Confirmation report of Bacterial Leaf Streak disease of rice caused by Xanthomonas oryzae pv. oryzicola in Senegal.

Xanthomonas oryzae pv. X. oryzicola (Xoc), the causal agent of Bacterial Leaf Streak (BLS), is considered as one of the most important emerging pathogens of rice in Africa. This disease is estimated as responsible of 20 to 30% yield loss (Sileshi et Gebeyehu 2021) and has been characterized in several west African countries including Mali and Burkina Faso since 2003 and more recently in Ivory Coast (Wonni et al. 2014, Diallo et al. 2021). Presence of BLS symptoms in Senegal were reported by Trinh in 1980 but, to our knowledge, BLS occurrence has never been validated further and no strain of Xoc have ever been isolated from Senegalese rice fields. Xoc is transmitted by seeds which contribute to its spread through the rice trade (Sileshi et Gebeyehu 2021). To confirm Trinh's observations, we surveyed rice fields between 2014 and 2016 in eight different regions where rice is produced in Senegal. Typical disease symptoms characterized by yellow-brown to black translucent leaf streaks sometimes along with exudates, were detected in fields of several regions and collected. Leaf pieces were successively sanitized, rinsed in sterile water, and symptomatic fragments were ground using the Qiagen Tissue Lyser System (QIAGEN, Courtaboeuf, France). The leaf powder was diluted in 1.5 ml of sterile water and incubated for 30 minutes at room temperature. Ten µl of the suspension was streaked on semi-selective PSA medium and incubated at 28°C for 3 to 7 days. Characteristic round, convex, mucous, straw-yellow Xoc candidate colonies were purified from six individual leaf samples from three distinct sites in the northern Senegal River Valley. To confirm their identity, isolated strains were tested for pathogenicity and molecular characterization. All isolates were subjected to the multiplex PCR developed for the identification of X. oryzae pathovars (Lang et al., 2010) and revealed the same PCR profile (two amplicons of 324 and 691 base pairs) similar to that of the Xoc reference strain BLS256. Leaves of 5-week-old plants of O. sativa cv. Kitaake were infiltrated with a needleless syringe containing a bacterial suspension set at an optical density of 0.5. Upon seven days of incubation under greenhouse conditions (27 ± 1°C with a 12-hour photoperiod), all infiltrated spots (2 spots on 3 plants per isolate) developed water-soaked lesions similar to those caused by control strain BLS256, except when leaves were infiltrated with water. Symptomatic leaf tissues were ground and plated on PSA medium, resulting in colonies with typical Xanthomonas morphology that were diagnosed as Xoc by multiplex PCR typing, thus fulfilling Koch's postulate. At last, four of the isolates were subjected to gyrB sequencing upon PCR amplification using the universal primers XgyrB1F and XgyrB1R (Young et al., 2008). Analysis of 780bp partial gyrB sequences of strains S18-3-4, S23-1-12, S52-1-4 and S52-1-10 highlighted 100% identity with the gyrB sequence of strain BLS256 (Acc. No. CP003057). To our knowledge, this is the first report of BLS in Senegal which is supported by molecular characterization methods. This study validates the presence of BLS in Senegal and will serve as a basis for future efforts of rice breeding for locally adapted resistance. More studies are needed to clarify the spatial distribution and prevalence of BLS in Senegal as rice cultivation is expanding rapidly in the country.

The Rice ILI2 Locus Is a Bidirectional Target of the African Xanthomonas oryzae pv. oryzae Major Transcription Activator-like Effector TalC but Does Not Contribute to Disease Susceptibility.

Xanthomonas oryzae pv. oryzae (Xoo) strains that cause bacterial leaf blight (BLB) limit rice (Oryza sativa) production and require breeding more resistant varieties. Transcription activator-like effectors (TALEs) activate transcription to promote leaf colonization by binding to specific plant host DNA sequences termed effector binding elements (EBEs). Xoo major TALEs universally target susceptibility genes of the SWEET transporter family. TALE-unresponsive alleles of clade III OsSWEET susceptibility gene promoter created with genome editing confer broad resistance on Asian Xoo strains. African Xoo strains rely primarily on the major TALE TalC, which targets OsSWEET14. Although the virulence of a talC mutant strain is severely impaired, abrogating OsSWEET14 induction with genome editing does not confer equivalent resistance on African Xoo. To address this contradiction, we postulated the existence of a TalC target susceptibility gene redundant with OsSWEET14. Bioinformatics analysis identified a rice locus named ATAC composed of the INCREASED LEAF INCLINATION 2 (ILI2) gene and a putative lncRNA that are shown to be bidirectionally upregulated in a TalC-dependent fashion. Gain-of-function approaches with designer TALEs inducing ATAC sequences did not complement the virulence of a Xoo strain defective for SWEET gene activation. While editing the TalC EBE at the ATAC loci compromised TalC-mediated induction, multiplex edited lines with mutations at the OsSWEET14 and ATAC loci remained essentially susceptible to African Xoo strains. Overall, this work indicates that ATAC is a probable TalC off-target locus but nonetheless documents the first example of divergent transcription activation by a native TALE during infection.

Resistance and susceptibility QTL identified in a rice MAGIC population by screening with a minor-effect virulence factor from Xanthomonas oryzae pv. oryzae.

Effective and durable disease resistance for bacterial blight (BB) of rice is a continuous challenge due to the evolution and adaptation of the pathogen, Xanthomonas oryzae pv. oryzae (Xoo), on cultivated rice varieties. Fundamental to this pathogens' virulence is transcription activator-like (TAL) effectors that activate transcription of host genes and contribute differently to pathogen virulence, fitness or both. Host plant resistance is predicted to be more durable if directed at strategic virulence factors that impact both pathogen virulence and fitness. We characterized Tal7b, a minor-effect virulence factor that contributes incrementally to pathogen virulence in rice, is a fitness factor to the pathogen and is widely present in geographically diverse strains of Xoo. To identify sources of resistance to this conserved effector, we used a highly virulent strain carrying a plasmid borne copy of Tal7b to screen an indica multi-parent advanced generation inter-cross (MAGIC) population. Of 18 QTL revealed by genome-wide association studies and interval mapping analysis, six were specific to Tal7b (qBB-tal7b). Overall, 150 predicted Tal7b gene targets overlapped with qBB-tal7b QTL. Of these, 21 showed polymorphisms in the predicted effector binding element (EBE) site and 23 lost the EBE sequence altogether. Inoculation and bioinformatics studies suggest that the Tal7b target in one of the Tal7b-specific QTL, qBB-tal7b-8, is a disease susceptibility gene and that the resistance mechanism for this locus may be through loss of susceptibility. Our work demonstrates that minor-effect virulence factors significantly contribute to disease and provide a potential new approach to identify effective disease resistance.

Functional analysis of African Xanthomonas oryzae pv. oryzae TALomes reveals a new susceptibility gene in bacterial leaf blight of rice.

Most Xanthomonas species translocate Transcription Activator-Like (TAL) effectors into plant cells where they function like plant transcription factors via a programmable DNA-binding domain. Characterized strains of rice pathogenic X. oryzae pv. oryzae harbor 9-16 different tal effector genes, but the function of only a few of them has been decoded. Using sequencing of entire genomes, we first performed comparative analyses of the complete repertoires of TAL effectors, herein referred to as TALomes, in three Xoo strains forming an African genetic lineage different from Asian Xoo. A phylogenetic analysis of the three TALomes combined with in silico predictions of TAL effector targets showed that African Xoo TALomes are highly conserved, genetically distant from Asian ones, and closely related to TAL effectors from the bacterial leaf streak pathogen Xanthomonas oryzae pv. oryzicola (Xoc). Nine clusters of TAL effectors could be identified among the three TALomes, including three showing higher levels of variation in their repeat variable diresidues (RVDs). Detailed analyses of these groups revealed recombination events as a possible source of variation among TAL effector genes. Next, to address contribution to virulence, nine TAL effector genes from the Malian Xoo strain MAI1 and four allelic variants from the Burkinabe Xoo strain BAI3, thus representing most of the TAL effector diversity in African Xoo strains, were expressed in the TAL effector-deficient X. oryzae strain X11-5A for gain-of-function assays. Inoculation of the susceptible rice variety Azucena lead to the discovery of three TAL effectors promoting virulence, including two TAL effectors previously reported to target the susceptibility (S) gene OsSWEET14 and a novel major virulence contributor, TalB. RNA profiling experiments in rice and in silico prediction of EBEs were carried out to identify candidate targets of TalB, revealing OsTFX1, a bZIP transcription factor previously identified as a bacterial blight S gene, and OsERF#123, which encodes a subgroup IXc AP2/ERF transcription factor. Use of designer TAL effectors demonstrated that induction of either gene resulted in greater susceptibility to strain X11-5A. The induction of OsERF#123 by BAI3Δ1, a talB knockout derivative of BAI3, carrying these designer TAL effectors increased virulence of BAI3Δ1, validating OsERF#123 as a new, bacterial blight S gene.

The overexpression of RXam1, a cassava gene coding for an RLK, confers disease resistance to Xanthomonas axonopodis pv. manihotis.

MAIN CONCLUSION: The overexpression of RXam1 leads to a reduction in bacterial growth of XamCIO136, suggesting that RXam1 might be implicated in strain-specific resistance. Cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam) is a prevalent disease in all regions, where cassava is cultivated. CBB is a foliar and vascular disease usually controlled through host resistance. Previous studies have found QTLs explaining resistance to several Xam strains. Interestingly, one QTL called XM5 that explained 13% of resistance to XamCIO136 was associated with a similar fragment of the rice Xa21-resistance gene called PCR250. In this study, we aimed to further identify and characterize this fragment and its role in resistance to CBB. Screening and hybridization of a BAC library using the molecular marker PCR250 as a probe led to the identification of a receptor-like kinase similar to Xa21 and were called RXam1 (Resistance to Xam 1). Here, we report the functional characterization of susceptible cassava plants overexpressing RXam1. Our results indicated that the overexpression of RXam1 leads to a reduction in bacterial growth of XamCIO136. This suggests that RXAM1 might be implicated in strain-specific resistance to XamCIO136.

Allelic variation for broad-spectrum resistance and susceptibility to bacterial pathogens identified in a rice MAGIC population.

Quantitative trait loci (QTL) that confer broad-spectrum resistance (BSR), or resistance that is effective against multiple and diverse plant pathogens, have been elusive targets of crop breeding programmes. Multiparent advanced generation intercross (MAGIC) populations, with their diverse genetic composition and high levels of recombination, are potential resources for the identification of QTL for BSR. In this study, a rice MAGIC population was used to map QTL conferring BSR to two major rice diseases, bacterial leaf streak (BLS) and bacterial blight (BB), caused by Xanthomonas oryzae pathovars (pv.) oryzicola (Xoc) and oryzae (Xoo), respectively. Controlling these diseases is particularly important in sub-Saharan Africa, where no sources of BSR are currently available in deployed varieties. The MAGIC founders and lines were genotyped by sequencing and phenotyped in the greenhouse and field by inoculation with multiple strains of Xoc and Xoo. A combination of genomewide association studies (GWAS) and interval mapping analyses revealed 11 BSR QTL, effective against both diseases, and three pathovar-specific QTL. The most promising BSR QTL (qXO-2-1, qXO-4-1 and qXO-11-2) conferred resistance to more than nine Xoc and Xoo strains. GWAS detected 369 significant SNP markers with distinguishable phenotypic effects, allowing the identification of alleles conferring disease resistance and susceptibility. The BSR and susceptibility QTL will improve our understanding of the mechanisms of both resistance and susceptibility in the long term and will be immediately useful resources for rice breeding programmes.

A resistance locus in the American heirloom rice variety Carolina Gold Select is triggered by TAL effectors with diverse predicted targets and is effective against African strains of Xanthomonas oryzae pv. oryzicola.

The rice pathogens Xanthomonas oryzae pathovar (pv.) oryzae and pv. oryzicola produce numerous transcription activator-like (TAL) effectors that increase bacterial virulence by activating expression of host susceptibility genes. Rice resistance mechanisms against TAL effectors include polymorphisms that prevent effector binding to susceptibility gene promoters, or that allow effector activation of resistance genes. This study identifies, in the heirloom variety Carolina Gold Select, a third mechanism of rice resistance involving TAL effectors. This resistance manifests through strong suppression of disease development in response to diverse TAL effectors from both X. oryzae pathovars. The resistance can be triggered by an effector with only 3.5 central repeats, is independent of the composition of the repeat variable di-residues that determine TAL effector binding specificity, and is independent of the transcriptional activation domain. We determined that the resistance is conferred by a single dominant locus, designated Xo1, that maps to a 1.09 Mbp fragment on chromosome 4. The Xo1 interval also confers complete resistance to the strains in the African clade of X. oryzae pv. oryzicola, representing the first dominant resistance locus against bacterial leaf streak in rice. The strong phenotypic similarity between the TAL effector-triggered resistance conferred by Xo1 and that conferred by the tomato resistance gene Bs4 suggests that monocots and dicots share an ancient or convergently evolved mechanism to recognize analogous TAL effector epitopes.

An operon for production of bioactive gibberellin A4 phytohormone with wide distribution in the bacterial rice leaf streak pathogen Xanthomonas oryzae pv. oryzicola.

Phytopathogens have developed elaborate mechanisms to attenuate the defense response of their host plants, including convergent evolution of complex pathways for production of the GA phytohormones, which were actually first isolated from the rice fungal pathogen Gibberella fujikuroi. The rice bacterial pathogen Xanthomonas oryzae pv. oryzicola (Xoc) has been demonstrated to contain a biosynthetic operon with cyclases capable of producing the universal GA precursor ent-kaurene. Genetic (knock-out) studies indicate that the derived diterpenoid serves as a virulence factor for this rice leaf streak pathogen, serving to reduce the jasmonic acid-mediated defense response. Here the functions of the remaining genes in the Xoc operon are elucidated and the distribution of the operon in X. oryzae is investigated in over 100 isolates. The Xoc operon leads to production of the bioactive GA4 , an additional step beyond production of the penultimate precursor GA9 mediated by the homologous operons recently characterized from rhizobia. Moreover, this GA biosynthetic operon was found to be widespread in Xoc (> 90%), but absent in the other major X. oryzae pathovar. These results indicate selective pressure for production of GA4 in the distinct lifestyle of Xoc, and the importance of GA to both fungal and bacterial pathogens of rice.

Characterization of the Xanthomonas translucens Complex Using Draft Genomes, Comparative Genomics, Phylogenetic Analysis, and Diagnostic LAMP Assays.

Prevalence of Xanthomonas translucens, which causes cereal leaf streak (CLS) in cereal crops and bacterial wilt in forage and turfgrass species, has increased in many regions in recent years. Because the pathogen is seedborne in economically important cereals, it is a concern for international and interstate germplasm exchange and, thus, reliable and robust protocols for its detection in seed are needed. However, historical confusion surrounding the taxonomy within the species has complicated the development of accurate and reliable diagnostic tools for X. translucens. Therefore, we sequenced genomes of 15 X. translucens strains representing six different pathovars and compared them with additional publicly available X. translucens genome sequences to obtain a genome-based phylogeny for robust classification of this species. Our results reveal three main clusters: one consisting of pv. cerealis, one consisting of pvs. undulosa and translucens, and a third consisting of pvs. arrhenatheri, graminis, phlei, and poae. Based on genomic differences, diagnostic loop-mediated isothermal amplification (LAMP) primers were developed that clearly distinguish strains that cause disease on cereals, such as pvs. undulosa, translucens, hordei, and secalis, from strains that cause disease on noncereal hosts, such as pvs. arrhenatheri, cerealis, graminis, phlei, and poae. Additional LAMP assays were developed that selectively amplify strains belonging to pvs. cerealis and poae, distinguishing them from other pathovars. These primers will be instrumental in diagnostics when implementing quarantine regulations to limit further geographic spread of X. translucens pathovars.

Characterization of a novel clade of Xanthomonas isolated from rice leaves in Mali and proposal of Xanthomonas maliensis sp. nov.

Four bacterial strains, designated M89, M92, M97(T), and M106, were isolated in a previous study from surface-sterilized leaves of rice (Oryza sativa) or murainagrass (Ischaemum rugosum) at three sites in Mali, Africa. Here they were examined by a polyphasic taxonomic approach and analysis of a whole-genome sequence. Phylogenetic analyses based on 16S rRNA sequence and multilocus sequence analysis of seven genes showed that these four strains formed a distinct lineage representing a novel species within the genus Xanthomonas. This was supported by whole-genome average nucleotide identity values calculated from comparisons of strain M97(T) with established Xanthomonas species. The strains can be differentiated from the known Xanthomonas species on the basis of their fatty acid and carbohydrate utilization profiles. Population growth studies on rice confirmed that these bacteria multiply in rice leaves without causing symptoms. Identification of this novel species can be accomplished by using diagnostic primer sets or by gyrB gene sequence analysis. We propose to classify these rice- and grass-associated bacteria as Xanthomonas maliensis sp. nov. with strain M97(T) = CFBP7942(T) = LMG27592(T) as the type strain.

High-throughput genomic sequencing of cassava bacterial blight strains identifies conserved effectors to target for durable resistance.

Cassava bacterial blight (CBB), incited by Xanthomonas axonopodis pv. manihotis (Xam), is the most important bacterial disease of cassava, a staple food source for millions of people in developing countries. Here we present a widely applicable strategy for elucidating the virulence components of a pathogen population. We report Illumina-based draft genomes for 65 Xam strains and deduce the phylogenetic relatedness of Xam across the areas where cassava is grown. Using an extensive database of effector proteins from animal and plant pathogens, we identify the effector repertoire for each sequenced strain and use a comparative sequence analysis to deduce the least polymorphic of the conserved effectors. These highly conserved effectors have been maintained over 11 countries, three continents, and 70 y of evolution and as such represent ideal targets for developing resistance strategies.

Sensitive detection of Xanthomonas oryzae Pathovars oryzae and oryzicola by loop-mediated isothermal amplification.

Molecular diagnostics for crop diseases can enhance food security by enabling the rapid identification of threatening pathogens and providing critical information for the deployment of disease management strategies. Loop-mediated isothermal amplification (LAMP) is a PCR-based tool that allows the rapid, highly specific amplification of target DNA sequences at a single temperature and is thus ideal for field-level diagnosis of plant diseases. We developed primers highly specific for two globally important rice pathogens, Xanthomonas oryzae pv. oryzae, the causal agent of bacterial blight (BB) disease, and X. oryzae pv. oryzicola, the causal agent of bacterial leaf streak disease (BLS), for use in reliable, sensitive LAMP assays. In addition to pathovar distinction, two assays that differentiate X. oryzae pv. oryzae by African or Asian lineage were developed. Using these LAMP primer sets, the presence of each pathogen was detected from DNA and bacterial cells, as well as leaf and seed samples. Thresholds of detection for all assays were consistently 10(4) to 10(5) CFU ml(-1), while genomic DNA thresholds were between 1 pg and 10 fg. Use of the unique sequences combined with the LAMP assay provides a sensitive, accurate, rapid, simple, and inexpensive protocol to detect both BB and BLS pathogens.

Development of a Genomics-Based LAMP (Loop-Mediated Isothermal Amplification) Assay for Detection of Pseudomonas fuscovaginae from Rice.

The vast amount of data available through next-generation sequencing technology is facilitating the design of diagnostic marker systems. This study reports the use of draft genome sequences from the bacterial plant pathogen Pseudomonas fuscovaginae, the cause of sheath brown rot of rice, to describe the genetic diversity within a worldwide collection of strains representing the species. Based on a comparative analysis with the draft sequences, primers for a loop-mediated isothermal amplification (LAMP) assay were developed to identify P. fuscovaginae. The assay reported here reliably differentiated strains of P. fuscovaginae isolated from rice from a range of other bacteria that are commonly isolated from rice and other plants using a primer combination designated Pf8. The LAMP assay identified P. fuscovaginae purified DNA, live or heat-killed cells from pure cultures, and detected the bacterium in extracts or exudates from infected host plant material. The P. fuscovaginae LAMP assay is a suitable diagnostic tool for the glasshouse and laboratory and could be further developed for in-field surveys.

Genome mining reveals the genus Xanthomonas to be a promising reservoir for new bioactive non-ribosomally synthesized peptides.

BACKGROUND: Various bacteria can use non-ribosomal peptide synthesis (NRPS) to produce peptides or other small molecules. Conserved features within the NRPS machinery allow the type, and sometimes even the structure, of the synthesized polypeptide to be predicted. Thus, bacterial genome mining via in silico analyses of NRPS genes offers an attractive opportunity to uncover new bioactive non-ribosomally synthesized peptides. Xanthomonas is a large genus of Gram-negative bacteria that cause disease in hundreds of plant species. To date, the only known small molecule synthesized by NRPS in this genus is albicidin produced by Xanthomonas albilineans. This study aims to estimate the biosynthetic potential of Xanthomonas spp. by in silico analyses of NRPS genes with unknown function recently identified in the sequenced genomes of X. albilineans and related species of Xanthomonas. RESULTS: We performed in silico analyses of NRPS genes present in all published genome sequences of Xanthomonas spp., as well as in unpublished draft genome sequences of Xanthomonas oryzae pv. oryzae strain BAI3 and Xanthomonas spp. strain XaS3. These two latter strains, together with X. albilineans strain GPE PC73 and X. oryzae pv. oryzae strains X8-1A and X11-5A, possess novel NRPS gene clusters and share related NRPS-associated genes such as those required for the biosynthesis of non-proteinogenic amino acids or the secretion of peptides. In silico prediction of peptide structures according to NRPS architecture suggests eight different peptides, each specific to its producing strain. Interestingly, these eight peptides cannot be assigned to any known gene cluster or related to known compounds from natural product databases. PCR screening of a collection of 94 plant pathogenic bacteria indicates that these novel NRPS gene clusters are specific to the genus Xanthomonas and are also present in Xanthomonas translucens and X. oryzae pv. oryzicola. Further genome mining revealed other novel NRPS genes specific to X. oryzae pv. oryzicola or Xanthomonas sacchari. CONCLUSIONS: This study revealed the significant potential of the genus Xanthomonas to produce new non-ribosomally synthesized peptides. Interestingly, this biosynthetic potential seems to be specific to strains of Xanthomonas associated with monocotyledonous plants, suggesting a putative involvement of non-ribosomally synthesized peptides in plant-bacteria interactions.

Genome sequence of Xanthomonas fuscans subsp. fuscans strain 4834-R reveals that flagellar motility is not a general feature of xanthomonads.

BACKGROUND: Xanthomonads are plant-associated bacteria responsible for diseases on economically important crops. Xanthomonas fuscans subsp. fuscans (Xff) is one of the causal agents of common bacterial blight of bean. In this study, the complete genome sequence of strain Xff 4834-R was determined and compared to other Xanthomonas genome sequences. RESULTS: Comparative genomics analyses revealed core characteristics shared between Xff 4834-R and other xanthomonads including chemotaxis elements, two-component systems, TonB-dependent transporters, secretion systems (from T1SS to T6SS) and multiple effectors. For instance a repertoire of 29 Type 3 Effectors (T3Es) with two Transcription Activator-Like Effectors was predicted. Mobile elements were associated with major modifications in the genome structure and gene content in comparison to other Xanthomonas genomes. Notably, a deletion of 33 kbp affects flagellum biosynthesis in Xff 4834-R. The presence of a complete flagellar cluster was assessed in a collection of more than 300 strains representing different species and pathovars of Xanthomonas. Five percent of the tested strains presented a deletion in the flagellar cluster and were non-motile. Moreover, half of the Xff strains isolated from the same epidemic than 4834-R was non-motile and this ratio was conserved in the strains colonizing the next bean seed generations. CONCLUSIONS: This work describes the first genome of a Xanthomonas strain pathogenic on bean and reports the existence of non-motile xanthomonads belonging to different species and pathovars. Isolation of such Xff variants from a natural epidemic may suggest that flagellar motility is not a key function for in planta fitness.

Identification of non-TAL effectors in Xanthomonas oryzae pv. oryzae Chinese strain 13751 and analysis of their role in the bacterial virulence.

Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of rice bacterial leaf blight, one of the most important rice bacterial diseases in China and many other countries. The upstream portions of 41 candidate genes encoding non-transcription activator-like effectors of Xoo Chinese strain 13751 were fused with the coding sequence of AvrBs159-445 in a broad host-range vector. The constructed plasmids were respectively introduced into Xoo strain 13751 and avrBs1 deletion mutant of X. campestris pv. campestris strain 8004 by tri-parental conjugation. The resultant transconjugants were respectively tested for hypersensitive response (HR) elicitation on pepper ECW-10R. Nine strains were able to elicit HR on pepper, indicating that the nine genes (XOO0037, XOO0103, XOO0110, XOO0315, XOO1488, XOO2875, XOO3150, XOO3222 and XOO4134) encoded effectors. Among them, xopAE 13751 (XOO0110), expressed in Xoo strain 13751 growing in rice leaves, was a new experimentally confirmed effector gene. XopAE13751 contains 11 leucine rich repeats. Furthermore, mutants for the nine effector genes were created in Xoo strain 13751 and subsequently tested for virulence in rice. As a result, only the xopR 13751 (XOO4134) deletion mutant GXMxopR showed a significant reduction in virulence in hybrid rice cv. Teyou63 compared to the wild type. However, the growth of GXMxopR in host plant rice was not affected. These results indicated that xopR 13751 was required for full virulence of Xoo strain 13751 by inducing rice disease tolerance.

Aging of myelinating glial cells predominantly affects lipid metabolism and immune response pathways.

Both the central and the peripheral nervous systems are prone to multiple age-dependent neurological deficits, often attributed to still unknown alterations in the function of myelinating glia. To uncover the biological processes affected in glial cells by aging, we analyzed gene expression of the Schwann cell-rich mouse sciatic nerve at 17 time points throughout life, from day of birth until senescence. By combining these data with the gene expression data of myelin mouse mutants carrying deletions of either Pmp22, SCAP, or Lpin1, we found that the majority of age-related transcripts were also affected in myelin mutants (54.4%) and were regulated during PNS development (59.5%), indicating a high level of overlap in implicated molecular pathways. The expression profiles in aging copied the direction of transcriptional changes observed in neuropathy models; however, they had the opposite direction when compared with PNS development. The most significantly altered biological processes in aging involved the inflammatory/immune response and lipid metabolism. Interestingly, both these pathways were comparably changed in the aging optic nerve, suggesting that similar biological processes are affected in aging of glia-rich parts of the central and peripheral nervous systems. Our comprehensive comparison of gene expression in three distinct biological conditions including development, aging, and myelin disease thus revealed a previously unanticipated relationship among themselves and identified lipid metabolism and inflammatory/immune response pathways as potential therapeutical targets to prevent or delay so far incurable age-related and inherited forms of neuropathies.

Genomic insights into strategies used by Xanthomonas albilineans with its reduced artillery to spread within sugarcane xylem vessels.

BACKGROUND: Xanthomonas albilineans causes leaf scald, a lethal disease of sugarcane. X. albilineans exhibits distinctive pathogenic mechanisms, ecology and taxonomy compared to other species of Xanthomonas. For example, this species produces a potent DNA gyrase inhibitor called albicidin that is largely responsible for inducing disease symptoms; its habitat is limited to xylem; and the species exhibits large variability. A first manuscript on the complete genome sequence of the highly pathogenic X. albilineans strain GPE PC73 focused exclusively on distinctive genomic features shared with Xylella fastidiosa-another xylem-limited Xanthomonadaceae. The present manuscript on the same genome sequence aims to describe all other pathogenicity-related genomic features of X. albilineans, and to compare, using suppression subtractive hybridization (SSH), genomic features of two strains differing in pathogenicity. RESULTS: Comparative genomic analyses showed that most of the known pathogenicity factors from other Xanthomonas species are conserved in X. albilineans, with the notable absence of two major determinants of the "artillery" of other plant pathogenic species of Xanthomonas: the xanthan gum biosynthesis gene cluster, and the type III secretion system Hrp (hypersensitive response and pathogenicity). Genomic features specific to X. albilineans that may contribute to specific adaptation of this pathogen to sugarcane xylem vessels were also revealed. SSH experiments led to the identification of 20 genes common to three highly pathogenic strains but missing in a less pathogenic strain. These 20 genes, which include four ABC transporter genes, a methyl-accepting chemotaxis protein gene and an oxidoreductase gene, could play a key role in pathogenicity. With the exception of hypothetical proteins revealed by our comparative genomic analyses and SSH experiments, no genes potentially involved in any offensive or counter-defensive mechanism specific to X. albilineans were identified, supposing that X. albilineans has a reduced artillery compared to other pathogenic Xanthomonas species. Particular attention has therefore been given to genomic features specific to X. albilineans making it more capable of evading sugarcane surveillance systems or resisting sugarcane defense systems. CONCLUSIONS: This study confirms that X. albilineans is a highly distinctive species within the genus Xanthomonas, and opens new perpectives towards a greater understanding of the pathogenicity of this destructive sugarcane pathogen.

Transcription activator-like (TAL) effectors targeting OsSWEET genes enhance virulence on diverse rice (Oryza sativa) varieties when expressed individually in a TAL effector-deficient strain of Xanthomonas oryzae.

Genomes of the rice (Oryza sativa) xylem and mesophyll pathogens Xanthomonas oryzae pv. oryzae (Xoo) and pv. oryzicola (Xoc) encode numerous secreted transcription factors called transcription activator-like (TAL) effectors. In a few studied rice varieties, some of these contribute to virulence by activating corresponding host susceptibility genes. Some activate disease resistance genes. The roles of X. oryzae TAL effectors in diverse rice backgrounds, however, are poorly understood. Xoo TAL effectors that promote infection by activating SWEET sucrose transporter genes were expressed in TAL effector-deficient X. oryzae strain X11-5A, and assessed in 21 rice varieties. Some were also tested in Xoc on variety Nipponbare. Several Xoc TAL effectors were tested in X11-5A on four rice varieties. Xoo TAL effectors enhanced X11-5A virulence on most varieties, but to varying extents depending on the effector and variety. SWEET genes were activated in all tested varieties, but increased virulence did not correlate with activation level. SWEET activators also enhanced Xoc virulence on Nipponbare. Xoc TAL effectors did not alter X11-5A virulence. SWEET-targeting TAL effectors contribute broadly and non-tissue-specifically to virulence in rice, and their function is affected by host differences besides target sequences. Further, the utility of X11-5A for characterizing individual TAL effectors in rice was established.

Development of a variable number of tandem repeats typing scheme for the bacterial rice pathogen Xanthomonas oryzae pv. oryzicola.

Xanthomonas oryzae pv. oryzicola is an important bacterial pathogen responsible for outbreaks of bacterial leaf streak (BLS) on rice, mostly occurring in Asia and parts of Africa. To better monitor epidemics and assess population structures, efficient tools that allow the precise identification and diagnosis of pathogenic populations are needed. In this study, we explored variable numbers of tandem repeats (VNTR) as a fast, reliable, and cost-effective molecular typing tool. Screening of three X. oryzae pv. oryzicola genome sequences (Philippine strain BLS256, Chinese strain GX01, and Malian strain MAI10) predicted 28 candidate VNTR loci. Primer pairs for polymerase chain reaction (PCR) amplification of all 28 loci were designed and applied to a panel of 20 X. oryzae pv. oryzicola strains originating from Asia and Africa. Sequencing of PCR amplicons revealed 25 robust and polymorphic VNTR loci that are shared among Asian and African X. oryzae pv. oryzicola strains. A dendrogram constructed from 25 VNTR loci indicated that most Asian strains are clearly discriminated from African strains. However, in agreement with previous reports, one strain from Mali is related to Asian strains, pointing to a possible introduction of Asian strains to the African continent. The new VNTR-based tool described here is useful for studies of population structures and epidemiological monitoring of X. oryzae pv. oryzicola.