Transgenic ‘Hamlin’ sweet orange expressing csd1 or d4e1 genes exhibits decreased susceptibility to citrus canker disease

Currently, there are no resistant sweet orange cultivars to the citrus canker ( cc ) [ Xanthomonas citri subsp. citri ( Xcc )], and the production of transgenic plants resistant to cc can be an efficient solution in the search of improved cultivars. Several strategies can be applied in this scenario, including the expression of genes that code for antimicrobial peptides and resistance genes from Xcc -tolerant cultivars. We evaluated the reaction of ‘Hamlin’ sweet orange ( Citrus sinensis L. Osbeck) transformed with a gene that codes for the D4E1 synthetic antimicrobial peptide, and ‘Hamlin’ sweet orange plants transformed with the gene that codes for a copper and zinc superoxide dismutase ( csd1 ), isolated from Poncirus trifoliata , after inoculation with Xcc . Transgenic plants were propagated through grafting in ‘Rangpur’ lime ( Citrus limonia Osbeck), and then inoculated with a Xcc suspension. Hydrogen peroxide production, activity of selected antioxidant enzymes and callose deposition were evaluated in the transgenic plants subjected to Xcc inoculation. The incidence and the severity of citrus canker were significantly reduced in several transgenic events, which also exhibited lower Xcc titers when compared with non-transgenic plants. Transgenic events which exhibited higher superoxide dismutase activities also showed increased accumulation of hydrogen peroxide in foliar tissues after Xcc inoculation. Epifluorescence microscopy analyses revealed a high callose deposition in the sieve tube elements of leaf tissues in some transgenic events. Callose accumulation is an important plant defense response to biotic stresses and, in csd1 transgenic lines, it was associated with lower Xcc populations.