Molecular characterization of XopAG effector AvrGf2 from Xanthomonas fuscans ssp. aurantifolii in grapefruit

Genomic insights into two new subspecies of Herbaspirillum huttiense strains isolated from diseased foliage in Florida

The genus Herbaspirillum comprises 13 species, the majority of which are plant colonizers. However, some species are occasionally isolated from environmental sources, including water and polluted soil, while others are opportunistic human pathogens. Four novel bacterial strains were isolated from diseased foliage of tomato and Boston fern in Florida, USA. Phylogenetic analysis based on the 16S rRNA gene sequence placed all strains into the genus Herbaspirillum. The Gram-negative strains produced opaque, creamy white, mucoid colonies, which is typical of the genus Herbaspirillum. Biolog biochemical profiling also identified those strains as members of Herbaspirillum. The strains were subjected to whole-genome sequencing, and their genomes were compared with those of reference strains of Herbaspirillum spp. using average nucleotide identity (ANI). The two strains isolated from Boston fern shared 99% pairwise ANI, as did the two strains isolated from tomato. Among all reference genomes tested, the novel strains shared the highest ANI to Herbaspirillum huttiense subsp. huttiense (G21-1742 and NC 40101, 96.76%; SE1, 97.23%; F1, 97.16%) and to H. huttiense subsp. putei. These values are above the established 95% threshold for species delineation based on ANI. As the ANI between members of the two currently described subspecies of H. huttiense, i.e. huttiense and putei, is also ~97%, it can be inferred that the two groups of novel strains described in this study should be considered as candidates for classification as two new subspecies of H. huttiense, given that the current H. huttiense subspecies also have ~97% with the fern and tomato strains. In silico DNA-DNA hybridization results were consistent with those of ANI; comparison of G21-1742 and NC 40101 with H. huttiense subsp. putei IAM 15032Tand H. huttiense subsp. huttiense LMG 2199T produced DNA-DNA hybridization (DDH) values of 66.1 and 73.6 %, respectively. Similarly, SE1 and F1 had DDH values of 68.9 and 68.8% with H. huttiense subsp. putei IAM 15032T and 77.1 and 76.7% with H. huttiense subsp. huttiense LMG 2199T, respectively. The genomes of all novel isolates carry genes involved in plant pathogenesis, including those of the type III secretion system, which are not present in other H. huttiense strains. Based on genomic and phenotypic data, we conclude that these strains represent the first phytopathogenic subspecies within H. huttiense and the names proposed are H. huttiense subsp. nephrolepidis for the two strains isolated from Nephrolepis exaltata (designated strain, G21-1742=LMG 33362=NCPPB 4765) and H. huttiense subsp. lycopersici (designated strain, SE1=LMG 3361=NCPPB 4764) for the two strains isolated from Solanum lycopersicum.

Whole-Genome-Sequence-Based Classification of Xanthomonas euvesicatoria pv. eucalypti and Computational Analysis of the Type III Secretion System

Xanthomonas spp. infect a wide range of annual and perennial plants. Bacterial blight in young seedlings of Eucalyptus spp. in Indonesia was originally identified as X. perforans. However, these strains failed to elicit a hypersensitive response (HR) on either tomatoes or peppers. Two of the strains, EPK43 and BCC 972, when infiltrated into tomato and pepper leaves, failed to grow to significant levels in comparison with well-characterized X. euvesicatoria pv. perforans (Xp) strains. Furthermore, spray inoculation of 'Bonny Best' tomato plants with a bacterial suspension of the Eucalyptus strains resulted in no obvious symptoms. We sequenced the whole genomes of eight strains isolated from two Eucalyptus species between 2007 and 2015. The strains had average nucleotide identities (ANIs) of at least 97.8 with Xp and X. euvesicatoria pv. euvesicatoria (Xeu) strains, both of which are causal agents of bacterial spot of tomatoes and peppers. A comparison of the Eucalyptus strains revealed that the ANI values were >99.99% with each other. Core genome phylogeny clustered all Eucalyptus strains with X. euvesicatoria pv. rosa. They formed separate clades, which included X. euvesicatoria pv. alangii, X. euvesicatoria pv. citrumelonis, and X. euvesicatoria pv. alfalfae. Based on ANI, phylogenetic relationships, and pathogenicity, we designated these Eucalyptus strains as X. euvesicatoria pv. eucalypti (Xee). Comparative analysis of sequenced strains provided unique profiles of type III secretion effectors. Core effector XopD, present in all pathogenic Xp and Xeu strains, was absent in the Xee strains. Comparison of the hrp clusters of Xee, Xp, and Xeu genomes revealed that HrpE in Xee strains was very different from that in Xp and Xeu. To determine if it was functional, we deleted the gene and complemented with the Xee hrpE, confirming it was essential for secretion of type III effectors. HrpE has a hypervariable N-terminus in Xanthomonas spp., in which the N-terminus of Xee strains differs significantly from those of Xeu and Xp strains.

A promoter trap in transgenic citrus mediates recognition of a broad spectrum of Xanthomonas citri pv. citri TALEs, including in planta-evolved derivatives

Citrus bacterial canker (CBC), caused by Xanthomonas citri subsp. citri (Xcc), causes dramatic losses to the citrus industry worldwide. Transcription activator-like effectors (TALEs), which bind to effector binding elements (EBEs) in host promoters and activate transcription of downstream host genes, contribute significantly to Xcc virulence. The discovery of the biochemical context for the binding of TALEs to matching EBE motifs, an interaction commonly referred to as the TALE code, enabled the in silico prediction of EBEs for each TALE protein. Using the TALE code, we engineered a synthetic resistance (R) gene, called the Xcc-TALE-trap, in which 14 tandemly arranged EBEs, each capable of autonomously recognizing a particular Xcc TALE, drive the expression of Xanthomonas avrGf2, which encodes a bacterial effector that induces plant cell death. Analysis of a corresponding transgenic Duncan grapefruit showed that transcription of the cell death-inducing executor gene, avrGf2, was strictly TALE-dependent and could be activated by several different Xcc TALE proteins. Evaluation of Xcc strains from different continents showed that the Xcc-TALE-trap mediates resistance to this global panel of Xcc isolates. We also studied in planta-evolved TALEs (eTALEs) with novel DNA-binding domains and found that these eTALEs also activate the Xcc-TALE-trap, suggesting that the Xcc-TALE-trap is likely to confer durable resistance to Xcc. Finally, we show that the Xcc-TALE-trap confers resistance not only in laboratory infection assays but also in more agriculturally relevant field studies. In conclusion, transgenic plants containing the Xcc-TALE-trap offer a promising sustainable approach to control CBC.

Suppression of citrus canker disease mediated by flagellin perception

Citrus cancer, caused by strains of Xanthomonas citri (Xc) and Xanthomonas aurantifolii (Xa), is one of the most economically important citrus diseases. Although our understanding of the molecular mechanisms underlying citrus canker development has advanced remarkably in recent years, exactly how citrus plants fight against these pathogens remains largely unclear. Using a Xa pathotype C strain that infects Mexican lime only and sweet oranges as a pathosystem to study the immune response triggered by this bacterium in these hosts, we herein report that the Xa flagellin C protein (XaFliC) acts as a potent defence elicitor in sweet oranges. Just as Xa blocked canker formation when coinfiltrated with Xc in sweet orange leaves, two polymorphic XaFliC peptides designated flgIII-20 and flgIII-27, not related to flg22 or flgII-28 but found in many Xanthomonas species, were sufficient to protect sweet orange plants from Xc infection. Accordingly, ectopic expression of XaFliC in a Xc FliC-defective mutant completely abolished the ability of this mutant to grow and cause canker in sweet orange but not Mexican lime plants. Because XaFliC and flgIII-27 also specifically induced the expression of several defence-related genes, our data suggest that XaFliC acts as a main immune response determinant in sweet orange plants.

Ralstonia solanacearum Effectors Localize to Diverse Organelles in Solanum Hosts

Phytopathogenic bacteria secrete type III effector (T3E) proteins directly into host plant cells. T3Es can interact with plant proteins and frequently manipulate plant host physiological or developmental processes. The proper subcellular localization of T3Es is critical for their ability to interact with plant targets, and knowledge of T3E localization can be informative for studies of effector function. Here we investigated the subcellular localization of 19 T3Es from the phytopathogenic bacteria Ralstonia pseudosolanacearum and Ralstonia solanacearum. Approximately 45% of effectors in our library localize to both the plant cell periphery and the nucleus, 15% exclusively to the cell periphery, 15% exclusively to the nucleus, and 25% to other organelles, including tonoplasts and peroxisomes. Using tomato hairy roots, we show that T3E localization is similar in both leaves and roots and is not impacted by Solanum species. We find that in silico prediction programs are frequently inaccurate, highlighting the value of in planta localization experiments. Our data suggest that Ralstonia targets a wide diversity of cellular organelles and provides a foundation for developing testable hypotheses about Ralstonia effector function.

Secrete or perish: The role of secretion systems in Xanthomonas biology

Bacteria of the Xanthomonas genus are mainly phytopathogens of a large variety of crops of economic importance worldwide. Xanthomonas spp. rely on an arsenal of protein effectors, toxins and adhesins to adapt to the environment, compete with other microorganisms and colonize plant hosts, often causing disease. These protein effectors are mainly delivered to their targets by the action of bacterial secretion systems, dedicated multiprotein complexes that translocate proteins to the extracellular environment or directly into eukaryotic and prokaryotic cells. Type I to type VI secretion systems have been identified in Xanthomonas genomes. Recent studies have unravelled the diverse roles played by the distinct types of secretion systems in adaptation and virulence in xanthomonads, unveiling new aspects of their biology. In addition, genome sequence information from a wide range of Xanthomonas species and pathovars have become available recently, uncovering a heterogeneous distribution of the distinct families of secretion systems within the genus. In this review, we describe the architecture and mode of action of bacterial type I to type VI secretion systems and the distribution and functions associated with these important nanoweapons within the Xanthomonas genus.

Plant responses underlying nonhost resistance of Citrus limon against Xanthomonas campestris pv. campestris

Citrus is an economically important fruit crop that is severely afflicted by citrus canker, a disease caused by Xanthomonas citri ssp. citri (X. citri); thus, new sustainable strategies to manage this disease are needed. Although all Citrus spp. are susceptible to this pathogen, they are resistant to other Xanthomonas species, exhibiting non-host resistance (NHR), for example, to the brassica pathogen X. campestris pv. campestris (Xcc) and a gene-for-gene host defence response (HDR) to the canker-causing X. fuscans ssp. aurantifolii (Xfa) strain C. Here, we examine the plant factors associated with the NHR of C. limon to Xcc. We show that Xcc induced asymptomatic type I NHR, allowing the bacterium to survive in a stationary phase in the non-host tissue. In C. limon, this NHR shared some similarities with HDR; both defence responses interfered with biofilm formation, and were associated with callose deposition, induction of the salicylic acid (SA) signalling pathway and the repression of abscisic acid (ABA) signalling. However, greater stomatal closure was seen during NHR than during HDR, together with different patterns of accumulation of reactive oxygen species and phenolic compounds and the expression of secondary metabolites. Overall, these differences, independent of Xcc type III effector proteins, could contribute to the higher protection elicited against canker development. We propose that Xcc may have the potential to steadily activate inducible defence responses. An understanding of these plant responses (and their triggers) may allow the development of a sustained and sustainable resistance to citrus canker.

Recent advances in the understanding of Xanthomonas citri ssp. citri pathogenesis and citrus canker disease management

Taxonomic status: Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; Species Xanthomonas citri ssp. citri (Xcc). Host range: Compatible hosts vary in their susceptibility to citrus canker (CC), with grapefruit, lime and lemon being the most susceptible, sweet orange being moderately susceptible, and kumquat and calamondin being amongst the least susceptible. Microbiological properties: Xcc is a rod-shaped (1.5-2.0 × 0.5-0.75 µm), Gram-negative, aerobic bacterium with a single polar flagellum. The bacterium forms yellow colonies on culture media as a result of the production of xanthomonadin. Distribution: Present in South America, the British Virgin Islands, Africa, the Middle East, India, Asia and the South Pacific islands. Localized incidence in the USA, Argentina, Brazil, Bolivia, Uruguay, Senegal, Mali, Burkina Faso, Tanzania, Iran, Saudi Arabia, Yemen and Bangladesh. Widespread throughout Paraguay, Comoros, China, Japan, Malaysia and Vietnam. Eradicated from South Africa, Australia and New Zealand. Absent from Europe.

Behind the lines-actions of bacterial type III effector proteins in plant cells

Pathogenicity of most Gram-negative plant-pathogenic bacteria depends on the type III secretion (T3S) system, which translocates bacterial effector proteins into plant cells. Type III effectors modulate plant cellular pathways to the benefit of the pathogen and promote bacterial multiplication. One major virulence function of type III effectors is the suppression of plant innate immunity, which is triggered upon recognition of pathogen-derived molecular patterns by plant receptor proteins. Type III effectors also interfere with additional plant cellular processes including proteasome-dependent protein degradation, phytohormone signaling, the formation of the cytoskeleton, vesicle transport and gene expression. This review summarizes our current knowledge on the molecular functions of type III effector proteins with known plant target molecules. Furthermore, plant defense strategies for the detection of effector protein activities or effector-triggered alterations in plant targets are discussed.