Comparative expression analysis of five caulimovirus promoters in citrus

Abstract

Four caulimovirus-derived constitutive promoters were evaluated for gene expression in citrus and their expression levels were compared with a 35S promoter. Chimeric promoters made with duplicated enhancer sequences from the cauliflower mosaic virus (D35S), the cassava vein mosaic virus (DCsVMV), the figwort mosaic virus (DFMV), the mirabilis mosaic virus (DMMV) and the peanut chlorotic streak virus (DPCLSV) were inserted into a transformation vector fused to the gus reporter gene. Gene expression patterns driven by these promoters were analyzed in the transgenic citrus cultivar Carrizo citrange (Citrus sinensis Osb. × Poncirus trifoliata L. Raf.). The histochemical and fluorometric measurement of GUS activity and the gene expression quantification by RT-qPCR analysis demonstrate that the DMMV promoter is able to direct gene expression in citrus as strongly as the D35S promoter and represents great application potential in citrus biotechnology. The DFMV, DCsVMV and DPCLSV constitutive promoters were weaker compared to the D35S promoter but can be considered for use in gene stacking strategies for the development of transgenic citrus. Our results also reveal the importance of the evaluation of specific promoter fragments for a particular crop cultivar due to the availability of species-specific transcription factors that can define the strength and tissue specificity of a determinate promoter.