Department of Plant Molecular Biology, University of Delhi South Campus, New Delhi-110021, India.
RNA silencing is an ancient mechanism of gene regulation with important antiviral roles in plants. The induction of this event has been well documented in plant RNA and DNA viruses (1). Double-stranded RNA from replication intermediates is processed into 21–24 nucleotide small-interfering RNAs (siRNAs) by the action of host encoded specialized Dicer-like proteins (DCLs), which are ribonucleases of the RNAse III family. Effector complexes termed RNA-induced silencing complexes (RISCs) are assembled upon the incorporation of one selected RNA strand into a member of the Argonaute (Ago) protein family, which mediate endonucleolytic cleavage of either the viral genomes or transcripts. Geminiviruses are single-stranded DNA viruses and are targets of RNA silencing. The annealing of sense and antisense transcripts originating from a bidirectional promoter results in the formation of dsRNA molecules, which act as substrates for DCL-mediated cleavage (2). Sri Lankan cassava mosaic virus (SLCMV) is a whitefly-transmitted virus of the genus Begomovirus (family Geminiviridae) and is the second virus to be associated with Cassava mosaic disease (CMD) in India (3). It has a bipartite genome organization comprising two circular, ssDNA designated as DNA and DNA B. In the present study we characterize the siRNAs associated with SLCMV-infected cassava plants. The study led to the identification of 22 and 26 nucleotide long virus-derived siRNAs; possibly implicating specialized DCL3 and DCL2 proteins in generating these small RNA species. Attempts are underway to identify the primary targets of RNA silencing within the viral genome.
