Overexpression of IbPAL1 promotes chlorogenic acid biosynthesis in sweetpotato

First systematic review of the last 30 years of research on sweetpotato: elucidating the frontiers and hotspots

Sweetpotato is an economically important crop, and it has various advantages over other crops in addressing global food security and climate change. Although substantial articles have been published on the research of various aspects of sweetpotato biology, there are no specific reports to systematically crystallize the research achievements. The current review takes the lead in conducting a keyword-centric spatiotemporal dimensional bibliometric analysis of articles on sweetpotato research using CiteSpace software to comprehensively clarify the development status, research hotspot, and development trend in the past 30 years (1993-2022). Quantitative analysis was carried out on the publishing countries, institutions, disciplines, and scholars to understand the basic status of sweetpotato research; then, visual analysis was conducted on high-frequency keywords, burst keywords, and keyword clustering; the evolution of major research hotspots and the development trend in different periods were summarized. Finally, the three main development stages-preliminary stage (1993-2005), rapid stage (2006-2013), and diversified mature stage (2014-2022)-were reviewed and analyzed in detail. Particularly, the development needs of sweetpotato production in improving breeding efficiency, enhancing stress tolerance, coordinating high yield with high quality and high resistance, and promoting demand were discussed, which will help to comprehensively understand the development dynamics of sweetpotato research from different aspects of biological exploration.

Integrated transcriptomic and CGAs analysis revealed IbGLK1 is a key transcription factor for chlorogenic acid accumulation in sweetpotato (Ipomoea batatas [L.] Lam.) blades

Sweetpotato blades are rich in the functional secondary metabolite chlorogenic acid (CGA), which deepen potential for effective utilization of the blade in industry. In this study, we evaluated the type and content of CGA in the blades of 16 sweetpotato genotypes and analyzed the correlation between CGA content and antioxidant capacity. Then we isolated and characterized IbGLK1, a GARP-type transcription factor, by comparative transcriptome analysis. A subcellular localization assay indicated that IbGLK1 is located in the nucleus. Overexpression and silencing of IbGLK1 in sweetpotato blade resulted in a 0.90-fold increase and 1.84-fold decrease, respectively, in CGA content compared to the control. Yeast one-hybrid and dual-luciferase assays showed that IbGLK1 binds and activates the promoters of IbHCT, IbHQT, IbC4H, and IbUGCT, resulting in the promotion of CGA biosynthesis. In conclusion, our study provides insights into a high-quality gene for the regulation of CGA metabolism and germplasm resources for breeding sweetpotato.

Review: Defense responses in sweetpotato (Ipomoea batatas L.) against biotic stress

Sweetpotato (Ipomoea batatas L.) is regarded as amongst the world's most important crops for food, vegetable, forage, and raw material for starch and alcohol production. Since pest attack and disease infection are the main limiting aspects frequently causing the yield loss and quality degradation of sweetpotato, it is a great demand to develop the effective defense strategies for maintaining productivity. In the past decade, many studies have focused on dynamic analysis at the physiological, biochemical, and molecular responses of sweetpotatoes to environmental challenges. This review offers an overview of the defense mechanisms against biotic stresses in sweetpotato observed so far, particularly insect herbivory and pathogen infections. The defenses of sweetpotato include the regulation of the toxic and anti-digestive proteins, plant-derived compounds, physical barrier formation, and sugar distribution. Ipomoelin and sporamin have been extensively researched for the defense against herbivore wounding. Herbivory-induced plant volatiles, chlorogenic acid, and latex phytochemicals play important roles in defenses for insect herbivory. Induction of IbSWEET10 reduces sugar content to mediate F. oxysporum resistance. Therefore, these researches provide the genetic strategies for improving resistance bioengineering and breeding of sweetpotato crops and future prospects for research in this field.

Flavonoid profile of Anoectochilus roxburghii (Wall.) Lindl. Under short-term heat stress revealed by integrated metabolome, transcriptome, and biochemical analyses

Global warming severely threatens plant growth, and could lead to yield reduction. Although findings suggest that flavonoids play important roles in biological process in plants, their response to heat stress in Anoectochilus roxburghii (Wall.) Lindl. remains unclear. Here, we aimed to examine the flavonoid profile of A. roxburghii under heat stress and assess the effect of exogenous application of quercetin on heat stress tolerance. Metabolome analysis showed that quercetin, tricetin, isorhamnetin, scutellarein, and 4',7-Isoflavandiol were the main upregulated flavonoids in A. roxburghii, based on variable importance in the projection >1 and with fold change >2. Determination of the concentrations of the flavonoids using a standard curve revealed that quercetin, kaempferol, and isorhamnetin contents increased by 8.24-, 7.55-, and 5.01-fold, respectively, during heat stress, whereas rutin concentration decreased from 83.04 to 80.89 mg/kg (dry weight). Additionally, transcriptome analysis indicated increased expression of several genes in flavonoid biosynthesis pathways, including phenylalanine ammonia-lyase and chalcone synthase. Moreover, exogenous application of quercetin improved the antioxidant capacity and physiological parameters, including photosynthetic rate and chlorophyll content, of A. roxburghii under heat stress. Overall, the flavonoid profile of A. roxburghii under short-term heat stress was characterized based on integrated metabolomic, transcriptomic, and biochemical analyses, providing new insights for improving the biological value of A. roxburghii.

Genetic fingerprint construction and genetic diversity analysis of sweet potato (Ipomoea batatas) germplasm resources

BACKGROUND

China is the largest producer of sweet potato in the world, accounting for 57.0% of the global output. Germplasm resources are the basis for promoting innovations in the seed industry and ensuring food security. Individual and accurate identification of sweet potato germplasm is an important part of conservation and efficient utilization.

RESULTS

In this study, nine pairs of simple sequence repeat molecular markers and 16 morphological markers were used to construct genetic fingerprints for sweet potato individual identification. Combined with basic information, typical phenotypic photographs, genotype peak graphs, and a two-dimensional code for detection and identification were generated. Finally, a genetic fingerprint database containing 1021 sweet potato germplasm resources in the "National Germplasm Guangzhou Sweet Potato Nursery Genebank in China" was constructed. Genetic diversity analysis of the 1021 sweet potato genotypes using the nine pairs of simple sequence repeat markers revealed a narrow genetic variation range of Chinese native sweet potato germplasm resources, and Chinese germplasm was close to that from Japan and the United States, far from that from the Philippines and Thailand, and the furthest from that from Peru. Sweet potato germplasm resources from Peru had the richest genetic diversity, supporting the view that Peru is the center of origin and domestication of sweet potato varieties.

CONCLUSIONS

Overall, this study provides scientific guidance for the conservation, identification, and utilization of sweet potato germplasm resources and offers a reference to facilitate the discovery of important genes to boost sweet potato breeding.

MePAL6 regulates lignin accumulation to shape cassava resistance against two-spotted spider mite

INTRODUCTION

The two-spotted spider mite (TSSM) is a devastating pest of cassava production in China. Lignin is considered as an important defensive barrier against pests and diseases, several genes participate in lignin biosynthesis, however, how these genes modulate lignin accumulation in cassava and shape TSSM-resistance is largely unknown.

METHODS

To fill this knowledge gap, while under TSSM infestation, the cassava lignin biosynthesis related genes were subjected to expression pattern analysis followed by family identification, and genes with significant induction were used for further function exploration.

RESULTS

Most genes involved in lignin biosynthesis were up-regulated when the mite-resistant cassava cultivars were infested by TSSM, noticeably, the MePAL gene presented the most vigorous induction among these genes. Therefore, we paid more attention to dissect the function of MePAL gene during cassava-TSSM interaction. Gene family identification showed that there are 6 MePAL members identified in cassava genome, further phylogenetic analysis, gene duplication, cis-elements and conserved motif prediction speculated that these genes may probably contribute to biotic stress responses in cassava. The transcription profile of the 6 MePAL genes in TSSM-resistant cassava cultivar SC9 indicated a universal up-regulation pattern. To further elucidate the potential correlation between MePAL expression and TSSM-resistance, the most strongly induced gene MePAL6 were silenced using virus-induced gene silencing (VIGS) assay, we found that silencing of MePAL6 in SC9 not only simultaneously suppressed the expression of other lignin biosynthesis genes such as 4-coumarate--CoA ligase (4CL), hydroxycinnamoyltransferase (HCT) and cinnamoyl-CoA reductase (CCR), but also resulted in decrease of lignin content. Ultimately, the suppression of MePAL6 in SC9 can lead to significant deterioration of TSSM-resistance.

DISCUSSION

This study accurately identified MePAL6 as critical genes in conferring cassava resistance to TSSM, which could be considered as promising marker gene for evaluating cassava resistance to insect pest.

Engineering of triterpene metabolism and overexpression of the lignin biosynthesis gene PAL promotes ginsenoside Rg3 accumulation in ginseng plant chassis

The ginsenoside Rg₃ found in Panax species has extensive pharmacological properties, in particular anti-cancer effects. However, its natural yield in Panax plants is limited. Here, we report a multi-modular strategy to improve yields of Rg₃ in a Panax ginseng chassis, combining engineering of triterpene metabolism and overexpression of a lignin biosynthesis gene, phenylalanine ammonia lyase (PAL). We first performed semi-rational design and site mutagenesis to improve the enzymatic efficiency of Pq3-O-UGT2, a glycosyltransferase that directly catalyzes the biosynthesis of Rg₃ from Rh₂ . Next, we used clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) gene editing to knock down the branch pathway of protopanaxatriol-type ginsenoside biosynthesis to enhance the metabolic flux of the protopanaxadiol-type ginsenoside Rg₃ . Overexpression of PAL accelerated the formation of the xylem structure, significantly improving ginsenoside Rg₃ accumulation (to 6.19-fold higher than in the control). We combined overexpression of the ginsenoside aglycon synthetic genes squalene epoxidase, Pq3-O-UGT2, and PAL with CRISPR/Cas9-based knockdown of CYP716A53v2 to improve ginsenoside Rg₃ accumulation. Finally, we produced ginsenoside Rg₃ at a yield of 83.6 mg/L in a shake flask (7.0 mg/g dry weight, 21.12-fold higher than with wild-type cultures). The high-production system established in this study could be a potential platform to produce the ginsenoside Rg₃ commercially for pharmaceutical use.

Analysis of Chlorogenic Acid in Sweet Potato Leaf Extracts

Sweet potato (Ipomoea batatas L.) is one of the most important food crops worldwide, with leaves of different varieties showing purple, green and yellow, and these leaves provide a dietary source of nutrients and various bioactive compounds. The objective of this study was to identify the active constituents of chlorogenic acids (CGAs) in different methanolic extract of leaves of three varieties of sweet potato (purple CYY 98-59, green Taoyuan 2, and yellow CN 1927-16) using liquid chromatography–tandem mass spectrometry. Genotype-specific metabolite variations were observed; CGAs and three isomeric peaks were detected in sweet potato leaf extracts (SPLEs). Among them, the yellow SPLE contained the highest contents of 3,5-dicaffeoylquinic acid (3,5-di-CQA) and 3,4-dicaffeoylquinic acid (3,4-di-CQA), followed by the green SPLE, whereas the purple SPLE retained lower 3,5-di-CQA content compared to yellow and green SPLEs. All three SPLEs contained lower 4,5-dicaffeoylquinic acid (4,5-di-CQA) and CGA contents compared to 3,5-di-CQA and 3,4-di-CQA, although CGA constituents were not significantly different in genotypes, whereas purple SPLE contained higher 4,5-di-CQA content compared to yellow and green SPLEs. This study indicates that SPLs marketed in Taiwan vary widely in their biological potentials and may impart different health benefits to consumers.

Probing the transcriptome of Boehmeria nivea reveals candidate genes associated with the biosynthesis of chlorogenic acid

Boehmeria nivea (L.) Gaudich is used in traditional Chinese medicine. Chlorogenic acids are major medically active components of Boehmeria nivea, which can be used clinically to treat hyperglycemia, pneumonia, and cancer. To identify the genes involved in chlorogenic acid biosynthesis, we analyzed transcriptome data from leaf, root, and stem tissues of Boehmeria nivea using the Illumina Hi-Seq 4000 platform. A total of 146,790 unigenes were obtained from Boehmeria nivea, of which 106,786 were annotated in public databases. In analyses of the KEGG (Kyoto Encyclopedia of Genes and Genome) database, 484 unigenes that encode the five key enzymes involved in chlorogenic acid biosynthesis were identified, and shikimate O-hydroxycinnamoyl transferase was spatially simulated. Some of these key enzyme unigenes expression levels were verified by RT-qPCR (real-time quantitative Polymerase Chain Reaction). Furthermore, multiple genes encoding plant resistance proteins or transcription factors were identified and analyzed. Differentially expressed genes were identified by performing pairwise comparison of genes between tissues. This study increases the number of public transcript datasets of this species and identifies candidate genes related to the biosynthesis of chlorogenic acid, laying a foundation for the further exploration of this pathway in Boehmeria nivea.

The Biological Activity Mechanism of Chlorogenic Acid and Its Applications in Food Industry: A Review

Chlorogenic acid (CGA), also known as coffee tannic acid and 3-caffeoylquinic acid, is a water-soluble polyphenolic phenylacrylate compound produced by plants through the shikimic acid pathway during aerobic respiration. CGA is widely found in higher dicotyledonous plants, ferns, and many Chinese medicine plants, which enjoy the reputation of “plant gold.” We have summarized the biological activities of CGA, which are mainly shown as anti-oxidant, liver and kidney protection, anti-bacterial, anti-tumor, regulation of glucose metabolism and lipid metabolism, anti-inflammatory, protection of the nervous system, and action on blood vessels. We further determined the main applications of CGA in the food industry, including food additives, food storage, food composition modification, food packaging materials, functional food materials, and prebiotics. With a view to the theoretical improvement of CGA, biological activity mechanism, and subsequent development and utilization provide reference and scientific basis.

Combining Metabolomics and Transcriptomics to Reveal the Mechanism of Coloration in Purple and Cream Mutant of Sweet Potato (Ipomoea batatas L.)

Purple sweet potato is considered as a healthy food because of its high anthocyanins. To understand the coloring mechanism and quality change between purple-fleshed sweet potato (cv. Xuzi201) and its cream fleshed mutant (M1001), a combined metabolomic and transcriptomic analysis was performed. The metabolome data showed that 4 anthocyanins, 19 flavones, 6 flavanones, and 4 flavonols dramatically decreased in M1001, while the contents of 3 isoflavones, 3 flavonols, 4 catechins, and 2 proanthocyanins increased. Transcriptomic analyses indicated that the expression of 49 structural genes in the flavonoid pathway and transcription factors (TFs) (e.g., bHLH2, R2R3-MYB, MYB1) inducting anthocyanin biosynthesis were downregulated, but the repressor MYB44 was upregulated. The IbMYB1-2 gene was detected as a mutation gene in M1001, which is responsible for anthocyanin accumulation in the storage roots. Thus, the deficiency of purple color in the mutant is due to the lack of anthocyanin accumulation which was regulated by IbMYB1. Moreover, the accumulation of starch and aromatic volatiles was significantly different between Xuzi201 and M1001. These results not only revealed the mechanism of color mutation but also uncovered certain health-promoting compounds in sweet potato.

Comparative transcriptome and weighted correlation network analyses reveal candidate genes involved in chlorogenic acid biosynthesis in sweet potato

Chlorogenic acids (CGAs) are important secondary metabolites produced in sweet potato. However, the mechanisms of their biosynthesis and regulation remain unclear. To identify potential genes involved in CGA biosynthesis, analysis of the dynamic changes in CGA components and RNA sequencing were performed on young leaves (YL), mature leaves (ML), young stems (YS), mature stems (MS) and storage roots (SR). Accordingly, we found that the accumulation of six CGA components varied among the different tissues and developmental stages, with YS and YL recording the highest levels, while SR exhibited low levels. Moreover, the transcriptome analysis yielded 59,287 unigenes, 3,767 of which were related to secondary-metabolite pathways. The differentially expressed genes (DEGs) were identified based on CGA content levels by comparing the different samples, including ML vs. YL, MS vs. YS, SR vs. YL and SR vs. YS. A total of 501 common DEGs were identified, and these were mainly implicated in the secondary metabolites biosynthesis. Additionally, eight co-expressed gene modules were identified following weighted gene co-expression network analysis, while genes in darkgrey module were highly associated with CGA accumulation. Darkgrey module analysis revealed that 12 unigenes encoding crucial enzymes (PAL, 4CL, C4H, C3H and HCT/HQT) and 42 unigenes encoding transcription factors (MYB, bHLH, WD40, WRKY, ERF, MADS, GARS, bZIP and zinc finger protein) had similar expression patterns with change trends of CGAs, suggesting their potential roles in CGA metabolism. Our findings provide new insights into the biosynthesis and regulatory mechanisms of CGA pathway, and will inform future efforts to build a genetically improve sweet potato through the breeding of high CGA content varieties.

Caffeoylquinic acids: chemistry, biosynthesis, occurrence, analytical challenges, and bioactivity

Caffeoylquinic acids (CQAs) are specialized plant metabolites we encounter in our daily life. Humans consume CQAs in mg-to-gram quantities through dietary consumption of plant products. CQAs are considered beneficial for human health, mainly due to their anti-inflammatory and antioxidant properties. Recently, new biosynthetic pathways via a peroxidase-type p-coumaric acid 3-hydroxylase enzyme were discovered. More recently, a new GDSL lipase-like enzyme able to transform monoCQAs into diCQA was identified in Ipomoea batatas. CQAs were recently linked to memory improvement; they seem to be strong indirect antioxidants via Nrf2 activation. However, there is a prevalent confusion in the designation and nomenclature of different CQA isomers. Such inconsistencies are critical and complicate bioactivity assessment since different isomers differ in bioactivity and potency. A detailed explanation regarding the origin of such confusion is provided, and a recommendation to unify nomenclature is suggested. Furthermore, for studies on CQA bioactivity, plant-based laboratory animal diets contain CQAs, which makes it difficult to include proper control groups for comparison. Therefore, a synthetic diet free of CQAs is advised to avoid interferences since some CQAs may produce bioactivity even at nanomolar levels. Biotransformation of CQAs by gut microbiota, the discovery of new enzymatic biosynthetic and metabolic pathways, dietary assessment, and assessment of biological properties with potential for drug development are areas of active, ongoing research. This review is focused on the chemistry, biosynthesis, occurrence, analytical challenges, and bioactivity recently reported for mono-, di-, tri-, and tetraCQAs.

Effects of exogenous phytohormones on chlorogenic acid accumulation and pathway-associated gene expressions in sweetpotato stem tips

Sweetpotato (Ipomoea batatas [L.] Lam.) stem tips, which contain high concentrations of chlorogenic acid (CGA), are useful as a physiologically functional food to protect against some serious diseases. According to previous studies, exogenous application of phytohormones may be an effective agrotechnical measure to control CGA biosynthesis through the transcriptional regulation of pathway gene expressions. To understand the mechanism of CGA biosynthesis in sweetpotato, we investigated the effects of exogenous phytohormones on CGA metabolism in stem tips of sweetpotato. A significantly elevated CGA content was observed in salicylic acid (SA)-treated sweetpotato stem tips at 72 h, as well as in those subjected to abscisic acid (ABA) or gibberellic acid (GA) treatments. Dynamic expression change of seven enzyme genes involved in sweetpotato CGA biosynthesis were analyzed to determine correlations between transcript levels and CGA accumulation. As revealed by the differential expression of these genes under distinct phytohormone treatments, the regulation of specific pathway genes is a critical determinant of the accumulation of CGA in sweetpotato stem tips. We also found that several hormone-responsive sites, such as those for ABA, GA, SA, and jasmonic acid (JA), were present in the promoter regions of sweetpotato CGA biosynthestic pathway genes. Collectively, phytohormones can regulate the transcription of CGA synthesis-related genes and ultimately affect CGA accumulation in sweetpotato stem tips, whereas the regulatory differences are mirrored by cis-acting elements in the corresponding pathway gene promoters.