Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2016
Publication Date: 7/22/2016
Citation: Hao, G., Stover, E.W., Gupta, G. 2016. Overexpression of a modified plant thionin enhances disease resistance to citrus canker and Huanglongbing (HLB). Frontiers in Plant Science. doi:10.3389/fpls.2016.01078.
Interpretive Summary: We are trying to develop citrus trees resistant to the critical diseases huanglongbing and citrus canker through using biotechnology to insert a modified plant defense gene. Canker development and the growth of the citrus canker bacterium was inhibited in some trees containing this gene. Nine months after grafting huanglongbing-infected shoots onto standard rootstocks and rootstocks with the modified defense gene, we showed that the trees with the defense gene had a much lower (0.5 percent) bacterial level in the rootstock compared to standard trees. Hopefully these modified plants will continue to inhibit bacterium growth under further test and will permit more normal citrus production even in the presence of huanglongbing.
Technical Abstract: Huanglongbing (HLB or citrus greening disease) caused by Candidatus Liberibacter asiaticus (Las) is a great threat to the United States citrus industry. There are no proven strategies to eliminate HLB disease and no cultivar has been identified with strong HLB resistance. Citrus canker is also an economically important disease associated with a bacterial pathogen (Xanthomonas citri). In this study, we characterized endogenous citrus thionins and investigated their expression in different citrus tissues. Since no HLB-resistant citrus cultivars have been identified, we attempted to develop citrus resistant to both HLB and citrus canker through overexpression of a modified plant thionin. To improve effectiveness for disease resistance, we modified and synthesized the sequence encoding a plant thionin and cloned into the binary vector pBinPlus/ARS. The construct was then introduced into Agrobacterium strain EHA105 for citrus transformation. Transgenic Carrizo plants expressing the modified plant thionin were generated by Agrobacterium-mediated transformation. Successful transformation and transgene gene expression was confirmed by molecular analysis. Transgenic Carrizo plants expressing the modified thionin gene were challenged with X. citri 3213 at a range of concentrations, and a significant reduction in canker symptoms and a decrease in bacterial growth were demonstrated compared to nontransgenic plants. Furthermore the transgenic citrus plants were challenged with HLB via graft inoculation. Our results showed significant Las titer reduction in roots of transgenic Carrizo compared with control plants. These data provide a promising approach to engineering citrus disease resistance against HLB and canker.