The concept of U’s triangle, which revealed the importance of polyploidization in plant genome evolution, described natural allopolyploidization events in Brassica using three diploids [B. rapa (A genome), B. nigra (B), and B. oleracea (C)] and derived allotetraploids [B. juncea (AB genome), B. napus (AC), and B. carinata (BC)]. However, comprehensive understanding of Brassica genome evolution has not been fully achieved. Here, we performed low-coverage (2–6×) whole-genome sequencing of 28 accessions of Brassica as well as of Raphanus sativus [R genome] to explore the evolution of six Brassica species based on chloroplast genome and ribosomal DNA variations. Our phylogenomic analyses led to two main conclusions. (1) Intra-species-level chloroplast genome variations are low in the three allotetraploids (2~7 SNPs), but rich and variable in each diploid species (7~193 SNPs). (2) Three allotetraploids maintain two 45SnrDNA types derived from both ancestral species with maternal dominance. Furthermore, this study sheds light on the maternal origin of the AC chloroplast genome. Overall, this study clarifies the genetic relationships of U’s triangle species based on a comprehensive genomics approach and provides important genomic resources for correlative and evolutionary studies.

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Nonstructural protein 3 (NS3) encoded by RNA3 of Rice stripe virus (RSV), known to be a suppressor of gene silencing, was cloned and sequenced. The cloned NS3 gene is composed of 636 nucleotides encoding 211 deduced amino acids, and showed a high degree of similarity with the equivalent genes isolated from Korea, Japan and China. The NS3 gene promoted the enhancement of transient gene expression and suppressed transgene co-silencing. In the transient GFP expression via agroinfiltration, GFP expression was dramatically enhanced in terms of both protein yield and expression period in the presence of NS3. The highest accumulation of GFP protein reached to 6.8% of total soluble proteins, which corresponded to a two-fold increase compared to that obtained in the absence of NS3. In addition, NS3 significantly suppressed the initiation of GFP co-silencing induced by the additive GFP infiltration in GFP-transgenic Nicotiana benthamiana. The NS3 gene was also found to be a stronger suppressor than Cucumber mosaic virus 2b. These observations are believed to be derived from the strong suppressive effect of NS3 on gene silencing, and indicate that NS3 could be used as an effective enhancer for the rapid production of foreign proteins in plants.

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