'Candidatus Liberibacter Asiaticus' SDE1 Effector Induces Huanglongbing Chlorosis by Downregulating Host DDX3 Gene

Area of Science:

• Plant Pathology
• Molecular Biology
• Genetics

Background:

• Huanglongbing (HLB) is a devastating citrus disease caused by 'Candidatus Liberibacter asiaticus' (CLas).
• CLas employs effector proteins, like Sec-delivered effector 1 (SDE1), to manipulate host plants, including inducing chlorosis and cell death.

Purpose of the Study:

• To investigate the molecular mechanism by which CLas SDE1 induces HLB symptoms.
• To identify host factors interacting with SDE1 and their role in symptom development.

Main Methods:

• Yeast two-hybrid assays to identify SDE1 interacting proteins.
• Gene silencing (RNAi) in *Nicotiana benthamiana* to assess the function of NbDDX3.
• Confocal microscopy to determine subcellular localization of SDE1-NbDDX3 interaction.
• Quantitative PCR to analyze gene expression levels in response to SDE1 overexpression and in HLB-affected citrus.

Main Results:

• DEAD-box RNA helicase (DDX3) was identified as an interactor of SDE1.
• Knockdown of *NbDDX3* in *N. benthamiana* induced chlorosis, mimicking HLB symptoms.
• SDE1 and NbDDX3 interact at the cell membrane, and SDE1 overexpression suppresses *NbDDX3* expression.
• Citrus DDX3 (CsDDX3) also interacts with SDE1, and *CsDDX3* expression is reduced in HLB-affected citrus leaves.

Conclusions:

• Downregulation of host DDX3 is a key mechanism driving HLB-induced leaf chlorosis.
• SDE1 targets CsDDX3, highlighting its crucial role in HLB pathogenesis.
• DDX3 represents a potential target for developing gene-editing strategies to enhance citrus resistance to HLB.