Characterisation of a satellite RNA of Cucumber mosaic virus that induces chlorosis in Capsicum annuum

Small non-coding satellite RNAs - the 'game changers' at the virus-host plant interaction?

Satellite RNAs (satRNAs) are RNA molecules associated with many plant viruses and fully dependent on them for replication, encapsidation, and movement within the plant or transmission from plant to plant. Their classification is based on their length, functional protein-coding capacity, and RNA structure (whether linear or circular). They have been of interest for a long time as some of them, in particular systems, cause significant changes in the pathogenesis and epidemiology of plant viruses. The outcomes of how satRNAs affect pathogenesis depend on the components of the pathosystem: host plant species or variety, virus species or even strain, and the sequence of satRNA. These can be additionally affected by biotic and abiotic factors, for example, environmental conditions such as the presence of their vectors or ambient temperature. satRNAs may interfere with primary metabolism, signalling, plant defence [including post-transcriptional gene silencing (PTGS)], as well as the efficiency of virus transmission from plant to plant. In recent years, due to wider access to high-throughput technologies and the extension of studies on satRNAs to include the involvement of external factors in plant-virus-satRNA systems, we are gaining a broader view of the consequences of the presence of these small molecules in viral infections. This review presents the state of the art of satRNA interactions with the helper virus and host plant as well as the influence of satRNAs on the insect vector's behaviour. Moreover, areas requiring further research are identified and knowledge gaps indicated.

Satellite RNAs and Satellite Viruses

Satellite RNAs and satellite viruses are extraviral components that can affect either the pathogenicity, the accumulation, or both of their associated viruses while themselves being dependent on the associated viruses as helper viruses for their infection. Most of these satellite RNAs are noncoding RNAs, and in many cases, have been shown to alter the interaction of their helper viruses with their hosts. In only a few cases have the functions of these satellite RNAs in such interactions been studied in detail. In particular, work on the satellite RNAs of Cucumber mosaic virus and Turnip crinkle virus have provided novel insights into RNAs functioning as noncoding RNAs. These effects are described and potential roles for satellite RNAs in the processes involved in symptom intensification or attenuation are discussed. In most cases, models describing these roles involve some aspect of RNA silencing or its suppression, either directly or indirectly involving the particular satellite RNA.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) is an important virus because of its agricultural impact in the Mediterranean Basin and worldwide, and also as a model for understanding plant-virus interactions. This review focuses on those areas where most progress has been made over the past decade in our understanding of CMV. Clearly, a deep understanding of the role of the recently described CMV 2b gene in suppression of host RNA silencing and viral virulence is the most important discovery. These findings have had an impact well beyond the virus itself, as the 2b gene is an important tool in the studies of eukaryotic gene regulation. Protein 2b was shown to be involved in most of the steps of the virus cycle and to interfere with several basal host defenses. Progress has also been made concerning the mechanisms of virus replication and movement. However, only a few host proteins that interact with viral proteins have been identified, making this an area of research where major efforts are still needed. Another area where major advances have been made is CMV population genetics, where contrasting results were obtained. On the one hand, CMV was shown to be prone to recombination and to show high genetic diversity based on sequence data of different isolates. On the other hand, populations did not exhibit high genetic variability either within plants, or even in a field and the nearby wild plants. The situation was partially clarified with the finding that severe bottlenecks occur during both virus movement within a plant and transmission between plants. Finally, novel studies were undertaken to elucidate mechanisms leading to selection in virus population, according to the host or its environment, opening a new research area in plant-virus coevolution.