A tobacco calmodulin-related protein suppresses sense transgene-induced RNA silencing but not inverted repeat-induced RNA silencing

A tobacco calmodulin-related protein, rgs-CaM, interacts with viral suppressors of RNA silencing and modulates host RNA silencing. Plants overexpressing the rgs-CaM gene were crossed with plants exhibiting sense transgene-induced RNA silencing (S-PTGS) or inverted repeat-induced RNA silencing (IR-PTGS). S44 plants harboring a sense transgene encoding a tobacco microsomal omega-3 fatty acide desaturase (NtFAD3) exhibited the S-PTGS phenotype. The frequency of the S-PTGS phenotype incidence was nearly 100 % in the hemizygous S44 plants, but was reduced to 30 % in crossbred plants with an rgs-CaM-overexpressing transgenic line. The remaining 70 % of crossbred plants successfully overexpressed the NtFAD3 transgene, and the amount of NtFAD3 small interfering RNAs (siRNAs) was largely decreased. In contrast, overexpression of rgs-CaM did not suppress siRNA production in the IR-PTGS that targeted the NtFAD3 gene. These results indicated that rgs-CaM suppresses RNA silencing at a step upstream of siRNA production and does not interfere with the later steps of RNA silencing, including siRNA-mediated RNA degradation.