Transgenic Diffusible Signaling Factor-Producing Rootstocks Confer Resistance to Xanthomonas citri subsp. citri in...

The rpfF gene from Xylella fastidiosa subsp. pauca, encoding the synthase for diffusible signal factor (DSF), was previously shown to reduce Xylella fastidiosa and Xanthomonas citri subsp. citri colonization and disease symptoms when expressed in transgenic Citrus sinensis plants. As DSF is a small, diffusible molecule, this study investigated whether its production in transgenic rootstocks expressing rpfF could confer resistance to Xanthomonas citri subsp. citri in nontransgenic scions. Using antibodies specific to RpfF, we confirmed the presence of this enzyme in various cellular compartments of the transgenic rootstock, validating rpfF translation in planta. Both bacterial populations and citrus canker symptoms were significantly reduced in 'Valencia' sweet orange scions grafted onto the transgenic rootstocks compared with that in nontransgenic control scions. Additionally, X. citri subsp. citri genes known to be differentially regulated in rpfF transgenic plants were also altered in the nontransgenic scions grafted onto the transgenic rootstocks. Further analysis of disease progression revealed that the area under the disease progress curve was significantly lower in the leaves of nontransgenic scions grafted onto transgenic rootstocks compared with those grafted onto nontransgenic controls. These results together suggest that DSF produced in the transgenic rootstock is translocated to the nontransgenic scion, resulting in enhanced tolerance. These findings highlight the potential of transgenic rootstocks to make more resistant nontransgenic scions.