Cocktail Therapy of Fosthiazate and Cupric-Ammoniun Complex for Citrus Huanglongbing

Chitosan Nanoparticles Encapsulate Antimicrobial Peptides for the Prevention and Control of Citrus reticulata Huanglongbing and Their Distribution in Plants

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), is a serious bacterial disease affecting citrus. This study found that the expression of long stable antimicrobial peptide (LSAMP) increases with citrus immune genes CsPR1, CsPR2, and CsNPR1, indicating a link to citrus immunity. Additionally, cut antimicrobial peptide (CAMP) functions similarly to stable antimicrobial peptide (SAMP) in inhibiting CLas proliferation and boosting immunity in periwinkle (Catharanthus roseus). Chitosan nanoparticles (CS-NPs) were synthesized to encapsulate CAMP (CAMP-NPs) and SAMP (SAMP-NPs), which improved their stability and bioavailability. Both CAMP-NPs and SAMP-NPs outperformed their unencapsulated forms in inhibiting CLas proliferation and enhancing immunity in HLB-infected periwinkle and citrus; they also enhance citrus photosynthesis. Confocal microscopy revealed that CS-NPs mainly localized in the leaf apoplast but also penetrated chloroplasts. This study offers a novel biological control strategy for HLB and provides a practical framework for utilizing CS-NPs loaded with antimicrobial peptides (AMPs) for plant disease resistance.

Exploring the dual roles of sec-dependent effectors from Candidatus Liberibacter asiaticus in immunity of citrus plants

KEY MESSAGE

The three SDEs of CLas were expressed in citrus leaves by AuNPs-PEI mediated transient expression system, and promoted the proliferation of CLas and inhibited citrus immunity. Huanglongbing (HLB) is the most severe bacterial disease of citrus caused by Candidatus Liberibacter asiaticus (CLas). CLas suppress host immune responses and promote infection by sec-dependent effectors (SDEs), thus insight into HLB pathogenesis is urgently needed to develop effective management strategies. In this study, we focused on the roles of SDE4310, SDE4435 and SDE4955 in citrus. We found that the expression of SDE4310, SDE4435 and SDE4955 to increase with increasing citrus immune genes CsPR1, CsPR2, CsPR5, CsNPR1, CsRBOHD, CsMAP3K and CsBIK1, suggesting that the level of citrus immunity could be judged by the expression of SDE. To further explore the relationship between these three SDEs and citrus immunity, we established a transient expression system in citrus leaves, using gold nanoparticle-polyethyleneimine (AuNPs-PEI) to deliver recombinant plasmid containing SDE4310, SDE4435 or SDE4955 respectively into citrus leaves. Results showed that SDE4310, SDE4435 and SDE4955 were successfully expressed in citrus leaves using this transient expression system, and found that SDE4310, SDE4435 and SDE4955 could promote the CLas proliferation by decreasing the immune gene expression of the citrus. Additionally, we used AuNPs-PEI to deliver siRNA4310 to citrus cells, significantly reducing the expression of SDE4310 within 3 days. Although the suppression of SDE4310 expression did not inhibit the CLas proliferation, it increased the expression level of CsPR1, CsNPR1 and CsBIK1. This is also the first time that AuNPs-PEI has been found to be able to deliver exogenous plasmids into citrus cells and express the target protein, providing a new method for future studies on citrus HLB.

Genome-Wide Identification of the DnaJ Gene Family in Citrus and Functional Characterization of ClDJC24 in Response to Citrus Huanglongbing

Citrus Huanglongbing (HLB) is the most destructive citrus disease worldwide. The etiological agent responsible for this disease is "Candidatus Liberibacter asiaticus" (CLas), a phloem-restricted bacterium transmitted by psyllid vectors. To date, effective practical strategies for curing Citrus HLB remain elusive. Additionally, no susceptibility genes associated with HLB have been identified in Citrus species, thereby complicating the application of gene-editing techniques such as CRISPR-Cas9 to enhance resistance to HLB. The co-chaperone DnaJ plays a crucial role in protein folding and the regulation of various physiological activities, and it is also associated with multiple pathological processes. DnaJ has been extensively studied in many species, including Arabidopsis, rice, and wheat. However, there is limited information available regarding the DnaJ gene family in citrus. In this study, we conducted a comprehensive genome-wide analysis of the DnaJ family genes in various Citrus species. The Citrus genome was identified to contain 86 DnaJ genes, which were unevenly distributed across nine chromosomes. Phylogenetic analysis indicated that these genes could be classified into six distinct groups. Furthermore, transcriptomic analysis revealed that nine DnaJ genes exhibited significantly higher induction in HLB-infected samples relative to non-HLB-infected Citrus. Cis-acting elements within the promoters of DnaJ genes were also examined, revealing the presence of hormone and defense/stress responsiveness elements (TC-rich) distributed on the ClDJC24 gene. The results were validated using quantitative real-time PCR (qRT-PCR). Additionally, the silencing of ClDJC24 suggested that this gene negatively regulates disease resistance in Citrus. Our study provided useful clues for further functional characterization and constructed a theoretical foundation for disease-resistant breeding in Citrus.

Comparative Analysis of Inhibitor Binding to Peroxiredoxins from Candidatus Liberibacter asiaticus and Its Host Citrus sinensis

The peroxiredoxins (Prxs), potential drug targets, constitute an important class of antioxidant enzymes present in both pathogen and their host. The comparative binding potential of inhibitors to Prxs from pathogen and host could be an important step in drug development against pathogens. Huanglongbing (HLB) is a most devastating disease of citrus caused by Candidatus Liberibacter asiaticus (CLa). In this study, the binding of conoidin-A (conoidin) and celastrol inhibitor molecules to peroxiredoxin of bacterioferritin comigratory protein family from CLa (CLaBCP) and its host plant peroxiredoxin from Citrus sinensis (CsPrx) was assessed. The CLaBCP has a lower specific activity than CsPrx and is efficiently inhibited by conoidin and celastrol molecules. The biophysical studies showed conformational changes and significant thermal stability of CLaBCP in the presence of inhibitor molecules as compared to CsPrx. The surface plasmon resonance (SPR) studies revealed that the conoidin and celastrol inhibitor molecules have a strong binding affinity (KD) with CLaBCP at 33.0 µM, and 18.5 µM as compared to CsPrx at 52.0 µM and 61.6 µM, respectively. The docked complexes of inhibitor molecules showed more structural stability of CLaBCP as compared to CsPrx during the run of molecular dynamics-based simulations for 100 ns. The present study suggests that the conoidin and celastrol molecules can be exploited as potential inhibitor molecules against the CLa to manage the HLB disease.

Three new discovery effector proteins from Candidatus Liberibacter asiaticus psy62 inhibit plant defense through interaction with AtCAT3 and AtGAPA

KEY MESSAGE

We identify three SDEs that inhibiting host defence from Candidatus Liberibacter asiaticus psy62, which is an important supplement to the pathogenesis of HLB. Candidatus Liberibacter asiaticus (CLas) is the main pathogen of citrus Huanglongbing (HLB). 38 new possible sec-dependent effectors (SDEs) of CLas psy62 were predicted by updated predictor SignalP 5.0, which 12 new SDEs were found using alkaline phosphate assay. Among them, SDE4310, SDE4435 and SDE4955 inhibited hypersensitivity reactions (HR) in Arabidopsis thaliana (Arabidopsis, At) and Nicotiana benthamiana leaves induced by pathogens, which lead to a decrease in cell death and reactive oxygen species (ROS) accumulation. And the expression levels of SDE4310, SDE4435, and SDE4955 genes elevated significantly in mild symptom citrus leaves. When SDE4310, SDE4435 and SDE4955 were overexpressed in Arabidopsis, HR pathway key genes pathogenesis-related 2 (PR2), PR5, nonexpressor of pathogenesis-related 1 (NPR1) and isochorismate synthase 1 (ICS1) expression significantly decreased and the growth of pathogen was greatly increased relative to control with Pst DC3000/AvrRps4 treatment. Our findings also indicated that SDE4310, SDE4435 and SDE4955 interacted with AtCAT3 (catalase 3) and AtGAPA (glyceraldehyde-3-phosphate dehydrogenase A). In conclusion, our results suggest that SDE4310, SDE4435 and SDE4955 are CLas psy62 effector proteins that may have redundant functions. They inhibit ROS burst and cell death by interacting with AtCAT3 and AtGAPA to negatively regulate host defense.

Expression of PEI-coated gold nanoparticles carrying exogenous gene in periwinkle mesophyll cells and its practice in Huanglongbing research

Huanglongbing (HLB) is a devastating disease of citrus, which is mostly caused by Candidatus Liberibacter asiaticus (CLas). To realize the specific application of nano-transgenic technology in HLB, AuNPs-PEI (Gold Nanoparticles-Polyethylenimine) was used to carry foreign genes into the leaves of periwinkle (Catharanthus roseus) by infiltration. Here, we demonstrated that NPR1-GFP protein expression was observed from the 12th hour to the 10th day after infiltrating AuNPs-PEI-pNPR1 (Arabidopsis thaliana nonexpressor of pathogenesis-related gene 1)-GFP. Fluorescence of mCherry was observed 6 h after AuNPs-PEI-pNLS (nuclear localization signal sequence)-mCherry infiltration and fluorescence of FAM was observed in the nucleus 4 h after AuNPs-PEI-FAM-siRNA_NPR1 infiltration. In addition, NPR1-GFP expression in CLas-infected periwinkle leaves was significantly higher than that in healthy periwinkle leaves after infiltration. Our work confirmed that the expression of exogenous NPR1-GFP could reduce the CLas titers by promoting the expression of PR (pathogenesis related) genes and ICS (isochorismate synthase) gene.

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AuNPs-PEI-FAM-siRNA_NPR1 entered the nucleus within 4 h after infiltration

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AuNPs-PEI-pNLS-mCherry expressed the corresponding protein within 6 h

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AuNPs-PEI-pNPR1-GFP continued to express the corresponding protein for 14 days

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After AuNPs-PEI-pNPR1-GFP infiltration for 2 days, CLas titer decreased significantly

Root-Associated Microbiota Response to Ecological Factors: Role of Soil Acidity in Enhancing Citrus Tolerance to Huanglongbing

The citrus orchards in southern China are widely threatened by low soil pH and Huanglongbing (HLB) prevalence. Notably, the lime application has been used to optimize soil pH, which is propitious to maintain root health and enhance HLB tolerance of citrus; however, little is known about the interactive effects of soil acidity on the soil properties and root-associated (rhizoplane and endosphere) microbial community of HLB-infected citrus orchard. In this study, the differences in microbial community structures and functions between the acidified and amended soils in the Gannan citrus orchard were investigated, which may represent the response of the host-associated microbiome in diseased roots and rhizoplane to dynamic soil acidity. Our findings demonstrated that the severity of soil acidification and aluminum toxicity was mitigated after soil improvement, accompanied by the increase in root activity and the decrease of HLB pathogen concentration in citrus roots. Additionally, the Illumina sequencing-based community analysis showed that the application of soil amendment enriched functional categories involved in host-microbe interactions and nitrogen and sulfur metabolisms in the HLB-infected citrus rhizoplane; and it also strongly altered root endophytic microbial community diversity and structure, which represented by the enrichment of beneficial microorganisms in diseased roots. These changes in rhizoplane-enriched functional properties and microbial composition may subsequently benefit the plant's health and tolerance to HLB disease. Overall, this study advances our understanding of the important role of root-associated microbiota changes and ecological factors, such as soil acidity, in delaying and alleviating HLB disease.

Controlling Citrus Huanglongbing: Green Sustainable Development Route Is the Future

Huanglongbing (HLB) is the most severe bacterial disease of citrus crops caused by Candidatus Liberibacter spp. It causes a reduction in fruit yield, poor fruit quality, and even plants death. Due to the lack of effective medicine, HLB is also called citrus "AIDS." Currently, it is essential for the prevention and control of HLB to use antibiotics and pesticides while reducing the spread of HLB by cultivating pathogen-free seedlings, removing disease trees, and killing Asian citrus psyllid (ACP). New compounds [e.g., antimicrobial peptides (AMPs) and nanoemulsions] with higher effectiveness and less toxicity were also found and they have made significant achievements. However, further evaluation is required before these new antimicrobial agents can be used commercially. In this review, we mainly introduced the current strategies from the aspects of physical, chemical, and biological and discussed their environmental impacts. We also proposed a green and ecological strategy for controlling HLB basing on the existing methods and previous research results.

Association between plant nutrients, the development of Huanglongbing and abnormal growth symptoms in navel orange

Huanglongbing (HLB) causes extensive damage in citrus orchards worldwide. Symptoms include blotchy mottle leaf (BML) and little leaf chlorosis (LLC), and nutrient deficiency usually occurs concurrently. However, the relationship between plant mineral content and infection with Candidatus Liberibacter asiaticus (CLas) is not clearly established. We sampled 7-month-old autumn shoots with three characteristic phenotypes, asymptomatic leaf (AL), BML and LLC, representing HLB disease progression, and further divided samples into CLas-infected and uninfected based on PCR analysis. HLB infection decreased transfer coefficients of Mg and K from leaf to phloem tissues through regulation of the transporter genes Cs3g03790.1 and PtrMGT5, increasing the content of leaf Mg and K. HLB infection also decreased leaf Zn, xylem Ca and phloem Ca and Zn content. Leaf Ca, Mg, Zn and B content decreased while leaf K content increased significantly as symptoms progressed from AL to LLC. The transport of P from leaf to phloem tissue, as evaluated by the transfer coefficient, was regulated by the transporter CsiPT2, resulting in irregular levels of leaf P. Our results provide insights into the nutrient dynamics in Citrus in response to CLas infection and the progression of HLB symptoms.