Abstract
Citrus Huanglongbing (HLB) or citrus greening, is the most destructive disease for citrus worldwide. It is caused by the psyllid-transmitted, phloem-limited bacteria “Candidatus Liberibacter asiaticus” (CLas). To date, there are still no effective practical strategies for curing citrus HLB. Understanding the mechanisms against CLas can contribute to the development of effective approaches for combatting HLB. However, the unculturable nature of CLas has hindered elucidating mechanisms against CLas. In this review, we summarize the main aspects that contribute to the understanding about the mechanisms against CLas, including (1) CLas virulence targets, focusing on inhibition of virulence genes; (2) activation of citrus host defense genes and metabolites of HLB-tolerant citrus triggered by CLas, and by agents; and (3) we also review the role of citrus microbiome in combatting CLas. Finally, we discuss novel strategies to continue studying mechanisms against CLas and the relationship of above aspects.
Highlights
Citrus Huanglongbing (HLB), or citrus greening, is the most destructive citrus disease worldwide. It is associated with three species of fastidious, phloem-restricted α-proteobacteria: “Candidatus Liberibacter asiaticus” (CLas), “Candidatus Liberibacter americanus”(CLam), and “Candidatus Liberibacter africanus” (CLaf), which are transmitted by the psyllids Diaphorina citri or Trioza erytreae (Jagoueix et al, 1994; Bové, 2006; Gottwald, 2010)
Induction of expression of PR-1 and PR-2 genes Induction of expression of PR-1 and PR-2 genes Induction of PR-2 gene expression Induction of PR-15 gene expression Involving in SAdepend pathway Functional analogs of salicylic acid (SA) Functional analogs of SA Interfering with biosynthesis of plant hormones and the signaling process Induction of some plant defense genes such as glutathione peroxidase, Jasmonate acid Induction of genes related to the metabolism of jasmonates, brassinosteroids, reactive oxygen species (ROS), and secondary metabolites Many genes involved in plant-bacterium interactions being upregulated post treatment, which may be contributed to host defense against CLas A strong upregulation of chaperones involved in reversing the effects of CLas infection in citrus plants
The development of novel approaches to understand the virulence mechanisms of the pathogen, the mode of action of antimicrobial therapies, the interactions with host and other endophytic microbes will aid in the search of effective and sustainable methods to combat CLas and HLB
Summary
Citrus Huanglongbing (HLB), or citrus greening, is the most destructive citrus disease worldwide. SAMPs effectively reduced CLas titer and disease symptoms in HLB-positive trees and prevented and inhibited infections by induction of defense response genes such as PR1 and PR2, an enzyme of SA biosynthesis, phenylpropanoid pathways, and phenylalanine ammonia-lyase 1(PAL; Huang et al, 2021a).
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