International Laboratory for Tropical Agricultural Biotechnology, Donald Danforth Plant Science Center, 975, N. Warson Road, St. Louis, MO-63132, USA.
Abstracts of the papers presented at the International Conference of Indian Virological Society on “Emerging and Re-emerging viral Diseases of the Tropics and Subtropics” at Indian Agricultural Research Institute, New Delhi, India, December 11–14, 2007.
Plant post-transcriptional gene silencing (PTGS) is a natural defense response against viruses, and as a counter-defense strategy, viruses encode PTGS suppressor proteins. Geminiviruses and their satellites encode at least 4 different PTGS suppressors; AC2, AC4, VP2 and C1. It has been shown that some of these suppressors played a major important role in the emergence of some diseases such as the cassava mosaic disease (CMD) in Africa or the cotton leaf curl disease in Pakistan (CLCuD). In the case of the CMD it is the differential and combinatorial effect of two PTGS suppressors AC2 and AC4 that promotes a synergistic effect between two viruses, and thereby enhance several folds the accumulation of the two viruses that are transmitted more efficiently by the natural vector, Bemisia tabacci. In the case of the CLCuD, it seems that the resistance breaking strain of CLCuV and its betasatellite CLCuMB are recombinants providing more efficient PTGS suppressors that broke the resistance of newly selected cotton genotypes. In Africa, new geminivirus satellites have been found capable of breaking the super resistant TME3 cultivar and it is possible that new PTGS suppressor elements may be involved.
The mode of action of all these suppressors is not fully elucidated but at least in the case of the AC4 PTGS suppressors, evidence has been shown that it is related to the binding of the protein to miRNAs and siRNAs. Symptoms are often associated with virus-encoded pathogenicity factors, many of which are PTGS suppressors. The transgenic expression of ACMV-AC4 proteins (PTGS suppressor) shows down-regulated host miRNAs correlated with an up-regulation of the corresponding mRNAs, and induces developmental abnormalities in Arabidopsis. In vitro and in vivo binding assays revealed the ability of ACMV-AC4 to bind miRNAs, but not to siRNAs. On the contrary similar experiments carried with the SLCMV-AC4 PTGS suppressor, indicated that this protein can bind to both ss and dsRNAs, therefore to miRNAs and siRNAs. In addition GFP fusion experiments showed a very different cell distribution of the two proteins and above all a very different movement capacity within the cells. It is clear that the PTGS suppressors of geminiviruses and their satellites play an important role in the virus infection process and consequently in the epidemiology of these viruses. It is also clear that the relationships between the viruses, their satellites and their host, through gene silencing interactions, was of crucial importance for their evolution as exemplified with the complex and numerous combinations of viruses, alpha- and betasatellites found in wild cotton species in Pakistan. Gene silencing can therefore be used to efficiently control geminiviruses as recently showed in cassava, tomato and cotton hosts.
