Seed coat pigmentation in transgenic soybean expressing the silencing suppressor 2b gene of Cucumber mosaic virus.

KEY MESSAGE: Soybean expressing the Cucumber mosaic virus 2b gene manifests seed coat pigmentation due to suppression of endogenous RNA silencing but no other morphological abnormality. This gene may help prevent transgene silencing. RNA silencing is an important mechanism for gene regulation and antiviral defense in plants. It is also responsible for transgene silencing, however, and thus hinders the establishment of transgenic plants. The 2b protein of Cucumber mosaic virus (CMV) functions as a suppressor of RNA silencing and therefore might prove beneficial for stabilization of transgene expression. We have now generated transgenic soybean that harbors the 2b gene of a CMV-soybean strain under the control of a constitutive promoter to investigate the effects of 2b expression. No growth abnormality was apparent in 2b transgenic plants, although the seed coat was pigmented in several of the transgenic lines. Genes for chalcone synthase (CHS), a key enzyme of the flavonoid pathway, are posttranscriptionally silenced by the inhibitor (I) locus in nonpigmented (yellow) soybean seeds. The levels of CHS mRNA and CHS small interfering RNA in strongly pigmented 2b transgenic seed coats were higher and lower, respectively, than those in the seed coat of a control transgenic line. The expression level of 2b also correlated with the extent of seed coat pigmentation. On the other hand, introduction of the 2b gene together with the DsRed2 gene into somatic embryos prevented the time-dependent decrease in transient DsRed2 expression. Our results indicate that the 2b gene alone is able to suppress RNA silencing of endogenous CHS genes regulated by the I locus, and that 2b is of potential utility for stabilization of transgene expression in soybean without detrimental effects other than seed coat pigmentation.