Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2016
Publication Date: 7/6/2016
Publication URL: https://handle.nal.usda.gov/10113/5425116
Citation: Pitino, M., Armstrong, C.M., Cano, L.M., Duan, Y. 2016. Transient expression of Candidatus Liberibacter asiaticus effector induces cell death in Nicotiana benthamiana. Frontiers in Plant Science. 7:982. doi: 10.3389/fpls.2016.00982.
Interpretive Summary: Candidatus Liberibacter asiaticus (Las), a bacterial plant pathogen, is the most prevalent species of Liberibacter associated with the devastating disease of citrus known as huanglongbing. Host-pathogen relationships encompass a myriad of protein-protein interactions including those that help the pathogen colonize and subsequently multiply within the plant. Effectors are proteins expressed by the pathogen that are known to aid in the infection process and understanding their roles often leads to more successful methods of pathogen control. However, the effector proteins of Las remain largely unknown. Because of this, a novel approach utilizing the Las secretome and a transient expression system in Nicotiana benthamiana was used to identify and assess the function of candidate effector proteins. In total, we revealed the diverse subcellular locations of the 16 candidate Las effectors. Moreover, one of the candidate effectors localized to the chloroplasts, Las5315, not only induced cell death in N. benthamiana at 3 days post inoculation but also suppressed pathogen-associated molecular pattern triggered immunity induced by the flagellin from Xanthomonas citri ssp. citri. This study helps understand how Las bacteria cause citrus HLB through their effectors, and therefore, we can control HLB by blocking the function of these effectors.
Technical Abstract: Candidatus Liberibacter asiaticus (Las) is a phloem-limited bacterial plant pathogen, and the most prevalent species of Liberibacter associated with citrus huanglongbing (HLB), a devastating disease of citrus worldwide. Although the complete sequence of the Las genome provides the basis for studying functional genomics of Las and molecular mechanisms of Las-plant interactions, the functional characterization of Las effectors remains a slow process since Las has a significantly reduced genome and remains to be cultured. Like other plant pathogens, Las may deliver effector proteins that modulate a variety of host cellular functions during the infection progress. In this study, we identified 16 candidate Las effectors via bioinformatics, and transiently expressed them in Nicotiana benthamiana plants. Using this heterologous expression system, diverse subcellular localization patterns with different shapes and aggregates of the effector candidates were revealed by UV- microscopy. Intriguingly, one of the sixteen candidates, Las5315, was found to localize to the chloroplasts and induced cell death at 3 days post inoculation in N. benthamiana. Moreover, Las5315 also suppressed pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) induced by Xcc-flg22. Overall, this study provides new insight into the localization and potential roles of these Las effectors in planta.