Plant Resistance to Viruses: Geminiviruses

Geminiviruses are a novel group of plant viruses that contain circular, single-stranded DNA of 2.7-5.2 kbp in size. Their unique 'geminate' or paired particle sets them apart from other viruses. Geminiviruses are an emergent group of plant viruses characterized by the debilitating diseases they cause, and their ready transmission by one of several phloem-feeding insects. The Geminiviridae contains four genera that are distinguished by host plant type and vector. The largest genus contains the whitefly-transmitted begomoviruses, which are distributed worldwide in the Tropics, and as far north as the Mediterranean Sea and the southern USA. Geminiviruses rely on a host DNA polymerase for replication, and shift cells back into replication phase of the cell cycle to enable their own replication. Geminivirus ORFs (6 and 7) encode multifunctional proteins, including the coat protein that is required for symptom development in some virus-host combinations, and for vector-mediated transmission. Geminiviruses encode a protein enabling cell-to-cell movement, and another that suppresses host defenses, while also serving as a transcriptional activator. Some monopartite begomovirus species associate with a single-stranded DNA (ssDNA) satellite molecule (1.5 kbp) that encodes a silencing suppressor, enabling their debilitated 'helper virus' to be pathogenic. Geminiviruses employ recombination and reassortment for diversification and adapt readily to new hosts.