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Sweetpotato (Ipomoea batatas(L.) Lam) is the world’s seventh most important food crop by production quantity. Cultivated sweetpotato is a hexaploid (2n = 6x = 90), and its genome (B1B1B2B2B2B2) is quite complex due to polyploidy, self-incompatibility, and high heterozygosity. Here we established a haploid-resolved and chromosome-scalede novoassembly of autohexaploid sweetpotato genome sequences. Before constructing the genome, we created chromosome-scale genome sequences inI. trifidausing a highly homozygous accession, Mx23Hm, with PacBio RSII and Hi-C reads. Haploid-resolved genome assembly was performed for a sweetpotato cultivar, Xushu18 by hybrid assembly with Illumina paired-end (PE) and mate-pair (MP) reads, 10X genomics reads, and PacBio RSII reads. Then, 90 chromosome-scale pseudomolecules were generated by aligning the scaffolds onto a sweetpotato linkage map.De novoassemblies were also performed for chloroplast and mitochondrial genomes inI. trifidaand sweetpotato. In total, 34,386 and 175,633 genes were identified on the assembled nucleic genomes ofI. trifidaand sweetpotato, respectively. Functional gene annotation and RNA-Seq analysis revealed locations of starch, anthocyanin, and carotenoid pathway genes on the sweetpotato genome. This is the first report of chromosome-scalede novoassembly of the sweetpotato genome. The results are expected to contribute to genomic and genetic analyses of sweetpotato.

Lignin is associated with cell wall rigidity, water and solute transport, and resistance to diverse stresses in plants. Lignin consists of polymerized monolignols (p-coumaryl, coniferyl, and sinapyl alcohols), which are synthesized by cinnamyl alcohol dehydrogenase (CAD) in the phenylpropanoid pathway. We previously investigated cold-induced IbCAD1 expression by transcriptome profiling of cold-stored tuberous roots of sweetpotato (Ipomoea batatas [L.] Lam). In this study, we confirmed that IbCAD1 expression levels depended on the sweetpotato root type and were strongly induced by several abiotic stresses. We generated transgenic sweetpotato plants overexpressing IbCAD1 (TC plants) to investigate CAD1 physiological functions in sweetpotato. TC plants displayed lower root weights and lower ratios of tuberous roots to pencil roots than non-transgenic (NT) plants. The lignin contents in tuberous roots of NT and TC plants differed slightly, but these differences were not significant. By contrast, monolignol levels and syringyl (S)/guaiacyl (G) ratios were higher in TC plants than NT plants, primarily owing to syringyl unit accumulation. Tuberous roots of TC plants displayed enhanced low-temperature (4 °C) storage with lower malondialdehyde and H2O2 contents than NT plants. We propose that high monolignol levels in TC tuberous roots served as substrates for increased peroxidase activity, thereby enhancing antioxidation capacity against cold stress–induced reactive oxygen species. Increased monolignol contents and/or increased S/G ratios might contribute to pathogen-induced stress tolerance as a secondary chilling-damage response in sweetpotato. These results provide novel information about CAD1 function in cold stress tolerance and root formation mechanisms in sweetpotato.

Class III peroxidases are involved in plant responses to various environmental conditions. Peroxidase production is triggered by various stimuli and activates stress-related signaling pathways to help plants cope with abiotic and biotic stresses. Recent evidence suggests that the interplay between diverse peroxidases plays a critical role in regulating stress responses. However, the underlying molecular mechanisms remain poorly understood. Previous research showed that transgenic sweetpotato overexpressing the swpa4 peroxidase gene exhibited increased tolerance to oxidative stress and high salinity. In this study, loss of function of the swpa4 peroxidase gene in sweetpotato was assessed in transgenic plants with constitutive RNAi-mediated suppression of the swpa4 gene. Compared with control plants, leaf discs from RNAi-suppressed lines exhibited increased damage following treatment with the oxidative stressors hydrogen peroxide and methyl viologen. Transgenic plants also showed reduced resistance to high salinity conditions compared with control plants. Repression of swpa4 also led to reduced expression of the endogenous stress-related peroxidase genes swpa1, swpa2, and swpn1. These findings suggest that swpa4 regulates signaling pathways involved in the regulation of peroxidase metabolism during the stress response in sweetpotato.

Sweetpotato (Ipomoea batatas [L.] Lam) is an important starch crop that ensures food and nutrition security in the era of climate change. Sweetpotato is tolerant to environmental stresses such as drought, high temperature, and high salt, and therefore, is well adapted to marginal lands; however, it is relatively vulnerable to flooding stress, which severely reduces its yield and commercial value. To understand the flooding stress response of sweetpotato, we performed comparative transcriptome analysis of the leaves of two sweetpotato cultivars with contrasting flooding stress tolerance levels: Yeonjami (YJM; flooding tolerant) and Jeonmi (JM; flooding sensitive). Both cultivars were partially submerged in water for 0, 0.5, and 3 days. RNA-seq data of both cultivars revealed 14,229 differentially expressed genes (DEGs), which were categorized into seven clusters and six groups, based on the expression pattern of co-expressed DEGs and expression duration of DEGs, respectively. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses showed that DEGs of distinguishing functions between the two cultivars were involved in plant hormone signaling, carbohydrate transport, and mitogen-activated protein kinase (MAPK) signaling. Based on these results, we predict that YJM promotes adventitious growth, whereas JM exhibits shoot elongation under flooding stress. The expression levels of several key candidate genes involved in flooding tolerance correlated well with the comparative transcriptomics data. Overall, this study provides further insights into the molecular mechanism of flooding stress response in sweetpotato, and reveals candidate genes that could be used for developing new flooding tolerant sweetpotato cultivars.

Sweetpotato (Ipomoea batatas [L.] Lam) is one of the most important food crops, as it ensures food and nutrition security in the face of climate change, given its high carbohydrate, antioxidant (carotenoid), dietary fiber, and mineral contents. The Orange protein of sweetpotato (IbOr) shows high molecular chaperone activity. Previously, we showed that transgenic sweetpotato plants overexpressing the IbOr gene exhibited strong tolerance to abiotic stresses such as high temperature (47 °C) and drought, owing to the increased contents of carotenoids including β-carotene. Interestingly, the expression of the sweetpotato carotenoid cleavage dioxygenase 4 (IbCCD4) gene was also significantly up-regulated in IbOr-overexpressing sweetpotato plants. In this study, we confirmed that the IbCCD4 gene is highly expressed in carotenoid-rich leaves and storage roots of three types of sweetpotato cultivars, each with a different storage root color (pale yellow, orange, and purple). The IbCCD4 protein showed interaction with IbOr in yeast two-hybrid and pull-down assays. Additionally, in vitro, carotenoid cleavage assays indicated that β-carotene is the major substrate of IbCCD4. Interestingly, the interaction between IbCCD4 and IbOr did not affect β-carotene degradation. Further characterization of the IbCCD4 gene in transgenic sweetpotato plants will provide new insights into carotenoid homeostasis under severe stress conditions.

We previously reported that transgenic Arabidopsis thaliana plants overexpressing the sweet potato peroxidase gene swpa4 under the control of the cauliflower mosaic virus (CaMV) 35 s promoter showed increased levels of reactive oxygen species (ROS) and nitric oxide (NO), and higher expression of ROS and NO related genes than control plants. Here, we investigated the effect of swpa4 overexpression on the abiotic and biotic stress tolerance levels of Arabidopsis plants. Methyl viologen (MV) treatment-induced oxidative stress caused visible damage to the seedlings and rosette leaves of all Arabidopsis plants, although the symptoms were more severe in control plants than in transgenic lines. Additionally, survival rates and ion leakage showed a slight decline in transgenic lines but a more severe decline in control plants after MV treatment. Transgenic plants also showed enhanced tolerance to drought stress. Dehydration treatment, followed by rehydration, resulted in a greater change in the relative water content and lipid peroxidation of control plants than in that of transgenic lines. However, transgenic plants did not show enhanced resistance to biotic stresses such as bacterial and fungal pathogens. These results indicate that transgenic Arabidopsis plants can efficiently regulate defense levels during oxidative stress via the overexpression of swpa4.

Environmental stress leads to the production of reactive oxygen species (ROS) and consequently oxidative stress, which limits plant productivity. Thus, new crop cultivars that are able to overcome oxidative stress are required to address the global issues of environmental, food, and energy security. Sweetpotato (Ipomoea batatas [L.] Lam) is considered an emerging multifunctional crop because of its high carbohydrate content and adaptation to marginal lands. Moreover, sweetpotato offers several health benefits for the aging population, as it is rich in low-molecular-weight antioxidants (LMWAs) (e.g., carotenoids, ascorbate, tocopherols, and polyphenols), dietary fiber, potassium, and minerals. We speculate that metabolic engineering of LMWAs in sweetpotato will lead to the development of smart cultivars with enhanced levels of antioxidants and abiotic stress tolerance. This review describes the current status of metabolic engineering of LMWAs in sweetpotato and the potential of sweetpotato in ensuring food and nutrition security in the twenty-first century. Here, we focus on carotenoids, ascorbate, tocopherols, and polyphenols including anthocyanins in transgenic sweetpotato plants. Additionally, we discuss the potential role of the sweetpotato Orange (IbOr) gene, involved in carotenoid accumulation and stress tolerance, in improving crop productivity under the changing global climate and alleviating the problem of vitamin A deficiency. Genetic engineering of the biosynthetic and metabolic genes of LMWAs in sweetpotato is currently under investigation via molecular breeding. We predict that metabolic engineering of antioxidants in sweetpotato will lead to the development of new cultivars with enhanced nutritional value and abiotic stress tolerance, which will ensure global food and nutrition security in the face of climate change.

The ERF (ethylene-responsive element-binding factor) transcription factor family plays important roles in abiotic and biotic stress responses in plants. An EAR motif-containing ERF gene, IbERF4, was isolated from sweetpotato [Ipomoea batatas (L.) Lam]. IbERF4 was highly expressed in the early stage of storage roots and can be induced by various stresses. By overexpressing IbERF4 in Arabidopsis, the transgenic Arabidopsis showed greater sensitivity to drought and salt stress by down-regulating stress-related genes, such as rd22, rd29, AtCBF1 and AtCBF2. Overexpressing IbERF4 in sweetpotato made the transgenic sweetpotato more sensitive to salt stress and made propagating new seedlings more difficult. Consistently, the transcriptomic profiling results showed that genes involved in oxidative stress were significantly down-regulated in sweetpotato overexpression lines. These results indicated that IbERF4 plays as key regulators in sweetpotato abiotic stress.

Flooding is harmful to almost all higher plants, including crop species. Most cultivars of the root crop sweet potato are able to tolerate environmental stresses such as drought, high temperature, and high salinity. They are, however, relatively sensitive to flooding stress, which greatly reduces yield and commercial value. Previous transcriptomic analysis of flood-sensitive and flood-resistant sweet potato cultivars identified genes that were likely to contribute to protection against flooding stress, including genes related to ethylene (ET), reactive oxygen species (ROS), and nitric oxide (NO) metabolism. Although each sweet potato cultivar can be classified as either tolerant or sensitive to flooding stress, the molecular mechanisms of flooding resistance in ET, ROS, and NO regulation-mediated responses have not yet been reported. Therefore, this study characterized the regulation of ET, ROS, and NO metabolism in two sweet potato cultivars-one flood-tolerant cultivar and one flood-sensitive cultivar-under early flooding treatment conditions. The expression of

Salinity stress is one of the major factors in the reduction of plant growth and crop yield worldwide. To improve our ability to grow crops in high-salinity environments, plant responses to salinity stress must be tightly controlled. Here, in order to further understand the precise regulation of plant responses under high salinity conditions, the function of the MYB3 transcription factor was studied as a repressor to control excessive accumulation of lignin and anthocyanin under salt stress conditions. Nuclear-localized MYB3 forms a homodimer. It is ubiquitously expressed, especially in vascular tissues, and its expression is highly induced by NaCl in various tissues such as roots, leaves, stems, and flowers. myb3 mutant plants exhibited longer root growth in high NaCl conditions than wild-type plants. However, several NaCl responsive or signaling genes were not significantly altered in myb3 compared to wild-type. Interestingly, high accumulation of lignin and anthocyanin was displayed in myb3, and the expression of genes involved in lignin and anthocyanin biosynthesis, such as PAL1, C4H, COMT, 4CL3, DFR and LDOX, sharply increased in myb3 compared to those of wild-type plants under NaCl treatment. According to yeast-two hybrid screenings, various transcription factors, including anthocyanin regulators TT8 and EGL3, were isolated as MYB3 interacting proteins. MYB3 was characterized as a transcriptional repressor and its repressor domain is located in the C-terminus of MYB3. Overall, these results suggest that nuclear-localized MYB3 functions as a transcriptional repressor for the control of lignin and anthocyanin accumulation under salinity stress conditions.

Root-knot nematodes (RKN) cause significant damage to sweetpotato plants and cause significant losses in yield and quality. Reactive oxygen species (ROS) play an important role in plant defenses, with levels of ROS-detoxifying antioxidant enzymes tightly regulated during pathogen infection. In this study, ROS metabolism was examined in three RKN-resistant and three RKN-susceptible sweetpotato cultivars. The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were assessed, as was lignin-related metabolism. In RKN-infected roots, both resistant and susceptible cultivars increased SOD activity to produce higher levels of hydrogen peroxide (H2O2). However, H2O2 removal by CAT activity differed between cultivars, with susceptible cultivars having higher CAT activity and lower overall H2O2 levels. In addition, the expression of phenylpropanoid-related phenylalanine ammonia-lyase and cinnamyl alcohol dehydrogenase genes, which encode enzymes involved in lignin metabolism, were higher in resistant cultivars, as were total phenolic and lignin contents. Enzyme activities and H2O2 levels were examined during the early (7 days) and late (28 days) phases of infection in representative susceptible and resistant cultivars, revealing contrasting changes in ROS levels and antioxidant responses in the different stages of infection. This study suggests that differences in antioxidant enzyme activities and ROS regulation in resistant and susceptible cultivars might explain reduced RKN infection in resistant cultivars, resulting in smaller RKN populations and overall higher resistance to infection and infestation by RKNs.

We generated an orange-colored (OC) rice callus line by targeted mutagenesis of the orange gene (OsOr) using the CRISPR-Cas9 system. The OC line accumulated more lutein, β-carotene, and two β-carotene isomers compared to the WT callus line. We also analyzed the expression levels of carotenoid biosynthesis genes by qRT-PCR. Among the genes encoding carotenoid metabolic pathway enzymes, the number of transcripts of the PSY2, PSY3, PDS, ZDS and β-LCY genes were higher in the OC line than in the WT line. In contrast, transcription of the ε-LCY gene was downregulated in the OC line compared to the WT line. In addition, we detected increases in the transcript levels of two genes involved in carotenoid oxidation in the OC lines. The developed OC lines also showed increased tolerance to salt stress. Collectively, these findings indicate that targeted mutagenesis of the OsOr gene via CRISPR/Cas9-mediated genome editing results in β-carotene accumulation in rice calli. Accordingly, we believe that this type of genome-editing technology could represent an effective alternative approach for enhancing the β-carotene content of plants.

In plants, the orange (Or) gene plays roles in regulating carotenoid biosynthesis and responses to environmental stress. The present study investigated whether the expression of rice Or (OsOr) gene could enhance rice tolerance to heat stress conditions. The OsOr gene was cloned and constructed with OsOr or OsOr-R115H (leading to Arg to His substitution at position 115 on the OsOr protein), and transformed into rice plants. The chlorophyll contents and proline contents of transgenic lines were significantly higher than those of non-transgenic (NT) plants under heat stress conditions. However, we found that the levels of electrolyte leakage and malondialdehyde in transgenic lines were significantly reduced compared to NT plants under heat stress conditions. In addition, the levels of expression of four genes related to reactive oxygen species (ROS) scavenging enzymes (OsAPX2, OsCATA, OsCATB, OsSOD-Cu/Zn) and five genes (OsLEA3, OsDREB2A, OsDREB1A, OsP5CS, SNAC1) responded to abiotic stress was showed significantly higher in the transgenic lines than NT plants under heat stress conditions. Therefore, OsOr-R115H could be exploited as a promising strategy for developing new rice cultivars with improved heat stress tolerance.

Sweetpotato is an emerging food crop that ensures food and nutrition security in the face of climate change. Alpha-linoleic acid (ALA) is one of the key factors affecting plant stress tolerance and is also an essential nutrient in humans. In plants, fatty acid desaturase 8 (FAD8) synthesizes ALA from linoleic acid (LA). Previously, we identified the cold-induced IbFAD8 gene from RNA-seq of sweetpotato tuberous roots stored at low-temperature. In this study, we investigated the effect of IbFAD8 on the low-temperature storage ability and ALA content of the tuberous roots of sweetpotato. Transgenic sweetpotato plants overexpressing IbFAD8 (TF plants) exhibited increased cold and drought stress tolerance and enhanced heat stress susceptibility compared with non-transgenic (NT) plants. The ALA content of the tuberous roots of TF plants (0.19 g/100 g DW) was ca. 3.8-fold higher than that of NT plants (0.05 g/100 g DW), resulting in 8–9-fold increase in the ALA/LA ratio in TF plants. Furthermore, tuberous roots of TF plants showed better low-temperature storage ability compared with NT plants. These results indicate that IbFAD8 is a valuable candidate gene for increasing the ALA content, environmental stress tolerance, and low-temperature storage ability of sweetpotato tuberous roots via molecular breeding.

Secretory class III peroxidases are involved in plant responses to a range of biological stress conditions, including oxidative stress. To investigate the stress-related functions of the swpa4 peroxidase gene in sweetpotato (Ipomoea batatas L.), transgenic plants overexpressing the swpa4 gene under the control of the CaMV 35S promoter were generated using Agrobacterium-mediated transformation. Peroxidase activity was 3- to 13-fold higher in transgenic lines than in control plants. Transgenic plants were tested for tolerance to stress. Following treatment with 400 mM hydrogen peroxide (H2O2), leaf discs from transgenic plants showed approximately 13–26% less damage than control plants. Transgenic plants also showed enhanced tolerance to high salinity conditions. Following treatment with NaCl, photosynthetic capacity and total chlorophyll contents were less severely impacted in swpa4 transgenic plants than in control plants. Aerial plant parts and storage root yields were not significantly different between transgenic and control plants following cultivation in field conditions. These results indicate that transgenic sweetpotato lines were able to respond efficiently to oxidative and saline stress via overexpression of swpa4.

Invertase inhibitor (INH) post-translationally regulates the activity of invertase, which hydrolyzes sucrose into glucose and fructose, and plays essential roles in plant growth and development. However, little is known about the role of INH in growth and responses to environmental challenges in sweetpotato. Here, we identified and characterized an INH-like gene (IbINH) from sweetpotato. IbINH belongs to the pectin methylesterase inhibitor super family. IbINH transcript was the most abundant in storage roots. IbINH mRNA levels were significantly up-regulated in response to drought, abscisic acid (ABA), salicyclic acid (SA) and jasmonic acid (JA) treatments. Overexpressing IbINH in sweetpotato (SI plants) led to the decrease of plant growth and the increase of drought tolerance, while down-regulation of IbINH (RI plants) by RNAi technology resulted in vigorous growth and drought sensitivity. Furthermore, sucrose was increased and hexoses was decreased in SI plants, but the opposite results were observed in RI plants. Moreover, higher levels of sugars were accumulated in SI plants in comparison to non-transgenic plants (NT plants) and RI plants during water deficit. In addition, ABA biosynthesis-involved and abiotic stress response-involved genes were prominently up-regulated in SI plants under drought stress. Taken together, these results indicate that IbINH mediates plant growth and drought stress tolerance in sweetpotato via induction of source-sink strength and ABA-regulated pathway.

Dehydroascorbate reductase (DHAR) plays a critical role in the regeneration of ascorbic acid (AsA), and widely involved in plant tolerance to biotic and abiotic stresses. In this study, the IbDHAR3 gene was cloned from sweetpotato cultivar Xushu 18 by RT-PCR. The full-length of this gene was 813 bp which encodes 270 amino acids. The IbDHAR3 protein contained two conserved domains of glutathione S-transferase (GST) and GST-C-DHAR, and one chloroplast transit peptide with 52 amino acids length. Transient expression in tobacco leaf epidermal cells indicated that IbDHAR3 protein is subcellular localized to chloroplast. The qRT-PCR results revealed that the relative expression level of IbDHAR3 in leaves is much higher than that in other tissues, and could be up-regulated by ABA, drought, salinity, and high-temperature stresses. The seed germination rate and root elongation were increased in contrast to wild type under mannitol and NaCl stresses in T3 transgenic Arabidopsis overexpressing IbDHAR3 gene. The soil drought experiments showed that the overexpression of IbDHAR3 gene in Arabidopsis reduced the malondialdehyde (MDA) and the H2O2 content, enhanced the level of AtGR gene expression, superoxide dismutase (SOD), ascorbate peroxidase (APX), DHAR activity, and the AsA content. Therefore, overexpression of IbDHAR3 gene could enhance the ability of scavenging reactive oxygen species such as H2O2 by promoting AsA-glutathione cycle and related antioxidant enzymes system, thereby contributing to increased stress tolerance in Arabidopsis.

IbOr-R96H resulted in carotenoid overaccumulation and enhanced abiotic stress tolerance in transgenic sweetpotato calli. The Orange (Or) protein is involved in the regulation of carotenoid accumulation and tolerance to various environmental stresses. Sweetpotato IbOr, with strong holdase chaperone activity, protects a key enzyme, phytoene synthase (PSY), in the carotenoid biosynthetic pathway and stabilizes a photosynthetic component, oxygen-evolving enhancer protein 2-1 (PsbP), under heat and oxidative stresses in plants. Previous studies of various plant species demonstrated that a single-nucleotide polymorphism (SNP) from Arg to His in Or protein promote a high level of carotenoid accumulation. Here, we showed that the substitution of a single amino acid at position 96 (Arg to His) of wild-type IbOr (referred to as IbOr-R96H) dramatically increases carotenoid accumulation. Sweetpotato calli overexpressing IbOr-WT or IbOr-Ins exhibited 1.8- or 4.3-fold higher carotenoid contents than those of the white-fleshed sweetpotato Yulmi (Ym) calli, and IbOr-R96H overexpression substantially increased carotenoid accumulation by up to 23-fold in sweetpotato calli. In particular, IbOr-R96H transgenic calli contained 88.4-fold higher levels of β-carotene than those in Ym calli. Expression levels of carotenogenesis-related genes were significantly increased in IbOr-R96H transgenic calli. Interestingly, transgenic calli overexpressing IbOr-R96H showed increased tolerance to salt and heat stresses, with similar levels of malondialdehyde to those in calli expressing IbOr-WT or IbOr-Ins. These results suggested that IbOr-R96H is a useful target for the generation of efficient industrial plants, including sweetpotato, to cope with growing food demand and climate change by enabling sustainable agriculture on marginal lands.

Sweetpotato [Ipomoea batatas (L.) Lam] is an important crop used for food, animal feed, and production of industrial materials. Although it is adapted to a wide range of unfavorable conditions, including drought and high salt, sweetpotato is vulnerable to low temperature, making it difficult to cultivate in low temperature regions. To understand the molecular responses occurring in sweetpotato leaves under low temperature stress, de novo transcriptome assembly was performed in leaves under low temperature stress (LT) and during recovery (RC). In comparison with non-treated controls (NT), 2461 and 1017 differentially expressed genes (DEGs) were identified in LT and RC leaves, respectively. When expression in RC and LT samples was directly compared, 2053 DEGs were detected. To increase understanding of the DEGs, the three datasets were analyzed using Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genome (KEGG) database. The CBF transcriptional cascade, a well-known cold response pathway, was investigated using transcriptomic analysis. In contrast with reports from the cold-tolerant Arabidopsis thaliana, none of the COR genes identified in sweetpotato showed increased expression in response to low temperature. Genes involved in antioxidant enzyme pathways mediating responses to reactive oxygen species (ROS) were investigated during low temperature response. This work provides insight into the molecular basis of the responses of sweetpotato to cold stress. This increased understanding of gene regulation in response to cold stress in sweetpotato will be beneficial for future research into molecular-assisted breeding.

Auxin is a major plant hormone that regulates many developmental processes in plants. Arabidopsis thaliana YUCCA6 (AtYUCCA6) mediates the tryptophan-dependent auxin biosynthesis pathway in A. thaliana. To study the effect of auxin in sweetpotato [Ipomoea batatas (L.) Lam], we generated transgenic sweetpotato plants expressing AtYUCCA6 under the control of the oxidative stress-inducible SWPA2 promoter (referred to as SY plants). SY plants displayed high-auxin phenotypic traits such as narrow downward-curled leaves, increased height, and increased node numbers. SY plants showed increased tolerance to 5 μM of methyl viologen-mediated oxidative stress by maintaining a lower ion leakage than wild-type (WT) plants. Moreover, SY plants exhibited higher relative water contents and lower malondialdehyde contents than WT plants under drought stress. Young SY plants also showed inhibition of adventitious root growth. SY plants showed significantly lower storage root formation than WT plants. Our results indicate that AtYUCCA6-mediated auxin overproduction is associated with tolerance to oxidative and drought stress. It also suggests that negatively regulates the formation of storage roots in sweetpotato.

Trehalose-6-phosphate synthase (TPS) plays an important regulatory role in the response of plants to multiple abiotic stresses. However, our knowledge of the stress tolerance functions of TPS genes in sweet potato (Ipomoea batatas [L.] Lam) remains limited. In the present study, we isolated and functionally characterized the sweet potato gene encoding TPS1, IbTPS1. Sequence analysis showed that IbTPS1 belongs to class I TPS proteins and harbors highly conserved acceptor (glucose-6-phosphate [Glc6P]) and donor (uridine diphosphoglucose [UDP-Glc]) binding sites. The IbTPS1 gene showed the highest level of constitutive expression in leaves and storage roots of sweet potato plants and was induced by several environmental stresses including drought, salt, and heat shock. The IbTPS1 protein might be localized to the cytosol. Complementation assay of yeast tps1Δ and tps1Δtps2Δ growth-defective mutants confirmed the TPS activity of IbTPS1, and truncation of the N-terminal extension of IbTPS1 (ΔNIbTPS1) increased the catalytic activity of the protein. Additionally, expression of IbTPS1 in yeast conferred abiotic stress tolerance to dehydration, salinity, and oxidation, and expression of ΔNIbTPS1 further enhanced the tolerance to abiotic stresses. The results of this study advance our understanding of the functions of IbTPS1 under abiotic stress conditions. Our results suggest that the IbTPS1 gene is an excellent candidate for improving the stress tolerance of different crop plants.

Sweetpotato (Ipomoea batatas [L.] Lam) is a prospective food crop that ensures food and nutrition security under the dynamic changes in global climate. Peroxidase (POD) is a multifunctional enzyme involved in diverse plant physiological processes, including stress tolerance and cell wall lignification. Although various POD genes were cloned and functionally characterized in sweetpotato, the role of POD in lignification and low-temperature storage ability of sweetpotato tuberous roots is yet to be investigated. In this study, we isolated the cold-induced lignin forming peroxidase (IbLfp) gene of sweetpotato, and analyzed its physiological functions. IbLfp showed more predominant expression in fibrous roots than in other tissues. Moreover, IbLfp expression was up-regulated in leaves and roots under cold stress, and was altered by other abiotic stresses. Tuberous roots of transgenic sweetpotato lines overexpressing IbLfp (LP lines) showed improved tolerance to low temperature, with lower malondialdehyde and hydrogen peroxide contents than non-transgenic sweetpotato plants under cold stress. The enhanced cold tolerance of LP lines could be attributed to the increased basal activity of POD, which is involved in reactive oxygen species (ROS) scavenging. Moreover, greater accumulation of lignin could also contribute to the enhanced cold tolerance of LP lines, as lignin acts as a protective barrier against invading pathogens, which is a secondary symptom of chilling injury in sweetpotato. Overall, the results of this study enhance our understanding of the function of POD in low-temperature storage of sweetpotato tuberous roots.

Tocopherols are lipid-soluble compounds regarded as vitamin E compounds and they function as antioxidants in scavenging lipid peroxyl radicals and quenching reactive oxygen species (ROS). In our previous studies, we isolated five tocopherol biosynthesis genes from sweetpotato (Ipomoea batatas [L.] Lam) plants including 4-hydroxyphenylpyruvate dioxygenase (IbHPPD). HPPD is the first regulatory enzyme in vitamin E biosynthesis and serves to catalyze in the first steps α-tocopherol and plastoquinone biosynthesis by converting 4-hydroxyphenylpyruvate (HPP) to homogentisic acid (HGA). In this study, we generated transgenic sweetpotato plants overexpressing IbHPPD under the control of cauliflower mosaic virus (CaMV) 35S promoter (referred to as HP plants) via Agrobacterium-mediated transformation to understand the function of IbHPPD in sweetpotato. Three transgenic lines (HP3, HP14 and HP15) with high transcript levels of IbHPPD were selected for further characterization. Compared with non-transgenic (NT) plants, HP plants exhibited enhanced tolerance to multiple environmental stresses, including salt, drought, and oxidative stresses. In addition, HP plants showed increased tolerance to the herbicide sulcotrione, which is involved in the inhibition of the HPPD. Interestingly, after stress treatments, HP plants also showed higher abscisic acid (ABA) contents than NT plants. Under dehydrated condition, HP plants displayed an elevated α-tocopherol content to 19-27% in leaves compared with NT plants. These results indicate that increased abiotic stress tolerance in HP plants is related to inducing enhancement of α-tocopherol and ABA contents.

Sweetpotato (Ipomoea batatas [L.] Lam) is an economically important, nutrient- and pigment-rich root vegetable used as both food and feed. Root-knot nematode (RKN), Meloidogyne incognita, causes major yield losses in sweetpotato and other crops worldwide. The identification of genes and mechanisms responsible for resistance to RKN will facilitate the development of RKN resistant cultivars not only in sweetpotato but also in other crops. In this study, we performed RNA-seq analysis of RKN resistant cultivars (RCs; Danjami, Pungwonmi and Juhwangmi) and susceptible cultivars (SCs; Dahomi, Shinhwangmi and Yulmi) of sweetpotato infected with M. incognita to examine the induced and constitutive defense response-related transcriptional changes. During induced defense, genes related to defense and secondary metabolites were induced in SCs, whereas those related to receptor protein kinase signaling and protein phosphorylation were induced in RCs. In the uninfected control, genes involved in proteolysis and biotic stimuli showed differential expression levels between RCs and SCs during constitutive defense. Additionally, genes related to redox regulation, lipid and cell wall metabolism, protease inhibitor and proteases were putatively identified as RKN defense-related genes. The root transcriptome of SCs was also analyzed under uninfected conditions, and several potential candidate genes were identified. Overall, our data provide key insights into the transcriptional changes in sweetpotato genes that occur during induced and constitutive defense responses against RKN infection.

Sweetpotato (Ipomoea batatas L. Lam) is an economically important crop that is cultivated for its storage roots. Storage roots provide a source of valuable nutrients, processed foods, animal feeds, and pigments. Sweetpotato storage roots spoil during post-harvest handling because of wounding, which makes them more susceptible to disease-causing microorganisms. Curing to promote wound healing is a common method to control microbial spoilage during post-harvest storage. However, molecular mechanisms underlying the process of curing in sweetpotato storage roots are unknown. To better understand the biology behind curing, the transcriptome of the sweetpotato cultivar, Pungwonmi, was studied using RNA-seq. Storage roots of sweetpotato were treated at 33 °C (Curing) and 13 °C (Control) for 3 days. RNA-seq data identified 78,781 unigenes and 3,366 differentially expressed genes by over log2 fold change (FC) 2 and-2. During curing, DEGs encoded genes related to drought/salt stress responses, phyto-hormones (e.g., auxin, ethylene and jasmonic acid), and proteolysis, were up-regulated, whereas those related to redox state, phyto-hormones (e.g., salicylic acid and brassinosteroids), and lignin and flavonoid biosynthesis were down-regulated. Additionally, among the candidate genes, DEGs encoded genes related to proteolysis and pathogen defense, such as protease inhibitors and lipid transfer proteins, were highly up-regulated during curing and storage. This study provides a valuable resource to further understand the molecular basis of curing-mediated wound healing in sweetpotato storage roots. Moreover, genes revealed in this work could present targets for the development of sweetpotato varieties with improved post-harvest storage characteristics.

Carotenoids function as photosynthetic accessory pigments, antioxidants, and vitamin A precursors. We recently showed that transgenic sweetpotato calli overexpressing the mutant sweetpotato (Ipomoea batatas [L.] Lam) Orange gene (IbOr-R96H), which carries a single nucleotide polymorphism responsible for Arg to His substitution at amino acid position 96, exhibited dramatically higher carotenoid content and abiotic stress tolerance than calli overexpressing the wild-type IbOr gene (IbOr-WT). In this study, we generated transgenic sweetpotato plants overexpressing IbOr-R96H under the control of the cauliflower mosaic virus (CaMV) 35S promoter via Agrobacterium-mediated transformation. The total carotenoid contents of IbOr-R96H storage roots (light-orange flesh) and IbOr-WT storage roots (light-yellow flesh) were 5.4–19.6 and 3.2-fold higher, respectively, than those of non-transgenic (NT) storage roots (white flesh). The β-carotene content of IbOr-R96H storage roots was up to 186.2-fold higher than that of NT storage roots. In addition, IbOr-R96H plants showed greater tolerance to heat stress (47 °C) than NT and IbOr-WT plants, possibly because of higher DPPH radical scavenging activity and ABA contents. These results indicate that IbOr-R96H is a promising strategy for developing new sweetpotato cultivars with improved carotenoid contents and heat stress tolerance.

The ethylene-responsive element binding factor (ERF) family is a large family of transcription factors involved in plant development and environmental stress responses. We previously reported the identification of 29 putative substrates of Mitogen-activated Protein Kinase3 (AtMPK3), AtMPK4 and AtMPK6, based on a solid-phase phosphorylation screening using a lambda phage expression library in Arabidopsis thaliana. In this study, a putative MPK substrate, AtERF72 (At3g16770), was strongly phosphorylated by AtMPK6 on the serine residue at position 151 (Ser151). AtERF72 binds to the GCC box (AGCCGCC) in the promoters of several pathogenesis-related (PR) genes and activates their transcription. We also show that the DNA-binding activity of AtERF72 is enhanced upon phosphorylation by AtMPK6 in vitro. In addition, transient co-expression experiments in Arabidopsis protoplasts revealed that effector constructs expressing a mutant variant of AtERF72, AtERF72S151D (carrying a Ser to aspartic acid [Asp] substitution at amino acid position 151) showed higher expression of the β-glucuronidase (GUS) reporter gene driven by the GCC box element than effector constructs expressing the wild-type AtERF72. Furthermore, yeast two-hybrid assays revealed that the interaction between AtERF72S151D and TGA4/OBF4 was stronger than that between wild-type AtERF72 and TGA4/OBF4. Since AtERF72S151D is equivalent to AtERF72 phosphorylated by AtMPK6 at Ser151, these results suggest that the phosphorylation of AtERF72 by AtMPK6 triggers an event of transcriptional regulation from defence signalling in Arabidopsis.

Sweetpotato [Ipomoea batatas (L.) Lam] serves as a sustainable food source and ensures nutrition security in the face of climate change. Recently, farmers have developed increased interest in replacing rice with sweetpotato in paddy fields for higher income. However, sweetpotato is more susceptible to flooding stress than other abiotic stresses including drought and salinity. Here, we selected flooding tolerant sweetpotato cultivars based on biochemical characterization. Young seedlings of 33 sweetpotato cultivars were subjected to flooding stress for 20 days, and Yeonjami (YJM) was identified as the most flooding tolerant sweetpotato cultivar. Plant growth and biochemical characteristics of YJM were compared with those of Jeonmi (JM), a flooding sensitive sweetpotato cultivar. Under flooding stress, YJM showed higher content of chlorophyll and lower inhibition of plant height and fibrous root length than JM. Biochemical characterization revealed that although malondialdehyde and hydrogen peroxide contents were increased in fibrous roots of both cultivars, the amount of increase was 4-fold lower in YJM than in JM. Additionally, leaves of YJM showed higher ascorbate peroxidase activity than those of JM under flooding stress. Our results suggest that high membrane stability and antioxidant capacity are important flooding tolerance factors in sweetpotato. Furthermore, several flooding tolerance-related genes involved in starch and sucrose metabolism, fermentation, and cell wall loosening showed earlier induction and higher transcript levels in YJM leaves and fibrous roots than in JM tissues under flooding stress. Thus, phenotypic and biochemical characterization suggests that YJM could be used as a flooding tolerant sweetpotato cultivar.

The anti-obesity effects of anthocyanin and carotenoid extracts from color-fleshed potatoes were studied with 3T3-L1 cells in vitro and high-fat diet (HFD)-induced obese mice in vivo. Treatment of 3T3-L1 adipocytes with anthocyanin and carotenoid extracts, respectively, after differentiation induction significantly inhibited fat accumulation by 63.1 and 83.5%. Studies of adipogenesis inhibition showed that the anthocyanin extract acts at intermediate stages, whereas the carotenoid extract influences all the stages. The extracts significantly diminished triglyceride (TG) content and peroxisome proliferator-activated receptor gamma (PPARγ) protein expression during adipogenesis of the intermediate stage. Oral administration of anthocyanin and carotenoid extracts, respectively, to HFD-fed mice significantly reduced weight gain and restored TG levels to normal or lower as compared to the HFD-fed group with improvement of a lipid profile, TG to HDL-C ratio. Histological differences in liver tissues revealed that the extracts protected the liver tissue from adipogenesis by HFD fed. This research presents the first direct demonstration that the two pigment extracts from sweet potato exhibit anti-obesity activities. PRACTICAL APPLICATIONS: Anthocyanins and carotenoids are the main pigments of purple- and orange-fleshed sweet potatoes, respectively, which are highly nutritious foods with antidiabetic and antioxidant properties. Obesity is a rapidly growing health problem that increases major risk factors of several serious diseases including cardiovascular diseases, diabetes, and cancer. The results of this research suggest that anthocyanin and carotenoid-rich extracts from color-fleshed sweet potatoes may be useful as supplementary ingredients for the treatment of obesity and related diseases.

Late embryogenesis abundant (LEA) proteins are small, highly hydrophilic proteins that act as protectors of macromolecules and increase abiotic stress tolerance in plants. We previously reported that overexpressing sweetpotato IbLEA14 under the control of the CaMV 35S promoter increased osmotic and salt stress tolerance in transgenic sweetpotato calli. In this study, we generated transgenic poplar plants (Populus alba × P. glandulosa) expressing IbLEA14 under the control of the oxidative stress-inducible SWPA2 promoter (referred to as SL plants). Among the 15 SL plants obtained, three lines (SL2, SL7, and SL12) were established based on IbLEA14 transcript levels, tolerance to salt stress and Southern blot analysis. The SL plants exhibited less damage in response to methyl viologen-mediated oxidative stress than non-transgenic (NT) plants. SL plants also showed enhanced tolerance to drought, salt, and heat stress, which was associated with higher photosystem II efficiency and lower malondialdehyde levels compared with NT plants. Furthermore, SL plants had higher levels of monolignol biosynthesis-related gene transcripts under drought stress compared with NT plants. Finally, SL plants exhibited increased tolerance to heat stress, which is associated with the high thermostability of IbLEA14 protein. SL plants might be useful for reforestation on global marginal lands, including desertification and reclaimed areas.

Sweetpotato [Ipomoea batatas (L.) Lam] represents an attractive starch crop that can be used to facilitate solving global food and environmental problems in the 21st century. It can be used as industrial bioreactors to produce various high value-added materials, including bio-ethanol, functional feed, antioxidants, as well as food resources. The non-profit Center for Science in the Public Interest (CSPI) announced sweetpotato as one of the ten ‘super foods’ for better health, since it contains high levels of low molecular weight antioxidants such as vitamin-C, vitamin-E and carotenoids, as well as dietary fiber and potassium. The United States Department of Agriculture (USDA) also reported that sweetpotato is the best bioenergy crop among starch crops on marginal lands, that does not affect food security. The Food and Agriculture Organization (FAO) estimated that world population in 2050 will be 9.7 billion, and require approximately 1.7 times more food than today. In this respect, sweetpotato will be a solution to solving problems such as food, energy, health, and environment facing the globe in the 21st century. In this paper, the current status of resources, and cultivation of sweetpotato in the world was first described. Development of a new northern route of the sweetpotato and its prior tasks of large scale cultivation of sweetpotato, were also described in terms of global food security, and production of high-value added biomaterials.

Dramatic increase in global population accompanied by rapid industrialization in developing countries has led to serious environmental, food, energy, and health problems. The Food and Agriculture Organization of the United Nations has estimated world population will increase to 9.7 billion by 2050 and require approximately 1.7 times more food, and more than 3.5 times energy than that of today. Particularly, sweetpotato is easy to cultivate in unfavorable conditions such as heat, drought, high salt, and marginal lands. In this respect, sweetpotato is an industrially valuable starch crop. To replace crops associated with these food and energy problems, it is necessary to develop new crops with improved nutrients and productivity, that can be grown on marginal lands, including desertification areas using plant biotechnology. For this purpose, exploring useful genes and developing genetically modified crops are essential strategies. Currently, sweetpotato [Ipomoea batatas (L.) Lam.] have been re-evaluated as the best health food and industrial crop that produces starch and low molecular weight antioxidants, such as vitamin A, vitamin E, anthocyanins and carotenoids. This review will focus on the current status of research on sweetpotato biotechnology on omics including genome sequencing, transcriptome, proteomics and molecular breeding. In addition, prospects on molecular breeding of sweetpotato on marginal lands for sustainable development were described.

Carotenoids play essential roles in various light-harvesting processes in plants and help protect the photosynthetic machinery from photo-oxidative damage. Orange genes, which play a role in carotenoid accumulation, have recently been isolated from several plant species, and their functions have been intensively investigated. The Orange gene (IbOr) of sweet potato [Ipomoea batatas (L.) Lam] helps maintain carotenoid homeostasis to improve plant tolerance to environmental stress. IbOr, a protein with strong holdase chaperone activity, directly interacts with phytoene synthase, a key enzyme involved in carotenoid biosynthesis, in plants under stress conditions, resulting in increased carotenoid accumulation and abiotic stress tolerance. In addition, IbOr interacts with the oxygen-evolving enhancer protein 2-1, a member of a protein complex in photosystem II that is denatured under heat stress. Transgenic sweet potato plants overexpressing IbOr showed enhanced tolerance to high temperatures (47 °C). These findings indicate that IbOr protects plants from environmental stress not only by controlling carotenoid biosynthesis, but also by directly stabilizing photosystem II. In this review, we discuss the functions of IbOr and Or proteins in other plant species and their possible biotechnological applications for molecular breeding for sustainable development on marginal lands.

Pollutant removal by dissolved air flotation (DAF) at combined sewer overflows (CSOs) was analyzed with operating parameters such as recycling ratio and CODcr concentration. The nozzle of the CO-Current DAF system is normally operated co-currently with the flocculated water. In this study, the Counter-Current DAF system was used to remove organic matter concentration and total phosphorus to prevent water pollution of river when CSOs occurred. Counter-Current flow with ejection of saturated water was studied, because the acceleration of the flow increased collisions of microbubbles with floc. The experiments showed higher removal efficiency of DAF with Counter-Current flow compared with other configurations. In addition, the direction of the DAF nozzle and variation of angle of the contact zone were studied in detail for better operation. Efficiencies were analyzed for T-P, CODcr, and turbidity. The optimized conditions for DAF were a flow rate of 4.3 L/min with the Counter-Current nozzle setting. The optimum angle of contact zone was 60° rather than 45°. The removal of T-P, CODcr, and turbidity at optimum conditions were shown as 82.5%, 72.9%, and 91.2%, respectively. In particular, phosphorous removal was enhanced by using the Counter-Current nozzle and by the angle of the contact zone. The removal of CODcr, turbidity, T-P and SS for pilot plant showed as 75%, 82%, 80% and 92% respectively at 25 m/h of the hydraulic loading. The recycle ratio was 25%, which is considered to be an effective technique for treating CSOs.