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Divergent evolution of multiple virus-resistance genes from a progenitor in Capsicum spp.
Plants have various defense mechanisms in their natural state in order to protect themselves from pathogens. Hypersensitive cell death is one of the strongest defense mechanisms of plants. Hypersensitive cell death prevents dissemination of pathogens by inducing local cell death at the invasion site after resistance proteins inside plants detect the proteins secreted by pathogens. The research team led by professor Do-il Choi uses chili pepper as a material for their experiment. More than 30 million tons of chili pepper are produced worldwide each year, and it has the largest cultivation area and highest production quantity among condiment vegetables in Korea. The capsaicin contained in chili pepper is responsible for the spicy flavor, and it secretes endorphins, thereby providing the effect of stress relief. In addition, it is known to have a vitamin C content that is 10 times higher than that of tomatoes. Thus, it is one of the crucial horticultural products with increasing consumption in Korea as well as in other countries.
Professor Choi’s research team conducted a study on chili pepper genes that show resistance against viruses. They isolated genes expressing resistance proteins for Pepper mottle virus (PepMoV) and Tomato spotted wilt virus (TSWV) from different species of chili pepper: Capsicum annuum and Capsicum chinense.
Based on the fact that these two plant viruses cause disease in wild tobacco plants, the team generated transformed tobacco overexpressing the resistance gene (Pvr4) of the isolated PepMoV (a gene of interest is inserted in the genome of tobacco plants and regulated to overexpress the gene). The transformed tobacco showed strong resistance to PepMoV, which belongs to the Potyvirus genus. In addition, overexpression of Pvr4 showed extensive resistance not only against PepMoV but also against Pepper severe mosaic virus (PepSMV) and Potato virus Y (PVY), which bring economic loss in regard to potatoes.
In a similar manner, the team found that transformed tobacco plants overexpressing Tsw showed resistance against TSWV. Pvr4, a resistance gene against PepMoV, and Tsw, a resistance gene against TSWV, are in alleles with identical loci on the same chromosome in different species, and the sequences of these two genes were similar in most parts. Amino acid sequence analysis revealed that such similarity came from the fact that these two proteins originated from the same ancestor after the differentiation of species of chili pepper.
These study results suggest that Pvr4 and Tsw genes will have important effects on the breeding of resistance cultivars in not only chili peppers but also solanaceous plants, including potatoes and tomatoes. In other words, developing a plant overexpressing Pvr4 among crops without the reported resistance to Potyvirus will yield virus-resistant crops.
These results were published in the New Phytologist journal in 2017. Professor Choi’s research team said that studies developing a sequence analysis program are needed in the future that allow easy discovery of critical genes and positional markers involved in the target traits of species with specific traits. We expect to see the development of new breeds with resistance to various viruses in the future.
(a) The expression level of Prv4, a resistance gene against PepMoV, was shown by its mRNA expression. (b) Transformed tobacco plants #20, #27, and #50 overexpressing Prv4 were infected with PepMoV-GFP, and the results show no detection of GFP fluorescence, which indicates resistance. The principle is that GFP fluorescence protein is fused into the PepMoV genome and the GFP fluorescence signal is detected when the plant is infected with the virus. (C) It shows the inhibition of viral replication in the upper leaf after the inoculation of the virus. (d) The expression level of Tsw was observed through its RNA expression. (e) Transformed tobacco plants #A, #E, and #F overexpressing Tsw were infected with TSWV, and hypersensitive cell necrosis, one of the resistance responses, was observed. (f) It shows the inhibition of viral replication in the upper leaf after the inoculation of the virus.
Student Reporter Seong, Ga-yeong / Park, Yu-bin
