus fruits at white and red pulp stages, respectively. A total of 1946 proteins were identified as the differentially expressed between the two samples, and 936 of them were significantly highly expressed at the red pulp stage of H. polyrhizus. RNA-seq and iTRAQ analyses showed that some transcripts and proteins were positively correlated; they belonged to “phenylpropanoid biosynthesis”, “tyrosine metabolism”, “flavonoid biosynthesis”, “ascorbate and aldarate metabolism”, “betalains biosynthesis” and “anthocyanin biosynthesis”. In betalains biosynthesis pathway, several proteins/enzymes such as polyphenol oxidase, CYP76AD3 and 4,5-dihydroxy-phenylalanine (DOPA) dioxygenase extradiol-like protein were identified. The present study provides a new insight into the molecular mechanism of the betalain biosynthesis at the posttranscriptional level.

Impact of extraction and processing conditions on betalains and comparison of properties with anthocyanins - A current review

The search for natural pigments has been driven by growing evidence indicating that synthetic colorants can cause deleterious health effects. Betalains, in addition to anthocyanins, have been proposed as an alternative to address this need. However, the incorporation of natural pigments poses some challenges to the food industry, such as reduced stability in comparison to their synthetic counterparts. Moreover, betalains are not well studied in comparison to anthocyanins and information about the effects of processing on their physicochemical properties and stability is scattered. Thus, this review will provide an overview of the recent research on the extraction and processing of betalains from natural sources, and comparison of their colorant and physicochemical properties with anthocyanins.