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

Abstract

‘Candidatus Liberibacter asiaticus’ (CLas) is the pathogenic bacterium that causes the disease Huanglongbing (HLB) in citrus and some model plants, such as Nicotiana benthamiana. After infection, CLas releases a set of effectors to modulate host responses. One of these critical effectors is Sec-delivered effector 1 (SDE1), which induces chlorosis and cell death in N. benthamiana. In this study, we revealed the DEAD-box RNA helicase (DDX3) interacts with SDE1. Gene silencing study revealed that knockdown of the NbDDX3 gene triggers leaf chlorosis, mimicking the primary symptom of CLas infection in N. benthamiana. The interactions between SDE1 and NbDDX3 were localized in the cell membrane. Overexpression of SDE1 resulted in suppression of NbDDX3 gene expression in N. benthamiana, which suggests a critical role of SDE1 in modulating NbDDX3 expression. Furthermore, we verified the interaction of SDE1 with citrus DDX3 (CsDDX3), and demonstrated that the expression of the CsDDX3 gene was significantly reduced in HLB-affected yellowing and mottled leaves of citrus. Thus, we provide molecular evidence that the downregulation of the host DDX3 gene is a crucial mechanism of leaf chlorosis in HLB-affected plants. The identification of CsDDX3 as a critical target of SDE1 and its association with HLB symptom development indicates that the DDX3 gene is an important target for gene editing, to interrupt the interaction between DDX3 and SDE1, and therefore interfere host susceptibility.

Comparative Phylogenomic Analysis Reveals Evolutionary Genomic Changes and Novel Toxin Families in Endophytic Liberibacter Pathogens

“…For instance, CLIBASIA_03875 (m3875) ( 20 ) is present in only one “ Ca. Liberibacter asiaticus” strain; other reported toxins, such as Sec-delivered effector 1 (SDE1; CLIBASIA_05315) ( 21 , 22 ), Las5315mp ( 23 ), SDE15 (CLIBASIA_04025) ( 24 ), and CLIBASIA_04405 (m4405) ( 25 ), are present only in “ Ca. Liberibacter asiaticus” stains but not in other HLB-associated pathogens (“ Ca.…”

Ralstonia solanacearum type III effector RipAA targets chloroplastic AtpB to modulate an incompatible interaction on Nicotiana benthamiana

“…Following a positive interaction that was identified as chloroplastic AtpB based on DNA sequencing, the complete ORF of chloroplastic AtpB was cloned into pGBKT7 to verify that pGADT7 harbored the factor responsible for the interaction with RipAA ( Supplementary Table 1 ). The interaction between NbDDX3 (pGADT7:NbDDX3) and SDE1 (pGBKT7:SDE1) was used as a positive control ( Zhou et al, 2020 ). The yeast two-hybrid experiment was conducted in triplicate.…”

Molecular signatures between citrus and Candidatus Liberibacter asiaticus

“…In a separate study, the effector Sec-delivered effector 1 (SDE1) encoded by CLIBASIA_05315 directly interacts with papain-like cysteine proteases (PLCPs) and inhibits protease activity [ 107 ]. Meanwhile, SDE1 interacts with DDX3 and down-regulates the expression of DDX3 in HLB-affected yellowing and mottled leaves of citrus, causing HLB typical chlorosis symptoms [ 108 ], which, however, was not observed by Clark and colleagues [ 107 ]. Severe yellowing and senescence signatures were observed in the mature leaves of SDE1-expressing Arabidopsis thaliana lines [ 109 ].…”