A sweetpotato SRD1 promoter confers strong root-, taproot-, and tuber-specific expression in Arabidopsis, carrot, and potato

A promoter toolbox for tissue-specific expression supporting translational research in cassava (Manihot esculenta)

There is an urgent need to stimulate agricultural output in many tropical and subtropical countries of the world to combat hunger and malnutrition. The starchy crop cassava (Manihot esculenta), growing even under sub-optimal conditions, is a key staple food in these regions, providing millions of people with food. Cassava biotechnology is an important technique benefiting agricultural progress, but successful implementation of many biotechnological concepts depends on the availability of the right spatiotemporal expression tools. Yet, well-characterized cassava promoters are scarce in the public domain. In this study, we investigate the promoter activity and tissue specificity of 24 different promoter elements in stably transformed cassava plants. We show that many of the investigated promoters, especially from other species, have surprisingly low activity and/or tissue specificity, but feature several promoter sequences that can drive tissue-specific expression in either autotrophic-, transport- or storage tissues. We especially highlight pAtCAB1, pMePsbR, and pSlRBCS2 as strong and specific source promoters, pAtSUC2, pMeSWEET1-like, and pMeSUS1 as valuable tools for phloem and phloem parenchyma expression, and pStB33, pMeGPT, pStGBSS1, as well as pStPatatin Class I, as strong and specific promoters for heterotrophic storage tissues. We hope that the provided information and sequences prove valuable to the cassava community by contributing to the successful implementation of biotechnological concepts aimed at the improvement of cassava nutritional value and productivity.

Identification of essential element determining fruit-specific transcriptional activity in the tomato HISTIDINE DECARBOXYLASE A gene promoter

In SlHDC-A promoter, SlHDC-A core-ES is an essential region for fruit-specific expression and interacts with GATA, HSF and AP1. Triplication of essential region was proposed as a minimal fruit-specific promoter. In plant biotechnology, fruit-specific promoter is an important tool for the improvement and utilization of tomato fruit. To expand our understanding on fruit-specific expression, it is necessary to determine the promoter region involved in fruit-specific transcriptional activity and transcriptional regulations of the promoter. In previous study, we isolated a fruit-specific SlHDC-A core promoter specifically expressed during tomato ripening stages. In this study, we identified SlHDC-A promoter region (SlHDC-A core-ES) that is essential for fruit-specific expression of the SlHDC-A. To understand the molecular mechanisms of fruit-specific expression of the SlHDC-A promoter, we first identified the putative transcription factor binding elements in the SlHDC-A core promoter region and corresponding putative transcription factors which are highly expressed during fruit maturation. Yeast one hybrid analysis confirmed that GATA, HSF, and AP1 interact with the SlHDC-A core-ES promoter region. Further transactivation analysis revealed that expression of the three transcription factors significantly activated expression of a reporter gene driven by SlHDC-A core-ES promoter. These results suggest that GATA, HSF, and AP1 are involved in the fruit-specific expression of SlHDC-A promoter. Furthermore, the synthetic promoter composed of three tandem repeats of SlHDC-A core-ES showed relatively higher activity than the constitutive 35S promoter in the transgenic tomato fruits at the orange stage. Taken together, we propose a new synthetic promoter that is specifically expressed during fruit ripening stage.

Enhancing plant growth and biomass production by overexpression of GA20ox gene under control of a root preferential promoter

Overexpression of GA20 oxidase gene has been a recent trend for improving plant growth and biomass. Constitutive expression of GA20ox has successfully improved plant growth and biomass in several plant species. However, the constitutive expression of this gene causes side-effects, such as reduced leaf size and stem diameter, etc. To avoid these effects, we identified and employed different tissue-specific promoters for GA20ox overexpression. In this study, we examined the utility of At1g promoter to drive the expression of GUS (β-glucuronidase) reporter and AtGA20ox genes in tobacco and Melia azedarach. Histochemical GUS assays and quantitative real-time-PCR results in tobacco showed that At1g was a root-preferential promoter whose expression was particularly strong in root tips. The ectopic expression of AtGA20ox gene under the control of At1g promoter showed improved plant growth and biomass of both tobacco and M. azedarach transgenic plants. Stem length as well as stem and root fresh weight increased by up to 1.5-3 folds in transgenic tobacco and 2 folds in transgenic M. azedarach. Both tobacco and M. azedarach transgenic plants showed increases in root xylem width with xylem to phloem ratio over 150-200% as compared to WT plants. Importantly, no significant difference in leaf shape and size was observed between At1g::AtGA20ox transgenic and WT plants. These results demonstrate the great utility of At1g promoter, when driving AtGA20ox gene, for growth and biomass improvements in woody plants and potentially some other plant species.

Tuber and Tuberous Root Development

Root and tuber crops have been an important part of human nutrition since the early days of humanity, providing us with essential carbohydrates, proteins, and vitamins. Today, they are especially important in tropical and subtropical regions of the world, where they help to feed an ever-growing population. Early induction and storage organ size are important agricultural traits, as they determine yield over time. During potato tuberization, environmental and metabolic status are sensed, ensuring proper timing of tuberization mediated by phloem-mobile signals. Coordinated cellular restructuring and expansion growth, as well as controlled storage metabolism in the tuber, are executed. This review summarizes our current understanding of potato tuber development and highlights similarities and differences to important tuberous root crop species like sweetpotato and cassava. Finally, we point out knowledge gaps that need to be filled before a complete picture of storage organ development can emerge.

A non-tandem CCCH-type zinc-finger protein, IbC3H18, functions as a nuclear transcriptional activator and enhances abiotic stress tolerance in sweet potato

CCCH-type zinc-finger proteins play essential roles in regulating plant development and stress responses. However, the molecular and functional properties of non-tandem CCCH-type zinc-finger (non-TZF) proteins have been rarely characterized in plants. Here, we report the biological and molecular characterization of a sweet potato non-TZF gene, IbC3H18. We show that IbC3H18 exhibits tissue- and abiotic stress-specific expression, and could be effectively induced by abiotic stresses, including NaCl, polyethylene glycol (PEG) 6000, H2 O2 and abscisic acid (ABA) in sweet potato. Accordingly, overexpression of IbC3H18 led to increased, whereas knock-down of IbC3H18 resulted in decreased tolerance of sweet potato to salt, drought and oxidation stresses. In addition, IbC3H18 functions as a nuclear transcriptional activator and regulates the expression of a range of abiotic stress-responsive genes involved in reactive oxygen species (ROS) scavenging, ABA signaling, photosynthesis and ion transport pathways. Moreover, our data demonstrate that IbC3H18 physically interacts with IbPR5, and that overexpression of IbPR5 enhances salt and drought tolerance in transgenic tobacco plants. Collectively, our data indicate that IbC3H18 functions in enhancing abiotic stress tolerance in sweet potato, which may serve as a candidate gene for use in improving abiotic stress resistance in crops.

An AP2/ERF gene, IbRAP2-12, from sweetpotato is involved in salt and drought tolerance in transgenic Arabidopsis

The manipulation of APETALA2/ethylene responsive factor (AP2/ERF) genes in plants makes great contributions on resistance to abiotic stresses. Here, we cloned an AP2/ERF gene from the salt-tolerant sweetpotato line ND98 and named IbRAP2-12. IbRAP2-12 protein expressed in nuclear revealed by transient expression in tobacco epidermal cells, and IbRAP2-12 exhibited transcriptional activation using heterologous expression assays in yeast. IbRAP2-12 was induced by NaCl (200 mM), 20% polyethylene glycol (PEG) 6000, 100 μM abscisic acid (ABA), 100 μM ethephon and 100 μM methyl jasmonate (MeJA). IbRAP2-12-overexpressing Arabidopsis lines were more tolerant to salt and drought stresses than wild type plants. Transcriptome analysis showed that genes involved in the ABA signalling, JA signalling, proline biosynthesis and reactive oxygen species (ROS) scavenging processes were up-regulated in IbRAP2-12 overexpression lines under salt and drought stresses. In comparing with WT, the contents of ABA, JA and proline were significantly increased, while hydrogen peroxide (H₂O₂) and the rate of water loss were significantly reduced in transgenic lines under salt and drought stresses. All these results demonstrated the roles of IbRAP2-12 in enhancing salt and drought tolerance in transgenic Arabidopsis lines. Thus, this IbRAP2-12 gene can be used to increase the tolerance ability during abiotic stresses in plants.

The Plasma Membrane-Localized Sucrose Transporter IbSWEET10 Contributes to the Resistance of Sweet Potato to Fusarium oxysporum

SWEET (Sugars Will Eventually be Exported Transporter) proteins, a novel family of sugar transporters, mediate the diffusion of sugars across cell membranes and acts as key players in sucrose phloem loading. Manipulation of SWEET genes in plants leads to various effects on resistance to biotic and abiotic stresses due to disruption of sugar efflux and changes in sugar distribution. In this study, a member of the SWEET gene family, IbSWEET10, was cloned from the sweet potato line ND98. mRNA expression analysis in sweet potato and promoter β-Glucuronidase analysis in Arabidopsis showed that IbSWEET10 is highly expressed in leaves, especially in vascular tissue. Transient expression in tobacco epidermal cells revealed plasma membrane localization of IbSWEET10, and heterologous expression assays in yeast indicated that IbSWEET10 encodes a sucrose transporter. The expression level of IbSWEET10 was significantly up-regulated in sweet potato infected with Fusarium oxysporum Schlecht. f. sp. batatas. Further characterization revealed IbSWEET10-overexpressing sweet potato lines to be more resistant to F. oxysporum, exhibiting better growth after infection compared with the control; conversely, RNA interference (RNAi) lines showed the opposite results. Additionally, the sugar content of IbSWEET10-overexpression sweet potato was significantly reduced, whereas that in RNAi plants was significantly increased compared with the control. Therefore, we suggest that the reduction in sugar content caused by IbSWEET10 overexpression is the major reason for the enhanced F. oxysporum resistance of the transgenic plants. This is the first report that the IbSWEET10 transporter contributes to the resistance of sweet potato to F. oxysporum. The IbSWEET10 gene has the great potential to be used for improving the resistance to F. oxysporum in sweet potato and other plants.

Isolation and characterization of a novel pollen-specific promoter in maize (Zea mays L.)

ZmSTK2_USP, located on the long arm of chromosome 4, belongs to the serine/threonine kinase gene in maize. The sequence analysis of 2100 bp upstream from the start codon ATG has shown that it contains cis-element motifs and two types of anther/pollen-specific promoter elements (GTGA and AGAAA), suggesting that it is the pollen-specific promoter. To investigate the function of ZmSTK2_USP promoter, the GUS gene fusion system was employed. In proZmSTK2_USP-GUS genetically modified plants, GUS activity was detected in mature pollen grains and pollen tubes but not found in other floral and vegetative tissues. These results show that proZmSTK2_USP is the pollen-specific promoter and drives pollen-specific activity during the middle stage of pollen development until pollen maturation.

Characterization of NtREL1, a novel root-specific gene from tobacco, and upstream promoter activity analysis in homologous and heterologous hosts

KEY MESSAGE

A novel root-specific gene and its upstream promoter were cloned and characterized for potential application in root-specific expression of transgenes. The root is an important plant organ subjected to many biotic and abiotic stresses, such as infection by Ralstonia solanacearum. To isolate tobacco root-specific promoters for genetic applications, microarray screening was performed to identify genes highly and specifically expressed in the root. One root-specific gene encoding an extensin-like protein (NtREL1) was isolated, and its expression pattern was further characterized by both microarray analysis and reverse transcription-polymerase chain reaction. NtREL1 was highly expressed only in roots but not in any other organ. NtREL1 expression was affected by hormone treatment (salicylic acid, abscisic acid, and ethephon) as well as low temperature, drought, and R. solanacearum infection. A full-length 849 bp cDNA containing a 657-nucleotide open reading frame was cloned by Rapid Amplification of cDNA Ends. Subsequently, a fragment of 1,574 bp upstream of NtREL1 was isolated by flanking PCR and named pNtREL1. This promoter fragment contains TATA, GATA, and CAAT-boxes, the basic elements of a promoter, and six root-specific expression elements, namely OSE1, OSE2, ROOTMOTIFTAPOX1, SURECOREATSULTR11, P1BS, and WUSATAg. A construct containing the bacterial uidA reporter gene (β-glucuronidase, GUS) driven by the pNtREL1 promoter was transformed into tobacco plants. GUS staining was only detected in the root, but not in leaves and stems. Additionally, transgenic tobacco plants containing peanut resveratrol synthase gene (AhRS) driven by the pNtREL1 promoter produced resveratrol only in the root. Thus, the pNtREL1 promoter can be used to direct root-specific expression of target genes to protect the root from stress or for biological studies.

Promoter isolation and characterization of GhAO-like1, a Gossypium hirsutum gene similar to multicopper oxidases that is highly expressed in reproductive organs

Cotton is one of the most economically important cultivated crops. It is the major source of natural fiber for the textile industry and an important target for genetic modification for both biotic stress and herbicide tolerance. Therefore, the characterization of genes and regulatory regions that might be useful for genetic transformation is indispensable. The isolation and characterization of new regulatory regions is of great importance to drive transgene expression in genetically modified crops. One of the major drawbacks in cotton production is pest damage; therefore, the most promising, cost-effective, and sustainable method for pest control is the development of genetically resistant cotton lines. Considering this scenario, our group isolated and characterized the promoter region of a MCO (multicopper oxidase) from Gossypium hirsutum, named GhAO-like1 (ascorbate oxidase-like1). The quantitative expression, together with the in vivo characterization of the promoter region reveals that GhAO-like1 has a flower- and fruit-specific expression pattern. The GUS activity is mainly observed in stamens, as expected considering that the GhAO-like1 regulatory sequence is enriched in cis elements, which have been characterized as a target of reproductive tissue specific transcription factors. Both histological and quantitative analyses in Arabidopsis thaliana have confirmed flower (mainly in stamens) and fruit expression of GhAO-like1. In the present paper, we isolated and characterized both in silico and in vivo the promoter region of the GhAO-like1 gene. The regulatory region of GhAO-like1 might be useful to confer tissue-specific expression in genetically modified plants.

GmPRP2 promoter drives root-preferential expression in transgenic Arabidopsis and soybean hairy roots

BACKGROUND

Promoters play important roles in gene expression and function. There are three basic types of promoters: constitutive, specific, and inducible. Constitutive promoters are widely used in genetic engineering, but these promoters have limitations. Inducible promoters are activated by specific inducers. Tissue-specific promoters are a type of specific promoters that drive gene expression in specific tissues or organs. Here, we cloned and characterized the GmPRP2 promoter from soybean. The expression pattern indicated that this promoter is root-preferential in transgenic Arabidopsis and the hairy roots of soybean. It can be used to improve the root resistance or tolerance to pathogens, pests, malnutrition and other abiotic stresses which cause extensive annual losses in soybean production.

RESULTS

The GmPRP2 promoter (GmPRP2p-1062) was isolated from soybean cv. Williams 82. Sequence analysis revealed that this promoter contains many cis-acting elements, including root-specific motifs. The GmPRP2p-1062 and its 5'-deletion fragments were fused with the GUS reporter gene and introduced into Arabidopsis and the hairy roots of soybean to further determine promoter activity. Histochemical analysis in transgenic Arabidopsis showed that GUS activity was mainly detected in roots and hypocotyls in all deletion fragments except GmPRP2p-471 (a 5'-deletion fragment of GmPRP2p-1062 with 471 bp length). GUS activity was higher in transgenic Arabidopsis and hairy roots with GmPRP2p-1062 and GmPRP2p-852 (a 5'-deletion fragment of GmPRP2p-1062 with 852 bp length) constructs than the other two constructs. GUS activity was enhanced by NaCl, PEG, IAA and JM treatments and decreased by SA, ABA and GA treatments in transgenic Arabidopsis.

CONCLUSIONS

GmPRP2p-1062 is a root-preferential promoter, and its core fragment for root-preferential expression might lie between -369 and +1. GmPRP2p-852 may be useful in the genetic engineering of novel soybean cultivars in the future.

A Novel Moderate Constitutive Promoter Derived from Poplar (Populus tomentosa Carrière)

A novel sequence that functions as a promoter element for moderate constitutive expression of transgenes, designated as the PtMCP promoter, was isolated from the woody perennial Populus tomentosa. The PtMCP promoter was fused to the GUS reporter gene to characterize its expression pattern in different species. In stable Arabidopsis transformants, transcripts of the GUS reporter gene could be detected by RT-PCR in the root, stem, leaf, flower and silique. Further histochemical and fluorometric GUS activity assays demonstrated that the promoter could direct transgene expression in all tissues and organs, including roots, stems, rosette leaves, cauline leaves and flowers of seedlings and maturing plants. Its constitutive expression pattern was similar to that of the CaMV35S promoter, but the level of GUS activity was significantly lower than in CaMV35S promoter::GUS plants. We also characterized the promoter through transient expression in transgenic tobacco and observed similar expression patterns. Histochemical GUS staining and quantitative analysis detected GUS activity in all tissues and organs of tobacco, including roots, stems, leaves, flower buds and flowers, but GUS activity in PtMCP promoter::GUS plants was significantly lower than in CaMV35S promoter::GUS plants. Our results suggested that the PtMCP promoter from poplar is a constitutive promoter with moderate activity and that its function is presumably conserved in different species. Therefore, the PtMCP promoter may provide a practical choice to direct moderate level constitutive expression of transgenes and could be a valuable new tool in plant genetic engineering.

Production of pharmaceutical proteins in solanaceae food crops

The benefits of increased safety and cost-effectiveness make vegetable crops appropriate systems for the production and delivery of pharmaceutical proteins. In particular, Solanaceae edible crops could be inexpensive biofactories for oral vaccines and other pharmaceutical proteins that can be ingested as minimally processed extracts or as partially purified products. The field of crop plant biotechnology is advancing rapidly due to novel developments in genetic and genomic tools being made available today for the scientific community. In this review, we briefly summarize data now available regarding genomic resources for the Solanaceae family. In addition, we describe novel strategies developed for the expression of foreign proteins in vegetable crops and the utilization of these techniques to manufacture pharmaceutical proteins.

Learning from nature: new approaches to the metabolic engineering of plant defense pathways

Biotechnological manipulation of plant defense pathways can increase crop resistance to herbivores and pathogens while also increasing yields of medicinal, industrial, flavor and fragrance compounds. The most successful achievements in engineering defense pathways can be attributed to researchers striving to imitate natural plant regulatory mechanisms. For example, the introduction of transcription factors that control several genes in one pathway is often a valuable strategy to increase flux in that pathway. The use of multi-gene cassettes which mimic natural gene clusters can facilitate coordinated regulation of a pathway and speed transformation efforts. The targeting of defense pathway genes to organs and tissues in which the defensive products are typically made and stored can also increase yield as well as defensive potential.

In silico identification and in vivo validation of a set of evolutionary conserved plant root-specific cis-regulatory elements

Marker genes are specifically expressed in a tissue, organ or time of development. Here we used a computational screen to identify marker genes of the root in Arabidopsis thaliana. We mined the existing transcriptome datasets for genes having high expression in roots while being low in all other organs under a wide range of growth conditions. We show that the root-specificity of these genes is conserved in the sister species Arabidopsis lyrata, indicating that their expression pattern is under selective pressure. We delineated the cis-regulatory elements responsible for root-specific expression and validated two third of those in planta as bona fide root-specific regulatory sequences. We identified three motifs over-represented in these sequences, which mutation resulted in alteration of root-specific expression, demonstrating that these motifs are functionally relevant. In addition, the three motifs are also over-represented in the cis-regulatory regions of the A. lyrata orthologs of our root-specific genes, and this despite an overall low degree of sequence conservation of these regions. Our results provide a resource to assess root-identity in the model genus Arabidopsis and shed light on the evolutionary history of gene regulation in plants.