A high-throughput system to identify inhibitors of Candidatus Liberibacter asiaticus transcription regulators

Study on the citrus greening disease: Current challenges and novel therapies

The unprecedented worldwide spread of the Citrus greening disorder, called Huanglongbing (HLB), has urged researchers for rapid interventions. HLB poses a considerable threat to global citriculture owing to its devastating impact on citrus species. This disease is caused by Candidatus Liberibacter species (CLs), primarily transferred through psyllid insects, such as Trioza erytreae and Diaphorina citri. It results in phloem malfunction, root decline, and altered plant source-sink relationships, leading to a deficient plant with minimal yield before it dies. Thus, many various techniques have been employed to eliminate HLB and control vector populations through the application of insecticides and antimicrobials. The latter have evidenced short-term efficiency. While nucleic acid-based analyses and symptom-based identification of the disease have been used for detection, they suffer from limitations such as false negatives, complex sample preparation, and high costs. To address these challenges, secreted protein-based biomarkers offer a promising solution for accurate, rapid, and cost-effective disease detection. This paper presents an overview of HLB symptoms in citrus plants, including leaf and fruit symptoms, as well as whole tree symptoms. The differentiation between HLB symptoms and those of nutrient deficiencies is discussed, emphasizing the importance of precise identification for effective disease management. The elusive nature of CLs and the challenges in culturing them in axenic cultures have hindered the understanding of their pathogenic mechanisms. However, genome sequencing has provided insights into CLs strains' metabolic traits and potential virulence factors. Efforts to identify potential host target genes for resistance are discussed, and a high-throughput antimicrobial testing method using Citrus hairy roots is introduced as a promising tool for rapid assessment of potential treatments. This review summarizes current challenges and novel therapies for HLB disease. It highlights the urgency of developing accurate and efficient detection methods and identifying the complex relations between CLs and their host plants. Transgenic citrus in conjunction with secreted protein-based biomarkers and innovative testing methodologies could revolutionize HLB management strategies toward achieving a sustainable citrus cultivation. It offers more reliable and practical solutions to combat this devastating disease and safeguard the global citriculture industry.

The swimming defect caused by the absence of the transcriptional regulator LdtR in Sinorhizobium meliloti is restored by mutations in the motility genes motA and motS

The flagellar motor is a powerful macromolecular machine used to propel bacteria through various environments. We determined that flagellar motility of the alpha-proteobacterium Sinorhizobium meliloti is nearly abolished in the absence of the transcriptional regulator LdtR, known to influence peptidoglycan remodeling and stress response. LdtR does not regulate motility gene transcription. Remarkably, the motility defects of the ΔldtR mutant can be restored by secondary mutations in the motility gene motA or a previously uncharacterized gene in the flagellar regulon, which we named motS. MotS is not essential for S. meliloti motility and may serve an accessory role in flagellar motor function. Structural modeling predicts that MotS comprised an N-terminal transmembrane segment, a long-disordered region, and a conserved β-sandwich domain. The C terminus of MotS is localized in the periplasm. Genetics based substitution of MotA with MotAG12S also restored the ΔldtR motility defect. The MotAG12S variant protein features a local polarity shift at the periphery of the MotAB stator units. We propose that MotS may be required for optimal alignment of stators in wild-type flagellar motors but becomes detrimental in cells with altered peptidoglycan. Similarly, the polarity shift in stator units composed of MotB/MotAG12S might stabilize its interaction with altered peptidoglycan.

Specifically targeting antimicrobial peptides for inhibition of Candidatus Liberibacter asiaticus

AIMS

Huanglongbing (citrus greening) is a plant disease putatively caused by the unculturable Gram-negative bacterium Candidatus Liberibacter asiaticus (CLas), and it has caused severe damage to citrus plantations worldwide. There are no definitive treatments for this disease, and conventional disease control techniques have shown limited efficacy. This work presents an in silico evaluation of using specifically targeting anti-microbial peptides (STAMPs) consisting of a targeting segment and an antimicrobial segment to inhibit citrus greening by inhibiting the BamA protein of CLas, which is an outer membrane protein crucial for bacterial viability.

METHODS AND RESULTS

Initially, a set of peptides with a high affinity toward BamA protein were screened and evaluated via molecular docking and molecular dynamics simulations and were verified in vitro via bio-layer interferometry (BLI). In silico studies and BLI experiments indicated that two peptides, HASP2 and HASP3, showed stable binding to BamA. Protein structures for STAMPs were created by fusing known anti-microbial peptides (AMPs) with the selected short peptides. The binding of STAMPs to BamA was assessed using molecular docking and binding energy calculations. The attachment of high-affinity short peptides significantly reduced the free energy of binding for AMPs, suggesting that it would make it easier for the STAMPs to bind to BamA. Efficacy testing in vitro using a closely related CLas surrogate bacterium showed that STAMPs had greater inhibitory activity than AMP alone.

CONCLUSIONS

In silico and in vitro results indicate that the STAMPs can inhibit CLas surrogate Rhizobium grahamii more effectively compared to AMPs, suggesting that STAMPs can achieve better inhibition of CLas, potentially via enhancing the site specificity of AMPs.

Inhibition of a conserved bacterial dual-specificity phosphatase confers plant tolerance to Candidatus Liberibacter spp

"Candidatus Liberibacter spp." are insect-vectored, fastidious, and vascular-limited phytopathogens. They are the presumptive causal agents of potato zebra chip, tomato vein clearing, and the devastating citrus greening disease worldwide. There is an urgent need to develop new strategies to control them. In this study, we characterized a dual-specificity serine/tyrosine phosphatase (STP) that is well conserved among thirty-three geographically diverse "Candidatus Liberibacter spp." and strains that infect multiple Solanaceaea and citrus spp. The STP is expressed in infected plant tissues, localized at the plant cytosol and plasma membrane, and interferes with plant cell death responses. We employed an in silico target-based molecular modeling and ligand screen to identify two small molecules with high binding affinity to STP. Efficacy studies demonstrated that the two molecules can inhibit "Candidatus Liberibacter spp." but not unrelated pathogens and confer plant disease tolerance. The inhibitors and strategies are promising means to control "Candidatus Liberibacter spp."

A protease and a lipoprotein jointly modulate the conserved ExoR-ExoS-ChvI signaling pathway critical in Sinorhizobium meliloti for symbiosis with legume hosts

Sinorhizobium meliloti is a model alpha-proteobacterium for investigating microbe-host interactions, in particular nitrogen-fixing rhizobium-legume symbioses. Successful infection requires complex coordination between compatible host and endosymbiont, including bacterial production of succinoglycan, also known as exopolysaccharide-I (EPS-I). In S. meliloti EPS-I production is controlled by the conserved ExoS-ChvI two-component system. Periplasmic ExoR associates with the ExoS histidine kinase and negatively regulates ChvI-dependent expression of exo genes, necessary for EPS-I synthesis. We show that two extracytoplasmic proteins, LppA (a lipoprotein) and JspA (a lipoprotein and a metalloprotease), jointly influence EPS-I synthesis by modulating the ExoR-ExoS-ChvI pathway and expression of genes in the ChvI regulon. Deletions of jspA and lppA led to lower EPS-I production and competitive disadvantage during host colonization, for both S. meliloti with Medicago sativa and S. medicae with M. truncatula. Overexpression of jspA reduced steady-state levels of ExoR, suggesting that the JspA protease participates in ExoR degradation. This reduction in ExoR levels is dependent on LppA and can be replicated with ExoR, JspA, and LppA expressed exogenously in Caulobacter crescentus and Escherichia coli. Akin to signaling pathways that sense extracytoplasmic stress in other bacteria, JspA and LppA may monitor periplasmic conditions during interaction with the plant host to adjust accordingly expression of genes that contribute to efficient symbiosis. The molecular mechanisms underlying host colonization in our model system may have parallels in related alpha-proteobacteria.

Versatility and Complexity: Common and Uncommon Facets of LysR-Type Transcriptional Regulators

LysR-type transcriptional regulators (LTTRs) form one of the largest families of bacterial regulators. They are widely distributed and contribute to all aspects of metabolism and physiology. Most are homotetramers, with each subunit composed of an N-terminal DNA-binding domain followed by a long helix connecting to an effector-binding domain. LTTRs typically bind DNA in the presence or absence of a small-molecule ligand (effector). In response to cellular signals, conformational changes alter DNA interactions, contact with RNA polymerase, and sometimes contact with other proteins. Many are dual-function repressor-activators, although different modes of regulation may occur at multiple promoters. This review presents an update on the molecular basis of regulation, the complexity of regulatory schemes, and applications in biotechnology and medicine. The abundance of LTTRs reflects their versatility and importance. While a single regulatory model cannot describe all family members, a comparison of similarities and differences provides a framework for future study.

'Candidatus Liberibacter asiaticus' secretory protein SDE3 inhibits host autophagy to promote Huanglongbing disease in citrus

Antimicrobial acroautophagy/autophagy plays a vital role in degrading intracellular pathogens or microbial molecules in host-microbe interactions. However, microbes evolved various mechanisms to hijack or modulate autophagy to escape elimination. Vector-transmitted phloem-limited bacteria, Candidatus Liberibacter (Ca. Liberibacter) species, cause Huanglongbing (HLB), one of the most catastrophic citrus diseases worldwide, yet contributions of autophagy to HLB disease proliferation remain poorly defined. Here, we report the identification of a virulence effector in "Ca. Liberibacter asiaticus" (Las), SDE3, which is highly conserved among the "Ca. Liberibacter". SDE3 expression not only promotes the disease development of HLB and canker in sweet orange (Citrus sinensis) plants but also facilitates Phytophthora and viral infections in Arabidopsis, and Nicotiana benthamiana (N. benthamiana). SDE3 directly associates with citrus cytosolic glyceraldehyde-3-phosphate dehydrogenases (CsGAPCs), which negatively regulates plant immunity. Overexpression of CsGAPCs and SDE3 significantly inhibits autophagy in citrus, Arabidopsis, and N. benthamiana. Intriguingly, SDE3 undermines autophagy-mediated immunity by the specific degradation of CsATG8 family proteins in a CsGAPC1-dependent manner. CsATG8 degradation is largely rescued by treatment with an inhibitor of the late autophagic pathway, E64d. Furthermore, ectopic expression of CsATG8s enhances Phytophthora resistance. Collectively, these results suggest that SDE3-CsGAPC interactions modulate CsATG8-mediated autophagy to enhance Las progression in citrus.Abbreviations: ACP: asian citrus psyllid; ACD2: ACCELERATED CELL DEATH 2; ATG: autophagy related; Ca. Liberibacter: Candidatus Liberibacter; CaMV: cauliflower mosaic virus; CMV: cucumber mosaic virus; Cs: Citrus sinensis; EV: empty vector; GAPC: cytosolic glyceraldehyde-3-phosphate dehydrogenase; HLB: huanglongbing; H2O2: hydrogen peroxide; Las: liberibacter asiaticus; Laf: liberibacter africanus; Lam: liberibacter americanus; Pst: Pseudomonas syringae pv. tomato; PVX: potato virus X; ROS: reactive oxygen species; SDE3: sec-delivered effector 3; TEM: transmission electron microscopy; VIVE : virus-induced virulence effector; WT: wild-type; Xcc: Xanthomonas citri subsp. citri.

A Perspective on Current Therapeutic Molecule Screening Methods Against 'Candidatus Liberibacter asiaticus', the Presumed Causative Agent of Citrus Huanglongbing

Huanglongbing (HLB), referred to as citrus greening disease, is a bacterial disease impacting citrus production worldwide and is fatal to young trees and mature trees of certain varieties. In some areas, the disease is devastating the citrus industry. A successful solution to HLB will be measured in economics: citrus growers need treatments that improve tree health, fruit production, and most importantly, economic yield. The profitability of citrus groves is the ultimate metric that truly matters when searching for solutions to HLB. Scientific approaches used in the laboratory, greenhouse, or field trials are critical to the discovery of those solutions and to estimate the likelihood of success of a treatment aimed at commercialization. Researchers and the citrus industry use a number of proxy evaluations of potential HLB solutions; understanding the strengths and limitations of each assay, as well as how best to compare different assays, is critical for decision-making to advance therapies into field trials and commercialization. This perspective aims to help the reader compare and understand the limitations of different proxy evaluation systems based on the treatment and evaluation under consideration. The researcher must determine the suitability of one or more of these metrics to identify treatments and predict the usefulness of these treatments in having an eventual impact on citrus production and HLB mitigation. As therapies advance to field trials in the next few years, a reevaluation of these metrics will be useful to guide future research efforts on strategies to mitigate HLB and vascular bacterial pathogens in other perennial crops.

Pathogenicity and Transcriptomic Analyses of Two "Candidatus Liberibacter asiaticus" Strains Harboring Different Types of Phages

"Candidatus Liberibacter asiaticus" is one of the putative causal agents of citrus Huanglongbing (HLB), a highly destructive disease threatening the global citrus industry. Several types of phages had been identified in "Ca. Liberibacter asiaticus" strains and found to affect the biology of "Ca. Liberibacter asiaticus." However, little is known about the influence of phages in "Ca. Liberibacter asiaticus" pathogenicity. In this study, two "Ca. Liberibacter asiaticus" strains, PYN and PGD, harboring different types of phages were collected and used for pathogenicity analysis in periwinkle (Catharanthus roseus). Strain PYN carries a type 1 phage (P-YN-1), and PGD harbors a type 2 phage (P-GD-2). Compared to strain PYN, strain PGD exhibited a faster reproduction rate and higher virulence in periwinkle: leaf symptoms appeared earlier, and there was a stronger inhibition in the growth of new flush. Estimation of phage copy numbers by type-specific PCR indicated that there are multiple copies of phage P-YN-1 in strain PYN, while strain PGD carries only a single copy of phage P-GD-2. Genome-wide gene expression profiling revealed the lytic activity of P-YN-1 phage, as evidenced by the unique expression of genes involved in lytic cycle, which may limit the propagation of strain PYN and lead to a delayed infection in periwinkle. However, the activation of genes involved in lysogenic conversion of phage P-GD-1 indicated it could reside within the "Ca. Liberibacter asiaticus" genome as a prophage form in strain PGD. Comparative transcriptome analysis showed that the significant differences in expression of virulence factor genes, including genes associated with pathogenic effectors, transcriptional factors, the Znu transport system, and the heme biosynthesis pathway, could be another major determinant of virulence variation between two "Ca. Liberibacter asiaticus" strains. This study expanded our knowledge of "Ca. Liberibacter asiaticus" pathogenicity and provided new insights into the differences in pathogenicity between "Ca. Liberibacter asiaticus" strains. IMPORTANCE Citrus Huanglongbing (HLB), also called citrus greening disease, is a highly destructive disease threatening citrus production worldwide. "Candidatus Liberibacter asiaticus" is one of the most common putative causal agents of HLB. Phages of "Ca. Liberibacter asiaticus" have recently been identified and found to affect "Ca. Liberibacter asiaticus" biology. Here, we found that "Ca. Liberibacter asiaticus" strains harboring different types of phages (type 1 or type 2) showed different levels of pathogenicity and multiplication patterns in the periwinkle plant (Catharanthus roseus). Transcriptome analysis revealed the possible lytic activity of type 1 phage in a "Ca. Liberibacter asiaticus" strain, which could limit the propagation of "Ca. Liberibacter asiaticus" and lead to the delayed infection in periwinkle. The heterogeneity in the transcriptome profiles, particularly the significant differences in expression of virulence factors genes, could be another major determinant of difference in virulence observed between the two "Ca. Liberibacter asiaticus" strains. These findings improved our understanding of "Ca. Liberibacter asiaticus"-phage interaction and provided insight into "Ca. Liberibacter asiaticus" pathogenicity.

Inoculation of Tomato With Plant Growth Promoting Rhizobacteria Affects the Tomato-Potato Psyllid-Candidatus Liberibacter Solanacearum Interactions

The Rio Grande Valley (RGV) in southern Texas is well-suited for vegetable production due to its relatively mild/warm weather conditions in the fall and winter. Consequently, insects inflict year-round, persistent damage to crops in the RGV and regions with similar climate. Bactericera cockerelli (Šulc) (Hemiptera: Triozidae), commonly known as the potato psyllid, is a known vector of Candidatus Liberibacter solanacearum (CLso) (Hyphomicrobiales: Rhizobiaceae), a fastidious phloem-limited bacterium associated to vein-greening in tomatoes and Zebra Chip in potatoes. Vector control is the primary approach of integrated pest management (IPM) strategies that aim to prevent plant diseases in commercial agricultural systems. However, resistance-selective pressures that decrease the effectiveness of chemical control (insecticide) applications over time are of increasing concern. Therefore, we explore an ecological approach to devising alternative IPM methodologies to manage the psyllid-transmitted CLso pathogen to supplement existing chemical products and application schedules without increasing resistance. In this study, our objective was to examine the effects of plant-growth promoting rhizobacteria (PGPR) on host-vector-pathogen interactions. Soil-drench applications of PGPRs to Solanum lycopersicum (Solanales: Solanaceae) seedlings revealed structural and possible physiological changes to the plant host and indirect changes on psyllid behavior: host plants had increased length and biomass of roots and exhibited delayed colonization by CLso, while psyllids displayed changes in parental (F0) psyllid behavior (orientation and oviposition) in response to treated hosts and in the sex ratio of their progeny (F1). Based on our results, we suggest that PGPR may have practical use in commercial tomato production.

Recombineering-Mediated Sinorhizobium meliloti Rm1021 Gene Deletion

Sinorhizobium meliloti Rm1021 (S. meliloti Rm1021) is a Gram-negative, soil-dwelling α-proteobacterium which serves as a model microorganism for the studies of symbiotic nitrogen fixation. The S. meliloti Rm1021 genome consists of one chromosome and two megaplasmids, pSymA and pSymB. Gene deletion is an essential tool for the elucidation of gene function and generation of mutants with improved properties. However, only two gene deletion methods, counterselectable marker sacB-based and FLP/FRT, Cre/loxP site-specific recombination, have been reported for S. meliloti Rm1021 gene deletion. Both methods require time-consuming and tedious gene cloning and conjugation steps. Herein, a λ Red recombineering-mediated gene deletion strategy is reported. The mutant was obtained via electroporating overlap-extension PCR-generated linear targeting DNA into Red-proficient cells. One gene each from the S. meliloti Rm1021 chromosome, megaplasmid SymA and pSymB was deleted, with deletion efficiency up to 100%. The straightforward and highly efficient recombineering procedure holds the promise to be a general gene manipulation method for S. meliloti Rm1021.

Evaluation of Candidatus Liberibacter Asiaticus Efflux Pump Inhibition by Antimicrobial Peptides

Citrus greening, also known as Huanglongbing (HLB), is caused by the unculturable bacterium Candidatus Liberibacter spp. (e.g., CLas), and has caused a devastating decline in citrus production in many areas of the world. As of yet, there are no definitive treatments for controlling the disease. Antimicrobial peptides (AMPs) that have the potential to block secretion-dependent effector proteins at the outer-membrane domains were screened in silico. Predictions of drug-receptor interactions were built using multiple in silico techniques, including molecular docking analysis, molecular dynamics, molecular mechanics generalized Born surface area analysis, and principal component analysis. The efflux pump TolC of the Type 1 secretion system interacted with natural bacteriocin plantaricin JLA-9, blocking the β barrel. The trajectory-based principal component analysis revealed the possible binding mechanism of the peptides. Furthermore, in vitro assays using two closely related culturable surrogates of CLas (Liberibacter crescens and Rhizobium spp.) showed that Plantaricin JLA-9 and two other screened AMPs inhibited bacterial growth and caused mortality. The findings contribute to designing effective therapies to manage plant diseases associated with Candidatus Liberibacter spp.

A serralysin-like protein of Candidatus Liberibacter asiaticus modulates components of the bacterial extracellular matrix

Huanglongbing (HLB), the current major threat for Citrus species, is caused by intracellular alphaproteobacteria of the genus Candidatus Liberibacter (CaL), with CaL asiaticus (CLas) being the most prevalent species. This bacterium inhabits phloem cells and is transmitted by the psyllid Diaphorina citri. A gene encoding a putative serralysin-like metalloprotease (CLIBASIA_01345) was identified in the CLas genome. The expression levels of this gene were found to be higher in citrus leaves than in psyllids, suggesting a function for this protease in adaptation to the plant environment. Here, we study the putative role of CLas-serralysin (Las1345) as virulence factor. We first assayed whether Las1345 could be secreted by two different surrogate bacteria, Rhizobium leguminosarum bv. viciae A34 (A34) and Serratia marcescens. The protein was detected only in the cellular fraction of A34 and S. marcescens expressing Las1345, and increased protease activity of those bacteria by 2.55 and 4.25-fold, respectively. In contrast, Las1345 expressed in Nicotiana benthamiana leaves did not show protease activity nor alterations in the cell membrane, suggesting that Las1345 do not function as a protease in the plant cell. Las1345 expression negatively regulated cell motility, exopolysaccharide production, and biofilm formation in Xanthomonas campestris pv. campestris (Xcc). This bacterial phenotype was correlated with reduced growth and survival on leaf surfaces as well as reduced disease symptoms in N. benthamiana and Arabidopsis. These results support a model where Las1345 could modify extracellular components to adapt bacterial shape and appendages to the phloem environment, thus contributing to virulence.

An Overview of the Mechanisms Against "Candidatus Liberibacter asiaticus": Virulence Targets, Citrus Defenses, and Microbiome

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.

Osmotic stress induces long-term biofilm survival in Liberibacter crescens

Citrus greening, also known as Huanglongbing (HLB), is a devastating citrus plant disease caused predominantly by Liberibacter asiaticus. While nearly all Liberibacter species remain uncultured, here we used the culturable L. crescens BT-1 as a model to examine physiological changes in response to the variable osmotic conditions and nutrient availability encountered within the citrus host. Similarly, physiological responses to changes in growth temperature and dimethyl sulfoxide concentrations were also examined, due to their use in many of the currently employed therapies to control the spread of HLB. Sublethal heat stress was found to induce the expression of genes related to tryptophan biosynthesis, while repressing the expression of ribosomal proteins. Osmotic stress induces expression of transcriptional regulators involved in expression of extracellular structures, while repressing the biosynthesis of fatty acids and aromatic amino acids. The effects of osmotic stress were further evaluated by quantifying biofilm formation of L. crescens in presence of increasing sucrose concentrations at different stages of biofilm formation, where sucrose-induced osmotic stress delayed initial cell attachment while enhancing long-term biofilm viability. Our findings revealed that exposure to osmotic stress is a significant contributing factor to the long term survival of L. crescens and, possibly, to the pathogenicity of other Liberibacter species.

Molecular signatures between citrus and Candidatus Liberibacter asiaticus

Citrus Huanglongbing (HLB), also known as citrus greening, is one of the most devastating citrus diseases worldwide. Candidatus Liberibacter asiaticus (CLas) is the most prevalent strain associated with HLB, which is yet to be cultured in vitro. None of the commercial citrus cultivars are resistant to HLB. The pathosystem of Ca. Liberibacter is complex and remains a mystery. In this review, we focus on the recent progress in genomic research on the pathogen, the interaction of host and CLas, and the influence of CLas infection on the transcripts, proteins, and metabolism of the host. We have also focused on the identification of candidate genes for CLas pathogenicity or the improvements of HLB tolerance in citrus. In the end, we propose potentially promising areas for mechanistic studies of CLas pathogenicity, defense regulators, and genetic improvement for HLB tolerance/resistance in the future.

Structure of Lipopolysaccharide from Liberibacter crescens Is Low Molecular Weight and Offers Insight into Candidatus Liberibacter Biology

Huanglongbing (HLB) disease, also known as citrus greening disease, was first reported in the US in 2005. Since then, the disease has decimated the citrus industry in Florida, resulting in billions of dollars in crop losses and the destruction of thousands of acres of citrus groves. The causative agent of citrus greening disease is the phloem limited pathogen Candidatus Liberibacter asiaticus. As it has not been cultured, very little is known about the structural biology of the organism. Liberibacter are part of the Rhizobiaceae family, which includes nitrogen-fixing symbionts of legumes as well as the Agrobacterium plant pathogens. To better understand the Liberibacter genus, a closely related culturable bacterium (Liberibacter crescens or Lcr) has attracted attention as a model organism for structural and functional genomics of Liberibacters. Given that the structure of lipopolysaccharides (LPS) from Gram-negative bacteria plays a crucial role in mediating host-pathogen interactions, we sought to characterize the LPS from Lcr. We found that the major lipid A component of the LPS consisted of a pentaacylated molecule with a β-6-GlcN disaccharide backbone lacking phosphate. The polysaccharide portion of the LPS was unusual compared to previously described members of the Rhizobiaceae family in that it contained ribofuranosyl residues. The LPS structure presented here allows us to extrapolate known LPS structure/function relationships to members of the Liberibacter genus which cannot yet be cultured. It also offers insights into the biology of the organism and how they manage to effectively attack citrus trees.

The role of PhoP/PhoQ two component system in regulating stress adaptation in Cronobacter sakazakii

Cronobacter sakazakii is an opportunistic foodborne bacterial pathogen that shows resistance to multiple stress conditions. The PhoP/PhoQ two component system is a key regulatory mechanism of stress response and virulence in various bacteria, but its role in C. sakazakii has not been thoroughly studied. In this study, we found the PhoP/PhoQ system in C. sakazakii ATCC BAA-894 enhanced bacterial growth in conditions with low Mg²⁺, acid pH, and the presence of polymyxin B. Moreover, the ΔphoPQ strain significantly reduced survival following exposure to heat, high osmotic pressure, oxidative or bile salts compared with WT strain. Furthermore, the RNA-seq analysis indicated that 1029 genes were upregulated and 979 genes were downregulated in ΔphoPQ strain. The bacterial secretion system, flagella assembly, beta-Lactam resistance and two-component system pathways were significantly downregulated, while the ABC transporters and microbial metabolism in diverse environments pathways were upregulated. qRT-PCR analysis further confirmed that twelve genes associated with stress tolerance were positively regulated by the PhoP/PhoQ system. Therefore, these findings suggest that the PhoP/PhoQ system is an important regulatory mechanism for C. sakazakii to resist various environmental stress.

Residue Dynamics of Streptomycin in Citrus Delivered by Foliar Spray and Trunk Injection and Effect on 'Candidatus Liberibacter asiaticus' Titer

Streptomycin (STR) has been used to control citrus huanglongbing (HLB) caused by 'Candidatus Liberibacter asiaticus' (CLas) via foliar spray. Here, we studied the residue dynamics of STR and its effect on CLas titers in planta applied by foliar spray and trunk injection of 3-year-old citrus trees that were naturally infected by CLas in the field. After foliar spray, STR levels in leaves peaked at 2 to 7 days postapplication (dpa) and gradually declined thereafter. The STR spray did not significantly affect CLas titers in leaves of treated plants as determined by quantitative PCR. After trunk injection, peak levels of STR were observed 7 to 14 dpa in the leaf and root tissues, and near-peak levels were sustained for another 14 days before significantly declining. At 12 months after injection, moderate to low or undetectable levels of STR were observed in the leaf, root, and fruit, depending on the doses of STR injected, with a residue level of 0.28 µg/g in harvested fruit at the highest injection concentration of 2.0 µg/tree. CLas titers in leaves were significantly reduced by trunk injection of STR at 1.0 or 2.0 g/tree, starting from 7 dpa and throughout the experimental period. The reduction of CLas titers was positively correlated with STR residue levels in leaves. The in planta minimum effective concentration of STR needed to suppress the CLas titer to an undetectable level (cycle threshold ≥36.0) was 1.92 µg/g fresh weight. Determination of the in planta minimum effective concentration of STR against CLas and its spatiotemporal residue levels in planta provides the guidance to use STR for HLB management.

Discovery of Novel GMPS Inhibitors of Candidatus Liberibacter Asiaticus by Structure Based Design and Enzyme Kinetic

Simple Summary

The spread of citrus Huanglongbing caused significant damage to the world’s citrus industry. Thermotherapy and chemical agents were used to control this disease; however, the effectiveness of these treatments is frequently inconsistent. In addition, CLas cannot be cultured in vitro. Therefore, structure-based virtual screening is a novel method to find compounds that work against CLas. This study used CLas GMPS as a target for high-throughput screening and selected some compounds which have a higher binding affinity to test their inhibition of CLas GMPS. Finally, two molecules were identified as the lead compound to control citrus HLB.

Abstract

Citrus production is facing an unprecedented problem because of huanglongbing (HLB) disease. Presently, no effective HLB-easing method is available when citrus becomes infected. Guanosine 5′-monophosphate synthetase (GMPS) is a key protein in the de novo synthesis of guanine nucleotides. GMPS is used as an attractive target for developing agents that are effective against the patogen infection. In this research, homology modeling, structure-based virtual screening, and molecular docking were used to discover the new inhibitors against CLas GMPS. Enzyme assay showed that folic acid and AZD1152 showed high inhibition at micromole concentrations, with AZD1152 being the most potent molecule. The inhibition constant (K_i) value of folic acid and AZD1152 was 51.98 µM and 4.05 µM, respectively. These results suggested that folic acid and AZD1152 could be considered as promising candidates for the development of CLas agents.

Engineering of citrus to obtain huanglongbing resistance

Huanglongbing (HLB) disease is threatening the sustainability of citriculture in affected regions because of its rapid spread and the severity of the symptoms it induces. Herein, we summarise the main research findings that can be exploited to develop HLB-resistant cultivars. A major bottleneck has been the lack of a system for the ex vivo cultivation of HLB-associated bacteria (CLs) in true plant hosts, which precludes the evaluation of target genes/metabolites in reliable plant/pathogen/vector environments. With regard to HLB vectors, several biotechnologies which have been proven in laboratory settings to be effective for insect control are presented. Finally, new genotypes that are resistant to CLs or their insect vectors are described, and the most relevant strategies for fighting HLB are highlighted.

Novel Small Molecule Growth Inhibitors of Xanthomonas spp. Causing Bacterial Spot of Tomato

Bacterial spot (BS) of tomato, caused by Xanthomonas gardneri, X. perforans, X. vesicatoria, and X. euvesicatoria, is difficult to control because of the high prevalence of copper- and streptomycin-resistant strains and the lack of resistance cultivars and effective bactericides. The objective of this study was to identify novel growth inhibitors of BS-causing Xanthomonas (BS-X) species by using small molecules (SM; n = 4,182). Several SMs (X1, X2, X5, X9, X12, and X16) completely inhibited the growth of BS-X isolates (n = 68 X. gardneri, 55 X. perforans, 4 X. vesicatoria, and 32 X. euvesicatoria) at ≥12.5 µM by disrupting Xanthomonas cell integrity through weakening of the cell membrane and formation of pores. These SMs were also effective against biofilm-embedded, copper- and streptomycin-resistant Xanthomonas strains while having minimal impact on other plant pathogenic (n = 20) and beneficial bacteria (n = 12). Furthermore, these SMs displayed equivalent antimicrobial activity against BS-X in seeds and X. gardneri in seedlings compared with conventional control methods (copper sulfate and streptomycin) at similar concentrations while having no detectable toxicity to tomato tissues. SMs X2, X5, and X12 reduced X. gardneri, X. perforans, X. vesicatoria, and X. euvesicatoria populations in artificially infested seeds ≤3.4-log CFU/seed 1 day postinfection (dpi) compared with the infested untreated control (P ≤ 0.05). SMs X1, X2, X5, and X12 reduced disease severity ≤72% and engineered bioluminescent X. gardneri populations ≤3.0-log CFU/plant in infected seedlings at 7 dpi compared with the infected untreated control (P ≤ 0.05). Additional studies are needed to increase the applicability of these SMs for BS management in tomato production.

A stable antimicrobial peptide with dual functions of treating and preventing citrus Huanglongbing

Citrus Huanglongbing (HLB), caused by a vector-transmitted phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas), is the most devastating citrus disease worldwide. Currently, there are no effective strategies to prevent infection or to cure HLB-positive trees. Here, using comparative analysis between HLB-sensitive citrus cultivars and HLB-tolerant citrus hybrids and relatives, we identified a novel class of stable antimicrobial peptides (SAMPs). The SAMP from Microcitrusaustraliasica can rapidly kill Liberibacter crescens (Lcr), a culturable Liberibacter strain, and inhibit infections of CLas and CL. solanacearum in plants. In controlled greenhouse trials, SAMP not only effectively reduced CLas titer and disease symptoms in HLB-positive trees but also induced innate immunity to prevent and inhibit infections. Importantly, unlike antibiotics, SAMP is heat stable, making it better suited for field applications. Spray-applied SAMP was taken up by citrus leaves, stayed stable inside the plants for at least a week, and moved systemically through the vascular system where CLas is located. We further demonstrate that SAMP is most effective on α-proteobacteria and causes rapid cytosol leakage and cell lysis. The α-helix-2 domain of SAMP is sufficient to kill Lcr Future field trials will help determine the efficacy of SAMP in controlling HLB and the ideal mode of application.

Defeating Huanglongbing Pathogen Candidatus Liberibacter asiaticus With Indigenous Citrus Endophyte Bacillus subtilis L1-21

Huanglongbing (HLB) has turned into a devastating botanical pandemic of citrus crops, caused by Candidatus Liberibacter asiaticus (CLas). However, until now the disease has remained incurable with very limited control strategies available. Restoration of the affected microbiomes in the diseased host through the introduction of an indigenous endophyte Bacillus subtilis L1-21 isolated from healthy citrus may provide an innovative approach for disease management. A novel half-leaf method was developed in vitro to test the efficacy of the endophyte L1-21 against CLas. Application of B. subtilis L1-21 at 10⁴ colony forming unit (cfu ml^-1) resulted in a 1,000-fold reduction in the CLas copies per gram of leaf midrib (10⁷ to 10⁴) in 4 days. In HLB-affected citrus orchards over a period of 2 years, the CLas incidence was reduced to < 3%, and CLas copies declined from 10⁹ to 10⁴ g^-1 of diseased leaf midribs in the endophyte L1-21 treated trees. Reduction in disease incidence may corroborate a direct or an indirect biocontrol effect of the endophytes as red fluorescent protein-labeled B. subtilis L1-21 colonized and shared niche (phloem) with CLas. This is the first large-scale study for establishing a sustainable HLB control strategy through citrus endophytic microbiome restructuring using an indigenous endophyte.

Plant hairy roots enable high throughput identification of antimicrobials against Candidatus Liberibacter spp

A major bottleneck in identifying therapies to control citrus greening and other devastating plant diseases caused by fastidious pathogens is our inability to culture the pathogens in defined media or axenic cultures. As such, conventional approaches for antimicrobial evaluation (genetic or chemical) rely on time-consuming, low-throughput and inherently variable whole-plant assays. Here, we report that plant hairy roots support the growth of fastidious pathogens like Candidatus Liberibacter spp., the presumptive causal agents of citrus greening, potato zebra chip and tomato vein greening diseases. Importantly, we leverage the microbial hairy roots for rapid, reproducible efficacy screening of multiple therapies. We identify six antimicrobial peptides, two plant immune regulators and eight chemicals which inhibit Candidatus Liberibacter spp. in plant tissues. The antimicrobials, either singly or in combination, can be used as near- and long-term therapies to control citrus greening, potato zebra chip and tomato vein greening diseases.

Evaluation of Bronopol and Disulfiram as Potential Candidatus Liberibacter asiaticus Inosine 5'-Monophosphate Dehydrogenase Inhibitors by Using Molecular Docking and Enzyme Kinetic

Citrus huanglongbing (HLB) is a destructive disease that causes significant damage to many citrus producing areas worldwide. To date, no strategy against this disease has been established. Inosine 5'-monophosphate dehydrogenase (IMPDH) plays crucial roles in the de novo synthesis of guanine nucleotides. This enzyme is used as a potential target to treat bacterial infection. In this study, the crystal structure of a deletion mutant of CLas IMPDHΔ98-201 in the apo form was determined. Eight known bioactive compounds were used as ligands for molecular docking. The results showed that bronopol and disulfiram bound to CLas IMPDHΔ98-201 with high affinity. These compounds were tested for their inhibition against CLas IMPDHΔ98-201 activity. Bronopol and disulfiram showed high inhibition at nanomolar concentrations, and bronopol was found to be the most potent molecule (Ki = 234 nM). The Ki value of disulfiram was 616 nM. These results suggest that bronopol and disulfiram can be considered potential candidate agents for the development of CLas inhibitors.

Challenging battles of plants with phloem-feeding insects and prokaryotic pathogens

For the past 4 decades, intensive molecular studies of mostly leaf mesophyll cell-infecting pathogens and chewing insects have led to compelling models of plant-pathogen and plant-insect interactions. Yet, some of the most devastating pathogens and insect pests live in or feed on the phloem, a systemic tissue belonging to the plant vascular system. Phloem tissues are difficult to study, and phloem-inhabiting pathogens are often impossible to culture, thus limiting our understanding of phloem-insect/pathogen interactions at a molecular level. In this Perspective, we highlight recent literature that reports significant advances in the understanding of phloem interactions with insects and prokaryotic pathogens and attempt to identify critical questions that need attention for future research. It is clear that study of phloem-insect/pathogen interactions represents an exciting frontier of plant science, and influx of new scientific expertise and funding is crucial to achieve faster progress in this important area of research that is integral to global food security.