Tomato Yellow Leaf Curl Viruses (Geminiviridae)

Tomato yellow leaf curl virus (TYLCV) is a member of the genus Begomovirus of the family Geminiviridae. It is transmitted in nature by the whitefly Bemisia tabaci in a circulative manner. It has a circular single-stranded DNA genomic molecule of 2787 nt encapsidated in a 20 × 30 nm geminate particle. The viral strand encodes four proteins while the viral complementary strand encodes two. The TYLCV genome replicates in host cell nuclei according to the rolling circle model. To-date TYLCVs can be found in the entire Middle East, Central and Southeast Asia, North and West Africa, Southeast Europe, the Caribbean Islands, Southeast USA and Australia. Sequence comparisons revealed that the name TYLCV encompasses a complex of closely, as well as distantly, related begomovirus species affecting tomato. In the field, TYLCVs cause important economic loss. Diagnosis includes immunodetection of the coat protein, DNA-DNA hybridization in vitro and in situ, and PCR. The virus is mostly phloem limited. It modifies considerably the transcriptome and the metabolome of the host plant, neutralizing the plant responses and acting with the cell replication machinery to ensure its own proliferation. Breeding programs based on the introgression of resistance from wild tomato species have produced varieties with adequate resistance. Six loci tightly linked to TYLCV resistance have been assigned to the tomato chromosomes and two genes providing resistance have been identified and mapped. Tomato plants with various levels of resistance have been genetically engineered by expressing functional as well as dysfunctional viral genes, by post-transcriptional gene silencing and by CRISPR/Cas strategies.