Comparative iTRAQ proteomic profiling of sweet orange fruit on sensitive and tolerant rootstocks infected by 'Candidatus Liberibacter asiaticus'

Intracellular Life Cycle of 'Candidatus Liberibacter asiaticus' Inside Psyllid Gut Cells

'Candidatus Liberibacter asiaticus' (CLas), the devastating pathogen related to Huanglongbing (HLB), is a phloem-limited, fastidious, insect-borne bacterium. Rapid spread of HLB disease relies on CLas-efficient propagation in the vector, the Asian citrus psyllid Diaphorina citri, in a circulative manner. Understanding the intracellular lifecycle of CLas in psyllid midgut, the major organ for CLas transmission, is fundamental to improving current management strategies. Using a microscopic approach within CLas-infected insect midgut, we observed the entry of CLas into gut cells inside vesicles, termed Liberibacter-containing vacuoles (LCVs), by endocytosis. Endocytosis is followed by the formation of endoplasmic reticulum-related and replication permissive vacuoles (rLCVs). Additionally, we observed the formation of double membrane autophagosome-like structure, termed autophagy-related vacuole (aLCV). Vesicles containing CLas egress from aLCV and fuse with the cell membrane. Immunolocalization studies showed that CLas uses endocytosis- and exocytosis-like mechanisms that mediates bacterial invasion and egress. Upregulation of autophagy-related genes indicated subversion of host autophagy by CLas in psyllid vector to promote infection. These results indicate that CLas interacts with host cellular machineries to undergo a multistage intracellular cycle through endocytic, secretory, autophagic, and exocytic pathways via complex machineries. Potential tactics for HLB control can be made depending on further investigations on the knowledge of the molecular mechanisms of CLas intracellular cycle.

'Candidatus Liberibacter asiaticus' Multimeric LotP Mediates Citrus sinensis Defense Response Activation

‘Candidatus Liberibacter asiaticus’ is known as the most pathogenic organism associated with citrus greening disease. Since its publicized emergence in Florida in 2005, ‘Ca. L. asiaticus’ remains unculturable. Currently, a limited number of potential disease effectors have been identified through in silico analysis. Therefore, these potential effectors remain poorly characterized and do not fully explain the complexity of symptoms observed in citrus trees infected with ‘Ca. L. asiaticus.’ LotP has been identified as a potential effector and have been partially characterized. This protein retains structural homology to the substrate binding domain of the Lon protease. LotP interacts with chaperones like GroEL, Hsp40, DnaJ, and ClpX and may exercise its biological role through interactions with different proteins involved in proteostasis networks. Here, we evaluate the interactome of LotP—revealing a new protein–protein interaction target (Lon-serine protease) and its effect on citrus plant tissue integrity. We found that via protein–protein interactions, LotP can enhance Lon protease activity, increasing the degradation rate of its specific targets. Infiltration of purified LotP strained citrus plant tissue causing photoinhibition and chlorosis after several days. Proteomics analysis of LotP tissues recovering after the infiltration revealed a large abundance of plant proteins associated with the stabilization and processing of mRNA transcripts, a subset of important transcription factors; and pathways associated with innate plant defense were highly expressed. Furthermore, interactions and substrate binding module of LotP suggest potential interactions with plant proteins, most likely proteases.

Comparative Proteomic Analysis of Sweet Orange Petiole Provides Insights Into the Development of Huanglongbing Symptoms

Huanglongbing (HLB) is the most destructive citrus disease worldwide. This is associated with the phloem-limited bacterium Candidatus Liberibacter, and the typical symptom is leaf blotchy mottle. To better understand the biological processes involved in the establishment of HLB disease symptoms, the comparative proteomic analysis was performed to reveal the global protein accumulation profiles in leaf petiole, where there are massive HLB pathogens of Ca. L. asiaticus-infected Newhall sweet orange (Citrus sinensis) plants at the asymptomatic and symptomatic stages compared to their healthy counterpart. Photosynthesis, especially the pathway involved in the photosystem I and II light reactions, was shown to be suppressed throughout the whole Ca. L. asiaticus infection cycle. Also, starch biosynthesis was induced after the symptom-free prodromal period. Many defense-associated proteins were more extensively regulated in the petiole with the symptoms than the ones from healthy plants. The change of salicylic and jasmonic acid levels in different disease stages had a positive correlation with the abundance of phytohormone biosynthesis-related proteins. Moreover, the protein-protein interaction network analysis indicated that an F-type ATPase and an alpha-1,4 glucan phosphorylase were the core nodes in the interactions of differentially accumulated proteins. Our study indicated that the infected citrus plants probably activated the non-unified and lagging enhancement of defense responses against Ca. L. asiaticus at the expense of photosynthesis and contribute to find out the key Ca. L. asiaticus-responsive genes for tolerance and resistance breeding.