The curious case of genome packaging and assembly in RNA viruses infecting plants.

Genome packaging is the crucial step for maturation of plant viruses containing an RNA genome. Viruses exhibit a remarkable degree of packaging specificity, despite the probability of co-packaging cellular RNAs. Three different types of viral genome packaging systems are reported so far. The recently upgraded type I genome packaging system involves nucleation and encapsidation of RNA genomes in an energy-dependent manner, which have been observed in most of the plant RNA viruses with a smaller genome size, while type II and III packaging systems, majorly discovered in bacteriophages and large eukaryotic DNA viruses, involve genome translocation and packaging inside the prohead in an energy-dependent manner, i.e., utilizing ATP. Although ATP is essential for all three packaging systems, each machinery system employs a unique mode of ATP hydrolysis and genome packaging mechanism. Plant RNA viruses are serious threats to agricultural and horticultural crops and account for huge economic losses. Developing control strategies against plant RNA viruses requires a deep understanding of their genome assembly and packaging mechanism. On the basis of our previous studies and meticulously planned experiments, we have revealed their molecular mechanisms and proposed a hypothetical model for the type I packaging system with an emphasis on smaller plant RNA viruses. Here, in this review, we apprise researchers the technical breakthroughs that have facilitated the dissection of genome packaging and virion assembly processes in plant RNA viruses.