Rapid transient protein production by the coat protein-deficient cucumber mosaic virus vector: non-packaged CMV system, NoPaCS

Trans-complementation of the viral movement protein mediates efficient expression of large target genes via a tobacco mosaic virus vector

The development of plant virus-based expression systems has expanded rapidly owing to their potential applications in gene functional and disease resistance research, and industrial production of pharmaceutical proteins. However, the low yield of certain proteins, especially high-molecular-mass proteins, restricts the production scale. In this study, we observed that the tobacco mosaic virus (TMV)-mediated expression of a foreign protein was correlated with the amount of the movement protein (MP) and developed a TMV-derived pAT-transMP vector system incorporating trans-complementation expression of MP. The system is capable of efficient expression of exogenous proteins, in particular those with a high molecular mass, and enables simultaneous expression of two target molecules. Furthermore, viral expression of competent CRISPR-Cas9 protein and construction of CRISPR-Cas9-mediated gene-editing system in a single pAT-transMP construct was achieved. The results demonstrated a novel role for TMV-MP in enhancing the accumulation of a foreign protein produced from the viral vector or a binary expression system. Further investigation of the mechanism underlying this role will be beneficial for optimization of plant viral vectors with broad applications.

Plant-made pharmaceuticals

Plant-made pharmaceuticals (PMP) have great potential in terms of production costs, scalability, safety, environmental protection, and consumer acceptability. The first PMP were antibodies and antigens produced in stably transformed transgenic plants in the around 90s. Even though the effort using stable transgenic plants is still going on, the mainstream of PMP production has shifted to transient expression in Nicotiana benthamiana. This system involves the expression vectors by Agrobacterium, and its efficiency has been improved by the development of new vector systems and host engineering. The COVID-19 outbreak accelerated this trend through efforts to produce vaccines in plants. Transient expression systems have been improved and diversified by the development of plant virus vectors, which can be classified as full and deconstructed vectors. Full virus vectors spread systemically, allowing for protein production in the entire plant. Compared with conventional agroinfiltration vectors, excellent virus vectors result in higher protein production. Engineering of host plants has included knocking out gene-silencing systems to increase protein production, and the introduction of glycan modification enzymes so that plant-made proteins more resemble animal-made proteins. Hydroponic cultivation systems in plant factories and environmental controls have contributed to efficient protein production in plants. Considering their advantages and small environmental impact, PMP should be more widely adopted for pharmaceuticals' production. However, the initial investment and running costs of plant factories are higher than open filed cultivation. The next objectives are to develop next-generation low-cost plant factories that use renewable energy and recycle materials based on the idea of circular economy.

Plant Viral Coat Proteins as Biochemical Targets for Antiviral Compounds

Coat proteins (CPs) of RNA plant viruses play a pivotal role in virus particle assembly, vector transmission, host identification, RNA replication, and intracellular and intercellular movement. Numerous compounds targeting CPs have been designed, synthesized, and screened for their antiviral activities. This review is intended to fill a knowledge gap where a comprehensive summary is needed for antiviral agent discovery based on plant viral CPs. In this review, major achievements are summarized with emphasis on plant viral CPs as biochemical targets and action mechanisms of antiviral agents. This review hopefully provides new insights and references for the further development of new safe and effective antiviral pesticides.

Virus-Mediated Targeted DNA Methylation Illuminates the Dynamics of Methylation in an Endogenous Plant Gene

DNA methylation maintains genome stability and regulates gene expression in plants. RNA-directed DNA methylation (RdDM) is critical for appropriate methylation. However, no efficient tools are available for the investigation of the functions of specific DNA methylation. In this study, the cucumber mosaic virus vector was used for targeted DNA methylation. Methylation was rapidly induced but gradually decreased from the 3' end of the target endogenous sequence in Nicotiana benthamiana, suggesting a mechanism to protect against the ectopic introduction of DNA methylation. Increasing 24-nt siRNAs blocked this reduction in methylation by down-regulating DCL2 and DCL4. RdDM relies on the sequence identity between RNA and genomic DNA; however, this identity does not appear to be the sole determinant for efficient DNA methylation. The current findings provide new insight into the regulation of DNA methylation and promote additional effort to develop efficient targeted DNA methylation in plants.

Role of salicylic acid glucosyltransferase in balancing growth and defence for optimum plant fitness

Salicylic acid (SA), an essential secondary messenger for plant defence responses, plays a role in maintaining a balance (trade-off) between plant growth and resistance induction, but the detailed mechanism has not been explored. Because the SA mimic benzothiadiazole (BTH) is a more stable inducer of plant defence than SA after exogenous application, we analysed expression profiles of defence genes after BTH treatment to better understand SA-mediated immune induction. Transcript levels of the salicylic acid glucosyltransferase (SAGT) gene were significantly lower in BTH-treated Nicotiana tabacum (Nt) plants than in SA-treated Nt control plants, suggesting that SAGT may play an important role in SA-related host defence responses. Treatment with BTH followed by SA suppressed SAGT transcription, indicating that the inhibitory effect of BTH is not reversible. In addition, in BTH-treated Nt and Nicotiana benthamiana (Nb) plants, an early high accumulation of SA and SA 2-O-β-d-glucoside was only transient compared to the control. This observation agreed well with the finding that SAGT-overexpressing (OE) Nb lines contained less SA and jasmonic acid (JA) than in the Nb plants. When inoculated with a virus, the OE Nb plants showed more severe symptoms and accumulated higher levels of virus, while resistance increased in SAGT-silenced (IR) Nb plants. In addition, the IR plants restricted bacterial spread to the inoculated leaves. After the BTH treatment, OE Nb plants were slightly larger than the Nb plants. These results together indicate that SAGT has a pivotal role in the balance between plant growth and SA/JA-mediated defence for optimum plant fitness.