1.11.16
VIRULENCE EVOLUTION OF SATELLITE RNA OF CUCUMBER MOSAIC VIRUS DURING AN EPIDEMIC IN TOMATO

F ESCRIU, A FRAILE, MA ARANDA and F GARCIA-ARENAL

Dpto Biotecnología, ETS Ingenieros Agrónomos, Universidad Politécnica de Madrid, Spain

Background and objectives
Some isolates of cucumber mosaic virus (CMV) encapsidate a satellite RNA (satRNA) that depends on CMV (helper virus) for its replication, accumulation in plants, encapsidation and transmission. SatRNA is able to aggravate or attenuate symptoms induced by CMV and/or depress its accumulation in the host plant. Symptom and accumulation modulation depend on the strains of CMV and of satRNA, and on the host plant species. Attenuating CMV-satRNA strains have been proposed as biocontrol agents for CMV-induced diseases. From 1986 to 1991 an epidemic of a necrotic syndrome in tomato crops occurred in eastern Spain [1]. Necrosis was found to be caused by CMV plus necrogenic satRNA variants. From 1989 onwards, the incidence of this syndrome decreased and it disappeared after 1992, when no necrogenic CMV-satRNA variants were found in this and other areas of Spain. The variability and genetic structure of the CMV-satRNA population in tomato have been described [2]. In order to evaluate the possible use of attenuating CMV-satRNA variants as biocontrol agents in field conditions, it is important to understand their population dynamics and evolution. The objective of this work is to discover what factors could explain the observed virulence evolution of CMV-satRNA in the Spanish population.

Results and conclusions
More than 80 CMV-plus-satRNA isolates were obtained from tomato plants sampled in eastern Spain. CMV particles were purified from field-infected plants, encapsidated RNAs were separated by polyacrylamide gel electrophoresis, and satRNA bands eluted from the gels. cDNA from these satRNA variants was cloned, and infectious transcripts from these clones were used to characterize the satRNA variants. When assayed with different isolates of CMV, the symptoms induced in tomato depended not on the isolate of CMV, but solely on the satRNA isolate. In mixed infections, necrosis always developed if a necrotic satRNA was present. Thus the observed decrease of necrosis incidence in the field must be due to the substitution of necrogenic for non-necrogenic variants within the satRNA population. To understand this evolution on the genetic structure of the population, the following fitness components were analysed for necrogenic and non-necrogenic satRNA variants: infectivity, accumulation in host plant, ability to compete in mixed infections, depression effect on the helper virus, and efficiency of encapsidation. All these experiments have involved several randomly chosen necrogenic and non-necrogenic satRNA variants.

The data obtained contribute to understanding of why, and under what conditions, attenuating satRNAs will be selected for in natural populations. This is relevant for the use of these variants in the control of CMV-induced diseases, either in cross-protection programmes or in expression of CMV-satRNA in transgenic plants.

References
1. Jordá C, Alfaro A, Aranda MA et al., 1992. Plant Disease 76, 363-366.
2. Aranda MA, Fraile A, García-Arenal F, 1993. Journal of Virology 67, 5896-5901.