Silencing signals in plants: a long journey for small RNAs

Abstract

Recent research shows that short RNA molecules act as mobile signals that direct mRNA cleavage and DNA methylation in recipient cells.

Figure 1

Schematic diagram of silencing pathways in plants. An overview of the (a) microRNA, (b) trans-acting siRNA, (c) viral siRNA and (d) heterochromatic siRNA pathways. sRNAs are processed from partially or perfectly double-stranded RNA (dsRNA) precursor molecules by Dicer-like (DCL) proteins. They are incorporated into Argonaute (AGO) complexes to target nucleic acids. RNA-dependent RNA polymerases (RDRs) convert transcripts generated by DNA-dependent RNA polymerase II (POL II) and IV (POL IV) into dsRNA. The associated RDR, DCL and AGO proteins that function in each pathway and the sizes and functions of the sRNAs produced are indicated. RISC refers to the RNA-induced silencing complex, a multiprotein complex that consists of an AGO protein, an sRNA and other protein factors. The different compositions of RISC complexes are indicated in different colors.

Figure 2

Genetic factors identified in short-distance movement of the silencing signal in the SUC-PDS system [30]. (a) The photobleaching phenotype seen in the leaves of JAP plants. (b) The SUC-PDS transgene (an inverted-repeat construct using the phloem-specific promoter from the SUC2 gene to target the PHYTOENE DESATURASE 3 (PDS) endogenous mRNAs) is expressed in companion cells and produces a silencing signal that is dependent on the POL IV pathway (NRPD1-RDR2-CLSY) as well as DCL4 and presumably DCL1 and AGO1. (c) The silencing signal moves to the mesophyll cells and produces photobleaching (a). AGO4 and DCL3 antagonize the spread of the silencing signal and probably act in the cells that produce the silencing signal. Where the POL IV pathway functions and whether DCL4 and AGO4/DCL3 are required in the cells that receive a silencing signal remains unknown. The circle indicates the nucleus and the red square corresponds to plasmodesmata. Aspects of this process that remain unknown are indicated by question marks.

Figure 3

Mobile silencing RNAs and their activity. Mature miRNAs (black) are probably mobile because they appear in recipient neighboring cells, phloem and distant tissue. Their physiologically active mobile form is not conclusively known, however, and might include a mobile precursor RNA. siRNAs (red) move from cell to cell and over long distances to direct RNA cleavage and DNA methylation. However, many aspects of this process remain unknown as indicated by question marks. There is evidence that miRNAs and siRNAs can move from the shoot to the root; however, very little is known about the reciprocal movement and activity of mobile sRNAs.