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Although the phloem is a highly specialized tissue, certain pathogens, including phytoplasmas, spiroplasmas, and viruses, have evolved to access and live in this sequestered and protected environment, causing substantial economic harm. In particular, Candidatus Liberibacter spp. are devastating citrus in many parts of the world. Given that most phloem pathogens are vectored, they are not exposed to applied chemicals and are therefore difficult to control. Furthermore, pathogens use the phloem network to escape mounted defenses. Our review summarizes the current knowledge of phloem anatomy, physiology, and biochemistry relevant to phloem/pathogen interactions. We focus on aspects of anatomy specific to pathogen movement, including sieve plate structure and phloem-specific proteins. Phloem sampling techniques are discussed. Finally, pathogens that cause particular harm to the phloem of crop species are considered in detail.

Expression studies have consistently identified tumor protein D52 (TPD52) overexpression in tumor cells. Murine TPD52 (mD52) shares 86% identity with the human orthologue. To study a possible role for TPD52 in transformation, 3T3 fibroblasts were transfected with the full-length cDNA for mD52. Expression of mD52 was confirmed by reverse transcription-PCR (RT-PCR), real-time PCR, and Western blot analysis compared with 3T3 and vector-transfected 3T3 (3T3.V), and the resultant cell line was designated 3T3.mD52. At 4 weeks, 3T3.mD52 gained a 2-fold increase in growth rate, lost contact inhibition, and exhibited a marked phenotype change. Further characterization revealed an acquired ability for anchorage-independent cell growth. To determine whether 3T3.mD52 had become tumorigenic, naïve, healthy, immunocompetent syngeneic mice were inoculated subcutaneously with varying cell doses. Tumors measuring >1 cm2 were detected 60 days postinoculation with 3T3.mD52, and a 50% subcutaneous tumor incidence was obtained with as few as 5 × 105 3T3.mD52 cells. Remarkably, when lungs from 3T3.mD52 tumor-bearing mice were analyzed, numerous tumor nodules were observed, ranging from nodules less than 10 to nodules too numerous to count (inoculation with 1 × 105 and 5 × 106 cells, respectively). Further support for the metastatic capacity of 3T3.mD52 was the demonstration that transforming growth factor (TGF)-βR1 (receptor) expression decreased and TGF-β1 secretion increased in 3T3.mD52 compared with 3T3 controls. cDNA microarray analysis showed a gene expression pattern that further supported mD52-induced transformation and metastasis. Together, these data suggest that mD52 expression in 3T3 cells initiated cellular transformation, tumorigenesis, and progression to metastasis. (Mol Cancer Res 2007;5(2):133–44)

Supplementary Data from Induction of Tumorigenesis and Metastasis by the Murine Orthologue of Tumor Protein D52

Oculomotor target selection often requires discriminating visual features, but it remains unclear how oculomotor substrates encoding saccade vectors functionally contribute to this process. One possibility is that oculomotor vector representations (observed directly as physiological activation or inferred from behavioral interference) of potential targets are continuously reweighted by task relevance computed elsewhere in specialized visual modules, whereas an alternative possibility is that oculomotor modules use local featural analyses to actively discriminate potential targets. Strengthening the former account, oculomotor vector representations have longer onset latencies for ventral- (i.e., color) than dorsal-stream features (i.e., luminance), suggesting that oculomotor vector representations originate from featurally relevant specialized visual modules. Here, we extended this reasoning by behaviorally examining whether the onset latency of saccadic interference elicited by visually complex stimuli is greater than is commonly observed for simple stimuli. We measured human saccade metrics (saccade curvature, endpoint deviations, saccade frequency, and error proportion) as a function of time after abrupt distractor onset. Distractors were novel, visually complex, and had to be discriminated from targets to guide saccades. The earliest saccadic interference latency was ∼110 ms, considerably longer than previous experiments, suggesting that sensory representations projected into the oculomotor system are gated to allow for sufficient featural processing to satisfy task demands. Surprisingly, initial oculomotor vector representations encoded features, as we manipulated the visual similarity between targets and distractors and observed increased vector modulation response magnitude and duration when the distractor was highly similar to the target. Oculomotor vector modulation was gradually extinguished over the time course of the experiment.

Phytopathogens use secreted effector proteins to suppress host immunity and promote pathogen virulence, and there is increasing evidence that the host-pathogen interactome comprises a complex network. To identify novel interactors of the Pseudomonas syringae effector HopZ1a, we performed a yeast two-hybrid screen that identified a previously uncharacterized Arabidopsis protein that we designate HopZ1a interactor 1 (ZIN1). Additional analyses in yeast and in planta revealed that ZIN1 also interacts with several other P. syringae effectors. We show that an Arabidopsis loss-of-function zin1 mutant is less susceptible to infection by certain strains of P. syringae, while overexpression of ZIN1 results in enhanced susceptibility. Functionally, ZIN1 exhibits topoisomerase-like activity in vitro. Transcriptional profiling of wild-type and zin1 Arabidopsis plants inoculated with P. syringae indicated that while ZIN1 regulates a wide range of pathogen-responsive biological processes, the list of genes more highly expressed in zin1 versus wild-type plants is particularly enriched for ribosomal protein genes. Altogether, these data illuminate ZIN1 as a potential susceptibility hub that interacts with multiple effectors to influence the outcome of plant-microbe interactions. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Pathogens secrete effector proteins into host cells to suppress host immunity and promote pathogen virulence, although many features at the molecular interface of host-pathogen interactions remain to be characterized. In a yeast two-hybrid assay, we found that the Pseudomonas syringae effector HopZ1a interacts with the Arabidopsis transcriptional regulator Abscisic Acid Repressor 1 (ABR1). Further analysis revealed that ABR1 interacts with multiple P. syringae effectors, suggesting that it may be targeted as a susceptibility hub. Indeed, loss-of-function abr1 mutants exhibit reduced susceptibility to a number of P. syringae strains. The ABR1 protein comprises a conserved APETALA2 (AP2) domain flanked by long regions of predicted structural disorder. We verified the DNA-binding activity of the AP2 domain and demonstrated that the disordered domains act redundantly to enhance DNA binding and to facilitate transcriptional activation by ABR1. Finally, we compared gene expression profiles from wild-type and abr1 plants following inoculation with P. syringae, which suggested that the reduced susceptibility of abr1 mutants is due to the loss of a virulence target rather than an enhanced immune response. These data highlight ABR1 as a functionally important component at the host-pathogen interface.

International audience; Plant diseases are an important threat to food production. While major pathogenicity determinants required for disease have been extensively studied, less is known on how pathogens thrive during host colonization, especially at early infection stages. Here, we used randomly barcoded-transposon insertion site sequencing (RB-TnSeq) to perform a genome-wide screen and identify key bacterial fitness determinants of the vascular pathogen Xanthomonas campestris pv campestris (Xcc) during infection of the cauliflower host plant (Brassica oleracea). This high-throughput analysis was conducted in hydathodes, the natural entry site of Xcc, in xylem sap and in synthetic media. Xcc did not face a strong bottleneck during hydathode infection. In total, 181 genes important for fitness were identified in plant-associated environments with functional enrichment in genes involved in metabolism but only few genes previously known to be involved in virulence. The biological relevance of 12 genes was independently confirmed by phenotyping single mutants. Notably, we show that XC_3388, a protein with no known function (DUF1631), plays a key role in the adaptation and virulence of Xcc possibly through c-di-GMP-mediated regulation. This study revealed yet unsuspected social behaviors adopted by Xcc individuals when confined inside hydathodes at early infection stages.

Pathogen pressure on hosts can lead to the evolution of genes regulating the innate immune response. By characterizing naturally occurring polymorphisms in immune receptors, we can better understand the molecular determinants of pathogen recognition. ZAR1 is an ancient Arabidopsis thaliana NLR (Nucleotide-binding [NB] Leucine-rich-repeat [LRR] Receptor) that recognizes multiple secreted effector proteins from the pathogenic bacteria Pseudomonas syringae and Xanthomonas campestris through its interaction with receptor-like cytoplasmic kinases (RLCKs). ZAR1 was first identified for its role in recognizing P. syringae effector HopZ1a, through its interaction with the RLCK ZED1. To identify additional determinants of HopZ1a recognition, we performed a computational screen for ecotypes from the 1001 Genomes project that were likely to lack HopZ1a recognition, and tested ~300 ecotypes. We identified ecotypes containing polymorphisms in ZAR1 and ZED1. Using our previously established Nicotiana benthamiana transient assay and Arabidopsis ecotypes, we tested for the effect of naturally occurring polymorphisms on ZAR1 interactions and the immune response. We identified key residues in the NB or LRR domain of ZAR1 that impact the interaction with ZED1. We demonstrate that natural diversity combined with functional assays can help define the molecular determinants and interactions necessary to regulate immune induction in response to pathogens.

Citrus huanglongbing (HLB), caused by phloem-limited ‘Candidatus Liberibacter’ bacteria, is a destructive disease threatening the worldwide citrus industry. The mechanisms of pathogenesis are poorly understood and no efficient strategy is available to control HLB. Here, we used a comparative genomics screen to identify candidate microbe-associated molecular patterns (MAMPs) from ‘Ca. Liberibacter’ spp. We identified the core genome from multiple ‘Ca. Liberibacter’ pathogens, and searched for core genes with signatures of positive selection. We hypothesized that genes encoding putative MAMPs would evolve to reduce recognition by the plant immune system, while retaining their essential functions. To efficiently screen candidate MAMP peptides, we established a high-throughput microtiter plate-based screening assay, particularly for citrus, that measured reactive oxygen species (ROS) production, which is a common immune response in plants. We found that two peptides could elicit ROS production in Arabidopsis and Nicotiana benthamiana. One of these peptides elicited ROS production and defense gene expression in HLB-tolerant citrus genotypes, and induced MAMP-triggered immunity against the bacterial pathogen Pseudomonas syringae. Our findings identify MAMPs that boost immunity in citrus and could help prevent or reduce HLB infection.

Plant diseases are an important threat to food production. While major pathogenicity determinants required for disease have been extensively studied, less is known on how pathogens thrive during host colonization especially at early infection stages. Here, we used randomly barcoded-transposon insertion site sequencing (RB-TnSeq) to perform a genome-wide screen and identify key bacterial fitness determinants of the vascular pathogenXanthomonas campestrispv.campestris(Xcc) during infection of the cauliflower host plant (Brassica oleracea). This high-throughput analysis was conducted in hydathodes, the natural entry site ofXcc, in xylem sap and in synthetic media.Xccdid not face a strong bottleneck during hydathode infection. 183 genes important for fitness were identified in plant-associated environments with functional enrichment in genes involved in metabolism when only few genes known to be involved in virulence were found to be affected. The biological relevance of 13 genes was independently confirmed by phenotyping single mutants. Notably, we show that the XC_3388, a protein with no known function (DUF1631), plays a key role in the adaptation and virulence ofXccpossibly through c-di-GMP-mediated regulation. This study thus revealed yet unsuspected social behaviors adopted byXccindividuals when confined inside hydathodes at early infection stages.

NLR (Nucleotide-binding [NB] Leucine-rich repeat [LRR] Receptor) proteins are critical for inducing immune responses in response to pathogen proteins, and must be tightly regulated to prevent spurious activation in the absence of a pathogen. The ZAR1 NLR recognizes diverse effector proteins from Pseudomonas syringae, including HopZ1a, and Xanthomonas species. Receptor-like cytoplasmic kinases (RLCKs) such as ZED1, interact with ZAR1 and provide specificity for different effector proteins, such as HopZ1a. We previously developed a transient expression system in Nicotiana benthamiana, that allowed us to demonstrate ZAR1 function is conserved from the Brassicaceae to the Solanaceae. Here, we combined structural modeling of ZAR1, with molecular and functional assays in our transient system, to show that multiple intramolecular and intermolecular interactions regulate ZAR1 activity. We identified new determinants required for the formation of the ZARCC dimer and its activity. Lastly, we characterized new intramolecular interactions between ZAR1 subdomains that participate in keeping ZAR1 immune complexes inactive. This work identifies molecular constraints on immune receptor function and activation.One sentence-summaryStructure-informed analyses reveal multiple finely-tuned intramolecular interactions that regulate the activity of the immune receptor ZAR1.FundingResearch on plant immunity in the Lewis laboratory was supported by the USDA ARS 2030-21000-046-00D and 2030-21000-050-00D (JDL), and the NSF Directorate for Biological Sciences IOS-1557661 (JDL). ECM and AJP acknowledge financial support from UEFISCDI grant PN-III-ID-PCE-2016-0650 and the Romanian Academy programs 1 & 2 of IBAR.

Phytopathogens use secreted effector proteins to suppress host immunity and promote pathogen virulence, and there is increasing evidence that the host-pathogen interactome comprises a complex network. To identify novel interactors of the

Successful oculomotor target selection often requires discriminating visual features but it remains contentious whether oculomotor substrates encoding saccade vectors functionally contribute to this process. One possibility is that visual features are discriminated cortically and oculomotor modules select the object with the highest activation in the set of all preprocessed cortical object representations, while an alternative possibility is that oculomotor modules actively discriminate potential targets based on visual features. If the latter view is correct, these modules should not require input from specialized visual cortices encoding the task relevant features. We therefore examined whether the latency of visual onset responses elicited by abrupt distractor onsets is consistent with input from specialized visual cortices by non-invasively measuring human saccade metrics (saccade curvature, endpoint deviations, saccade frequency, error proportion) as a function of distractor processing time for novel, visually complex distractors that had to be discriminated from a target to guide saccades. Visual onset response latencies were ~110 ms, consistent with projections from anterior cortical sites specialized for object processing. Surprisingly, oculomotor visual onset responses encoded features, as we manipulated the visual similarity between targets and distractors and observed an increased visual onset response magnitude and duration when the distractor was highly similar to the target, which was not attributable to an inhibitory processing delay. Visual onset responses were dynamically modulated by executive function, as these responses were anticipatorily extinguished over the time course of the experiment. As expected, the latency of distractor-related inhibition was modulated by the behavioral relevance of the distractor.Significance StatementWe provide novel insights into the role of the oculomotor system in saccadic target selection by challenging the convention that neural substrates that encode oculomotor vectors functionally contribute to target discrimination. Our data show that the oculomotor system selects a winner from amongst the preprocessed object representations output from specialized visual cortices as supposed to discriminating visual features locally. We also challenge the convention that oculomotor visual onset responses are feature-invariant, as they encoded task-relevance.

Pathogen pressure on hosts can lead to the evolution of genes regulating the innate immune response. By characterizing naturally occurring polymorphisms in immune receptors, we can better understand the molecular determinants of pathogen recognition. ZAR1 is an ancient Arabidopsis thaliana NLR (Nucleotide-binding [NB] Leucine-rich-repeat [LRR] Receptor) that recognizes multiple secreted effector proteins from the pathogenic bacteria Pseudomonas syringae and Xanthomonas campestris through its interaction with receptor-like cytoplasmic kinases (RLCKs). ZAR1 was first identified for its role in recognizing P. syringae effector HopZ1a, through its interaction with the RLCK ZED1. To identify additional determinants of HopZ1a recognition, we performed a computational screen for ecotypes from the 1001 Genomes project that were likely to lack HopZ1a recognition, and tested ~300 ecotypes. We identified ecotypes containing polymorphisms in ZAR1 and ZED1. Using our previously established Nicotiana benthamiana transient assay and Arabidopsis ecotypes, we tested for the effect of naturally occurring polymorphisms on ZAR1 interactions and the immune response. We identified key residues in the NB or LRR domain of ZAR1 that impact the interaction with ZED1. We demonstrate that natural diversity combined with functional assays can help define the molecular determinants and interactions necessary to regulate immune induction in response to pathogens.

Tomato is an agronomically important crop that can be infected by Pseudomonas syringae, a Gram-negative bacterium, resulting in bacterial speck disease. The tomato-P. syringae pv. tomato pathosystem is widely used to dissect the genetic basis of plant innate responses and disease resistance. While disease was successfully managed for many decades through the introduction of the Pto/Prf gene cluster from Solanum pimpinellifolium into cultivated tomato, race 1 strains of P. syringae have evolved to overcome resistance conferred by the Pto/Prf gene cluster and occur worldwide. Wild tomato species are important reservoirs of natural diversity in pathogen recognition, because they evolved in diverse environments with different pathogen pressures. In typical screens for disease resistance in wild tomato, adult plants are used, which can limit the number of plants that can be screened due to their extended growth time and greater growth space requirements. We developed a method to screen 10-day-old tomato seedlings for resistance, which minimizes plant growth time and growth chamber space, allows a rapid turnover of plants, and allows large sample sizes to be tested. Seedling outcomes of survival or death can be treated as discrete phenotypes or on a resistance scale defined by amount of new growth in surviving seedlings after flooding. This method has been optimized to screen 10-day-old tomato seedlings for resistance to two P. syringae strains and can easily be adapted to other P. syringae strains.

Citrus huanglongbing (HLB), caused by phloem-limited '

CRISPR-Cas technologies have provided programmable gene editing tools that have revolutionized research. The leading CRISPR-Cas9 and Cas12a enzymes are ideal for programmed genetic manipulation, however, they are limited for genome-scale interventions. Here, we utilized a Cas3-based system featuring a processive nuclease, expressed endogenously or heterologously, for genome engineering purposes. Using an optimized and minimal CRISPR-Cas3 system (Type I-C) programmed with a single crRNA, large deletions ranging from 7 - 424 kb were generated in Pseudomonas aeruginosa with high efficiency and speed. By comparison, Cas9 yielded small deletions and point mutations. Cas3-generated deletion boundaries were variable in the absence of a homology-directed repair (HDR) template, and successfully and efficiently specified when present. The minimal Cas3 system is also portable; large deletions were induced with high efficiency in Pseudomonas syringae and Escherichia coli using an “all-in-one” vector. Notably, Cas3 generated bi-directional deletions originating from the programmed cut site, which was exploited to iteratively reduce a P. aeruginosa genome by 837 kb (13.5%) using 10 distinct crRNAs. We also demonstrate the utility of endogenous Cas3 systems (Type I-C and I-F) and develop an “anti-anti-CRISPR” strategy to circumvent endogenous CRISPR-Cas inhibitor proteins. CRISPR-Cas3 could facilitate rapid strain manipulation for synthetic biological and metabolic engineering purposes, genome minimization, and the analysis of large regions of unknown function.

The identity of the active sites of hydrophobic Lewis acid zeolites was elucidated by solid-state 31P nuclear magnetic resonance (NMR) spectroscopy following adsorption of trimethylphosphine oxide (TMPO) probe molecules. The adsorption of TMPO on these materials resulted in distinct 31P NMR resonances between δiso = 59.9 and 54.9 ppm that correspond to unique chemical environments of the Lewis acidic heteroatoms in the Beta zeolite framework. The 31P NMR resonances were assigned to sites in Sn-Beta zeolites by correlating the variation of 31P NMR spectra during TMPO titration experiments with the corresponding changes in the 119Sn NMR spectra. This method allowed us to establish quantitative relationships between the assignments for each site and the catalytic activity for the glucose isomerization and aldol condensation reactions. The rate of glucose isomerization directly correlated with the combined integrated intensities of 31P MAS NMR resonances at δiso = 55.8 and 54.9 ppm, which amounted to 12–33% o...