Novel compounds that enhance Agrobacterium-mediated plant transformation by mitigating oxidative stress

CRISPR/Cas9 editing of Downy mildew resistant 6 (DMR6-1) in grapevine leads to reduced susceptibility to Plasmopara viticola

Downy mildew of grapevine (Vitis vinifera), caused by the oomycete Plasmopara viticola, is an important disease that is present in cultivation areas worldwide, and using resistant varieties provides an environmentally friendly alternative to fungicides. DOWNY MILDEW RESISTANT 6 (DMR6) from Arabidopsis is a negative regulator of plant immunity and its loss of function confers resistance to downy mildew. In grapevine, DMR6 is present in two copies, named VvDMR6-1 and VvDMR6-2. Here, we describe the editing of VvDMR6-1 in embryogenic calli using CRISPR/Cas9 and the regeneration of the edited plants. All edited plants were found to be biallelic and chimeric, and whilst they all showed reduced growth compared with non-transformed control plants, they also had reduced susceptibility to P. viticola. Comparison between mock-inoculated genotypes showed that all edited lines presented higher levels of salicylic acid than controls, and lines subjected to transformation presented higher levels of cis-resveratrol than controls. Our results identify VvDMR6-1 as a promising target for breeding grapevine cultivars with improved resistance to downy mildew.

Melatonin supplementation enhances browning suppression and improves transformation efficiency and regeneration of transgenic rough lemon plants (Citrus × jambhiri)

Enzymatic browning poses a significant challenge that limits in vitro propagation and genetic transformation of plant tissues. This research focuses on investigating how adding antioxidant substances can suppress browning, leading to improved efficiency in transforming plant tissues using Agrobacterium and subsequent plant regeneration from rough lemon (Citrus × jambhiri). When epicotyl segments of rough lemon were exposed to Agrobacterium, they displayed excessive browning and tissue decay. This was notably different from the 'Hamlin' explants, which did not exhibit the same issue. The regeneration process failed completely in rough lemon explants, and they accumulated high levels of total phenolic compounds (TPC) and polyphenol oxidase (PPO), which contribute to browning. To overcome these challenges, several antioxidant and osmoprotectant compounds, including lipoic acid, melatonin, glycine betaine, and proline were added to the tissue culture medium to reduce the oxidation of phenolic compounds and mitigate browning. Treating epicotyl segments with 100 or 200 μM melatonin led to a significant reduction in browning and phenolic compound accumulation. This resulted in enhanced shoot regeneration, increased transformation efficiency, and reduced tissue decay. Importantly, melatonin supplementation effectively lowered the levels of TPC and PPO in the cultured explants. Molecular and physiological analyses also confirmed the successful overexpression of the CcNHX1 transcription factor, which plays a key role in imparting tolerance to salinity stress. This study emphasizes the noteworthy impact of supplementing antioxidants in achieving successful genetic transformation and plant regeneration in rough lemon. These findings provide valuable insights for developing strategies to address enzymatic browning and enhance the effectiveness of plant tissue culture and genetic engineering methods with potential applications across diverse plant species.

Aztreonam is a novel chemical inducer that promotes Agrobacteium transformation and lateral root development in soybean

Agrobacterium-mediated soybean transformation is the simplest method of gene transfer. However, the low transformation due to the intractable nature of soybean genotypes hinders this process. The use of biochemicals (acetosyringone, cinnamic acid, flavonoids, etc.) plays an important role in increasing soybean transformation. These biochemicals induce chemotaxis and virulence gene activation during the infection process. Here we identified a biochemical, aztreonam (a monobactam), for high agrobacterium-mediated transformation in soybean. The soybean explants from three genotypes were inoculated with A. tumefaciens (GV3101) harboring the pMDC32 vector containing hpt or the GmUbi-35S-GUS vector containing the GUS gene during two separate events. High transient GUS expression was obtained during cotyledon explant culture on MS media supplemented with 2.5 mg/L aztreonam. The aztreonam-treated explants showed high efficiency in transient and stable transformation as compared to the untreated control. The transformation of aztreonam-treated explants during seed imbibition resulted in an average of 21.1% as compared to 13.2% in control by using the pMDC32 vector and 28.5 and 20.7% while using the GUS gene cassette, respectively. Based on these findings, the metabolic analysis of the explant after aztreonam treatment was assessed. The high accumulation of flavonoids was identified during an untargeted metabolic analysis. The quantification results showed a significantly high accumulation of the four compounds, i.e., genistein, apigenin, naringenin, and genistin, in cotyledon explants after 18 hours of aztreonam treatment. Alongside this, aztreonam also had some surprising effects on root elongation and lateral root formation when compared to indole-3-butyric acid (IBA). Our findings were limited to soybeans. However, the discovery of aztreonam and its effect on triggering flavonoids could lead to the potential role of aztreonam in the agrobacterium-mediated transformation of different crops.

Melatonin strongly enhances the Agrobacterium- mediated transformation of carnation in nitrogen-depleted media

With the rising demand for new cultivars of carnation, efficient transformation protocols are needed to enable the bioengineering of new traits. Here, we established a novel and efficient Agrobacterium-mediated transformation system using callus as the target explant for four commercial carnation cultivars. Leaf-derived calli of all cultivars were inoculated with Agrobacterium tumefaciens strain LBA4404 containing the plasmid pCAMBIA 2301 harboring genes for β-glucuronidase (uidA) and neomycin phosphotransferase (nptII). Polymerase chain reaction (PCR) and histochemical assays confirmed the presence of uidA and β-glucuronidase (GUS), respectively in transgenic shoots. The effect on transformation efficiency of medium composition and the presence of antioxidants during inoculation and co-cultivation was investigated. The transformation efficiency was increased in Murashige and Skoog (MS) medium lacking KNO₃ and NH₄NO_3, and also in MS medium lacking macro and micro elements and Fe to 5% and 3.1% respectively, compared to 0.6% in full-strength medium. Transformation efficiency was increased dramatically to 24.4% across all carnation cultivars by the addition of 2 mg/l melatonin to nitrogen-depleted MS medium. Shoot regeneration was also doubled in this treatment. The establishment of this efficient and reliable transformation protocol can advance the development of novel carnation cultivars through molecular breeding approaches.

A Synergistic Effect Based on the Combination of Melatonin with 1-Methylcyclopropene as a New Strategy to Increase Chilling Tolerance and General Quality in Zucchini Fruit

Zucchini fruit are highly sensitive to low temperatures leading to significant peel depressions, increasing weight loss and making them impossible to be commercialized. In this study the effect on the reduction of chilling injury (CI) assaying different postharvest treatments to cv. Cronos was evaluated. We have compared the application of substances such as 1-methylcyclopropene (1-MCP) with the application of a natural origin compound as melatonin (MT), both with demonstrated activity against CI in different vegetal products. The effects of MT (1 mM) by dipping treatment of 1 h and 1-MCP (2400 ppb) have been evaluated on zucchini fruit during 15 days of storage at 4 °C plus 2 days at 20 °C. Treatments applied independently improved some fruit quality parameters in comparison with control fruit but were not able to manage CI even though they mitigated the impact on several parameters. However, when these two separated strategies were combined, zucchini cold tolerance increased with a synergic trend. This synergic effect affected in general all parameters but specially CI, being also the only lot in which zucchini fruit were most effectively preserved. This is the first evidence in which a clear positive effect on zucchini chilling tolerance has been obtained combining these two different strategies. In this sense, the combined effect of 1-MCP and MT could be a suitable tool to reach high quality standards and increasing shelf life under suboptimal temperatures.

An Improved Procedure for Agrobacterium-Mediated Transformation of ‘Carrizo’ Citrange

Although several protocols for genetic transformation of citrus have been published, it is highly desirable to further improve its efficiency. Here we report treatments of Agrobacterium cells and citrus explants prior to and during co-cultivation process to enhance transformation efficiency using a commercially used rootstock ‘Carrizo’ citrange [Citrus sinensis (L.) Osb. × Poncirius trifoliata (L.) Raf.] as a model plant. We found explants from light-grown seedlings exhibited higher transformation efficiency than those from etiolated seedlings. We pre-cultured Agrobacterium cells in a 1/10 MS, 0.5 g/L 2-(N-morpholino) ethanesulfonic acid (MES) and 100 µM acetosyringone liquid medium for 6 h at 25 °C before used to infect citrus explants. We incubated epicotyl segments in an MS liquid medium containing 13.2 µM 6-BA, 4.5 µM 2,4-D, 0.5 µM NAA for 3 h at 25 °C prior to Agrobacterium infection. In the co-cultivation medium, we added 30 µM paclobutrazol and 10 µM lipoic acid. Each of these treatments significantly increased the efficiencies of transformation up to 30.4% (treating Agrobacterium with acetosyringone), 31.8% (treating explants with cytokinin and auxin), 34.9% (paclobutrazol) and 38.6% (lipoic acid), respectively. When the three treatments were combined, we observed that the transformation efficiency was enhanced from 11.5% to 52.3%. The improvement of genetic transformation efficiency mediated by these three simple treatments may facilitate more efficient applications of transgenic and gene editing technologies for functional characterization of citrus genes and for genetic improvement of citrus cultivars.

Effects of Different β-Lactam Antibiotics on Indirect Tomato (Solanum lycopersicum L.) Shoot Organogenesis and Agrobacterium tumefaciens Growth Inhibition In Vitro

A β-lactams that act by inhibiting the bacterial cell wall biosynthesis are one of the most common classes of antibiotics applied to suppress the growth of latent bacterial infection associated with the plant tissue culture, as well as in the Agrobacterium-mediated transformation techniques. Plant sensitivity to antibiotics usually is species-, genotype-, or even tissue-specific and mainly depends on concentrations, growth conditions, and culture system. In the presented article, we estimated a comparative effect of four β-lactam antibiotics (Claforan^®, timentin, amoxicillin, and Amoxiclav^®) at different concentrations in an agar-solidified Murashige and Skoog (MS) culture medium supplemented with 5 mg L^-1 6-benzylaminopurine (6-BA) and 0.1 mg L^-1 indole-3-acetic acid (IAA) on in vitro callus induction and shoot organogenesis from hypocotyl and cotyledon explants of two tomato cultivars (Rekordsmen, Moryana). The role of clavulanic acid in combination with amoxicillin (Amoxiclav^®) in the shoot organogenesis frequency and number of shoots per explant has been demonstrated. Additionally, the growth inhibition of Agrobacterium tumefaciens AGL0 strain according to agar disk-diffusion assay was studied. As a result, both stimulatory (timentin, amoxicillin, and Amoxiclav^®) and inhibitory (Claforan^®) effects of β-lactam antibiotics on in vitro morphogenetic responses of tomato were noted. It was found that clavulanic acid, which is part of the commercial antibiotic Amoxiclav^®, significantly increased the shoot regeneration frequency from cotyledon and hypocotyl explants of Rekordsmen tomato cultivar. Possible reasons for the stimulating effect of clavulanic acid on the induction of shoot organogenesis are discussed. According to agar disk-diffusion assay, the maximum diameter of growth inhibition zones (43.9 mm) was identified using 200 mg L^-1 timentin. The in vitro antibacterial activity of tested β-lactam antibiotics was arranged in the following order: timentin > Claforan^® > amoxicillin ≥ Amoxiclav^®. Thus, to suppress the growth of internal and latent bacterial infection of tomato plant tissue culture, as well as for transformation of Moryana and Rekordsmen cultivars by A. tumefaciens strain AGL0, we recommend adding of 100-200 mg L^-1 timentin or 400-800 mg L^-1 Amoxiclav^® to the shoot induction medium.

A Systematic Review of Melatonin in Plants: An Example of Evolution of Literature

Melatonin (N-acetyl-5-methoxy-tryptamine) is a mammalian neurohormone, antioxidant and signaling molecule that was first discovered in plants in 1995. The first studies investigated plant melatonin from a human perspective quantifying melatonin in foods and medicinal plants and questioning whether its presence could explain the activity of some plants as medicines. Starting with these first handful of studies in the late 1990s, plant melatonin research has blossomed into a vibrant and active area of investigation and melatonin has been found to play critical roles in mediating plant responses and development at every stage of the plant life cycle from pollen and embryo development through seed germination, vegetative growth and stress response. Here we have utilized a systematic approach in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) protocols to reduce bias in our assessment of the literature and provide an overview of the current state of melatonin research in plants, covering 1995-2021. This review provides an overview of the biosynthesis and metabolism of melatonin as well as identifying key themes including: abiotic stress responses, root development, light responses, interkingdom communication, phytohormone and plant signaling. Additionally, potential biases in the literature are investigated and a birefringence in the literature between researchers from plant and medical based which has helped to shape the current state of melatonin research. Several exciting new opportunities for future areas of melatonin research are also identified including investigation of non-crop and non-medicinal species as well as characterization of melatonin signaling networks in plants.

Agrobacterium strains and strain improvement: Present and outlook

Almost 40 years ago the first transgenic plant was generated through Agrobacterium tumefaciens-mediated transformation, which, until now, remains the method of choice for gene delivery into plants. Ever since, optimized Agrobacterium strains have been developed with additional (genetic) modifications that were mostly aimed at enhancing the transformation efficiency, although an optimized strain also exists that reduces unwanted plasmid recombination. As a result, a collection of very useful strains has been created to transform a wide variety of plant species, but has also led to a confusing Agrobacterium strain nomenclature. The latter is often misleading for choosing the best-suited strain for one's transformation purposes. To overcome this issue, we provide a complete overview of the strain classification. We also indicate different strain modifications and their purposes, as well as the obtained results with regard to the transformation process sensu largo. Furthermore, we propose additional improvements of the Agrobacterium-mediated transformation process and consider several worthwhile modifications, for instance, by circumventing a defense response in planta. In this regard, we will discuss pattern-triggered immunity, pathogen-associated molecular pattern detection, hormone homeostasis and signaling, and reactive oxygen species in relationship to Agrobacterium transformation. We will also explore alterations that increase agrobacterial transformation efficiency, reduce plasmid recombination, and improve biocontainment. Finally, we recommend the use of a modular system to best utilize the available knowledge for successful plant transformation.

Genetic Transformation in Peach (Prunus persica L.): Challenges and Ways Forward

Almost 30 years have passed since the first publication reporting regeneration of transformed peach plants. Nevertheless, the general applicability of genetic transformation of this species has not yet been established. Many strategies have been tested in order to obtain an efficient peach transformation system. Despite the amount of time and the efforts invested, the lack of success has significantly limited the utility of peach as a model genetic system for trees, despite its relatively short generation time; small, high-quality genome; and well-studied genetic resources. Additionally, the absence of efficient genetic transformation protocols precludes the application of many biotechnological tools in peach breeding programs. In this review, we provide an overview of research on regeneration and genetic transformation in this species and summarize novel strategies and procedures aimed at producing transgenic peaches. Promising future approaches to develop a robust peach transformation system are discussed, focusing on the main bottlenecks to success including the low efficiency of A. tumefaciens-mediated transformation, the low level of correspondence between cells competent for transformation and those that have regenerative competence, and the high rate of chimerism in the few shoots that are produced following transformation.

A Short History and Perspectives on Plant Genetic Transformation

Plant genetic transformation is an important technological advancement in modern science, which has not only facilitated gaining fundamental insights into plant biology but also started a new era in crop improvement and commercial farming. However, for many crop plants, efficient transformation and regeneration still remain a challenge even after more than 30 years of technical developments in this field. Recently, FokI endonuclease-based genome editing applications in plants offered an exciting avenue for augmenting crop productivity but it is mainly dependent on efficient genetic transformation and regeneration, which is a major roadblock for implementing genome editing technology in plants. In this chapter, we have outlined the major historical developments in plant genetic transformation for developing biotech crops. Overall, this field needs innovations in plant tissue culture methods for simplification of operational steps for enhancing the transformation efficiency. Similarly, discovering genes controlling developmental reprogramming and homologous recombination need considerable attention, followed by understanding their role in enhancing genetic transformation efficiency in plants. Further, there is an urgent need for exploring new and low-cost universal delivery systems for DNA/RNA and protein into plants. The advancements in synthetic biology, novel vector systems for precision genome editing and gene integration could potentially bring revolution in crop-genetic potential enhancement for a sustainable future. Therefore, efficient plant transformation system standardization across species holds the key for translating advances in plant molecular biology to crop improvement.

Suppression of Melatonin 2-Hydroxylase Increases Melatonin Production Leading to the Enhanced Abiotic Stress Tolerance against Cadmium, Senescence, Salt, and Tunicamycin in Rice Plants

Melatonin 2-hydroxylase (M2H) catalyzes the conversion of melatonin into 2hydroxymelatonin (2OHM), which is present in plants at a higher concentration than melatonin. Although M2H has been cloned, the in vivo function of its product is unknown. Here, we generated stable T₂ homozygous transgenic rice plants in which expression of endogenous M2H was suppressed (RNAi lines). However, we failed to generate M2H overexpression transgenic rice due to failure of somatic embryogenesis. The M2H transcript level showed a diurnal rhythm with a peak at night concomitantly with the peak concentration of 2OHM. RNAi rice showed a reduced M2H mRNA level and 2OHM and melatonin concentrations. The unexpected decrease in the melatonin concentration was caused by redirection of melatonin into cyclic 3hydroxymelatonin via a detour catabolic pathway. Thus, the decrease in the melatonin concentration in M2H RNAi rice led to slowed seedling growth and delayed germination. By contrast, the transient increase in the melatonin concentration was of greater magnitude in the M2H RNAi than the wild-type rice upon cadmium treatment due to possible suppression of melatonin degradation. Due to its higher concentration of melatonin, the M2H RNAi rice displayed tolerance to senescence, salt, and tunicamycin stresses. Therefore, the increase in the melatonin concentration caused by suppression of melatonin degradation or by overexpression of melatonin biosynthetic genes enhances stress tolerance in rice.

Development of Agrobacterium-mediated transient expression system in Caragana intermedia and characterization of CiDREB1C in stress response

BACKGROUND

The Agrobacterium-mediated transient transformation is a versatile and indispensable way of rapid analyzing gene function in plants. Despite this transient expression system has been successfully applied in a number of plant species, it is poorly developed in Caragana intermedia.

RESULTS

In this study, we established an Agrobacterium-mediated transient expression system in C. intermedia leaves and optimized the effect of different Agrobacterial strains, several surfactants and the concentration of Silwet L-77, which would affect transient expression efficiency. Among the 5 Agrobacterial strains examined, GV3101 produced the highest GUS expression level. Besides, higher level of transient expression was observed in plants infiltrated with Silwet L-77 than with Triton X-100 or Tween-20. Silwet L-77 at a concentration of 0.001% greatly improved the level of GUS transient expression. Real-time PCR showed that expression of CiDREB1C was highly up-regulated in transiently expressed plants and reached the highest level at the 2nd day after infiltration. Based on this optimized transient transformation method, we characterized CiDREB1C function in response to drought, salt and ABA treatment. The results showed that transiently expressed CiDREB1C in C. intermedia leaves could enhance the survival rate and chlorophyll content, and reduce the lodging rate compared with the control seedlings under drought, salt and ABA treatments. Furthermore, the rate of leaf shedding of CiDREB1C transient expression seedlings was lower than that of the control under ABA treatment.

CONCLUSIONS

The optimized transient expression condition in C. intermedia leaves were infiltrated with Agrobacterial strains GV3101 plus Silwet L-77 at a concentration of 0.001% added into the infiltration medium. Transiently expressed CiDREB1C enhanced drought, salt and ABA stress tolerance, indicated that it was a suitable and effective tool to determine gene function involved in abiotic stress response in C. intermedia.

Robust genetic transformation of sorghum (Sorghum bicolor L.) using differentiating embryogenic callus induced from immature embryos

BACKGROUND

Sorghum (Sorghum bicolor L.) is one of the world's most important cereal crops grown for multiple applications and has been identified as a potential biofuel crop. Despite several decades of study, sorghum has been widely considered as a recalcitrant major crop for transformation due to accumulation of phenolic compounds, lack of model genotypes, low regeneration frequency and loss of regeneration potential through sub-cultures. Among different explants used for genetic transformation of sorghum, immature embryos are ideal over other explants. However, the continuous supply of quality immature embryos for transformation is labour intensive and expensive. In addition, transformation efficiencies are also influenced by environmental conditions (light and temperature). Despite these challenges, immature embryos remain the predominant choice because of their success rate and also due to non-availability of other dependable explants without compromising the transformation efficiency.

RESULTS

We report here a robust genetic transformation method for sorghum (Tx430) using differentiating embryogenic calli (DEC) with nodular structures induced from immature embryos and maintained for more than a year without losing regeneration potential on modified MS media. The addition of lipoic acid (LA) to callus induction media along with optimized growth regulators increased callus induction frequency from 61.3 ± 3.2 to 79 ± 6.5% from immature embryos (1.5-2.0 mm in length) isolated 12-15 days after pollination. Similarly, the regeneration efficiency and the number of shoots from DEC tissue was enhanced by LA. The optimized regeneration system in combination with particle bombardment resulted in an average transformation efficiency (TE) of 27.2 or 46.6% based on the selection strategy, 25% to twofold higher TE than published reports in Tx430. Up to 100% putative transgenic shoots were positive for npt-II by PCR and 48% of events had < 3 copies of transgenes as determined by digital droplet PCR. Reproducibility of this method was demonstrated by generating ~ 800 transgenic plants using 10 different gene constructs.

CONCLUSIONS

This protocol demonstrates significant improvements in both efficiency and ease of use over existing sorghum transformation methods using PDS, also enables quick hypothesis testing in the production of various high value products in sorghum.

Melatonin is required for H2 O2 - and NO-mediated defense signaling through MAPKKK3 and OXI1 in Arabidopsis thaliana

Melatonin influences plant innate immunity through the mitogen-activated protein kinase (MAPK) pathway. However, the most upstream MAPK component in melatonin signaling and the dependence of generation of a reactive oxygen species (ROS) burst on melatonin synthesis and signaling remain unclear. In this study, treatment of several mekk (alias mapkkk)-knockout Arabidopsis mutants with melatonin revealed that the MAPKKK3 and OXI1 (oxidative signal-inducible1) kinases are responsible for triggering melatonin-induced defense signaling pathways. In addition, melatonin induction upon infection with the avirulent pathogen Pseudomonas syringae DC3000 (avrRpt2) was independent of H₂ O₂ and NO individually, but dependent on the combination of H₂ O₂ and NO. Moreover, melatonin-mediated induction of the expression of defense-related genes, such as PR1 and ICS1, was not altered in the H₂ O₂ -deficient rbohD/F-knockout mutant cotreated with an NO scavenger, indicating that melatonin functions downstream of the ROS and NO burst. Collectively, the data indicate that melatonin-mediated induction of an innate immune response requires multiple signaling molecules and activation of MAPKKK3 and OXI1, followed by triggering of downstream MAPK cascades, such as MAPK3 and MAPK6.

Transcriptome Analysis of Maize Immature Embryos Reveals the Roles of Cysteine in Improving Agrobacterium Infection Efficiency

Maize Agrobacterium-mediated transformation efficiency has been greatly improved in recent years. Antioxidants, such as, cysteine, can significantly improve maize transformation frequency through improving the Agrobacterium infection efficiency. However, the mechanism underlying the transformation improvement after cysteine exposure has not been elucidated. In this study, we showed that the addition of cysteine to the co-cultivation medium significantly increased the Agrobacterium infection efficiency of hybrid HiII and inbred line Z31 maize embryos. Reactive oxygen species contents were higher in embryos treated with cysteine than that without cysteine. We further investigated the mechanism behind cysteine-related infection efficiency increase using transcriptome analysis. The results showed that the cysteine treatment up-regulated 939 genes and down-regulated 549 genes in both Z31 and HiII. Additionally, more differentially expressed genes were found in HiII embryos than those in Z31 embryos, suggesting that HiII was more sensitive to the cysteine treatment than Z31. GO analysis showed that the up-regulated genes were mainly involved in the oxidation reduction process. The up-regulation of these genes could help maize embryos to cope with the oxidative stress stimulated by Agrobacterium infection. The down-regulated genes were mainly involved in the cell wall and membrane metabolism, such as, aquaporin and expansin genes. Decreased expression of these cell wall integrity genes could loosen the cell wall, thereby improving the entry of Agrobacterium into plant cells. This study offers insight into the role of cysteine in improving Agrobacterium-mediated transformation of maize immature embryos.

2-Hydroxymelatonin promotes the resistance of rice plant to multiple simultaneous abiotic stresses (combined cold and drought)

We investigated the physiological roles of 2-hydroxymelatonin (2-OHMel) in rice seedlings. When they were challenged with simultaneous multiple abiotic stressors, such as a combination of cold and drought, those pretreated with 2-OHMel were resistant, whereas no tolerance was observed in seedlings treated with either melatonin or water (control). The tolerance phenotype was associated with the induction of several transporter proteins, including the proton transporter (UCP1), potassium transporter (HKT1), and water channel protein (PIP2;1). Treatment with 2-OHMel increased the content of the osmoprotectant proline and maintained mitochondrial structure when plants were subjected to a combination of cold and drought stress. We screened the corresponding transcription factors (TFs) for 2-OHMel-mediated resistance to the combined stressors through analysis of large numbers of cold- and drought-related TFs. Two TFs, Myb4 and AP37, were only induced by 2-OHMel treatment. Transgenic rice lines overexpressing rice Myb4 were not resistant to the combined stressors; however, the expression of UCP1, HKT1, and PIP2;1 transcripts was slightly enhanced. These data show that 2-OHMel alleviates the effects of simultaneous abiotic stressors via the actions of multiple TFs, including Myb4 and AP37.

A Perspective on Hypericum perforatum Genetic Transformation

Hypericum perforatum (St John's wort) is a reservoir of diverse classes of biologically active and high value secondary metabolites, which captured the interest of both researchers and the pharmaceutical industry alike. Several studies and clinical trials have shown that H. perforatum extracts possess an astounding array of pharmacological properties. These properties include antidepressant, anti-inflammatory, antiviral, anti-cancer, and antibacterial activities; and are largely attributed to the naphtodianthrones and xanthones found in the genus. Hence, improving their production via genetic manipulation is an important strategy. In spite of the presence of contemporary genome editing tools, genetic improvement of this genus remains challenging without robust transformation methods in place. In the recent past, we found that H. perforatum remains recalcitrant to Agrobacterium tumefaciens mediated transformation partly due to the induction of plant defense responses coming into play. However, H. perforatum transformation is possible via a non-biological method, biolistic bombardment. Some research groups have observed the induction of hairy roots in H. perforatum after Agrobacterium rhizogenes co-cultivation. In this review, we aim at updating the available methods for regeneration and transformation of H. perforatum. In addition, we also propose a brief perspective on certain novel strategies to improve transformation efficiency in order to meet the demands of the pharmaceutical industry via metabolic engineering.

Low melatonin production by suppression of either serotonin N-acetyltransferase or N-acetylserotonin methyltransferase in rice causes seedling growth retardation with yield penalty, abiotic stress susceptibility, and enhanced coleoptile growth under anoxic conditions

Serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT) are the last two key enzymes for melatonin biosynthesis in living organisms. In this study, we demonstrated that transgenic rice (Oryza sativa L.) plants, in which expression of either endogenous SNAT or ASMT was suppressed, had reduced melatonin synthesis, confirming that both SNAT and ASMT are functionally involved in melatonin synthesis. The melatonin-deficient SNAT rice had retarded seedling growth, which was partially restored by exogenous melatonin application, suggesting melatonin's role in seedling growth. In addition, the plants were more sensitive to various abiotic stresses, including salt and cold, compared with the wild type. Melatonin-deficient SNAT rice had increased coleoptile growth under anoxic conditions, indicating that melatonin also inversely regulates plant growth under anaerobic conditions with the concomitant high expression of alcohol dehydrogenase genes. Similarly, the melatonin-deficient ASMT rice exhibited accelerated senescence in detached flag leaves, as well as significantly reduced yield. These loss-of-function studies on the melatonin biosynthetic genes confirmed most previous pharmacological reports that melatonin not only promotes plant growth but also mitigates various abiotic stresses.

Melatonin production in Escherichia coli by dual expression of serotonin N-acetyltransferase and caffeic acid O-methyltransferase

Melatonin is a well-known bioactive molecule produced in animals and plants and a well-studied natural compound. Two enzymatic steps are required for the biosynthesis of melatonin from serotonin. First, serotonin N-acetyltransferase (SNAT) catalyzes serotonin to N-acetylserotonin (NAS) followed by the action of N-acetylserotonin O-methyltransferase (ASMT), resulting in the synthesis of O-methylated NAS, also known as melatonin. Attempts to document melatonin production in Escherichia coli have been unsuccessful to date due to either low enzyme activity or inactive ASMT expression. Here, we employed caffeic acid O-methyltransferase (COMT) instead of ASMT, as COMT is a multifunctional enzyme that has ASMT activity as well. Among several combinations of dual expression cassettes, recombinant E. coli that expressed sheep SNAT with rice COMT produced a high quantity of melatonin, which was measured in a culture medium (1.46 mg/L in response to 1 mM serotonin). This level was several orders of magnitude higher than that produced in transgenic rice and tomato overexpressing sheep SNAT and ASMT, respectively. This heterologous expression system can be widely employed to screen various putative SNAT or ASMT genes from animals and plants as well as to overproduce melatonin in various useful microorganisms.

Functions of melatonin in plants: a review

The number of studies on melatonin in plants has increased significantly in recent years. This molecule, with a large set of functions in animals, has also shown great potential in plant physiology. This review outlines the main functions of melatonin in the physiology of higher plants. Its role as antistress agent against abiotic stressors, such as drought, salinity, low and high ambient temperatures, UV radiation and toxic chemicals, is analyzed. The latest data on their role in plant-pathogen interactions are also discussed. Both abiotic and biotic stresses produce a significant increase in endogenous melatonin levels, indicating its possible role as effector in these situations. The existence of endogenous circadian rhythms in melatonin levels has been demonstrated in some species, and the data, although limited, suggest a central role of this molecule in the day/night cycles in plants. Finally, another aspect that has led to a large volume of research is the involvement of melatonin in aspects of plant development regulation. Although its role as a plant hormone is still far of from being fully established, its involvement in processes such as growth, rhizogenesis, and photosynthesis seems evident. The multiple changes in gene expression caused by melatonin point to its role as a multiregulatory molecule capable of coordinating many aspects of plant development. This last aspect, together with its role as an alleviating-stressor agent, suggests that melatonin is an excellent prospect for crop improvement.