Using Plant Model Systems to Characterize Geminivirus Infections that Reduce Crop Productivity

Abstract

Geminiviruses are a group of single-stranded DNA viruses that cause devastating diseases in a wide variety of crops world wide. The family Geminiviridae consists of nine genera, many of which have been shown to be transmitted by insect vectors. In tropical and subtropical areas of the world, geminiviruses affect cassava and tomato, which are important food and cash crops for small holder farmers. To design management strategies, information about geminivirus-host interactions and endogenous host resistance is required. The use of model host systems can facilitate acquisition of key information in a timely and cost-effective manner. In the first aim, 'Florida Lanai', a small fast-growing tomato variety, was broadly characterized to establish its suitability as a model system for studying geminiviruses. Infection by three begomoviruses (Tomato yellow leaf curl virus, TYLCV; Tomato mottle virus, ToMoV; Tomato golden mosaic virus, TGMV) and a curtovirus (Beet curly top virus, BCTV) was examined. Agroinoculation of TYLCV, ToMoV and BCTV, mechanical inoculation of ToMoV or TGMV using a microsprayer, and whitefly transmission of TYLCV or ToMoV resulted in 100% infection efficiency and rapid disease progress reflected by strong disease symptoms and viral DNA accumulation. Infection had measurable impacts on plant height, flowering and fruit number. Florida Lanai’s small size is suitable for limited laboratory space, and its short life cycle allows completion of experiments in a short time. These properties established Florida Lanai as a suitable model host for studying geminiviruses infecting tomato. In the second aim, a virus-induced gene silencing (VIGS) vector derived from the begomovirus, Cabbage leaf curl virus (CaLCuV), was used to assess natural variation in virus–host interactions in 2003 Arabidopsis thaliana accessions. The screen identified the Pla-1 accession as the only ecotype to show strong resistance to CaLCuV. A major resistance QTL was mapped onto Chromosome 1 using Pla-1 x Col-0 crosses. The resistance is recessive, quantitative and broad-based. Identification of this QTL provides a platform for future research identifying and deploying the corresponding resistance gene in crop plants. In the third aim, SEGS-1, a sequence from the cassava genome that enhances geminivirus symptoms, was characterized in wild-type Arabidopsis Sei-0 plants, plants carrying a SEGS-1 transgene, and Nicotiana tabacum suspension cells. SEGS-1 increased symptoms development, viral DNA levels and the number of infected cells in both wild-type and transgenic Arabidopsis co-inoculated with African cassava mosaic virus (ACMV). SEGS-1 also broke host resistance to CaLCuV in co-inoculation experiments of Pla-1 accession, similar to previous results when a resistant cassava cultivar was co-inoculated with a cassava begomovirus and SEGS-1. However, unlike cassava, no SEGS-1 episomes were detected in infected Arabidopsis plants. Nicotiana tabacum cells also accumulated higher levels of ACMV DNA-A when co-inoculated with SEGS-1. Together, these results demonstrated that SEGS-1 can function with a heterologous host and begomovirus to increase disease and break resistance. Moreover, SEGS-1 can function in a genomic context, indicating that SEGS-1 episomes are not required for disease xxii enhancement. This information sheds light on the role of SEGS-1 in virus infection and will be useful while formulating management strategies