Activation of the pathogen-inducible Gst1 promoter of potato after elicitation by Venturia inaequalis and Erwinia amylovora in transgenic apple (Malus x domestica)

Plant Promoters: Their Identification, Characterization, and Role in Gene Regulation

One of the strategies to overcome diseases or abiotic stress in crops is the use of improved varieties. Genetic improvement could be accomplished through different methods, including conventional breeding, induced mutation, genetic transformation, or gene editing. The gene function and regulated expression through promoters are necessary for transgenic crops to improve specific traits. The variety of promoter sequences has increased in the generation of genetically modified crops because they could lead to the expression of the gene responsible for the improved trait in a specific manner. Therefore, the characterization of the promoter activity is necessary for the generation of biotechnological crops. That is why several analyses have focused on identifying and isolating promoters using techniques such as reverse transcriptase-polymerase chain reaction (RT-PCR), genetic libraries, cloning, and sequencing. Promoter analysis involves the plant genetic transformation method, a potent tool for determining the promoter activity and function of genes in plants, contributing to understanding gene regulation and plant development. Furthermore, the study of promoters that play a fundamental role in gene regulation is highly relevant. The study of regulation and development in transgenic organisms has made it possible to understand the benefits of directing gene expression in a temporal, spatial, and even controlled manner, confirming the great diversity of promoters discovered and developed. Therefore, promoters are a crucial tool in biotechnological processes to ensure the correct expression of a gene. This review highlights various types of promoters and their functionality in the generation of genetically modified crops.

Establishment of Highly Efficient Plant Regeneration, Callus Transformation and Analysis of Botrytis cinerea-Responsive PR Promoters in Lilium brownii var. viridulum

Lilium brownii var. viridulum, commonly called Longya lily, is a well-known flower and vegetable plant in China that has poor tolerance to Botrytis fungal disease. The molecularimprovement has mainly been restricted to an efficient regeneration and transformation system. In this study, the highly efficient regeneration of Longya lily was established through the optimization of embryogenic callus, adventitious shoot and rooting induction. The major factors influencing transformation (antibiotics, Agrobacterium concentration, infection time, suspension solution and coculture medium) were examined. The expression responses of PR promoters (ZmPR4 and BjCHI1) to B. cinerea were assessed in transgenic calli. The results showed that Murashige and Skoog (MS) medium with 1.0 mg·L-1 picloram (PIC) and 0.2 mg·L-1 1-naphthaleneacetic acid (NAA) under light conditions and MS with 0.5 mg·L-1 6-benzylaminopurine (6-BA) and 1.0 mg·L-1 NAA under darkness were optimal for embryogenic callus induction (64.67% rate) and proliferation (3.96 coefficient). Callus inoculation into MS containing 2.0 mg·L-1 thidiazuron (TDZ), 0.4 mg·L-1 NAA, 1.0 mg·L-1 TDZ and 0.5 mg·L-1 NAA led to shooting induction (92.22 of rate) and proliferation (3.28 of coefficient) promotion, respectively. The rooting rate reached 99.00% on MS with 0.3 mg·L-1 NAA. Moreover, a transformation rate of 65.56% was achieved by soaking the callus in Agrobacterium at an OD600 of 0.4 for 10 min in modified MS without NH4NO3 as the suspension solution and coculture medium before selecting 75 mg·L-1 hygromycin and 300 mg·L-1 cefotaxime. Only the BjCHI1 promoter was obviously expressed in transgenic calli. These results could facilitate the generation of Longya lily transgenic plants with improved B. cinerea resistance.

The Sclerotinia sclerotiorum-inducible promoter pBnGH17D7 in Brassica napus: isolation, characterization, and application in host-induced gene silencing

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is among the most devastating diseases in Brassica napus worldwide. Conventional breeding for SSR resistance in Brassica species is challenging due to the limited availability of resistant germplasm. Therefore, genetic engineering is an attractive approach for developing SSR-resistant Brassica crops. Compared with the constitutive promoter, an S. sclerotiorum-inducible promoter would avoid ectopic expression of defense genes that may cause plant growth deficits. In this study, we generated a S. sclerotiorum-inducible promoter. pBnGH17D7, from the promoter of B. napus glycosyl hydrolase 17 gene (pBnGH17). Specifically, 5'-deletion and promoter activity analyses in transgenic Arabidopsis thaliana plants defined a 189 bp region of pBnGH17 which was indispensable for S. sclerotiorum-induced response. Compared with pBnGH17, pBnGH17D7 showed a similar response upon S. sclerotiorum infection, but lower activity in plant tissues in the absence of S. sclerotiorum infection. Moreover, we revealed that the transcription factor BnTGA7 directly binds to the TGACG motif in pBnGH17D7 to activate BnGH17. Ultimately, pBnGH17D7 was exploited for engineering Sclerotinia-resistant B. napus via host-induced gene silencing. It induces high expression of siRNAs against the S. sclerotiorum pathogenic factor gene specifically during infection, leading to increased resistance.

Expression Activity of Artificial Promoters for Disease Resistance in Transgenic Eucalyptus urophylla

The transcriptional properties of artificial promoters are closely related to the type and arrangement position of cis-elements. GWSF (374-bp) was an effective SPIP with four cis-element dimers. There were four pathogen-inducible cis-elements in the GWSF promoter (GST1-boxes, W-boxes, S-boxes, and F-boxes) and a minimal cauliflower mosaic virus 35S promoter. V-element dimers were inserted into the upstream (VGWSF), midstream (GWVSF), and downstream (GWSFV) regions of the original GWSF promoter sequence to examine their affect on the position. The expression activity of promoters was analyzed and estimated using the histochemical staining of leaf discs of eucalyptus with transient expression, an image digitization method to extract the color features, and the induction treatment by a plant pathogenic microorganism/inducer and qPCR assays. The histochemical staining results of the adventitious buds indicated that the promoters had been successfully integrated into the E. urophylla genome and that they drove the expression of the gus gene. There was a noticeable difference in the intensity of color between the adventitious buds on the same callus block, as well as the intensity of color within the same adventitious bud. According to the established two-factor model of blue value, there was a greater difference between the levels of the genotype factor than the promoter factor in eucalyptus leaf discs. Further, the basal and inducible transcriptional levels of the three improved promoters were investigated by qPCR. With the basal transcriptional level of the GWSF promoter normalized to one, the relative basal levels of VGWSF, GWVSF, and GWSFV were 1.40, 1.45, and 4.15, respectively. The qPCR results were consistent with the staining results of GUS histochemical staining. The three improved promoters all had the properties of being induced by salicylic acid, Ralstonia solanacearum, and Phytophthora capsici. The three improved promoters demonstrated a significantly higher TMV induction activity: their induction activity from high to low was GWSFV > GWVSF > VGWSF. The findings will be beneficial to the construction and optimization of artificial promoters for transgenic plants.

Functional characterization of late embryogenesis abundant genes and promoters in pearl millet (Pennisetum glaucum L.) for abiotic stress tolerance

Late embryogenesis abundant (LEA) genes display distinct functions in response to abiotic stresses in plants. In pearl millet (Pennisetum glaucum L.), a total of 21 PgLEA genes were identified and classified into six groups including LEA1, LEA2, LEA3, LEA5, LEA7, and dehydrins (DHN). Open reading frames (ORFs) of PgLEAs range from 291 bp (PgLEA1-1) to 945 bp (PgLEA2-11) and distributed randomly among the seven chromosomes. Phylogenetic analysis revealed that all PgLEA proteins are closely related to sorghum LEA proteins. The PgLEAs were found to be expressed differentially under high progressive vapor pressure deficit (VPD), PgLEA7 was significantly expressed under high VPD and was selected for functional validation. In silico analysis of the PgLEA promoter regions revealed abiotic stress-specific cis-acting elements such as ABRE, CCAAT, MYBS, and LTRE. Based on the type of motifs, PgLEAPC promoter (758 bp), its deletion 1 (PgLpd1, 349 bp) and deletion 2 (PgLpd2, 125 bp) were cloned into the plant expression vector pMDC164 having the promoter-less uidA gene. All the three plant expression vectors were introduced into tobacco through Agrobacterium tumefaciens-mediated transformation to obtain T₁ and T₂ generations of transgenic plants. Based on expression of the uidA gene, tissue-specific expression was observed in mature stems, roots and seedlings of PgLEAPC and PgLpd1 carrying transgenics only. While the transgenic PgLEAPC plants displayed significantly higher uidA expression in the stem and root tissues under salt, drought, heat, and cold stresses, very low or no expression was observed in PgLpd1 and PgLpd2 transgenics under the tested stress conditions. The results of this study indicate that the complete promoter of PgLEAPC plays a role in developing abiotic stress tolerance in plants.

The Arabidopsis pattern recognition receptor EFR enhances fire blight resistance in apple

Fire blight disease, caused by the bacterium Erwinia amylovora (E. amylovora), is responsible for substantial losses in cultivated apples worldwide. An important mechanism of plant immunity is based on the recognition of conserved microbial molecules, named pathogen-associated or microbe-associated molecular patterns (PAMPs or MAMPs), through pattern recognition receptors (PRRs), leading to pattern-triggered immunity (PTI). The interspecies transfer of PRRs represents a promising strategy to engineer broad-spectrum and durable disease resistance in crops. EFR, the Arabidopsis thaliana PRR for the PAMP elf18 derived from the elongation factor thermal unstable (EF-Tu) proved to be effective in improving bacterial resistance when expressed into Solanaceae and other plant species. In this study, we tested whether EFR can affect the interaction of apple with E. amylovora by its ectopic expression in the susceptible apple rootstock M.26. Stable EFR expression led to the activation of PAMP-triggered immune response in apple leaves upon treatment with supernatant of E. amylovora, as measured by the production of reactive oxygen species and the induction of known defense genes. The amount of tissue necrosis associated with E. amylovora infection was significantly reduced in the EFR transgenic rootstock compared to the wild-type. Our results show that the expression of EFR in apple rootstock may be a valuable biotechnology strategy to improve the resistance of apple to fire blight.

Identification and characterization of a new soybean promoter induced by Phakopsora pachyrhizi, the causal agent of Asian soybean rust

BACKGROUND

Phakopsora pachyrhizi is a biotrophic fungal pathogen responsible for the Asian soybean rust disease causing important yield losses in tropical and subtropical soybean-producing countries. P. pachyrhizi triggers important transcriptional changes in soybean plants during infection, with several hundreds of genes being either up- or downregulated.

RESULTS

Based on published transcriptomic data, we identified a predicted chitinase gene, referred to as GmCHIT1, that was upregulated in the first hours of infection. We first confirmed this early induction and showed that this gene was expressed as early as 8 h after P. pachyrhizi inoculation. To investigate the promoter of GmCHIT1, transgenic soybean plants expressing the green fluorescence protein (GFP) under the control of the GmCHIT1 promoter were generated. Following inoculation of these transgenic plants with P. pachyrhizi, GFP fluorescence was detected in a limited area located around appressoria, the fungal penetration structures. Fluorescence was also observed after mechanical wounding whereas no variation in fluorescence of pGmCHIT1:GFP transgenic plants was detected after a treatment with an ethylene precursor or a methyl jasmonate analogue.

CONCLUSION

We identified a soybean chitinase promoter exhibiting an early induction by P. pachyrhizi located in the first infected soybean leaf cells. Our results on the induction of GmCHIT1 promoter by P. pachyrhizi contribute to the identification of a new pathogen inducible promoter in soybean and beyond to the development of a strategy for the Asian soybean rust disease control using biotechnological approaches.

Dissecting the Role of Promoters of Pathogen-sensitive Genes in Plant Defense

Plants inherently show resistance to pathogen attack but are susceptible to multiple bacteria, viruses, fungi, and phytoplasmas. Diseases as a result of such infection leads to the deterioration of crop yield. Several pathogen-sensitive gene activities, promoters of such genes, associated transcription factors, and promoter elements responsible for crosstalk between the defense signaling pathways are involved in plant resistance towards a pathogen. Still, only a handful of genes and their promoters related to plant resistance have been identified to date. Such pathogen-sensitive promoters are accountable for elevating the transcriptional activity of certain genes in response to infection. Also, a suitable promoter is a key to devising successful crop improvement strategies as it ensures the optimum expression of the required transgene. The study of the promoters also helps in mining more details about the transcription factors controlling their activities and helps to unveil the involvement of new genes in the pathogen response. Therefore, the only way out to formulate new solutions is by analyzing the molecular aspects of these promoters in detail. In this review, we provided an overview of the promoter motifs and cis-regulatory elements having specific roles in pathogen attack response. To elaborate on the importance and get a vivid picture of the pathogen-sensitive promoter sequences, the key motifs and promoter elements were analyzed with the help of PlantCare and interpreted with available literature. This review intends to provide useful information for reconstructing the gene networks underlying the resistance of plants against pathogens.

Recent Advances of In Vitro Culture for the Application of New Breeding Techniques in Citrus

Citrus is one of the most important fruit crops in the world. This review will discuss the recent findings related to citrus transformation and regeneration protocols of juvenile and adult explants. Despite the many advances that have been made in the last years (including the use of inducible promoters and site-specific recombination systems), transformation efficiency, and regeneration potential still represent a bottleneck in the application of the new breeding techniques in commercial citrus varieties. The influence of genotype, explant type, and other factors affecting the regeneration and transformation of the most used citrus varieties will be described, as well as some examples of how these processes can be applied to improve fruit quality and resistance to various pathogens and pests, including the potential of using genome editing in citrus. The availability of efficient regeneration and transformation protocols, together with the availability of the source of resistance, is made even more important in light of the fast diffusion of emerging diseases, such as Huanglongbing (HLB), which is seriously challenging citriculture worldwide.

Cloning and functional analysis of the promoter of a stress-inducible gene (Zmap) in maize

The anionic peroxidases play an important role in a variety of plant physiological processes. We characterized and isolated the Zmap promoter (PZmap) at the 5' flanking region in order to better understand the regulatory mechanisms of Zmap gene expression. A series of PZmap deletion derivatives, termed a1 -a6, at positions -1694, -1394, -1138, -784, -527 and -221 from the translation start site were blended to the β-glucuronidase reporter gene. Agrobacterium-mediated transformation method was used to study each deletion construct in tobaccos. Sequence analysis showed that several cis-acting elements (MYB binding site, Box-II, a TGACG-element, a CGTCA-element and a low temperature responsive element) were located within the promoter. Deletion analysis suggested the sequence between -1,694 and -1394bp may contain cis-elements associated with GUS up regulation. The MYB binding site (-757) might act as a negative drought-responsive element. There might be repressor elements located in the region (-1,694 to -1394bp) to repress Zmap expression under 4°C. The characterized promoter would be an ideal candidate for genetic engineering for improving the resistance of maize to different stressors.

HIPM Is a Susceptibility Gene of Malus spp.: Reduced Expression Reduces Susceptibility to Erwinia amylovora

Fire blight, a devastating disease caused by the bacterium Erwinia amylovora, is a major threat to apple crop production. To improve our understanding of the fire blight disease and to identify potential strategies to control the pathogen, we studied the apple protein HIPM (for HrpN-interacting protein from Malus spp.), which has previously been identified as interacting with the E. amylovora effector protein HrpN. Transgenic apple plants were generated with reduced HIPM expression, using an RNA interference construct, and were subsequently analyzed for susceptibility to E. amylovora infection. Lines exhibiting a greater than 50% silencing of HIPM expression showed a significant decrease in susceptibility to E. amylovora infection. Indeed, a correlation between HIPM expression and E. amylovora infection was identified, demonstrating the crucial role of HIPM during fire blight disease progression. Furthermore, an apple oxygen-evolving enhancer-like protein (MdOEE) was identified via a yeast two-hybrid screen to interact with HIPM. This result was confirmed with bimolecular fluorescence complementation assays and leads to new hypotheses concerning the response mechanism of the plant to E. amylovora as well as the mechanism of infection of the bacterium. These results suggest that MdOEE and, particularly, HIPM are promising targets for further investigations toward the genetic improvement of apple.

Isolation, characterization, and evaluation of three Citrus sinensis-derived constitutive gene promoters

Regulatory sequences from the citrus constitutive genes cyclophilin (CsCYP), glyceraldehyde-3-phosphate dehydrogenase C2 (CsGAPC2), and elongation factor 1-alpha (CsEF1) were isolated, fused to the uidA gene, and qualitatively and quantitatively evaluated in transgenic sweet orange plants. The 5' upstream region of a gene (the promoter) is the most important component for the initiation and regulation of gene transcription of both native genes and transgenes in plants. The isolation and characterization of gene regulatory sequences are essential to the development of intragenic or cisgenic genetic manipulation strategies, which imply the use of genetic material from the same species or from closely related species. We describe herein the isolation and evaluation of the promoter sequence from three constitutively expressed citrus genes: cyclophilin (CsCYP), glyceraldehyde-3-phosphate dehydrogenase C2 (CsGAPC2), and elongation factor 1-alpha (CsEF1). The functionality of the promoters was confirmed by a histochemical GUS assay in leaves, stems, and roots of stably transformed citrus plants expressing the promoter-uidA construct. Lower uidA mRNA levels were detected when the transgene was under the control of citrus promoters as compared to the expression under the control of the CaMV35S promoter. The association of the uidA gene with the citrus-derived promoters resulted in mRNA levels of up to 60-41.8% of the value obtained with the construct containing CaMV35S driving the uidA gene. Moreover, a lower inter-individual variability in transgene expression was observed amongst the different transgenic lines, where gene constructs containing citrus-derived promoters were used. In silico analysis of the citrus-derived promoter sequences revealed that their activity may be controlled by several putative cis-regulatory elements. These citrus promoters will expand the availability of regulatory sequences for driving gene expression in citrus gene-modification programs.

Inducible expression of Bs2 R gene from Capsicum chacoense in sweet orange (Citrus sinensis L. Osbeck) confers enhanced resistance to citrus canker disease

Transgenic expression of the pepper Bs2 gene confers resistance to Xanthomonas campestris pv. vesicatoria (Xcv) pathogenic strains which contain the avrBs2 avirulence gene in susceptible pepper and tomato varieties. The avrBs2 gene is highly conserved among members of the Xanthomonas genus, and the avrBs2 of Xcv shares 96% homology with the avrBs2 of Xanthomonas citri subsp. citri (Xcc), the causal agent of citrus canker disease. A previous study showed that the transient expression of pepper Bs2 in lemon leaves reduced canker formation and induced plant defence mechanisms. In this work, the effect of the stable expression of Bs2 gene on citrus canker resistance was evaluated in transgenic plants of Citrus sinensis cv. Pineapple. Interestingly, Agrobacterium-mediated transformation of epicotyls was unsuccessful when a constitutive promoter (2× CaMV 35S) was used in the plasmid construction, but seven transgenic lines were obtained with a genetic construction harbouring Bs2 under the control of a pathogen-inducible promoter, from glutathione S-transferase gene from potato. A reduction of disease symptoms of up to 70% was observed in transgenic lines expressing Bs2 with respect to non-transformed control plants. This reduction was directly dependent on the Xcc avrBs2 gene since no effect was observed when a mutant strain of Xcc with a disruption in avrBs2 gene was used for inoculations. Additionally, a canker symptom reduction was correlated with levels of the Bs2 expression in transgenic plants, as assessed by real-time qPCR, and accompanied by the production of reactive oxygen species. These results indicate that the pepper Bs2 resistance gene is also functional in a family other than the Solanaceae, and could be considered for canker control.

Transgenic apple expressing an antigenic protein of the human respiratory syncytial virus

A gene coding for the human respiratory syncytial virus (RSV)-F protein, driven by the constitutively expressed CaMV 35S promoter, was introduced into leaf tissues of apple, Malusxdomestica Borkh. cv. Royal Gala, via Agrobacterium-mediated transformation. Two putative transgenic lines were identified, and the presence of the RSV-F gene was confirmed by polymerase chain reaction (PCR). A total of 25 plants from these different transgenic events were successfully rooted, acclimatized, and transferred to the greenhouse. Stable integration of the transgene was confirmed and transgene copy number was determined by DNA gel blot analysis. Expression of the npt-II selectable marker and RSV-F was determined using reverse-transcription polymerase chain reaction (RT-PCR). Furthermore, enzyme-linked immunosorbent assay (ELISA) revealed varying levels of protein expression of the RSV-F transgene, ranging from 0 to 20 microg/g tissue. This is a first step in an effort to assess the efficacy of using apple for developing a plant-based vaccine against RSV.

A proteinase inhibitor from Nicotiana alata inhibits the normal development of light-brown apple moth, Epiphyas postvittana in transgenic apple plants

Insecticidal proteins are a potential resource to enhance resistance to insect pests in transgenic plants. Here, we describe the generation and analysis of the apple cultivar 'Royal Gala' transgenic for Nicotiana alata (N. alata) proteinase inhibitor (PI) and the impact of this PI on the growth and development of the Epiphyas postvittiana (light-brown apple moth). A cDNA clone encoding a proteinase inhibitor precursor from N. alata (Na-PI) under the control of either a double 35S promoter or a promoter from a ribulose-1,5-bisphosphate carboxylase small sub-unit gene (rbcS-E9 promoter) was stably incorporated into 'Royal Gala' apple using Agrobacterium-mediated transformation. A 40.3 kDa Na-PI precursor protein was expressed and correctly processed into 6-kDa proteinase inhibitors in the leaves of transgenic apple lines. The 6-kDa polypeptides accumulated to levels of 0.05 and 0.1% of the total soluble protein under the control of the rbc-E9 promoter and the double 35S promoter, respectively. Light-brown apple moth larvae fed with apple leaves expressing Na-PI had significantly reduced body weight after 7 days of feeding and female pupae were 19-28% smaller than controls. In addition, morphological changes such as pupal cases attached to the wing, deformed wings, deformed body shape, and pupal cases and curled wings attached to a deformed body were observed in adults that developed from larvae fed with apple leaves expressing Na-PI, when compared to larvae fed with the non-transformed apple leaves.

Recombinant DNA technology in apple

This review summarizes the achievements of almost 20 years of recombinant DNA technology applied to apple, grouping the research results into the sections: developing the technology, insect resistance, fungal disease resistance, self-incompatibility, herbicide resistance, fire blight resistance, fruit ripening, allergens, rooting ability, and acceptance and risk assessment. The diseases fire blight, caused by Erwinia amylovora, and scab, caused by Venturia inaequalis, were and still are the prime targets. Shelf life improvement and rooting ability of rootstocks are also relevant research areas. The tools to create genetically modified apples of added value to producers, consumers, and the environment are now available.