Viroids: how to infect a host and cause disease without encoding proteins

Area of Science:

• Plant Pathology
• Molecular Biology
• RNA Biology

Background:

• Viroids are small, non-protein-coding RNA molecules that induce disease symptoms in plants, similar to larger DNA and RNA viruses.
• Satellite RNAs share structural similarities with viroids but require helper viruses for replication and transmission, often altering induced symptoms.
• The precise molecular mechanisms by which these non-coding RNAs cause pathogenesis have remained largely enigmatic.

Purpose of the Study:

• To investigate the role of RNA silencing in viroid and satellite RNA-induced plant pathogenesis.
• To provide mechanistic evidence for the hypothesis that viroid-derived small RNAs (vd-sRNAs) and satellite RNA-derived small RNAs target host genes.

Main Methods:

• Analysis of small RNA populations generated during infection.
• Identification of specific small RNAs associated with disease symptoms.
• Bioinformatic prediction and experimental validation of small RNA targets in host mRNA.

Main Results:

• A 22-nt small RNA from a satellite RNA was identified as the cause of yellow symptoms by targeting the chlorophyll biosynthetic gene CHLI mRNA.
• Two 21-nt vd-sRNAs were found to induce albinism by targeting the mRNA of chloroplastic heat-shock protein 90.
• Both instances involved the host's RNA silencing machinery mediating the cleavage of target host mRNAs.

Conclusions:

• RNA silencing is a key mechanism underlying pathogenesis induced by non-coding RNAs like viroids and satellite RNAs.
• Specific small RNAs derived from these pathogens can directly target and cleave essential host mRNAs, leading to distinct phenotypic alterations.
• This provides compelling evidence for RNA silencing-based pathogenesis, although alternative mechanisms cannot be entirely excluded.