Reduction in susceptibility to Xanthomonas axonopodis pv. citri in transgenic Citrus sinensis expressing the rice Xa21 gene

This work reports the evaluation of citrus canker resistance in transgenic lines of four sweet orange cultivars Hamlin, Natal, Pera and Valencia expressing the rice Xa21 R-gene. The Agrobacterium-mediated genetic transformation was performed and transgene integration was confirmed by Southern blot analyses. These analyses showed that the plants had undergone 1-3 independent transgene integration events. The genetic transformation efficiency was 18 center dot 6% for Hamlin, 8 center dot 3% for Natal, 3% for Pera and 11 center dot 3% for Valencia sweet orange. Transgene transcription was confirmed by northern blot analyses in most transgenic lines analysed. Transgenic lines of each of the cultivars were selected and propagated for resistance evaluation to Xanthomonas axonopodis pv. citri. Transgenic lines with a higher resistance to citrus canker were identified for cvs Hamlin, Natal and Pera. The lowest diseased leaf area values were detected for transgenic lines of Natal and Pera, cultivars which are considered moderately resistant to citrus canker in the field, indicating an influence of cultivar genetic background on the effectiveness of the Xa21 gene in controlling the pathogen. Transgenic lines in which transgene transcription was not detected did not show any improvement in resistance to the pathogen. These results show the integration and expression of the rice Xa21 gene in citrus and the possibility of using R-genes in controlling diseases in a heterologous genetic background.