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1、RNA Silencing in PlantsDefense and CounterdefenseVicki Vance1* and Herve Vaucheret2RNA silencing is a remarkable type of gene regulation based onsequence-specific targeting and degradation of RNA. The term encom- passes related pathways found in a broad range of eukaryotic organ- isms, including fun
2、gi, plants, and animals. In plants, it serves as an antiviral defense, and many plant viruses encode suppressors of silenc- ing. The emerging view is that RNA silencing is part of a sophisticated network of interconnected pathways for cellular defense, RNA surveil- lance, and development and that it
3、 may become a powerful tool to manipulate gene expression experimentally.RNA silencing was first discovered in trans- genic plants, where it was termed cosuppres- sion or posttranscriptional gene silencing (PTGS). Sequence-specific RNA degradation processes related to PTGS have also been found in ci
4、liates, fungi (quelling), and a va- riety of animals from Caenorhabditis elegans to mice (RNA interference RNAs (siRNAs) for recent reviews on RNA silencing, see (13). A key feature uniting the RNA silenc- ing pathways in different organisms is the importanceofdouble-strandedRNA (dsRNA) as a trigger
5、 or an intermediate. The dsRNA is cleaved into small interfering RNAs (siRNAs) (21 to 25 nucleotides) of both polarities, and these are thought to act as guides to direct the RNA degradation ma- chinery to the target RNAs (4, 5). An intrigu- ing aspect of RNA silencing in plants is that it can be tr
6、iggered locally and then spread via a mobile silencing signal (6, 7). The signal- ing molecule is currently unknown but is expected to contain a nucleic acid component to account for the sequence-specificity. Sys- temic spread of silencing also occurs in other organisms, though the mechanism may not
7、 be the same as in plants. Finally, in plants, RNA silencing is correlated with methylation of homologous transgene DNA in the nucleus (8, 9). Other types of epigenetic modifica- tions may be associated with silencing in other organisms. Three major avenues of re- search have contributed to a recent
8、 burst of information about these different aspects of RNA silencing. (i) Mutant analyses have identified a number of genes that are required for RNA silencing in multiple organisms. (ii) Plant viral suppressors of silencing have pro- vided a tool to identify steps in the pathway and an alternate ap
9、proach to find cellularproteins that are involved in the process. (iii) The development of an in vitro RNA silenc- ing system from Drosophila has allowed a biochemical analysis of some steps in the pathway.RNA Silencing as a Defense Against Viruses Several lines of research indicate that RNA silenci
10、ng is a general antiviral defense mech- anism in plants. The first indication came from studies of pathogen-derived resistance (PDR) in plants. In PDR, resistance to a particular virus is engineered by stably trans- forming plants with a transgene derived from the virus. Eventually, it became clear
11、that one class of PDR was the result of RNA silencing of the viral transgene. Once RNA silencing of the transgene had been established, all RNAs with homology to the transgene were degrad- ed, including those derived from an infecting virus (10). Thus, plant viruses could be the target of RNA silenc
12、ing induced by a trans- gene. The same work demonstrated that plant viruses could also induce RNA silencing. Virus-induced gene silencing (VIGS) can be targeted to either transgenes or endogenous genes (11) and the technique has been used to screen for gene function using libraries of endogenous seq
13、uences cloned into a viral vector. The idea that RNA silencing is an antivi- ral defense pathway is strengthened by obser- vations of natural plant-virus interactions. First, plants recover from certain plant viral infections, and the recovered plants are resis- tant to reinfection by the initial vi
14、rus (and to closely related viruses) because of an RNA silencing mechanism (12, 13). Second, many plant viruses encode proteins that suppress RNA silencing (14), suggesting a coevolu- tion of defense and counterdefense between the host and the invading viruses. These viral suppressors target differe
15、nt steps in the si- lencing pathway and have provided a new approach to understand the mechanism of RNA silencing in plants. It remains to be seenif viruses of fungi or animals use a similar counterdefensive strategy against RNA si- lencing, or indeed, if RNA silencing serves as a defense against vi
16、ruses in organisms other than plants. In plants, RNA silencing can be induced locally and then spread throughout the organ- ism (6, 7), and this aspect of the process likely reflects its role in viral defense. Plant viruses generally enter a cell at a small wound, replicate within that cell, and then move cell-to-cell until they reach the vascular tissue, which serves as a conduit to all parts of the plant body. The movement of the mobile silencing signal in the plant parallels that of the
