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Increase of sweet orange resistance against Xanthomonas citri pv citri through translocation of DSF molecules from transgenic rootstocks

Raquel Caserta: Centro de Citricultura Sylvio Moreira

<div>Rootstocks are widely used to improve citrus scions characteristics. We have shown that transgenic sweet orange scions producing <em>Xf</em>DSF, a <em>diffusible signal factor</em> molecule used by <em>Xylella fastidiosa (Xf)</em> for Quorum Sensing (QS), were more resistant for both citrus variegated chlorosis and citrus canker diseases. Citrus canker is caused by <em>Xanthomonas citri</em> subsp. <em>citri</em> (<em>Xcc</em>), and the resistance of these transgenic plants was attributed to the antagonist effect of the <em>Xf</em>DSF-producing plants. As the production of <em>Xf</em>DSF increased the resistance in sweet orange scions, we hypothesized that transgenic rootstocks could increase the resistance of non-transgenic grafted scions by the translocation of DSF molecules. Our goal was to obtain transgenic plants of the rootstock Carrizo producing DSF and to evaluate the resistance of grafted Valencia and Hamlin sweet orange varieties against <em>Xcc</em>. The challenge of plants grafted onto transgenic rootstocks demonstrated reduced symptoms of canker lesions. Expression of key virulence-related genes of <em>Xcc</em>, known to be positively regulated by the <em>Xcc </em>DSF, such as bacterial movement, cellulose degradation and type III secretion system were downregulated in <em>Xcc</em> isolated from transgenic plants. This strategy is innovative for citrus once a molecule produced by a transgenic rootstock increased the resistance of non-transgenic scions through the alteration of bacterial behavior.</div>