Transgenic Arabidopsis plants expressing Agrobacterium tumefaciens VirD2 protein are less susceptible to Agrobacterium transformation

Genomic consequences associated with Agrobacterium-mediated transformation of plants

Attenuated strains of the naturally occurring plant pathogen Agrobacterium tumefaciens can transfer virtually any DNA sequence of interest to model plants and crops. This has made Agrobacterium-mediated transformation (AMT) one of the most commonly used tools in agricultural biotechnology. Understanding AMT, and its functional consequences, is of fundamental importance given that it sits at the intersection of many fundamental fields of study, including plant-microbe interactions, DNA repair/genome stability, and epigenetic regulation of gene expression. Despite extensive research and use of AMT over the last 40 years, the extent of genomic disruption associated with integrating exogenous DNA into plant genomes using this method remains underappreciated. However, new technologies like long-read sequencing make this disruption more apparent, complementing previous findings from multiple research groups that have tackled this question in the past. In this review, we cover progress on the molecular mechanisms involved in Agrobacterium-mediated DNA integration into plant genomes. We also discuss localized mutations at the site of insertion and describe the structure of these DNA insertions, which can range from single copy insertions to large concatemers, consisting of complex DNA originating from different sources. Finally, we discuss the prevalence of large-scale genomic rearrangements associated with the integration of DNA during AMT with examples. Understanding the intended and unintended effects of AMT on genome stability is critical to all plant researchers who use this methodology to generate new genetic variants.

Resistance analysis of cherry rootstock 'CDR-1' (Prunus mahaleb) to crown gall disease

BACKGROUND

Crown gall disease, caused by the pathogenic bacterium Agrobacterium tumefaciens, is responsible for extensive economic losses in orchards. Cherry rootstock 'CDR-1' (Prunus mahaleb) shows high resistance but the mechanism remains unclear. Here, we examined the morphology of pathogen-infected root neck surface, determined the activity of 10 defense-related enzymes and the content of salicylic acid (SA) and jasmonic acid (JA), and also applied transcriptome analysis, transient expression and transgenic verification to explore the crown gall resistance genes in 'CDR-1' plants.

RESULTS

In our study, peroxidase increased in the first 10 days, while phenylalanine ammonialyase and lipoxygenase increased in the first 15 days post-infection. Four key enzymes in the AsA-GSH cycle also responded, to a certain extent; although JA content increased significantly after the treatment, the SA content did not. In a follow-up transcriptome analysis, the differentially expressed genes Pm4CL2, PmCYP450, PmHCT1, PmHCT2, and PmCAD were up-regulated. Based on the above results, we focused on the lignin biosynthetic pathway, and further measured lignin content, and found it increased significantly. The Pm4CL2 gene was used to conduct transient expression and transgenic experiments to verify its function in crown gall disease resistance. It showed the relative expression of the treatment group was almost 14-fold that of the control group at 12 h post-treatment. After the infection treatment, clear signs of resistance were found in the transgenic lines; this indicated that under the higher expression level and earlier activation of Pm4CL2, plant resistance was enhanced.

CONCLUSIONS

The crown gall resistance of 'CDR-1' is likely related to the lignin biosynthetic pathway, in which Pm4CL2 functions crucially during the plant defense response to the pathogen A. tumefaciens. The results thus offer novel insights into the defense responses and resistance mechanism of cherry rootstock 'CDR-1' against crown gall disease.

Silencing of Agrobacterium tumefaciens oncogenes ipt and iaaM induces resistance to crown gall disease in plum but not in apricot

BACKGROUND

In this study, two vectors with short-length chimeric transgenes were used to produce Prunus rootstocks resistant to crown gall disease through RNA-interference-mediated gene silencing of the Agrobacterium tumefaciens oncogenes ipt and iaaM.

RESULTS

Transgenic plum and apricot lines were produced with efficiencies of up to 7.7 and 1.1% respectively. An in vitro evaluation method allowed identification of susceptible lines and reduction in the number of lines to be evaluated in the greenhouse. Five transgenic plum lines, expressing transgene-derived small interfering RNA (siRNA) and low levels of transgene hairpin RNA (hpRNA), showed a significant reduction in the development of the disease after infection with Agrobacterium strains C58 and A281 under greenhouse conditions. However, unexpectedly, all transgenic apricot lines were gall susceptible. The infection of apricot plants with a binary vector containing only the 6b oncogene demonstrated that the expression of this gene is involved in the induction of tumours in the apricot species.

CONCLUSION

RNAi-mediated gene silencing can be used for inducing crown gall resistance in plum rootstocks. These could be used to graft non-genetically modified commercial fruit cultivars reducing, or eliminating, the disease symptoms. © 2017 Society of Chemical Industry.

Novel plant transformation vectors containing the superpromoter

We developed novel plasmids and T-DNA binary vectors that incorporate a modified and more useful form of the superpromoter. The superpromoter consists of a trimer of the octopine synthase transcriptional activating element affixed to the mannopine synthase2' (mas2') transcriptional activating element plus minimal promoter. We tested a superpromoter-beta-glucuronidaseA fusion gene in stably transformed tobacco (Nicotiana tabacum) and maize (Zea mays) plants and in transiently transformed maize Black Mexican Sweet protoplasts. In both tobacco and maize, superpromoter activity was much greater in roots than in leaves. In tobacco, superpromoter activity was greater in mature leaves than in young leaves, whereas in maize activity differed little among the tested aerial portions of the plant. When compared with other commonly used promoters (cauliflower mosaic virus 35S, mas2', and maize ubiquitin), superpromoter activity was approximately equivalent to those of the other promoters in both maize Black Mexican Sweet suspension cells and in stably transformed maize plants. The addition of a maize ubiquitin intron downstream of the superpromoter did not enhance activity in stably transformed maize.

Caspase-resistant VirD2 protein provides enhanced gene delivery and expression in plants

Agrobacterium tumefaciens VirD2 protein is one of the key elements of Agrobacterium-mediated plant transformation, a process of transfer of T-DNA sequence from the Agrobacterium tumour inducing plasmid into the nucleus of infected plant cells and its integration into the host genome. The VirD2 protein has been shown to be a substrate for a plant caspase-like protease activity (PCLP) in tobacco. We demonstrate here that mutagenesis of the VirD2 protein to prevent cleavage by PCLP increases the efficiency of reporter gene transfer and expression. These results indicate that PCLP cleavage of the Agrobacterium VirD2 protein acts to limit the effectiveness of T-DNA transfer and is a novel resistance mechanism that plants utilise to combat Agrobacterium infection.