AGROBEST: an efficient Agrobacterium-mediated transient expression method for versatile gene function analyses in Arabidopsis seedlings

Knockout of the Agrobacterium ILVC gene generates a valine-isoleucine auxotrophic strain for plant transformation

The Agrobacterium tumefaciens mediated transformation is the prevailing methodology for plant genetic manipulation; however, A. tumefaciens overgrowth is a common constraint in the process. Exploring auxotrophic A. tumefaciens could reduce overgrowth and enhance plant transformation efficiency. The ILVC gene, which encodes the ketol-acid isomeroreductase, is critical for Valine (Val) and isoleucine (Ile) biosynthesis in some microorganisms. However, its function in A. tumefaciens is unclear. To ascertain the function of ILVC and generate an auxotrophic A. tumefaciens, this study employed an allelic exchange to disrupt the ILVC in A. tumefaciens strain GV3101. This resulted in the loss of ketol-acid isomeroreductase activity and the prevention of Val and Ile biosynthesis, creating a dual-auxotrophic GV3101∆ILVC. Transient expression assays in Nicotiana benthamiana transformation demonstrated that the GV3101∆ILVC was capable of T-DNA transfer. Moreover, stable genetic transformation analysis in N. benthamiana indicated that the introduction of GV3101∆ILVC led to a reduction in overgrowth within infected plant tissues. Additionally, an enhancement in transformation efficiency was observed with the prolongation of the co-cultivation time of the explant-infected strain. This study revealed the function of ILVC and explored a dual-auxotrophic A. tumefaciens for Val and Ile, potentially broadening the utilization of auxotrophic strains in plant genetic transformation.

Distinct strategies of diguanylate cyclase domain proteins on inhibition of virulence and interbacterial competition by agrobacteria

Diguanylate cyclases (DGCs) synthesize bis-(3',5')-cyclic diguanylic acid (c-di-GMP), a critical bacterial second messenger that coordinates diverse biological processes. Agrobacterium tumefaciens, a plant pathogen causing crown gall disease, relies on type IV secretion system for pathogenesis and type VI secretion system (T6SS) for interbacterial competition. Our study identified two putative DGCs, named diguanylate cyclase domain proteins regulating virulences A and B (DcvA and DcvB), that negatively regulate virulence through distinct mechanisms. DcvA suppresses virulence by targeting the VirA/VirG two-component system downstream of VirA. This inhibition is independent of c-di-GMP levels. DcvB positively regulates biofilm formation, inhibits T6SS-mediated interbacterial competition, and suppresses virulence via the ChvG/ChvI two-component system downstream of ChvG. These effects are dependent on its cyclase activity and the associated increase in intracellular c-di-GMP levels. These findings suggest that DcvA and DcvB control virulence and interbacterial competition using different mechanisms in Agrobacterium. DcvA suppresses virulence, independent of c-di-GMP, and DcvB enhances global c-di-GMP concentration to promote biofilm formation and inhibits virulence and T6SS antibacterial activity. The findings provide understanding of how DGC domain proteins orchestrate complex regulatory networks to balance virulence, biofilm formation, and interbacterial competition, enabling them to adapt to changing environments.IMPORTANCEBacteria produce second messengers, such as c-di-GMP, to regulate various cellular processes, including biofilm formation, virulence, and bacterial antagonism. Diguanylate cyclases (DGCs) catalyze the biosynthesis of c-di-GMP and function to cope with changing environments through targeting specific effector proteins. In this study, we uncover that phytopathogenic agrobacteria deploy two DGC domain proteins to suppress virulence and interbacterial competition through two different regulatory pathways. One exhibits the DGC activity, enhancing global c-di-GMP concentration to elevate biofilm formation and inhibit virulence and antibacterial activity, while the other specifically suppresses virulence, independent of c-di-GMP biosynthesis. Our findings provide new insight into the distinct regulatory mechanisms of DGC domain proteins on regulating virulence and interbacterial competition, highlighting potential new strategies for controlling Agrobacterium pathogenicity.

Rhizobium rhizogenes A4-derived strains mediate hyper-efficient transient gene expression in Nicotiana benthamiana and other solanaceous plants

Agroinfiltration, a method utilizing agrobacteria to transfer DNA into plant cells, is widely used for transient gene expression in plants. Besides the commonly used Agrobacterium strains, Rhizobium rhizogenes can also introduce foreign DNA into host plants for gene expression. While many R. rhizogenes strains have been known for inducing hairy root symptoms, their use for transient expression has not been fully explored. Here, we showed that R. rhizogenes A4 outperformed all other tested agrobacterial strains in agroinfiltration experiments on leaves of Nicotiana benthamiana and other solanaceous plants. By conducting an agroinfiltration screening in N. benthamiana leaves using various agrobacterial strains carrying the RUBY reporter gene cassette, we discovered that A4 mediates the strongest and fastest transient expression. Utilizing the genomic information, we developed a collection of disarmed and modified strains derived from A4. By performing vacuum infiltration assays, we demonstrated that these A4-derived strains efficiently transiently transform 6-week-old N. benthamiana leaves, showing less sensitivity to the age of plants compared to the laboratory strain GV3101. Furthermore, we performed agroinfiltration using AS109, an A4-derived disarmed strain, on the leaves of tomato, pepper, and eggplant. Remarkably, AS109 mediated transient gene expression on tested solanaceous plants more effectively than all the tested commonly used agrobacterial strains. This discovery paves the way for establishing R. rhizogenes A4-derived strains as a new option for enhancing transient expression in N. benthamiana and facilitating the functional study of plant genes in other solanaceous species.

Two redox-responsive LysR-type transcription factors control the oxidative stress response of Agrobacterium tumefaciens

Pathogenic bacteria often encounter fluctuating reactive oxygen species (ROS) levels, particularly during host infection, necessitating robust redox-sensing mechanisms for survival. The LysR-type transcriptional regulator (LTTR) OxyR is a widely conserved bacterial thiol-based redox sensor. However, members of the Rhizobiales also encode LsrB, a second LTTR with potential redox-sensing function. This study explores the roles of OxyR and LsrB in the plant-pathogen Agrobacterium tumefaciens. Through single and combined deletions, we observed increased H2O2 sensitivity, underscoring their function in oxidative defense. Genome-wide transcriptome profiling under H2O2 exposure revealed that OxyR and LsrB co-regulate key antioxidant genes, including katG, encoding a bifunctional catalase/peroxidase. Agrobacterium tumefaciens LsrB possesses four cysteine residues potentially involved in redox sensing. To elucidate the structural basis for redox-sensing, we applied single-particle cryo-EM (cryogenic electron microscopy) to experimentally confirm an AlphaFold model of LsrB, identifying two proximal cysteine pairs. In vitro thiol-trapping coupled with mass spectrometry confirmed reversible thiol modifications of all four residues, suggesting a functional role in redox regulation. Collectively, these findings reveal that A. tumefaciens employs two cysteine-based redox sensing transcription factors, OxyR and LsrB, to withstand oxidative stress encountered in host and soil environments.

Tissue-Specific Regulation of Vesicular Trafficking Mediated by Rab-GEF Complex MON1/CCZ1 From Solanum chilense Increases Salt Stress Tolerance in Arabidopsis thaliana

Salt stress constrains the development and growth of plants. To tolerate it, mechanisms of endocytosis and vacuolar compartmentalization of Na+ are induced. In this work, the genes that encode a putative activator of vesicular trafficking called MON1/CCZ1 from Solanum chilense, SchMON1 and SchCCZ1, were co-expressed in roots of Arabidopsis thaliana to determine whether the increase in prevacuolar vesicular trafficking also increases the Na+ compartmentalization capacity and tolerance. Initially, we demonstrated that both SchMON1 and SchCCZ1 genes rescued the dwarf phenotype of both A. thaliana mon1-1 and ccz1a/b mutants associated with the loss of function, and both proteins colocalized with their functional targets, RabF and RabG, in endosomes. Transgenic A. thaliana plants co-expressing these genes improved salt stress tolerance compared to wild type plants, with SchMON1 contributing the most. At the sub-cellular level, co-expression of SchMON1/SchCCZ1 reduced ROS levels and increased endocytic activity, and number of acidic structures associated with autophagosomes. Notably, greater Na+ accumulation in vacuoles of cortex and endodermis was evidenced in the SchMON1 genotype. Molecular analysis of gene expression in each genotype supported these results. Altogether, our analysis shows that root activation of prevacuolar vesicular trafficking mediated by MON1/CCZ1 emerges as a promising physiological molecular mechanism to increase tolerance to salt stress in crops of economic interest.

Identification of U6 Promoter and Establishment of Gene-Editing System in Larix kaempferi (Lamb.) Carr

This study aimed to establish a CRISPR/Cas9 gene-editing system for Larix kaempferi (Lamb.) Carr. (Japanese larch). We screened L. kaempferi U6 promoters and used them to drive sgRNA expression in the CRISPR/Cas9 gene-editing system. The L. kaempferi embryogenic callus was used as the receptor material for genetic transformation, and the frequency and types of gene editing were then analyzed. The results showed various mutations in the transgenic materials, including base substitutions and deletions, and the editing frequency ranged from 5% to 14.29%. In summary, we established a CRISPR/Cas9 gene-editing system for L. kaempferi. Our results demonstrate that the CRISPR/Cas9 system can efficiently edit genes in L. kaempferi, with significantly higher editing frequencies observed when sgRNA expression is driven by endogenous LaU6 promoters compared to the exogenous promoter ProAtU6-26. This work provides technical support for the study of L. kaempferi gene functions and genetic improvement.

Suppression of defense gene expression under nutrient-rich condition in Arabidopsis seedlings

Plant hormones like salicylic acid (SA) and jasmonic acid (JA) play crucial roles in regulating defense gene expression systems. SA mainly regulates defense against biotrophic pathogens, while JA mediates defense against necrotrophic pathogens. Compounds called plant activators including probenazole, acibenzolar-s-methyl and 2,6-dichloroisonicotinic acid (INA) activate plant immune systems, providing protection against pathogens. Unlike conventional pesticides that directly target pathogens, plant activators boost the host's defense mechanisms, potentially reducing the likelihood of drug resistance development. Various high-throughput screening systems (HTS) have been developed with the aim of searching for plant activators. Transgenic Arabidopsis lines expressing luciferase under the control of defense gene promoters allow us to monitor the activity of defense-related gene in vivo. To investigate the influence of nutrients on the HTS system, we conducted luciferase assays using Arabidopsis seedlings and observed the suppression of defense gene expression in response to the treatment of plant activators. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) was employed to monitor the expression levels of endogenous genes in response to nutrient-rich conditions and confirmed the suppression effect of defense gene expression as observed in the luciferase reporter assays. The findings highlight the importance of considering nutrient effects when evaluating plant activators and screening for compounds that induce defense gene expression under nutrient-rich conditions.

A simple, highly efficient Agrobacterium tumefaciens-mediated moss transformation system with broad applications

Mosses, particularly desiccation-tolerant (DT) species, are important model organisms for studying genes involved in plant development and stress resistance. The lack of a simple and efficient stable moss transformation system has hindered progress in deciphering the genetic mechanisms underlying traits of interest in these organisms. Here, we present an Agrobacterium tumefaciens-mediated transformation system for DT mosses that uses Agrobacterium strain EHA105 harboring the binary vector pCAMBIA1301-GUS. This system achieved transformation efficiencies of 74% and 81% in Physcomitrium patens and Bryum argenteum protonemata, respectively, without the need for culture and callus formation prior to regeneration. We detected GUS enzyme activity in the regenerated transgenic moss via histochemical staining. Southern blot, PCR, and RT-qPCR analyses confirmed the presence of the GUS gene. In addition, we successfully used this system to transform wild DT Syntrichia caninervis. Furthermore, P. patens and B. argenteum transformed using this system with the stress resistance gene EsDREB from the desert plant Eremosparton songoricum (Litv.) exhibited improved salt tolerance. We thus present an efficient tool for the genetic analysis of DT moss species, paving the way for the development of stress-resistant crop cultivars.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-024-00174-4.

Establishment of Agrobacterium-Mediated Transient Transformation System in Desert Legume Eremosparton songoricum (Litv.) Vass

Eremosparton songoricum (Litv.) Vass. is a desert legume exhibiting extreme drought tolerance and the ability to withstand various harsh environments, making it a good candidate for investigating stress tolerance mechanisms and exploring valuable stress-resistant genes. However, the absence of a genetic transformation system for E. songoricum poses significant limitations for functionally validating these stress-resistant genes in situ. In this study, we developed an Agrobacterium-mediated transient transformation system for E. songoricum utilizing the β-glucuronidase (GUS) gene as a reporter. We investigated three types of explants (seedlings, assimilated branches and callus) and the effects of different Agrobacterium strains, seedling ages, OD600 values, acetosyringone (AS) concentrations, sucrose concentrations and infection times on the transformation efficiency. The results reveal that the optimal transformation system was infecting one-month-old regenerating assimilated branches with the Agrobacterium strain C58C1. The infection solution comprised 1/2 MS medium with 3% sucrose and 200 μM AS at an OD600 of 0.8, infection for 3 h and then followed by 2 days of dark cultivation, which achieving a maximum transformation rate of 97%. The maximum transformation rates of the seedlings and calluses were 57.17% and 39.51%, respectively. Moreover, we successfully utilized the assimilated branch transient transformation system to confirm the role of the previously reported transcription factor EsDREB2B in E. songoricum. The overexpression of EsDREB2B enhanced drought tolerance by increasing the plant's reactive oxygen species (ROS) scavenging capacity in situ. This study established the first transient transformation system for a desert legume woody plant, E. songoricum. This efficient system can be readily applied to investigate gene functions in E. songoricum. It will expedite the exploration of genetic resources and stress tolerance mechanisms in this species, offering valuable insights and serving as a reference for the transformation of other desert plants and woody legumes.

Improved Protocol for Efficient Agrobacterium-Mediated Transient Gene Expression in Medicago sativa L

Medicago sativa L. (Alfalfa) is a globally recognized forage legume that has recently gained attention for its high protein content, making it suitable for both human and animal consumption. However, due to its perennial nature and autotetraploid genetics, conventional plant breeding requires a longer timeframe compared to other crops. Therefore, genetic engineering offers a faster route for trait modification and improvement. Here, we describe a protocol for achieving efficient transient gene expression in alfalfa through genetic transformation with the Agrobacterium tumefaciens pCAMBIA1304 vector. This vector contains the reporter genes β-glucuronidase (GUS) and green fluorescent protein (GFP), along with a selectable hygromycin B phosphotransferase gene, all driven by the CaMV 35s promoter. Various transformation parameters-such as different explant types, leaf ages, leaf sizes, wounding types, bacterial concentrations (OD600nm), tissue preculture periods, infection periods, co-cultivation periods, and different concentrations of acetosyringone, silver nitrate, and calcium chloride-were optimized using 3-week-old in vitro-grown plantlets. Results were attained from data based on the semi-quantitative observation of the percentage and number of GUS spots on different days of agro-infection in alfalfa explants. The highest percentage of GUS positivity (76.2%) was observed in 3-week-old, scalpel-wounded, segmented alfalfa leaf explants after 3 days of agro-infection at a bacterial concentration of 0.6, with 2 days of preculture, 30 min of co-cultivation, and the addition of 150 µM acetosyringone, 4 mM calcium chloride, and 75 µM silver nitrate. The transient expression of genes of interest was confirmed via histochemical GUS and GFP assays. The results based on transient reporter gene expression suggest that various factors influence T-DNA delivery in the Agrobacterium-mediated transformation of alfalfa. The improved protocol can be used in stable transformation techniques for alfalfa.

The U1 small nuclear RNA enhances drought tolerance in Arabidopsis

Alternative splicing (AS) is an important posttranscriptional regulatory mechanism that improves plant tolerance to drought stress by modulating gene expression and generating proteome diversity. The interaction between the 5' end of U1 small nuclear RNA (U1 snRNA) and the conserved 5' splice site of precursor messenger RNA (pre-mRNA) is pivotal for U1 snRNP involvement in AS. However, the roles of U1 snRNA in drought stress responses remain unclear. This study provides a comprehensive analysis of AtU1 snRNA in Arabidopsis (Arabidopsis thaliana), revealing its high conservation at the 5' end and a distinctive four-leaf clover structure. AtU1 snRNA is localized in the nucleus and expressed in various tissues, with prominent expression in young floral buds, flowers, and siliques. The overexpression of AtU1 snRNA confers enhanced abiotic stress tolerance, as evidenced in seedlings by longer seedling primary root length, increased fresh weight, and a higher greening rate compared with the wild-type. Mature AtU1 snRNA overexpressor plants exhibit higher survival rates and lower water loss rates under drought stress, accompanied by a significant decrease in H2O2 and an increase in proline. This study also provides evidence of altered expression levels of drought-related genes in AtU1 snRNA overexpressor or genome-edited lines, reinforcing the crucial role of AtU1 snRNA in drought stress responses. Furthermore, the overexpression of AtU1 snRNA influences the splicing of downstream target genes, with a notable impact on SPEECHLESS (SPCH), a gene associated with stomatal development, potentially explaining the observed decrease in stomatal aperture and density. These findings elucidate the critical role of U1 snRNA as an AS regulator in enhancing drought stress tolerance in plants, contributing to a deeper understanding of the AS pathway in drought tolerance and increasing awareness of the molecular network governing drought tolerance in plants.

Wheat TaNACα18 functions as a positive regulator of high-temperature adaptive responses and improves cell defense machinery

Global wheat production amounted to >780 MMT during 2022-2023 whose market size are valued at >$128 billion. Wheat is highly susceptible to high-temperature stress (HTS) throughout the life cycle and its yield declines 5-7% with the rise in each degree of temperature. Previously, we reported an array of HTS-response markers from a resilient wheat cv. Unnat Halna and described their putative role in heat acclimation. To complement our previous results and identify the key determinants of thermotolerance, here we examined the cytoplasmic proteome of a sensitive cv. PBW343. The HTS-triggered metabolite reprograming highlighted how proteostasis defects influence the formation of an integrated stress-adaptive response. The proteomic analysis identified several promising HTS-responsive proteins, including a NACα18 protein, designated TaNACα18, whose role in thermotolerance remains unknown. Dual localization of TaNACα18 suggests its crucial functions in the cytoplasm and nucleus. The homodimerization of TaNACα18 anticipated its function as a transcriptional coactivator. The complementation of TaNACα18 in yeast and overexpression in wheat demonstrated its role in thermotolerance across the kingdom. Altogether, our results suggest that TaNACα18 imparts tolerance through tight regulation of gene expression, cell wall remodeling and activation of cell defense responses.

A high-efficiency transient expression system mediated by Agrobacterium tumefaciens in Spinacia oleracea leaves

BACKGROUND

Optimization of a highly efficient transient expression system is critical for the study of gene function, particularly in those plants in which stable transformation methods are not widely available. Agrobacterium tumefaciens‑mediated transient transformation is a simple and low-cost method that has been developed and applied to a wide variety of plant species. However, the transient expression in spinach (Spinacia oleracea L.) is still not reported.

RESULTS

We developed a transient expression system in spinach leaves of the Sp75 and Sp73 varieties. Several factors influencing the transformation efficiency were optimized such as Agrobacterium strain, spinach seedling stage, leaf position, and the expression time after injection. Agrobacterium strain GV3101 (pSoup-p19) was more efficient than AGL1 in expressing recombinant protein in spinach leaves. In general, Sp75 leaves were more suitable than Sp73 leaves, regardless of grow stage. At four-leaf stage, higher intensity and efficiency of transient expression were observed in group 1 (G1) of Sp75 at 53 h after injection (HAI) and in G1 of Sp73 at 64 HAI. At six-leaf stage of Sp75, group 3 (G3) at 72 HAI were the most effective condition for transient expression. Using the optimized expression system, we detected the subcellular localization of a transcriptional co-activator SoMBF1c and a NADPH oxidase SoRbohF. We also detected the interaction of the protein kinase SoCRK10 and the NADPH oxidase SoRbohB.

CONCLUSION

This study established a method of highly efficient transient expression mediated by Agrobacterium in spinach leaves. The transient expression system will facilitate the analysis of gene function and lay a solid foundation for molecular design breeding of spinach.

Agrobacterium tumefaciens-Mediated Plant Transformation: A Review

Agrobacterium tumefaciens-mediated plant transformation is the most dominant technique for the transformation of plants. It is used to transform monocotyledonous and dicotyledonous plants. A. tumefaciens apply for stable and transient transformation, random and targeted integration of foreign genes, as well as genome editing of plants. The Advantages of this method include cheapness, uncomplicated operation, high reproducibility, a low copy number of integrated transgenes, and the possibility of transferring larger DNA fragments. Engineered endonucleases such as CRISPR/Cas9 systems, TALENs, and ZFNs can be delivered with this method. Nowadays, Agrobacterium-mediated transformation is used for the Knock in, Knock down, and Knock out of genes. The transformation effectiveness of this method is not always desirable. Researchers applied various strategies to improve the effectiveness of this method. Here, a general overview of the characteristics and mechanism of gene transfer with Agrobacterium is presented. Advantages, updated data on the factors involved in optimizing this method, and other useful materials that lead to maximum exploitation as well as overcoming obstacles of this method are discussed. Moreover, the application of this method in the generation of genetically edited plants is stated. This review can help researchers to establish a rapid and highly effective Agrobacterium-mediated transformation protocol for any plant species.

Arabidopsis AGB1 participates in salinity response through bZIP17-mediated unfolded protein response

BACKGROUND

Plant heterotrimeric G proteins respond to various environmental stresses, including high salinity. It is known that Gβ subunit AGB1 functions in maintaining local and systemic Na+/K+ homeostasis to accommodate ionic toxicity under salt stress. However, whether AGB1 contributes to regulating gene expression for seedling's survival under high salinity remains unclear.

RESULTS

We showed that AGB1-Venus localized to nuclei when facing excessive salt, and the induction of a set of bZIP17-dependent salt stress-responsive genes was reduced in the agb1 mutant. We confirmed both genetic and physical interactions of AGB1 and bZIP17 in plant salinity response by comparing salt responses in the single and double mutants of agb1 and bzip17 and by BiFC assay, respectively. In addition, we show that AGB1 depletion decreases nuclei-localization of transgenic mRFP-bZIP17 under salt stress, as shown in s1p s2p double mutant in the Agrobacteria-mediated transient mRFP-bZIP17 expression in young seedlings.

CONCLUSIONS

Our results indicate that AGB1 functions in S1P and/or S2P-mediated proteolytic processing of bZIP17 under salt stress to regulate the induction of salinity-responsive gene expression.

Critical parameters for robust Agrobacterium-mediated transient transformation and quantitative promoter assays in Catharanthus roseus seedlings

Agrobacterium-mediated transient expression methods are widely used to study gene function in both model and non-model plants. Using a dual-luciferase assay, we quantified the effect of Agrobacterium-infiltration parameters on the transient transformation efficiency of Catharanthus roseus seedlings. We showed that transformation efficiency is highly sensitive to seedling developmental state and a pre- and post-infiltration dark incubation and is less sensitive to the Agrobacterium growth stage. For example, 5 versus 6 days of germination in the dark increased seedling transformation efficiency by seven- to eight-fold while a dark incubation pre- and post-infiltration increased transformation efficiency by five- to 13-fold. Agrobacterium in exponential compared with stationary phase increased transformation efficiency by two-fold. Finally, we quantified the variation in our Agrobacterium-infiltration method in replicate infiltrations and experiments. Within a given experiment, significant differences of up to 2.6-fold in raw firefly luciferase (FLUC) and raw Renilla luciferase (RLUC) luminescence occurred in replicate infiltrations. These differences were significantly reduced when FLUC was normalized to RLUC values, highlighting the utility of including a reference reporter to minimize false positives. Including a second experimental replicate further reduced the potential for false positives. This optimization and quantitative validation of Agrobacterium infiltration in C. roseus seedlings will facilitate the study of this important medicinal plant and will expand the application of Agrobacterium-mediated transformation methods in other plant species.

Exploring the frontier of rapid prototyping technologies for plant synthetic biology and what could lie beyond

Realizing the full potential of plant synthetic biology both to elucidate the relationship between genotype and phenotype and to apply these insights to engineer traits requires rapidly iterating through design-build-test cycles. However, the months-long process of transgenesis, the long generation times, and the size-based limitations on experimentation have stymied progress by limiting the speed and scale of these cycles. Herein, we review a representative sample of recent studies that demonstrate a variety of rapid prototyping technologies that overcome some of these bottlenecks and accelerate progress. However, each of them has caveats that limit their broad utility. Their complementary strengths and weaknesses point to the intriguing possibility that these strategies could be combined in the future to enable rapid and scalable deployment of synthetic biology in plants.

Paradise by the far-red light: Far-red and red:blue ratios independently affect yield, pigments, and carbohydrate production in lettuce, Lactuca sativa

In controlled environment agriculture, customized light treatments using light-emitting diodes are crucial to improving crop yield and quality. Red (R; 600-700 nm) and blue light (B; 400-500 nm) are two major parts of photosynthetically active radiation (PAR), often preferred in crop production. Far-red radiation (FR; 700-800 nm), although not part of PAR, can also affect photosynthesis and can have profound effects on a range of morphological and physiological processes. However, interactions between different red and blue light ratios (R:B) and FR on promoting yield and nutritionally relevant compounds in crops remain unknown. Here, lettuce was grown at 200 µmol m-2 s-1 PAR under three different R:B ratios: R:B87.5:12.5 (12.5% blue), R:B75:25 (25% blue), and R:B60:40 (40% blue) without FR. Each treatment was also performed with supplementary FR (50 µmol m-2 s-1; R:B87.5:12.5+FR, R:B75:25+FR, and R:B60:40+FR). White light with and without FR (W and W+FR) were used as control treatments comprising of 72.5% red, 19% green, and 8.5% blue light. Increasing the R:B ratio from R:B87.5:12.5 to R:B60:40, there was a decrease in fresh weight (20%) and carbohydrate concentration (48% reduction in both sugars and starch), whereas pigment concentrations (anthocyanins, chlorophyll, and carotenoids), phenolic compounds, and various minerals all increased. These results contrasted the effects of FR supplementation in the growth spectra; when supplementing FR to different R:B backgrounds, we found a significant increase in plant fresh weight, dry weight, total soluble sugars, and starch. Additionally, FR decreased concentrations of anthocyanins, phenolic compounds, and various minerals. Although blue light and FR effects appear to directly contrast, blue and FR light did not have interactive effects together when considering plant growth, morphology, and nutritional content. Therefore, the individual benefits of increased blue light fraction and supplementary FR radiation can be combined and used cooperatively to produce crops of desired quality: adding FR increases growth and carbohydrate concentration while increasing the blue fraction increases nutritional value.

The phosphorylation of carboxyl-terminal eIF2α by SPA kinases contributes to enhanced translation efficiency during photomorphogenesis

Light triggers an enhancement of global translation during photomorphogenesis in Arabidopsis, but little is known about the underlying mechanisms. The phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2α) at a conserved serine residue in the N-terminus has been shown as an important mechanism for the regulation of protein synthesis in mammalian and yeast cells. However, whether the phosphorylation of this residue in plant eIF2α plays a role in regulation of translation remains elusive. Here, we show that the quadruple mutant of SUPPRESSOR OF PHYA-105 family members (SPA1-SPA4) display repressed translation efficiency after light illumination. Moreover, SPA1 directly phosphorylates the eIF2α C-terminus under light conditions. The C-term-phosphorylated eIF2α promotes translation efficiency and photomorphogenesis, whereas the C-term-unphosphorylated eIF2α results in a decreased translation efficiency. We also demonstrate that the phosphorylated eIF2α enhances ternary complex assembly by promoting its affinity to eIF2β and eIF2γ. This study reveals a unique mechanism by which light promotes translation via SPA1-mediated phosphorylation of the C-terminus of eIF2α in plants.

Establishment of an efficient callus transient transformation system for Vitis vinifera cv. 'Chardonnay'

Grape (Vitis L.), a highly valued fruit crop, poses significant challenges in genetic transformation and functional characterization of genes. Therefore, there is an urgent need for the development of a rapid and effective method for grape transformation and gene function identification. Here, we introduce a streamlined Agrobacterium-mediated transient transformation system for grape calli. Optimal conditions were established with a leaf-derived callus induction medium; chiefly B5 medium supplemented with 0.05 mg/L NAA, 0.5 mg/L 2,4-D, and 2.0 mg/L KT; and a callus proliferation medium (B5 medium supplemented with 0.5 mg/L NAA and 2.0 mg/L 6-BA), respectively. Notably, GUS enzyme activity peaked (352.96 ± 33.95 mol 4-MU/mg/min) by sonication with Agrobacterium tumefaciens EHA105 and 100 μM AS for 4 min, followed by vacuum infection for 5 min, and co-culture at 25 °C in the dark for 1 day using callus as explants at an optical density (OD600) of 0.8. VaCIPK18 gene was transiently transformed into calli, and transcripts of the gene (endogenous and exogenous) were detected at higher levels than in non-transformed calli (endogenous). Moreover, after 10 days of treatment at 4 °C or -4 °C, the callus net weight of transformed callus was significantly higher than that of the untransformed callus, indicating that the VaCIPK18-overexpressing grape callus could improve cold tolerance. Overall, we establish a simple but effective transient transformation approach for grape callus, which could serve as a useful tool for the rapid assessment of gene function in this important crop.

The WOX Genes from the Intermediate Clade: Influence on the Somatic Embryogenesis in Medicago truncatula

Transcription factors from the WOX family are well-known regulators of cell proliferation and differentiation in plants. Herein, we focused on several WOX genes from the intermediate clade and checked their impact on somatic embryogenesis using the model legume object Medicago truncatula. As a result, we show that MtWOX9-1 overexpression not only stimulates somatic embryogenesis in the embryogenic M. truncatula line, as it was shown previously, but can also induce somatic embryogenesis in the non-embryogenic line. Other intermediate clade WOX, including the close paralog of MtWOX9-1, as well as WOX11 homologs, did not have any significant impact on somatic embryogenesis in our in vitro cultivation system. Together, our results give new information about the diversity of the WOX family proteins and their specific functions. These data can be used for the search of new regeneration stimulators.

Helper NLRs Nrc2 and Nrc3 act codependently with Prf/Pto and activate MAPK signaling to induce immunity in tomato

Plant intracellular immune receptors, primarily nucleotide-binding, leucine-rich repeat proteins (NLRs), detect pathogen effector proteins and activate NLR-triggered immunity (NTI). Recently, 'sensor' NLRs have been reported to function with 'helper' NLRs to activate immunity. We investigated the role of two helper NLRs, Nrc2 and Nrc3, on immunity in tomato to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) mediated by the sensor NLR Prf and the Pto kinase. An nrc2/nrc3 mutant no longer activated Prf/Pto-mediated NTI to Pst containing the effectors AvrPto and AvrPtoB. An nrc3 mutant showed intermediate susceptibility between wild-type plants and a Prf mutant, while an nrc2 mutant developed only mild disease. These observations indicate that Nrc2 and Nrc3 act additively in Prf-/Pto-mediated immunity. We examined at what point Nrc2 and Nrc3 act in the Prf/Pto-mediated immune response. In the nrc2/3 mutant, programmed cell death (PCD) normally induced by constitutively active variants of AvrPtoB, Pto, or Prf was abolished, but that induced by M3Kα or Mkk2 was not. PCD induced by a constitutively active Nrc3 was also abolished in a Nicotiana benthamiana line with reduced expression of Prf. MAPK activation triggered by expression of AvrPto in the wild-type tomato plants was completely abolished in the nrc2/3 mutant. These results indicate that Nrc2 and Nrc3 act with Prf/Pto and upstream of MAPK signaling. Nrc2 and Nrc3 were not required for PCD triggered by Ptr1, another sensor NLR-mediating Pst resistance, although these helper NLRs do appear to be involved in resistance to certain Pst race 1 strains.

Boosting grapevine breeding for climate-smart viticulture: from genetic resources to predictive genomics

The multifaceted nature of climate change is increasing the urgency to select resilient grapevine varieties, or generate new, fitter cultivars, to withstand a multitude of new challenging conditions. The attainment of this goal is hindered by the limiting pace of traditional breeding approaches, which require decades to result in new selections. On the other hand, marker-assisted breeding has proved useful when it comes to traits governed by one or few genes with great effects on the phenotype, but its efficacy is still restricted for complex traits controlled by many loci. On these premises, innovative strategies are emerging which could help guide selection, taking advantage of the genetic diversity within the Vitis genus in its entirety. Multiple germplasm collections are also available as a source of genetic material for the introgression of alleles of interest via adapted and pioneering transformation protocols, which present themselves as promising tools for future applications on a notably recalcitrant species such as grapevine. Genome editing intersects both these strategies, not only by being an alternative to obtain focused changes in a relatively rapid way, but also by supporting a fine-tuning of new genotypes developed with other methods. A review on the state of the art concerning the available genetic resources and the possibilities of use of innovative techniques in aid of selection is presented here to support the production of climate-smart grapevine genotypes.

Effects of different light conditions on transient expression and biomass in Nicotiana benthamiana leaves

In the process of the production of recombinant proteins by using an Agrobacterium-mediated transient gene expression system, the effectiveness of the control of light conditions pre- and post-agroinfiltration on efficiency of transient expression is worth being evaluated. In this study, Nicotiana benthamiana plants were used as a bioreactor to investigate the effects of different light conditions pre- and post-agroinfiltration on the transient expression of green fluorescent protein (GFP). The results showed that the plants grown under light condition for 5 weeks had the highest level of transient expression among those grown for 4-8 weeks. In the pre-agroinfiltration, the level of transient expression of GFP was obviously decreased by the increase in light intensity or by the shortening of the photoperiod. Although the shortening of the photoperiod post-agroinfiltration also decreased the level of transient expression, moderate light intensity post-agroinfiltration was needed for higher level of transient expression efficiency. However, there was no strong correlation between the transient expression efficiency and plant growth. The results suggested that light condition was an important factor affecting the level of transient expression in plants. Hence, light conditions should be optimized to obtain higher productivity of recombinant protein from transient expression systems.

Evaluation of cucumber UBL5 promoter as a tool for transgene expression and genome editing in plants

Transgene expression and genome editing can help improve cucumber varieties to better respond to climate change. This study aimed to evaluate the applicability of the CsUBL5 promoter in transgene expression and genome editing in cucumber. The CsUBL5 promoter was cloned and analyzed to identify cis-elements that respond to abiotic signals, hormones, signal molecules, and nutrient treatments. 5' deletion constructs of the promoter were tested for their ability to drive GUS reporter expression in cucumber cotyledons, Arabidopsis seedlings, and tobacco leaves, and their response to various treatments including SA, light, drought, IAA, and GA was determined. The results showed that the CsUBL5 promoter effectively drove transgene expression in these plants, and their expressions under treatments were consistent with the predicted cis-elements, with some exceptions. Furthermore, the pCsUBL5-749 deletion construct can improve genome editing efficiency in cucumber when driving Cas9 expression. The editing efficiency of two sgRNAs targeting the ATG6 gene in cucumber was up to 4.6-fold higher using pCsUBL5-749 compared to a rice UBI promoter, although the effects of changing promoter on the editing efficiency is sgRNA specific. These findings highlight the potential utility of the CsUBL5 promoter for improving cucumber varieties through genetic engineering and genome editing. It also demonstrates the importance of modulating Cas9 expression to increase genome editing efficiency in cucumbers.

A SNARE-like protein from Solanum lycopersicum increases salt tolerance by modulating vesicular trafficking in tomato

Intracellular vesicular trafficking ensures the exchange of lipids and proteins between endomembrane compartments. This is relevant under high salinity conditions, since both the removal of transporters and ion channels from the plasma membrane and the compartmentalization of toxic ions require the formation of vesicles, which can be maintained as multivesicular bodies or be fused to the central vacuole. SNARE proteins (Soluble N-ethylmaleimide-sensitive factor attachment receptor) participate in the vesicle fusion process and give specificity to their destination. Plant genome studies have revealed a superfamily of genes that encode for proteins called SNARE-like. These proteins appear to be participating in vesicular trafficking with similar functions to those of SNARE proteins. A SNARE-like, named SlSLSP6, in Solanum lycopersicum plants has been shown to be induced under high salinity conditions. A phylogenetic relationship of SlSLSP6 with SNARE-like proteins of salinity-tolerant plants, including Salicornia brachiata, Zostera marina and Solanum pennelli, was determined. Considering its amino acid sequence, a putative clathrin adapter complex domain and palmitoylation site was predicted. Subcellular localization analysis evidenced that SlSLSP6 is mostly localized in the plasma membrane. Using transgenic tomato plants, we identified that overexpression of SlSLSP6 increased tolerance to salt stress. This tolerance was evident when we quantified an improvement in physiological and biochemical parameters, such as higher chlorophyll content, performance index, efficiency of photosystem II and relative water content, and lower malondialdehyde content, compared to control plants. At the subcellular level, the overexpression of SlSLSP6 reduced the presence of H2O2 in roots and increased the compartmentalization of sodium in vacuoles during salt stress. These effects appear to be associated with the higher endocytic rate of FM4-64, determined in the plant root cells. Taken together, these results indicate that SlSLSP6 increases tolerance to salt stress by modulating vesicular trafficking through over-induction of the endocytic pathway. This work contributes to understanding the role of this type of SNARE-like protein during salt stress and could be a potential candidate in breeding programs for tolerance to salt stress in tomato plants.

The Ti Plasmid, Driver of Agrobacterium Pathogenesis

The phytopathogenic bacterium Agrobacterium tumefaciens causes crown gall disease in plants, characterized by the formation of tumor-like galls where wounds were present. Nowadays, however, the bacterium and its Ti (tumor-inducing) plasmid is better known as an effective vector for the genetic manipulation of plants and fungi. In this review, I will briefly summarize some of the major discoveries that have led to this bacterium now playing such a prominent role worldwide in plant and fungal research at universities and research institutes and in agricultural biotechnology for the production of genetically modified crops. I will then delve a little deeper into some aspects of Agrobacterium biology and discuss the diversity among agrobacteria and the taxonomic position of these bacteria, the diversity in Ti plasmids, the molecular mechanism used by the bacteria to transform plants, and the discovery of protein translocation from the bacteria to host cells as an essential feature of Agrobacterium-mediated transformation.

Engineered Agrobacterium improves transformation by mitigating plant immunity detection

Agrobacterium tumefaciens microbe-associated molecular pattern elongation factor Tu (EF-Tu) is perceived by orthologs of the Arabidopsis immune receptor EFR activating pattern-triggered immunity (PTI) that causes reduced T-DNA-mediated transient expression. We altered EF-Tu in A. tumefaciens to reduce PTI and improved transformation efficiency. A robust computational pipeline was established to detect EF-Tu protein variation in a large set of plant bacterial species and identified EF-Tu variants from bacterial pathogen Pseudomonas syringae pv. tomato DC3000 that allow the pathogen to escape EFR perception. Agrobacterium tumefaciens strains were engineered to substitute EF-Tu with DC3000 variants and examined their transformation efficiency in plants. Elongation factor Tu variants with rarely occurred amino acid residues were identified within DC3000 EF-Tu that mitigates recognition by EFR. Agrobacterium tumefaciens strains were engineered by expressing DC3000 EF-Tu instead of native agrobacterial EF-Tu and resulted in decreased plant immunity detection. These engineered A. tumefaciens strains displayed an increased efficiency in transient expression in both Arabidopsis thaliana and Camelina sativa. The results support the potential application of these strains as improved vehicles to introduce transgenic alleles into members of the Brassicaceae family.

The Chloroplast Envelope of Angiosperms Contains a Peptidoglycan Layer

Plastids in plants are assumed to have evolved from cyanobacteria as they have maintained several bacterial features. Recently, peptidoglycans, as bacterial cell wall components, have been shown to exist in the envelopes of moss chloroplasts. Phylogenomic comparisons of bacterial and plant genomes have raised the question of whether such structures are also part of chloroplasts in angiosperms. To address this question, we visualized canonical amino acids of peptidoglycan around chloroplasts of Arabidopsis and Nicotiana via click chemistry and fluorescence microscopy. Additional detection by different peptidoglycan-binding proteins from bacteria and animals supported this observation. Further Arabidopsis experiments with D-cycloserine and AtMurE knock-out lines, both affecting putative peptidoglycan biosynthesis, revealed a central role of this pathway in plastid genesis and division. Taken together, these results indicate that peptidoglycans are integral parts of plastids in the whole plant lineage. Elucidating their biosynthesis and further roles in the function of these organelles is yet to be achieved.

The ScAPD1-like gene from the desert moss Syntrichia caninervis enhances resistance to Verticillium dahliae via phenylpropanoid gene regulation

Soloist is a member of a distinct and small subfamily within the AP2/ERF transcriptional factor family that play important roles in plant biotic and abiotic stress responses. There are limited studies of Soloist genes and their functions are poorly understood. We characterized the abiotic and biotic stress tolerance function of the ScSoloist gene (designated as ScAPD1-like) from the desert moss Syntrichia caninervis. ScAPD1-like responded to multiple abiotic, biotic stresses and plant hormone treatments. ScAPD1-like protein located to the nucleus and bound to several DNA elements. Overexpression of ScAPD1-like in Arabidopsis did not alter abiotic stress resistance or inhibit Pseudomonas syringae pv. tomato (Pst) DC3000 infection. However, overexpression of ScAPD1-like significantly increased the resistance of transgenic Arabidopsis and S. caninervis to Verticillium dahliae infection, decreased reactive oxygen species accumulation and improved reactive oxygen species scavenging activity. ScAPD1-like overexpression plants altered the abundance of transcripts for lignin synthesis and promoted lignin accumulation in Arabidopsis. ScAPD1-like directly bind to RAV1, AC elements, and TATA-box in the promoters of AtPAL1 and AtC4H genes, respectively, in vitro. Chromatin immunoprecipitation-quantitative polymerase chain reaction assays demonstrated ScAPD1-like directly bound to PAL and C4H genes promoters in Arabidopsis and their homologs in S. caninervis. In S. caninervis, ScAPD1-like overexpression and RNAi directly regulated the abundance of ScPAL and ScC4H transcripts and modified the metabolites of phenylpropanoid pathway. We provide insight into the function of Soloist in plant defense mechanisms that likely occurs through activation of the phenylpropanoid biosynthesis pathway. ScAPD1-like is a promising candidate gene for breeding strategies to improve resistance to Verticillium wilt.

Relative Protein Lifetime Measurement in Plants Using Tandem Fluorescent Protein Timers

Targeted protein degradation plays a wide range of important roles in plant growth and development, but analyzing protein turnover in vivo is technically challenging. Until recently, there has been no straightforward methodology for quantifying protein dynamics at subcellular resolution during cellular transitions in plants. A tandem fluorescent protein timer (tFT) is a fusion of two different fluorescent proteins with distinct fluorophore maturation kinetics, which allows estimation of relative protein age from the ratio of fluorescence intensities of the two fluorescent proteins. Here, we describe approaches to use this technology to report relative protein lifetime in both transient and stable plant transformation systems. tFTs enable in vivo, real-time protein lifetime assessment within subcellular compartments and across tissues, permitting the analysis of protein degradation dynamics in response to stresses or developmental cues and in different genetic backgrounds.

CRISPR RNA-guided integrase enables high-efficiency targeted genome engineering in Agrobacterium tumefaciens

Agrobacterium tumefaciens, the causal agent of plant crown gall disease, has been widely used to genetically transform many plant species. The inter-kingdom gene transfer capability made Agrobacterium an essential tool and model system to study the mechanism of exporting and integrating a segment of bacterial DNA into the plant genome. However, many biological processes such as Agrobacterium-host recognition and interaction are still elusive. To accelerate the understanding of this important plant pathogen and further improve its capacity in plant genetic engineering, we adopted a CRISPR RNA-guided integrase system for Agrobacterium genome engineering. In this work, we demonstrate that INsertion of Transposable Elements by Guide RNA-Assisted TargEting (INTEGRATE) can efficiently generate DNA insertions to enable targeted gene knockouts. In addition, in conjunction with Cre-loxP recombination system, we achieved precise deletions of large DNA fragments. This work provides new genetic engineering strategies for Agrobacterium species and their gene functional analyses.

A Biotechnological Approach for the Production of Pharmaceutically Active Human Interferon-α from Raphanus sativus L. Plants

Human interferon (IFN) is a type of cytokine that regulates the immune system’s response to viral and bacterial infections. Recombinant IFN-α has been approved for use in the treatment of a variety of viral infections as well as an anticancer medication for various forms of leukemia. The objective of the current study is to produce a functionally active recombinant human IFN-α2a from transgenic Raphanus sativus L. plants. Therefore, a binary plant expression construct containing the IFN-α2a gene coding sequence, under the regulation of the cauliflower mosaic virus 35SS promoter, was established. Agrobacterium-mediated floral dip transformation was used to introduce the IFN-α2a expression cassette into the nuclear genome of red and white rooted Raphanus sativus L. plants. From each genotype, three independent transgenic lines were established. The anticancer and antiviral activities of the partially purified recombinant IFN-α2a proteins were examined. The isolated IFN-α2a has been demonstrated to inhibit the spread of the Vesicular Stomatitis Virus (VSV). In addition, cytotoxicity and cell apoptosis assays against Hep-G2 cells (Human Hepatocellular Carcinoma) show the efficacy of the generated IFN-α2a as an anticancer agent. In comparison to bacterial, yeast, and animal cell culture systems, the overall observed results demonstrated the efficacy of using Raphanus sativus L. plants as a safe, cost-effective, and easy-to-use expression system for generating active human IFN-α2a.

Arabidopsis cryptochrome 2 forms photobodies with TCP22 under blue light and regulates the circadian clock

Cryptochromes are blue light receptors that regulate plant growth and development. They also act as the core components of the central clock oscillator in animals. Although plant cryptochromes have been reported to regulate the circadian clock in blue light, how they do so is unclear. Here we show that Arabidopsis cryptochrome 2 (CRY2) forms photobodies with the TCP22 transcription factor in response to blue light in plant cells. We provide evidence that PPK kinases influence the characteristics of these photobodies and that together these components, along with LWD transcriptional regulators, can positively regulate the expression of CCA1 encoding a central component of the circadian oscillator.

Cryptochrome signaling has been reported to regulate circadian oscillations in plants. Here the authors show that CRY2 and the TCP22 transcription factors can form photobodies in a blue light dependent manner and induce expression of CCA1, a core component of the circadian oscillator.

Agrobacterium expressing a type III secretion system delivers Pseudomonas effectors into plant cells to enhance transformation

Agrobacterium-mediated plant transformation (AMT) is the basis of modern-day plant biotechnology. One major drawback of this technology is the recalcitrance of many plant species/varieties to Agrobacterium infection, most likely caused by elicitation of plant defense responses. Here, we develop a strategy to increase AMT by engineering Agrobacterium tumefaciens to express a type III secretion system (T3SS) from Pseudomonas syringae and individually deliver the P. syringae effectors AvrPto, AvrPtoB, or HopAO1 to suppress host defense responses. Using the engineered Agrobacterium, we demonstrate increase in AMT of wheat, alfalfa and switchgrass by ~250%-400%. We also show that engineered A. tumefaciens expressing a T3SS can deliver a plant protein, histone H2A-1, to enhance AMT. This strategy is of great significance to both basic research and agricultural biotechnology for transient and stable transformation of recalcitrant plant species/varieties and to deliver proteins into plant cells in a non-transgenic manner.

Characterization of T-Circles and Their Formation Reveal Similarities to Agrobacterium T-DNA Integration Patterns

Agrobacterium transfers T-DNA to plants where it may integrate into the genome. Non-homologous end-joining (NHEJ) has been invoked as the mechanism of T-DNA integration, but the role of various NHEJ proteins remains controversial. Genetic evidence for the role of NHEJ in T-DNA integration has yielded conflicting results. We propose to investigate the formation of T-circles as a proxy for understanding T-DNA integration. T-circles are circular double-strand T-DNA molecules, joined at their left (LB) and right (RB) border regions, formed in plants. We characterized LB-RB junction regions from hundreds of T-circles formed in Nicotiana benthamiana or Arabidopsis thaliana. These junctions resembled T-DNA/plant DNA junctions found in integrated T-DNA: Among complex T-circles composed of multiple T-DNA molecules, RB-RB/LB-LB junctions predominated over RB-LB junctions; deletions at the LB were more frequent and extensive than those at the RB; microhomology was frequently used at junction sites; and filler DNA, from the plant genome or various Agrobacterium replicons, was often present between the borders. Ku80 was not required for efficient T-circle formation, and a VirD2 ω mutation affected T-circle formation and T-DNA integration similarly. We suggest that investigating the formation of T-circles may serve as a surrogate for understanding T-DNA integration.

An Agrobacterium strain auxotrophic for methionine is useful for switchgrass transformation

Auxotrophic strains of Agrobacterium tumefaciens can contribute to the development of more efficient transformation systems, especially for crops historically considered recalcitrant. Homologous recombination was used to derive methionine auxotrophs of two common A. tumefaciens strains, LBA4404 and EHA105. The EHA105 strains were more efficient for switchgrass transformation, while both the EHA105 and LBA4404 strains worked equally well for the rice control. Event quality, as measured by transgene copy number, was not affected by auxotrophy, but was higher for the LBA4404 strains than the EHA105 strains. Ultimately, the use of auxotrophs reduced bacterial overgrowth during co-cultivation and decreased the need for antibiotics.

Stress associated proteins coordinate the activation of comprehensive antiviral immunity in Phalaenopsis orchids

Viruses cause severe damage on crops, and identification of key gene(s) that can comprehensively activate antiviral immunity will provide insights for designing effective antiviral strategies. Salicylic acid (SA)-mediated antiviral immunity and RNA interference (RNAi) are two independently discovered antiviral pathways. Previously, we identified the orchid stress-associated protein (SAP), Pha13, which serves as a hub in SA-mediated antiviral immunity. As SAPs exist as a protein family, whether duplicated SAPs have redundant or distinctive functions in antiviral immunity remains elusive. We performed functional assays on orchid Pha21, a homolog of Pha13, using transient and transgenic approaches on orchid, Arabidopsis and Nicotiana benthamiana to overexpress and/or silence Pha21. The SA treatment induced the expression of both Pha13 and Pha21, whereas Pha21 was found to play a key role in the initiation of the RNAi pathway in Phalaenopsis orchids. We demonstrated that Pha21-mediated antiviral immunity and enhancement of the RNAi pathway is conserved between dicotyledons and monocotyledons. We provide new insight that orchid SAPs confer distinctive functions to coordinate both SA-signaling and RNAi for comprehensive activation of antiviral immunity, and this information will help us develop antiviral strategies on crops.

Gene targeting in polymerase theta-deficient Arabidopsis thaliana

Agrobacterium tumefaciens-mediated transformation has been for decades the preferred tool to generate transgenic plants. During this process, a T-DNA carrying transgenes is transferred from the bacterium to plant cells, where it randomly integrates into the genome via polymerase theta (Polθ)-mediated end joining (TMEJ). Targeting of the T-DNA to a specific genomic locus via homologous recombination (HR) is also possible, but such gene targeting (GT) events occur at low frequency and are almost invariably accompanied by random integration events. An additional complexity is that the product of recombination between T-DNA and target locus may not only map to the target locus (true GT), but also to random positions in the genome (ectopic GT). In this study, we have investigated how TMEJ functionality affects the biology of GT in plants, by using Arabidopsis thaliana mutated for the TEBICHI gene, which encodes for Polθ. Whereas in TMEJ-proficient plants we predominantly found GT events accompanied by random T-DNA integrations, GT events obtained in the teb mutant background lacked additional T-DNA copies, corroborating the essential role of Polθ in T-DNA integration. Polθ deficiency also prevented ectopic GT events, suggesting that the sequence of events leading up to this outcome requires TMEJ. Our findings provide insights that can be used for the development of strategies to obtain high-quality GT events in crop plants.

AGROBEST: A Highly Efficient Agrobacterium-Mediated Transient Expression System in Arabidopsis Seedlings

Agrobacterium-mediated transient transformation for gene expression is a simple and fast method to analyze transgene functions in plants. Agroinfiltration in leaves of Nicotiana benthamiana is a common method for transient expression. However, agroinfiltration in leaves of Arabidopsis thaliana is challenging due to the low and variable efficiency. Here, we describe procedures of a highly efficient and robust Agrobacterium-mediated transient expression system, named AGROBEST (Agrobacterium-mediated enhanced seedling transformation) for gene expression in A. thaliana seedlings. High efficiency of AGROBEST has been achieved by virulence (vir) gene pre-induction of a specific disarmed Agrobacterium tumefaciens strain C58C1(pTiB6S3ΔT)^H followed by co-cultivation with Arabidopsis seedlings in an optimized medium with AB salts and buffered acidic plant culture medium. The stable acidic medium largely increases Agrobacterium-mediated transient expression levels and reduces plant defense responses, suggesting that AGROBEST enables high transient expression efficiency by compromising plant immunity. In summary, AGROBEST is a simple, fast, reliable, and robust transient expression system offering a quick and convenient method to observe protein localization, protein-protein interactions, promoter activities, and gene functional studies in Arabidopsis seedlings.

Agrobacterium-Mediated Seedling Transformation to Measure Circadian Rhythms in Arabidopsis

Circadian clocks are endogenous timing mechanisms that allow an organism to adapt cellular processes in anticipation of predictable changes in the environment. Luciferase reporters are well utilized as an effective, nondestructive method to measure circadian rhythms of promoter activity in Arabidopsis. Obtaining stable transgenic reporter lines can be laborious. Here, we report a protocol for Agrobacterium-mediated seedling transformation tailored for plant circadian studies. We show that period estimates generated from wild-type and clock-mutant seedlings transformed with circadian luciferase reporters are similar to rhythms obtained from equivalent stable transgenic seedlings. These experiments demonstrate the versatility and robustness of the protocol for testing new constructs or quickly assessing circadian effects in any genotype of interest.

Agrobacterium tumefaciens Type IV and Type VI Secretion Systems Reside in Detergent-Resistant Membranes

Cell membranes are not homogenous but compartmentalized into lateral microdomains, which are considered as biochemical reaction centers for various physiological processes in eukaryotes and prokaryotes. Due to their special lipid and protein composition, some of these microdomains are resistant to treatment with non-ionic detergents and can be purified as detergent-resistant membranes (DRMs). Here we report the proteome of DRMs from the Gram-negative phytopathogen Agrobacterium tumefaciens. Using label-free liquid chromatography-tandem mass spectrometry, we identified proteins enriched in DRMs isolated under normal and virulence-mimicking growth conditions. Prominent microdomain marker proteins such as the SPFH (stomatin/prohibitin/flotillin/HflKC) proteins HflK, HflC and Atu3772, along with the protease FtsH were highly enriched in DRMs isolated under any given condition. Moreover, proteins involved in cell envelope biogenesis, transport and secretion, as well as motility- and chemotaxis-associated proteins were overrepresented in DRMs. Most strikingly, we found virulence-associated proteins such as the VirA/VirG two-component system, and the membrane-spanning type IV and type VI secretion systems enriched in DRMs. Fluorescence microscopy of the cellular localization of both secretion systems and of marker proteins was in agreement with the results from the proteomics approach. These findings suggest that virulence traits are micro-compartmentalized into functional microdomains in A. tumefaciens.

Arabidopsis RAB8A, RAB8B and RAB8D Proteins Interact with Several RTNLB Proteins and are Involved in the Agrobacterium tumefaciens Infection Process

Arabidopsis thaliana small GTP-binding proteins, AtRAB8s, associate with the endomembrane system and modulate tubulovesicular trafficking between compartments of the biosynthetic and endocytic pathways. There are five members in Arabidopsis, namely AtRAB8A-8E. Yeast two-hybrid assays, bimolecular fluorescence complementation assays and glutathione-S-transferase pull-down assays showed that RAB8A, 8B and 8D interacted with several membrane-associated reticulon-like (AtRTNLB) proteins in yeast, plant cells and in vitro. Furthermore, RAB8A, 8B and 8D proteins showed interactions with the Agrobacterium tumefaciens virulence protein, VirB2, a component of a type IV secretion system (T4SS). A. tumefaciens uses a T4SS to transfer T-DNA and Virulence proteins to plants, which causes crown gall disease in plants. The Arabidopsis rab8A, rab8B and rab8D single mutants showed decreased levels of Agrobacterium-mediated root and seedling transformation, while the RAB8A, 8B and 8D overexpression transgenic Arabidopsis plants were hypersusceptible to A. tumefaciens and Pseudomonas syringae infections. RAB8A-8E transcripts accumulated differently in roots, rosette leaves, cauline leaves, inflorescence and flowers of wild-type plants. In summary, RAB8A, 8B and 8D interacted with several RTNLB proteins and participated in A. tumefaciens and P. syringae infection processes.

A high-efficiency Agrobacterium-mediated transient expression system in the leaves of Artemisia annua L

BACKGROUND

The Agrobacterium-mediated transient transformation, which proved effective in diverse plant species, has been widely applied for high-throughput gene function studies due to its simplicity, rapidity, and high efficiency. Despite the efforts have made on Artemisia annua transient expression, achieving high-throughput gene functional characterization basing on a fast and easy-manipulated transient transformation system in A. annua remains challenging.

RESULTS

The first pair of true leaves of A. annua is an ideal candidate for Agrobacterium injection. EHA105 was the optimal strain that can be used for the development of the transient expression system. The supplementation of Triton X-100 at a concentration of 0.005% greatly improved the transient expression frequency. According to the histochemical β-Glucuronidase (GUS) staining assay, high transient expression level of the reporter gene (GUS) maintained at least a week. Dual-luciferase (Dual-LUC) transient assays showed that the activity of cauliflower mosaic virus 35S (CaMV35S) promoter and its derivates varied between A. annua and tobacco. In A. annua, the CaMV35S promoter had comparable activity with double CaMV35S promoter, while in tobacco, CaMV35S exhibited approximately 50% activity of double CaMV35S promoter. Otherwise, despite the CaMV35S promoter and double CaMV35S promoter from GoldenBraid Kit 2.0 displayed high activity strength in tobacco, they demonstrated a very low activity in transiently expressed A. annua. The activity of UBQ10 promoter and endogenous UBQb promoter was investigated as well. Additionally, using our transient expression system, the transactivation of AaGSW1 and AaORA on AaCYP71AV1 promoter was confirmed. Dual-LUC assays demonstrated that AaHD8 activated the expression of two glandular secreting trichomes-specific lipid transfer protein genes AaLTP1 and AaLTP2, indicating that AaLTP1 and AaLTP2 might serve as downstream components of AaHD8-involved glandular trichome initiation and cuticle formation, as well as artemisinin secretion in A. annua.

CONCLUSIONS

A simple, rapid, good-reproducibility, high-efficiency and low-cost transient transformation system in A. annua was developed. Our method offered a new way for gene functional characterization studies such as gene subcellular localization, promoter activity and transcription activation assays in A. annua, avoiding the aberrant phenotypes resulting from gene expression in a heterologous system.

Agroinfiltration Mediated Scalable Transient Gene Expression in Genome Edited Crop Plants

Agrobacterium-mediated transformation is one of the most commonly used genetic transformation method that involves transfer of foreign genes into target plants. Agroinfiltration, an Agrobacterium-based transient approach and the breakthrough discovery of CRISPR/Cas9 holds trending stature to perform targeted and efficient genome editing (GE). The predominant feature of agroinfiltration is the abolishment of Transfer-DNA (T-DNA) integration event to ensure fewer biosafety and regulatory issues besides showcasing the capability to perform transcription and translation efficiently, hence providing a large picture through pilot-scale experiment via transient approach. The direct delivery of recombinant agrobacteria through this approach carrying CRISPR/Cas cassette to knockout the expression of the target gene in the intercellular tissue spaces by physical or vacuum infiltration can simplify the targeted site modification. This review aims to provide information on Agrobacterium-mediated transformation and implementation of agroinfiltration with GE to widen the horizon of targeted genome editing before a stable genome editing approach. This will ease the screening of numerous functions of genes in different plant species with wider applicability in future.

Protoplast: A Valuable Toolbox to Investigate Plant Stress Perception and Response

Plants are constantly facing abiotic and biotic stresses. To continue to thrive in their environment, they have developed many sophisticated mechanisms to perceive these stresses and provide an appropriate response. There are many ways to study these stress signals in plant, and among them, protoplasts appear to provide a unique experimental system. As plant cells devoid of cell wall, protoplasts allow observations at the individual cell level. They also offer a prime access to the plasma membrane and an original view on the inside of the cell. In this regard, protoplasts are particularly useful to address essential biological questions regarding stress response, such as protein signaling, ion fluxes, ROS production, and plasma membrane dynamics. Here, the tools associated with protoplasts to comprehend plant stress signaling are overviewed and their potential to decipher plant defense mechanisms is discussed.

Identification of MaWRKY40 and MaDLO1 as Effective Marker Genes for Tracking the Salicylic Acid-Mediated Immune Response in Bananas

Bananas are among the world's most important cash and staple crops but are threatened by various devastating pathogens. The phytohormone salicylic acid (SA) plays a key role in the regulation of plant immune response. Tracking the expression of SA-responsive marker genes under pathogen infection is important in pathogenesis elucidation. However, the common SA-responsive marker genes are not consistently induced in different banana cultivars or different organs. Here, we conducted transcriptome analysis for SA response of a banana cultivar, 'Pei-Chiao' (Cavendish, AAA genome), and identified three genes, MaWRKY40, MaWRKY70, and Downy Mildew Resistant 6 (DMR6)-Like Oxygenase 1 (MaDLO1) that are robustly induced upon SA treatment in both the leaves and roots. Consistent induction of these three genes by SA treatment was also detected in both the leaves and roots of bananas belonging to different genome types such as 'Tai-Chiao No. 7' (Cavendish, AAA genome), 'Pisang Awak' (ABB genome), and 'Lady Finger' (AA genome). Furthermore, the biotrophic pathogen cucumber mosaic virus elicited the expression of MaWRKY40 and MaDLO1 in infected leaves of susceptible cultivars. The hemibiotrophic fungal pathogen Fusarium oxysporum f. sp. cubense tropical race 4 (TR4) also consistently induced the expression of MaWRKY40 and MaDLO1 in the infected roots of the F. oxysporum f. sp. cubense TR4-resistant cultivar. These results indicate that MaWRKY40 and MaDLO1 can be used as reliable SA-responsive marker genes for the study of plant immunity in banana. Revealing SA-responsive marker genes provides a stepping stone for further studies in banana resistance to pathogens.

MKK4/5-MPK3/6 Cascade Regulates Agrobacterium-Mediated Transformation by Modulating Plant Immunity in Arabidopsis

Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.

Development of an efficient transient expression system for Siraitia grosvenorii fruit and functional characterization of two NADPH-cytochrome P450 reductases

Siraitia grosvenorii (Luo hanguo or monk fruit) is a valuable medicinal herb for which the market demand has increased dramatically worldwide. As promising natural sweeteners, mogrosides have received much attention from researchers because of their extremely high sweetness and lack of calories. Nevertheless, owing to the absence of genetic transformation methods, the molecular mechanisms underlying the regulation of mogroside biosynthesis have not yet been fully elucidated. Therefore, an effective method for gene function analysis needs to be developed for S. grosvenorii fruit. As a powerful approach, transient expression has become a versatile method to elucidate the biological functions of genes and proteins in various plant species. In this study, PBI121 with a β-glucuronidase (GUS) marker and tobacco rattle virus (TRV) were used as vectors for overexpression and silencing, respectively, of the SgCPR1 and SgCPR2 genes in S. grosvenorii fruit. The effectiveness of transient expression was validated by GUS staining in S. grosvenorii fruit, and the expression levels of SgCPR1 and SgCPR2 increased significantly after infiltration for 36 h. In addition, TRV-induced gene silencing suppressed the expression of SgCPR1 and SgCPR2 in S. grosvenorii fruit. More importantly, the production of the major secondary metabolites mogrol, mogroside IIE (MIIE) and mogroside III (MIII) was activated by the overexpression of SgCPR1 and SgCPR2 in S. grosvenorii fruit, with levels 1-2 times those in the control group. Moreover, the accumulation of mogrol, MIIE and MIII was decreased in the SgCPR1 and SgCPR2 gene silencing assays. Therefore, this transient expression approach was available for S. grosvenorii fruit, providing insight into the expression of the SgCPR1 and SgCPR2 genes involved in the mogroside biosynthesis pathway. Our study also suggests that this method has potential applications in the exploration of the molecular mechanisms, biochemical hypotheses and functional characteristics of S. grosvenorii genes.

A Core35S Promoter of Cauliflower Mosaic Virus Drives More Efficient Replication of Turnip Crinkle Virus

The 35S promoter with a duplicated enhancer (frequently referred to as 2X35S) is a strong dicotyledonous plant-specific promoter commonly used in generating transgenic plants to enable high-level expression of genes of interest. It is also used to drive the initiation of RNA virus replication from viral cDNA, with the consensus understanding that high levels of viral RNA production powered by 2X35S permit a more efficient initiation of virus replication. Here, we showed that the exact opposite is true. We found that, compared to the Core35S promoter, the 2X35S promoter-driven initiation of turnip crinkle virus (TCV) infection was delayed by at least 24 h. We first compared three versions of 35S promoter, namely 2X35S, 1X35S, and Core35S, for their ability to power the expression of a non-replicating green fluorescent protein (GFP) gene, and confirmed that 2X35S and Core35S correlated with the highest and lowest GFP expression, respectively. However, when inserted upstream of TCV cDNA, 2X35S-driven replication was not detected until 72 h post-inoculation (72 hpi) in inoculated leaves. By contrast, Core35S-driven replication was detected earlier at 48 hpi. A similar delay was also observed in systemically infected leaves (six versus four days post-inoculation). Combining our results, we hypothesized that the stronger 2X35S promoter might enable a higher accumulation of a TCV protein that became a repressor of TCV replication at higher cellular concentration. Extending from these results, we propose that the Core35S (or mini35S) promoter is likely a better choice for generating infectious cDNA clones of TCV.