Abstract:
Viroids are the smallest plant pathogens, consisting of a single-stranded circular RNA
of less than 500 ribonucleotides in length. Despite their noncoding nature, viroids elicit disease
symptoms in many economically important plant hosts, and are, thus, a class of pathogens of great
interest. How these viroids establish disease within host plants, however, is not yet fully understood.
Recent transcriptomic studies have revealed that viroid infection influences the expression of genes
in several pathways and processes in plants, including defence responses, phytohormone signalling,
cell wall modification, photosynthesis, secondary metabolism, transport, gene expression and protein
modification. There is much debate about whether affected pathways signify a plant response to
viroid infection, or are associated with the appearance of disease symptoms in these interactions. In
this review, we consolidate the findings of viroid–host transcriptome studies to provide an overview
of trends observed in the data. When considered together, changes in the gene expression of different
hosts upon viroid infection reveal commonalities and differences in diverse interactions. Here,
we discuss whether trends in host gene expression can be correlated to plant defence or disease
development during viroid infection, and highlight avenues for future research in this field.
