Subviral agents associated with plant single-stranded DNA viruses

Begomoviruses (family Geminiviridae) are responsible for many economically important crop diseases worldwide. The majority of these diseases are caused by bipartite begomovirus infections, although a rapidly growing number of diseases of the Old World are associated with monopartite begomoviruses. With the exception of several diseases of tomato, most of these are caused by a monopartite begomovirus in association with a recently discovered essential satellite component (DNA-β). These begomovirus/satellite disease complexes are widespread and diverse and collectively infect a wide variety of crops, weeds and ornamental plants. Non-essential subviral components (DNA-1) originating from nanoviruses are frequently associated with these disease complexes, and there are tantalizing hints that further novel satellites may also be associated with some begomovirus diseases. DNA-β components can be maintained in permissive plants by more than one distinct begomovirus, reflecting less stringent requirements for trans-replication that will undoubtedly encourage diversification and adaptation as a consequence of component exchange and recombination. In view of their impact on agriculture, there is a pressing need to develop a more comprehensive picture of the diversity and distribution of the disease complexes. A greater understanding of how they elicit the host response may provide useful information for their control as well as an insight into plant developmental processes.