High meiotic stability of a foreign gene introduced into tobacco by Agrobacterium-mediated transformation

Ralstonia solanacearum effector RipAK suppresses homodimerization of the host transcription factor ERF098 to enhance susceptibility and the sensitivity of pepper plants to dehydration

Plants have evolved a sophisticated immune system to defend against invasion by pathogens. In response, pathogens deploy copious effectors to evade the immune responses. However, the molecular mechanisms used by pathogen effectors to suppress plant immunity remain unclear. Herein, we report that an effector secreted by Ralstonia solanacearum, RipAK, modulates the transcriptional activity of the ethylene-responsive factor ERF098 to suppress immunity and dehydration tolerance, which causes bacterial wilt in pepper (Capsicum annuum L.) plants. Silencing ERF098 enhances the resistance of pepper plants to R. solanacearum infection not only by inhibiting the host colonization of R. solanacearum but also by increasing the immunity and tolerance of pepper plants to dehydration and including the closure of stomata to reduce the loss of water in an abscisic acid signal-dependent manner. In contrast, the ectopic expression of ERF098 in Nicotiana benthamiana enhances wilt disease. We also show that RipAK targets and inhibits the ERF098 homodimerization to repress the expression of salicylic acid-dependent PR1 and dehydration tolerance-related OSR1 and OSM1 by cis-elements in their promoters. Taken together, our study reveals a regulatory mechanism used by the R. solanacearum effector RipAK to increase virulence by specifically inhibiting the homodimerization of ERF098 and reprogramming the transcription of PR1, OSR1, and OSM1 to boost susceptibility and dehydration sensitivity. Thus, our study sheds light on a previously unidentified strategy by which a pathogen simultaneously suppresses plant immunity and tolerance to dehydration by secreting an effector to interfere with the activity of a transcription factor and manipulate plant transcriptional programs.

The CaPti1-CaERF3 module positively regulates resistance of Capsicum annuum to bacterial wilt disease by coupling enhanced immunity and dehydration tolerance

Bacterial wilt, a severe disease involving vascular system blockade, is caused by Ralstonia solanacearum. Although both plant immunity and dehydration tolerance might contribute to disease resistance, whether and how they are related remains unclear. Herein, we showed that immunity against R. solanacearum and dehydration tolerance are coupled and regulated by the CaPti1-CaERF3 module. CaPti1 and CaERF3 are members of the serine/threonine protein kinase and ethylene-responsive factor families, respectively. Expression profiling revealed that CaPti1 and CaERF3 were upregulated by R. solanacearum inoculation, dehydration stress, and exogenously applied abscisic acid (ABA). They in turn phenocopied each other in promoting resistance of pepper (Capsicum annuum) to bacterial wilt not only by activating salicylic acid-dependent CaPR1, but also by activating dehydration tolerance-related CaOSM1 and CaOSR1 and inducing stomatal closure to reduce water loss in an ABA signaling-dependent manner. Our yeast two hybrid assay showed that CaERF3 interacted with CaPti1, which was confirmed using co-immunoprecipitation, bimolecular fluorescence complementation, and pull-down assays. Chromatin immunoprecipitation and electrophoretic mobility shift assays showed that upon R. solanacearum inoculation, CaPR1, CaOSM1, and CaOSR1 were directly targeted and positively regulated by CaERF3 and potentiated by CaPti1. Additionally, our data indicated that the CaPti1-CaERF3 complex might act downstream of ABA signaling, as exogenously applied ABA did not alter regulation of stomatal aperture by the CaPti1-CaERF3 module. Importantly, the CaPti1-CaERF3 module positively affected pepper growth and the response to dehydration stress. Collectively, the results suggested that immunity and dehydration tolerance are coupled and positively regulated by CaPti1-CaERF3 in pepper plants to enhance resistance against R. solanacearum.

The alcohol dehydrogenase gene family in sugarcane and its involvement in cold stress regulation

BACKGROUND

Alcohol dehydrogenases (ADHs) in plants are encoded by a multigene family. ADHs participate in growth, development, and adaptation in many plant species, but the evolution and function of the ADH gene family in sugarcane is still unclear.

RESULTS

In the present study, 151 ADH genes from 17 species including 32 ADH genes in Saccharum spontaneum and 6 ADH genes in modern sugarcane cultivar R570 were identified. Phylogenetic analysis demonstrated two groups of ADH genes and suggested that these genes underwent duplication during angiosperm evolution. Whole-genome duplication (WGD)/segmental and dispersed duplications played critical roles in the expansion of ADH family in S. spontaneum and R570, respectively. ScADH3 was cloned and preferentially expressed in response to cold stress. ScADH3 conferred improved cold tolerance in E. coli cells. Ectopic expression showed that ScADH3 can also enhance cold tolerance in transgenic tobacco. The accumulation of reactive oxygen species (ROS) in leaves of transgenic tobacco was significantly lower than in wild-type tobacco. The transcript levels of ROS-related genes in transgenic tobacco increased significantly. ScADH3 seems to affect cold tolerance by regulating the ROS-related genes to maintain the ROS homeostasis.

CONCLUSIONS

This study depicted the size and composition of the ADH gene family in 17 species, and investigated their evolution pattern. Comparative genomics analysis among the ADH gene families of S. bicolor, R570 and S. spontaneum revealed their close evolutionary relationship. Functional analysis suggested that ScADH3, which maintained the steady state of ROS by regulating ROS-related genes, was related to cold tolerance. These findings will facilitate research on evolutionary and functional aspects of the ADH genes in sugarcane, especially for the understanding of ScADH3 under cold stress.

Overexpression of the peanut CLAVATA1-like leucine-rich repeat receptor-like kinase AhRLK1 confers increased resistance to bacterial wilt in tobacco

Bacterial wilt caused by Ralstonia solanacearum is a devastating disease affecting hundreds of plant species, yet the host factors remain poorly characterized. The leucine-rich repeat receptor-like kinase gene AhRLK1, characterized as CLAVATA1, was found to be up-regulated in peanut upon inoculation with R. solanacearum. The AhRLK1 protein was localized in the plasma membrane and cell wall. qPCR results showed AhRLK1 was induced in a susceptible variety but little changed in a resistant cultivar after inoculated with R. solanacearum. Hormones such as salicylic acid, abscisic acid, methyl jasmonate, and ethephon induced AhRLK1 expression. In contrast, AhRLK1 expression was down-regulated under cold and drought treatments. Transient overexpression of AhRLK1 led to a hypersensitive response (HR) in Nicotiana benthamiana. Furthermore, AhRLK1 overexpression in tobacco significantly increased the resistance to R. solanacearum. Besides, the transcripts of most representative defense responsive genes in HR and hormone signal pathways were significantly increased in the transgenic lines. EDS1 and PAD4 in the R gene signaling pathway were also up-regulated, but NDR1 was down-regulated. Accordingly, AhRLK1 may increase the defense response to R. solanacearum via HR and hormone defense signaling, in particular through the EDS1 pathway of R gene signaling. These results provide a new understanding of the CLAVATA1 function and will contribute to genetic enhancement of peanut.

The CaAP2/ERF064 Regulates Dual Functions in Pepper: Plant Cell Death and Resistance to Phytophthora capsici

Phytophthora blight is one of the most destructive diseases of pepper (Capsicum annuum L.) globally. The APETALA2/Ethylene Responsive Factors (AP2/ERF) genes play a crucial role in plant response to biotic stresses but, to date, have not been studied in the context of Phytophthora resistance in pepper. Here, we documented potential roles for the pepper CaAP2/ERF064 gene in inducing cell death and conferring resistance to Phytophthora capsici (P. capsici) infection. Results revealed that the N-terminal, AP2 domain, and C-terminal of CaAP2/ERF064 protein is responsible for triggering cell death in Nicotiana benthamiana (N. benthamiana). Moreover, the transcription of CaAP2/ERF064 in plant is synergistically regulated by the Methyl-Jasmonate (MeJA) and ethephon (ET) signaling pathway. CaAP2/ERF064 was found to regulate the expression of CaBPR1, which is a pathogenesis-related (PR) gene of pepper. Furthermore, the silencing of CaAP2/ERF064 compromised the pepper plant resistance to P.capsici by reducing the transcript level of defense-related genes CaBPR1, CaPO2, and CaSAR82, while the ectopic expression of CaAP2/ERF064 in N. benthamiana plant elevated the expression level of NbPR1b and enhanced resistance to P.capsici. These results suggest that CaAP2/ERF064 could positively regulate the defense response against P. capsici by modulating the transcription of PR genes in the plant.

Functional and Promoter Analysis of ChiIV3, a Chitinase of Pepper Plant, in Response to Phytophthora capsici Infection

Despite the involvement of many members of the chitinase family in plant immunity, the precise functions of the majority of the members remain poorly understood. Herein, the gene ChiIV3 in Capsicum annuum encoding a chitinase protein containing a chitin binding domain and targeting to the plasma membrane was found to be induced by Phytophthora capsici inoculation (PCI) and applied chitin treatment. Besides its direct inhibitory effect on growth of Phytophthora capsici (P. capsici), ChiIV3 was also found by virus-induced gene silencing (VIGS) and transient overexpression (TOE) in pepper plants to act as a positive regulator of plant cell death and in triggering defense signaling and upregulation of PR (pathogenesis related) genes against PCI. A 5' deletion assay revealed that pChiIV3-712 to -459 bp was found to be sufficient for ChiIV3' response to PCI. Furthermore, a mutation assay indicated that W-box-466 to -461 bp in pChiIV3-712 to -459 bp was noted to be the PCI-responsible element. These results collectively suggest that ChiIV3 acts as a likely antifungal protein and as a receptor for unidentified chitin in planta to trigger cell death and defense signaling against PCI.

Stability of the MON 810 transgene in maize

We analysed the DNA variability of the transgene insert and its flanking regions in maize MON 810 commercial varieties. Southern analysis demonstrates that breeding, since the initial transformation event more than 10 years ago, has not resulted in any rearrangements. A detailed analysis on the DNA variability at the nucleotide level, using DNA mismatch endonuclease assays, showed the lack of polymorphisms in the transgene insert. We conclude that the mutation rate of the transgene is not significantly different from that observed in the maize endogenous genes. Six SNPs were observed in the 5'flanking region, corresponding to a Zeon1 retrotransposon long terminal repeat. All six SNPs are more than 500 bp upstream of the point of insertion of the transgene and do not affect the reliability of the established PCR-based transgene detection and quantification methods. The mutation rate of the flanking region is similar to that expected for a maize repetitive sequence. We detected low levels of cytosine methylation in leaves of different transgenic varieties, with no significant differences on comparing different transgenic varieties, and minor differences in cytosine methylation when comparing leaves at different developmental stages. There was also a reduction in cryIAb mRNA accumulation during leaf development.

Transgenic plants as tools to study the molecular organization of plant genes

Transgenic plants are generated in nature by Agrobacterium tumefaciens, a pathogen that produces disease through the transfer of some of its own DNA into susceptible plants. The genes are carried on a plasmid. Much has been learned about how the plasmid is transferred, how the plasmid-borne genes are organized, regulated, and expressed, and how the bacteria's pathogenic effects are produced. The A. tumefaciens plasmid has been manipulated for use as a general vector for the transfer of specific segments of foreign DNA of interest (from plants and other sources) into plants; the activities of various genes and their regulation by enhancer and silencer sequences have been assessed. Future uses of the vector (or others like it that have different host ranges) by the agriculture industry are expected to aid in moving into vulnerable plants specific genes that will protect them from such killers as nonselective herbicides, insects, and viruses.

Expression of the sweet protein brazzein in maize for production of a new commercial sweetener

The availability of foods low in sugar content yet high in flavour is critically important to millions of individuals conscious of carbohydrate intake for diabetic or dietetic purposes. Brazzein is a sweet protein occurring naturally in a tropical plant that is impractical to produce economically on a large scale, thus limiting its availability for food products. We report here the use of a maize expression system for the production of this naturally sweet protein. High expression of brazzein was obtained, with accumulation of up to 4% total soluble protein in maize seed. Purified corn brazzein possessed a sweetness intensity of up to 1200 times that of sucrose on a per weight basis. In addition, application tests demonstrated that brazzein-containing maize germ flour could be used directly in food applications, providing product sweetness. These results demonstrate that high-intensity sweet protein engineered into food products can give sweetener attributes useful in the food industry.

Expression and inheritance of foreign genes in transgenic peanut plants generated byAgrobacterium-mediated transformation

To evaluate and characterize the stability of traits transferred viaAgrobacterium transformation, foreign gene expression must be examined in sexually derived progeny. The objective of this study was to analyze three transgenic peanut plants, 1-10, 12-1, and 17-1, for the inheritance and expression of their foreign genes. Segregation ratios for the introduced genes in T₂ plants gave either 100% or 3:1 expression of the β-glucuronidase (GUS) gene, demonstrating recovery of both homozygous and heterozygous T₁ plants. Fluorometric GUS assay in T₁ and T₂ generations of all three plants showed that the GUS gene was stably expressed in the progeny. DNA analyses showed 100% concordance between the presence of the foreign gene and enzyme activity. Our results demonstrate that transgenes in peanut introduced byAgrobacterium can be inherited in a Mendelian manner.

Inheritance of gusA and neo genes in transgenic rice

Inheritance of foreign genes neo and gusA in rice (Oryza sativa L. cv. IR54 and Radon) has been investigated in three different primary (T0) transformants and their progeny plants. T0 plants were obtained by co-transforming protoplasts from two different rice suspension cultures with the neomycin phosphotransferase II gene [neo or aph (3') II] and the beta-glucuronidase gene (uidA or gusA) residing on separate chimeric plasmid constructs. The suspension cultures were derived from callus of immature embryos of indica variety IR54 and japonica variety Radon. One transgenic line of Radon (AR2) contained neo driven by the CaMV 35S promoter and gusA driven by the rice actin promoter. A second Radon line (R3) contained neo driven by the CaMV 35S promoter and gusA driven by a promoter of the rice tungro bacilliform virus. The third transgenic line, IR54-1, contained neo driven by the CaMV 35S promoter and gusA driven by the CaMV 35S. Inheritance of the transgenes in progeny of the transgenic rice was investigated by Southern blot analysis and enzyme assays. Southern blot analysis of genomic DNA showed that, regardless of copy numbers of the transgenes in the plant genome and the fact that the two transgenes resided on two different plasmids before transformation, the introduced gusA and neo genes were stably transmitted from one generation to another and co-inherited together in transgenic rice progeny plants derived from self-pollination. Analysis of GUS and NPT II activities in T1 to T2 plants provided evidence that inheritance of the gusA and neo genes was in a Mendelian fashion in one plant line (AR2), and in an irregular fashion in the two other plant lines (R3 and IR54-1). Homozygous progeny plants expressing the gusA and neo genes were obtained in the T2 generation of AR2, but the homozygous state was not found in the other two lines of transgenic rice.

High frequency, heat treatment-induced inactivation of the phosphinothricin resistance gene in transgenic single cell suspension cultures of Medicago sativa

One descendant of the Medicago sativa Ra-3 transformant T304 was analysed with respect to the somatic stability of the synthetic phosphinothricin-N-acetyltransferase (pat) gene which was used as a selective marker and was under the control of the 5'/3' expression signals of the cauliflower mosaic virus (CaMV) gene VI. In order to quantify gene instability, we developed a system for culturing and regenerating individual cells. Single cell suspension cultures derived from T304 and the ancestral non-transgenic M. sativa cultivar Ra-3, were established. The cells were regenerated into monoclonal calli. In transgenic calli, the phosphinothricin (Pt)-resistance phenotype was retained after more than 2 months of non-selective growth. In contrast, up to 12% of the suspension culture cells grown under nonselective conditions and at constant temperature (25 degrees C) lost the herbicide-resistance phenotype within 150 days. Surprisingly, a heat treatment (37 degrees C), lasting for 10 days, during the culture period resulted in an almost complete (95%) loss of the Pt resistance of the suspension culture cells. However, the frequency of cell division was identical in cultures grown under normal and heat treatment conditions. A biochemical test revealed that no phosphinothricin-N-acetyltransferase activity was present in heat treated, Pt-sensitive cells. The resistance level of the Pt-sensitive transgenic cells was equivalent to that of the wild-type cells. A PCR analysis confirmed the presence of the pat gene in heat treated, Pt-sensitive cells. From these results it is concluded that the Pt resistance gene was heat-inactivated at a high frequency in the M. sativa suspension cultures.

Characterization of transgenic sulfonylurea-resistant flax (Linum usitatissimum)

Fourteen transgenic flax (Linum usitatissimum) lines, carrying a mutant Arabidopsis acetolactate synthase (ALS) gene selected for resistance to chlorsulfuron, were characterized for resistance to two sulfonylurea herbicides. Progeny of 10 of the 14 lines segregated in a ratio of 3 resistant to 1 susceptible, indicating a single insertion. Progeny of 1 line segregated in a 15∶1 ratio, indicating two insertions of the ALS gene at independent loci. Progeny from 3 lines did not segregate in a Mendelian fashion and were likely the products of chimeric shoots. Resistance to chlorsulfuron was stably inherited in all lines. At the enzyme level, the transgenic lines were 2.5 to more than 60 times more resistant to chlorsulfuron than the parental lines. The transgenic lines were 25-260 times more resistant to chlorsulfuron than the parental lines in root growth experiments and demonstrated resistance when grown in soil treated with 20 g ha(-1) chlorsulfuron. The lines demonstrated less resistance to metsulfuron methyl; in root growth experiments, the transgenic lines were only 1.6-4.8 times more resistant to metsulfuron methyl than the parental lines. Resistance was demonstrated in the field at half (2.25 g ha(-1)) and full (4.5 g ha(-1)) rates of metsulfuron methyl.

Segregation of transgenes in maize

Progeny recovered from backcrossed transgenic maize tissue culture regenerants (R0) were analyzed to determine the segregation, expression, and stability of the introduced genes. Transgenic A188 x B73 R0 plants (regenerated from embryogenic suspension culture cells transformed by microprojectile bombardment; see [9]) were pollinated with nontransformed B73 pollen. Inheritance of a selectable marker gene, bar, and a nonselectable marker gene, uidA, was analyzed in progeny (R1) representing four independent transformation events. Activity of the bar gene product, phosphinothricin acetyltransferase (PAT), was assessed in plants comprising the four R1 populations. The number of R1 plants containing PAT activity per total number of R1 plants recovered for each population was 2/7, 19/34, 3/14 and 73/73. Molecular analysis confirmed the segregation of bar in three R1 populations and the lack of segregation in one R1 population. Cosegregation analysis indicated genetic linkage of bar and uidA in all four R1 populations. Analysis of numerous R2 plants derived from crossing transformed R1 plants with nontransformed inbreds revealed 1:1 segregation of PAT activity in three of four lines, including the line that failed to segregate in the R1 generation. Integrated copies of bar in one line appeared to be unstable or poorly transmitted.

Reversible inactivation of a transgene in Arabidopsis thaliana

Fifty percent of Arabidopsis thaliana plants transgenic for a hygromycin resistance gene failed to transmit the resistance phenotype to the progeny. The complete transgene was, however, inherited in all cases according to Mendelian laws as observed by Southern analysis. This discrepancy between genotype and phenotype was the result of a reduced level of transcript in the sensitive transformants. The gene inactivation occurred in plants with multicopy integration of the foreign DNA. No definite correlation was found between gene inactivity and methylation of cytidine residues in the transgene sequence. Explants from several sensitive transformed plants regained a low level of hygromycin resistance on callus induction medium. Subsequent generations obtained by self-pollination were sensitive. In contrast, spontaneous restoration of hygromycin tolerance was observed in seedlings originating from out-crosses with wild-type plants or a different sensitive transformant. A reduction of the copy number was not a prerequisite for spontaneous reactivation. The resistance was often lost again in the next generation. Inactivation and reactivation of the transgene are therefore reversible.

Quantitation of chloramphenicol acetyl transferase in transgenic tobacco plants by ELISA and correlation with gene copy number

A monoclonal antibody to chloramphenicol acetyl transferase (CAT) was used in an indirect competitive enzyme immunoassay (ELISA) for the quantitation of CAT in leaf extracts of eighteen transgenic tobacco plants containing the CAT gene fused to the cauliflower mosaic virus 35S promoter. The ELISA could be used to quantify CAT when present in extracts at 20 ng/ml. Enzymatic activity and electrophoretic mobility of CAT in these extracts was not different from CAT from Escherichia coli. Concentrations of CAT in these transgenic plants ranged from 79 to 732 ng CAT/mg protein. The average coefficient of variation among three replicate samples was 15%. All plants were sampled on two separate occasions. The CAT concentrations often varied between the two sampling dates. We determined the CAT gene copy number and the number of independently segregating loci in each plant by Southern blot analysis and progeny testing. We found no significant differences in CAT expression among all ten plants with a single CAT gene. We also found a significant correlation between CAT gene copy number and the level of CAT expressed in each plant, although plants with one gene copy sometimes had more CAT than plants with more than one gene copy. In this population, therefore, gene copy number contributed more to the variation in CAT expression than did position effects.

A Dwarf Mutant of Arabidopsis Generated by T-DNA Insertion Mutagenesis

Most plant genes that control complex traits of tissues, organs, and whole plants are uncharacterized. Plant height, structure of reproductive organs, seed development and germination, for example, are traits of great agronomic importance. However, in the absence of knowledge of the gene products, current molecular approaches to isolate these important genes are limited. Infection of germinatng seeds of Arabidopsis thaliana with Agrobacterium results in transformed lines in which the integrated T-DNA from Agrobacterium and its associated kanamycin-resistance trait cosegregate with stable, phenotypic alterations. A survey of 136 transformed lines produced plants segregating in a manner consistent with Mendelian predictions for phenotypes altered in height, flower structure, trichomes, gametogenesis, embryogenesis, and seedling development. This report is the characterization of a dwarf mutant in which the phenotype is inherited as a single recessive nuclear mutation that cosegregates with both the kanamycin-resistance trait and the T-DNA insert.