Viroid and viroid-like elements in plants and plant-associated microbiota: a new layer of biodiversity for plant holobionts.

The functional relevance of plant-associated microorganisms is theoretically framed within the holobiont concept. The role of viruses in plant holobionts is being recognized both for their direct effects when hosted in plants (cryptic plant viruses) and for their indirect effects when infecting microorganisms associated with plants in tripartite interactions (e.g.