A method to generate siRNAs in vivo is described, as are constructs and compositions useful in the method. The method does not depend on the use of DNA or synthetic constructs that contain inverted duplications or dual promoters so as to form perfect or largely double -stranded RNA. Rather, the method depends on constructs that yield single-stranded RNA transcripts, and exploits endogenous or in vivo-produced miRNAs or siRNAs to initiate production of siRNAs. The miRNAs or siRNAs guide cleavage of the transcript and set the register for production of siRNAs (usually 21 nucleotides in length) encoded adjacent to the initiation cleavage site within the construct. The method results in specific formation of siRNAs of predictable size and register (phase) relative to the initiation cleavage site. The method can be used to produce specific siRNAs in vivo for inactivation or suppression of one or more target genes or other entities, such as pathogens.