Cassava Bacterial Blight Research Articles

Silicon Induces Resistance to Bacterial Blight by Altering the Physiology and Antioxidant Enzyme Activities in Cassava

Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam) is a devastating disease limiting cassava production. The potential effect of Si application on the physiological and biochemical mechanisms attributed to Si-mediated resistance of cassava to Xam was evaluated. The optimal concentration of Si in enhancing resistance to CBB without detrimental effects on plant growth was determined using cultivars TME14 and TMS60444 known for their susceptibility to Xam. Varied concentrations of Si (0.7 to 2.1 mM) were administered by watering the plants three times per week before and after Xam inoculation. The optimized Si concentration was used to evaluate the effect of Si supplementation on resistance to CBB disease using eight farmer-preferred cassava cultivars. The population of Xam, cultivar resistance, chlorophyll content, lipid peroxidation, H2O2 content, activity of antioxidant enzymes and total Si content in cassava cultivars were quantified 21 days post inoculation. Silicon concentration of 1.4 mM was optimal in enhancing cassava resistance to Xam. Silicon-treated plants of all cassava cultivars showed significantly (P ≤ 0.05) lower Xam population ranging from 5% to 26.7% compared to non-Si treated control plants. Activities of antioxidant enzymes, malondialdehyde, H2O2 and chlorophyll contents were significantly (P ≤ 0.05) higher in Si treated plants than non-Si treated plants. Silicon accumulation in leaves of Si treated plants was higher compared to non-Si treated control plants.

Phenotypic and genotypic diversity of Xanthomonas axonopodis pv. manihotis causing bacterial blight disease of cassava in Kenya

Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam), is the most important bacterial disease of cassava. There is no information available on the morphological and genetic variability of Xam isolates from Kenya. The aim of this study therefore, was to determine the diversity of Xam isolates from different cassava growing regions of Kenya using phenotypic characteristics and repetitive DNA polymerase chain reaction-based fingerprinting (rep-PCR). Thirty three isolates were recovered from infected cassava leaf samples collected from farmers’ fields in cassava growing regions. The dendrogram generated from analysis of phenotypic characteristics of the isolates produced two major clusters at 75% similarity level. Analysis of 19 isolates with repetitive extragenic palindromic (rep) primers yielded characteristic fingerprint pattern with bands ranging between 400 and 2000 bp in size and their numbers ranged from 1 to 6 bands per isolate. Cluster analysis using unweighted pair group method with arithmetic averages (UPGMA) did not reveal any significant differences in clustering and relationship to the geographical origin, with exception of a single isolate that had unique fingerprints. These findings indicate that Xam population in Kenya evolved from the same origin and is a uniform population, and this may prove useful in future breeding programmes.

QTL identification for cassava bacterial blight resistance under natural infection conditions

La yuca, Manihot esculenta Crantz, representa la principal fuente de alimento para cerca de 1000 millones de personas. La producción de yuca se ve afectada por diversas enfermedades, una de las más serias es la bacteriosis vascular (CBB) causada por Xanthomonas axonopodis pv. manihotis (Xam). En este estudio se realizó un análisis de loci de rasgos cuantitativos (QTL) para la resistencia a CBB en condiciones naturales de infección, usando una población de mapeo constituida por 99 genotipos de hermanos completos segregantes y un mapa genético altamente denso basado en SNPs. La evaluación fenotípica se llevó a cabo en Puerto López (Meta), Colombia, durante la época de lluvias durante el segundo semestre de 2015. En la población de mapeo fueron detectados individuos con una segregación transgresiva tanto resistentes como susceptibles. A través de un análisis no paramétrico de intervalo simple, se detectaron dos QTL que explican el 10,9 y el 12,6 % de la varianza fenotípica de la resistencia en campo a CBB. Mediante análisis bioinformáticos se identificaron cuatro genes candidatos presentes en los intervalos de los QTL. Este trabajo representa un esfuerzo por dilucidar los mecanismos moleculares implicados en la resistencia de yuca a CBB.

Evaluation of Some Improved Varieties of Cassava (Manihot esculenta) (Crantz) for Resistance to cassava Bacterial Blight (CBB) in a Humid Forest Zone of Nigeria

Ten improved varieties of cassava Manihot esculenta (Crantz) obtained from the International Institute of Tropical Agriculture (IITA) Ibadan and a local variety (Efuwa) which served as a local check were evaluated for resistance to Cassava Bacterial Blight (CCB) at the Rivers State Agricultural Development Programme (ADP) Research Farm Degema, Rivers State, Nigeria. The varieties were laid out in a randomized complete block design and replicated three times. Field observation showed that seven varieties were resistant to CBB infection with proportional increase in tuber yield except 91/02324, 97/3200 and 98/2101. Two categories of varieties were observed in the study. The first category consists 97/0162, 96/1642, 96/1632, 99/6012 and 94/0026 which had high resistance to CBB and good tuber yield (26.8�� ℎ�� −1 − 35.32�� ℎ�� −1 ) thus having potential in reducing foliar diseases as well as increasing tuber yield. The second category: TMS 30572, 96/1642, 99/6012 and 94/0026 produced relatively high biomass (10.68�� ℎ�� −1 − 23.32�� ℎ�� −1 ) and good tuber Yield (27.71�� ℎ�� −1 − 35.04�� ℎ�� −1 ). This group cantherefore be cultivatedfor good foliage and tuber yield. The

Quantitative, Image-Based Phenotyping Methods Provide Insight into Spatial and Temporal Dimensions of Plant Disease.

Plant disease symptoms exhibit complex spatial and temporal patterns that are challenging to quantify. Image-based phenotyping approaches enable multidimensional characterization of host-microbe interactions and are well suited to capture spatial and temporal data that are key to understanding disease progression. We applied image-based methods to investigate cassava bacterial blight, which is caused by the pathogen Xanthomonas axonopodis pv. manihotis (Xam). We generated Xam strains in which individual predicted type III effector (T3E) genes were mutated and applied multiple imaging approaches to investigate the role of these proteins in bacterial virulence. Specifically, we quantified bacterial populations, water-soaking disease symptoms, and pathogen spread from the site of inoculation over time for strains with mutations in avrBs2, xopX, and xopK as compared to wild-type Xam ∆avrBs2 and ∆xopX both showed reduced growth in planta and delayed spread through the vasculature system of cassava. ∆avrBs2 exhibited reduced water-soaking symptoms at the site of inoculation. In contrast, ∆xopK exhibited enhanced induction of disease symptoms at the site of inoculation but reduced spread through the vasculature. Our results highlight the importance of adopting a multipronged approach to plant disease phenotyping to more fully understand the roles of T3Es in virulence. Finally, we demonstrate that the approaches used in this study can be extended to many host-microbe systems and increase the dimensions of phenotype that can be explored.

Infección de callo embriogénico friable de yuca con Xanthomonas axonopodis pv. manihotis (Xam)

Las nuevas tecnologías para la edición de genomas, como los TALEN y el sistema CRISPR/Cas9, representan una gran oportunidad para mejorar características deseables en diferentes organismos. Los TALEN son el resultado del acoplamiento de nucleasas a los TALE (Transcription Activator-Like Effectors), los cuales son efectores naturales de gran importancia en la patogénesis de las especies de Xanthomonas. Xanthomonas axonopodis pv. manihotis (Xam) es el agente causal del añublo bacteriano de la yuca, quien durante el proceso patogénico es capaz de translocar sus efectores a la célula vegetal mediante el sistema de secreción tipo tres (SSTT). Actualmente no hay protocolos estándar para la edición de genomas en yuca. En este estudio se exploró la posibilidad de translocar efectores sobre callo embriogénico friable (CEF) a través de la inoculación con Xam, con el fin de determinar el potencial de este patógeno como sistema de entrega de TALEN. El CEF de dos variedades de yuca susceptibles (COL2215 y cv. 60444) se cocultivaron con la cepa Xam668 a diferentes tiempos. Posteriormente, se evaluó la expresión de marcadores correspondientes a los genes blanco conocidos para los TALE presentes en esta cepa bacteriana. Aunque no se logró demostrar la translocación de los mismos en el tejido embriogénico, sí se lograron establecer condiciones adecuadas de cocultivo con Xam y el efecto que la infección bacteriana tiene sobre la regeneración de embriones a partir de este tejido. Palabras clave: cultivo de tejidos vegetales, edición de genomas, sistema de secreción tipo tres, efectores TALE, transformación.

First Report of Cassava Bacterial Blight Caused by Xanthomonas axonopodis pv. manihotis in Burkina Faso

HomePlant DiseaseVol. 99, No. 4First Report of Cassava Bacterial Blight Caused by Xanthomonas axonopodis pv. manihotis in Burkina Faso PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Cassava Bacterial Blight Caused by Xanthomonas axonopodis pv. manihotis in Burkina FasoI. Wonni, L. Ouedraogo, S. Dao, C. Tekete, O. Koita, G. Taghouti, P. Portier, B. Szurek, and V. VerdierI. WonniSearch for more papers by this author, L. OuedraogoSearch for more papers by this author, S. DaoSearch for more papers by this author, C. TeketeSearch for more papers by this author, O. KoitaSearch for more papers by this author, G. TaghoutiSearch for more papers by this author, P. PortierSearch for more papers by this author, B. SzurekSearch for more papers by this author, and V. VerdierSearch for more papers by this authorAffiliationsAuthors and Affiliations I. Wonni L. Ouedraogo , INERA Farako-Ba, Bobo-Dioulasso, Burkina-Faso S. Dao C. Tekete O. Koita , Université des Sciences Techniques et Technologiques, Faculté des Sciences et Techniques, LBMA, Bamako, Mali G. Taghouti P. Portier , CIRM-CFBP, UMR 1345 IRHS INRA, ACO, UA, 49070 Beaucouzé Cedex, France B. Szurek V. Verdier , UMR186, Interactions Plantes Microorganismes Environnement (IPME), IRD-Cirad-UM, 34394 Montpellier Cedex 5, France. Published Online:1 Apr 2015https://doi.org/10.1094/PDIS-03-14-0302-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Xanthomonas axonopodis pv. manihotis is the causal agent of cassava bacterial blight (CBB), a major disease of cassava in tropical and subtropical areas. CBB is a foliar and vascular disease characterized by angular leaf lesions, blight, wilting, stem exudates, and stem cankers. Since cassava is propagated clonally from stem cuttings, CBB plays a major role in limiting productivity, with losses between 12 and 100% affecting both yield and planting material. In August of 2011 and October of 2012, CBB symptoms were observed on 10-month-old field cassava grown in Bonfeba and in Takeledougou, Burkina-Faso (Cascades region). Symptoms consisted of angular, water-soaked leaf lesions, wilt, and visible exudates on the stem. White Xanthomonas-like strains were isolated from leaf tissues on LPGA medium (yeast, 5 g; peptone, 5 g; glucose, 5 g; bacto agar, 15 g; and distilled water, 1,000 ml). A PCR assay developed for the identification of X. axonopodis pv. manihotis (2) was used to determine the identity of Xanthomonas-like strains. X. axonopodis pv. manihotis strain CFBP7661 was used as a positive control. The expected DNA fragment (898 bp) was obtained from all the strains. No fragment was observed for negative controls (distilled water as the template). Three X. axoponodis pv. manihotis strains were further analyzed by sequence analysis using the gyrB and rpoD housekeeping genes. Comparison of rpoD and gyrB sequences showed that strains were 99 to 100% identical to 65 different strains of X. axonopodis pv. manihotis. Pathogenicity tests were performed on greenhouse-grown 4-week-old cassava plants, cv. MCOL 1522. Cultures were grown overnight in LPGA medium, adjusted in sterile water to 1 × 108 CFU/ml, and inoculated into cassava leaves and stems as previously described (1). Control plants were inoculated with sterile water. X. axonopodis pv. manihotis strain CFBP7661 was used as a positive control. After 7 days of incubation in the greenhouse at 28 ± 1°C with a 12-h photoperiod, inoculated leaves developed water-soaked lesions. Wilted leaves and stem exudates were visible at 30 days after stem inoculation. Symptoms were identical to those seen in the field. Control plants remained symptomless. Koch’s postulates were fulfilled after re-isolation of Xanthomonas-like white strains from leaf symptoms on inoculated cassava and confirmation as X. axonopodis pv. manihotis by PCR assay as described above. Three strains (CFBP7945, CFBP7946, CFBP7947) were deposited in the French Collection for Plant-Associated Bacteria (CIRM-CFBP). Information on Xam strains as well as gyrB and rpoD sequences are available through CIRM-CFBP http://www6.inra.fr/cirm_eng/CFBP-Plant-Associated-Bacteria. To our knowledge, this is the first report of CBB in Burkina Faso. Since cassava is becoming a crop of importance for human consumption in Burkina-Faso, CBB may limit productivity. Further surveys will be necessary to evaluate the geographic distribution and prevalence of CBB in Burkina-Faso and neighboring countries.

Assays to Assess Virulence of Xanthomonas axonopodis pv. manihotis on Cassava

Cassava (Manihot esculenta) is a root crop that provides calories for people living in more than 100 tropical and subtropical countries and serves as a raw material for processing into starch and biofuels as well as feed for livestock (Howeler et al., 2013). Xanthomonas axonopodis pv. manihotis (Xam), the causal agent of cassava bacterial blight (CBB), can cause extensive crop damage (reviewed in Lopez et al., 2012; Lozano, 1986). Bacterial movement, growth in planta and the ability to cause disease symptoms are all important measures of bacterial fitness and plant susceptibility to CBB. Here we present a protocol for visualizing the movement of Xam within the plant. We also provide a detailed method of assaying bacterial growth in the cassava leaf midvein, and bacterial growth and disease symptom development in the leaf apoplast. These methods will be important tools for determining Xam strain pathogenicity and for developing cassava varieties that are resistant to CBB.

Removal of Symptomatic Cassava Leaves as Cultural Practice to Control Cassava Bacterial Blight

The cassava landrace BEN 86052, susceptible to cassava bacterial blight (CBB), was chosen for the field experiments conducted in the forest savannah transition zone at Abomey-Calavi, Benin Republic. One month old plants were spray-inoculated with Xanthomona axonopodiss pv. manihotis (Xam) strain GSPB 2506 to achieve a homogenous infection across all plots. Disease development and cassava growth parameters were monitored over 12 months. Diseased leaves were removed four times at an interval of three weeks. A detailed symptom evaluation of percentage of spots, blight, wilt and dieback revealed significant reduction of blight and wilt symptoms in removed leaves plots. Disease severity was reduced by 71% in plots with leaf removal compared to non-removed leaves plots. Growth parameters leaf, stem and root weight at 6 and 12 months after planting were not significantly different between removed leaves and non-removed leaves plots. In fact, removal of infected leaves had no significant effect on root yield and reduced the epidemic potential of cassava bacterial blight in the field in the same season and the transfer of the infection by contaminated cuttings to the next season. Removal of diseased leaves should be highly effective in cassava fields with low disease incidence and specifically recommended for moderately resistant or resistant cultivars as part of an integrated management to control the cassava bacterial blight.

Evaluation of Growth, Yield and Susceptibility of Five Cassava Cultivars in Ilorin, Southern Guinea Savanna, Nigeria

Development of high yielding and disease/pest resistance cultivars are major ways of overcoming threats to cassava production in Africa. A field study aimed at evaluating the growth performance and susceptibility rating of five cassava cultivars was conducted at the University of Ilorin Teaching and Research Farm, Ilorin, Nigeria. The cassava cultivars included :  TMS 92/0057, TMS 92/0326, TMS 92/30572, 'Odongbo', 'Oko-Iyawo' - selected for high yield and tolerance to cassava mosaic disease (CMD), cassava bacterial blight (CBB) and cassava green spider mite (CGM). The experiment was laid out in Randomized Complete Block Design (RCBD) with three replications. Symptom expression of CMD was moderate in 'Odongbo' while TMS 92/30572 had the least severity symptom. 'Odongbo' recorded the least CGM severity of damage which was at par with TMS 92/0326 and 'Oko-Iyawo'. Symptoms expression of CMD and CBB were generally higher with the local checks - 'Odongbo' and 'Oko-Iyawo' respectively while CGM symptom was significantly (p< 0.05) higher with TMS 92/30572.  Results on growth parameters showed that least mean number of branches/plant was recorded in the local check- 'Oko-Iyawo' which was significantly different from TMS 92/30572. Mean weight of tubers/plant was positively correlated with number of tubers/plant and negatively correlated with CMD. TMS 92/0326 recorded high harvestable tuber/ha and its pest/disease severity scores were low, suggesting that this cultivar could be recommended to farmers in the studied agro-ecological zone.

RNAseq analysis of cassava reveals similar plant responses upon infection with pathogenic and non-pathogenic strains of Xanthomonas axonopodis pv. manihotis.

An RNAseq-based analysis of the cassava plants inoculated with Xam allowed the identification oftranscriptional upregulation of genes involved in jasmonate metabolism, phenylpropanoid biosynthesis and putative targets for a TALE. Cassava bacterial blight, a disease caused by the gram-negative bacterium Xanthomonas axonopodis pv. manihotis (Xam), is a major limitation to cassava production worldwide and especially in developing countries. The molecular mechanisms underlying cassava susceptibility to Xam are currently unknown. To identify host genes and pathways leading to plant susceptibility, we analyzed the transcriptomic responses occurring in cassava plants challenged with either the non-pathogenic Xam strain ORST4, or strain ORST4(TALE1 Xam ) which is pathogenic due to the major virulence transcription activator like effector TALE1 Xam . Both strains triggered similar responses, i.e., induction of genes related to photosynthesis and phenylpropanoid biosynthesis, and repression of genes related to jasmonic acid signaling. Finally, to search for TALE1 Xam virulence targets, we scanned the list of cassava genes induced upon inoculation of ORST4(TALE1 Xam ) for candidates harboring a predicted TALE1 Xam effector binding element in their promoter. Among the six genes identified as potential candidate targets of TALE1 Xam a gene coding for a heat shock transcription factor stands out as the best candidate based on their induction in presence of TALE1 Xam and contain a sequence putatively recognized by TALE1 Xam .

Population typing of the causal agent of cassava bacterial blight in the Eastern Plains of Colombia using two types of molecular markers.

BackgroundMolecular typing of pathogen populations is an important tool for the development of effective strategies for disease control. Diverse molecular markers have been used to characterize populations of Xanthomonas axonopodis pv. manihotis (Xam), the main bacterial pathogen of cassava. Recently, diversity and population dynamics of Xam in the Colombian Caribbean coast were estimated using AFLPs, where populations were found to be dynamic, diverse and with haplotypes unstable across time. Aiming to examine the current state of pathogen populations located in the Colombian Eastern Plains, we also used AFLP markers and we evaluated the usefulness of Variable Number Tandem Repeats (VNTRs) as new molecular markers for the study of Xam populations.ResultsThe population analyses showed that AFLP and VNTR provide a detailed and congruent description of Xam populations from the Colombian Eastern Plains. These two typing strategies clearly separated strains from the Colombian Eastern Plains into distinct populations probably because of geographical distance. Although the majority of analyses were congruent between typing markers, fewer VNTRs were needed to detect a higher number of genetic populations of the pathogen as well as a higher genetic flow among sampled locations than those detected by AFLPs.ConclusionsThis study shows the advantages of VNTRs over AFLPs in the surveillance of pathogen populations and suggests the implementation of VNTRs in studies that involve large numbers of Xam isolates in order to obtain a more detailed overview of the pathogen to improve the strategies for disease control.

A complex population structure of the cassava pathogen Xanthomonas axonopodis pv. manihotis in recent years in the Caribbean Region of Colombia.

Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihotis (Xam), is the most important bacterial disease affecting this crop. A continuous surveillance of the pathogen population dynamics is required to develop an efficient disease management program. During the 1990s, Xam populations showed high levels of genetic variation and relevant migratory processes that were important determinants of the distribution of the pathogen diversity in Colombia. Aiming to characterize the current population structure of the pathogen and the evolutionary forces that shape these populations, sampling collections were carried out from September 2008 until November 2010 in the Colombian Caribbean Region. One hundred and sixty bacterial isolates were characterized using amplified fragment length polymorphism (AFLP) markers. Additionally, a subset of effector genes were sequenced in some isolates to determine their usefulness in Xam population studies and to provide additional information to that obtained with AFLPs. Virulence patterns of ten isolates were determined in nine cassava accessions. Our results show a complex architecture of population and confirm migratory process previously reported in the Caribbean Region. Chinú, one of the locations sampled, presented remarkable features in population dynamics such as longer genetic distances, higher diversity indices, and a genetically differentiated population when it was compared with other locations. Virulence tests showed that MCOL2215, one of the most cultivated cassava varieties in the Caribbean coast, was susceptible to the majority of Xam isolates tested. This study shows the current condition of populations of Xam in the Caribbean Region of Colombia, and it contributes to improve the existing bacterial blight control practices.

Characterization of Congolese Strains of <i>Xanthomonas axonopodis</i> pv. <i>manihotis</i> Associated with Cassava Bacterial Blight

Cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis has been reported in several African countries since 1970. Knowledge of the virulence and diversity of Xanthomonas axonopodis pv. manihotis strains is important for an integrated control of CBB. The main objective of the present study was to characterize strains of Xanthomonas axonopodis collected from various regions in the DR-Congo. There was variability among strains for shape (form), contour (margin) and elevation. Bacterial cell size for the strains analyzed varied from 24.1 μm × 11.3 μm to 11.4 μm × 4.2 μm. All the Xanthomonas axonopodis pv. manihotis strains but one was motile. Two distinctive groups were identified based on radial growth of their colonies. The first group grows faster (7.8-10.5 mm/d) compared to the second group (4.8-6.9 mm/d). Five strains (Gandajika, Inera/Stat, Kansasa, Mulumba and Musakatshi) were classified as virulent with a damage rating ≤ 1 and four were aggressive (Luputa, M'vuazi, Boketa and Kiyaka) with a damage rating > 1. Significant differences were also observed among strains for disease onset, incidence and plant mortality. The highest incidence (33%) of bacterial blight 21 days after infestation (DAI) resulted from the Boketa strain inoculation and the lowest (0 % disease incidence) from INERA/STAT and Musakatshi strains. There was no clear association between geographic origin of the strains and their aggressiveness.

Cassava Pests and Diseases’ Prevalence and Performance as Revealed by Adaptive Trial Sites in North Western Agro-Ecological Zone of Uganda

Cassava adaptive trial was planted in ten sub counties across West Nile Agro-ecological Zone (WNAEZ) with six cassava varieties to test their performance and reactions to major pests and diseases present in the Zone. Th six cassava varieties comprised of improved (TME 14 and 204, NASE 13 and 14 and Akena – TMS I92/00067) and a local (Abiria) were selected based on their availability and preference in the region. The experiment was planted in RCBD design with three replicates. The experiment was planted in ten locations with the plot size was 6x6 metres. Results showed Cassava green mite (CGM), Cassava mosaic disease (CMD) and cassava bacterial blight (CBB) as major pest and diseases across all sites. Cassava Anthracnose (CA) and Cassava mealy bug (CM) were not present in the study sites. Cassava brown streak disease (CBSD) foliar symptoms was observed in three locations of Dranya s/c, Gimara s/c and Nyaravuru s/c on the three varieties of TME 204, TMS-I92/00067 and TME 14, whereas CBSD root necrosis was seen across all sites on TME 204, TMS-I92/00067, NASE 13, TME 14 and the Local except on NASE 14. In terms of yields, results showed that it was highest in TMS-I92/00067 (53.0 t/ha), TME 204 (46.0 t/ha), NASE 14 (39.4 t/ha), TME 14 (34.6 t/ha), NASE 13 (33.4 t/ha) and the local (22.7 t/ha) in that order. Farmers’ ranking of the studied cassava varieties in order of preference was in the order of NASE 14, TME 204, TMS-I92/00067, TME 14, NASE 13 and the Local. In conclusion, absence of both foliar and root symptoms on NASE 14 across all sites indicated that this variety is still tolerant to CBSD and can still be multiplied for production in West Nile Agro-ecological Zone.

Construction of a cassava PR protein‐interacting network during Xanthomonas axonopodis pv. manihotis infection

Pathogenesis‐related (PR) proteins are inducible antimicrobial defence proteins whose specific function in immunity or in other plant processes have not been deeply studied. Genes coding for PRs have been reported in different plant species with numerous homologous proteins belonging to the same functional protein family. Previously some PR candidate genes were identified in response to cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam): the genes for hevamine (HEV), chitinase (CHI) and sulphite reductase (SiR) were induced in a resistant cassava cultivar during the response to Xam. To understand their functional roles in defence responses, a protein–protein interaction map was generated based on yeast two‐hybrid (Y2H) assays of these candidate PR proteins. The results showed that the cassava PRs interact with each other and, although hevamine and chitinase belong to the same class of chitinases, they share only four interactors. Co‐regulated expression of PRs and their interactors was observed and similar ontology terms were identified for interactors. Furthermore, an overlap between immune and other metabolic pathways was noticed based on other bioinformatics studies. These results therefore shed more light on the possible function of PR proteins.

Correction for Bart et al., High-throughput genomic sequencing of cassava bacterial blight strains identifies conserved effectors to target for durable resistance

Correction for Bart et al., High-throughput genomic sequencing of cassava bacterial blight strains identifies conserved effectors to target for durable resistance

Combining ability of cassava genotypes for cassava mosaic disease and cassava bacterial blight, yield and its related components in two ecological zones in Ghana

Breeding for resistant genotypes is the best strategy to offset the destructive effects of cassava mosaic disease (CMD) and cassava bacterial blight (CBB). Two sets of diallel parents were selected for the forest and the savannah ecological zones in Ghana based on good levels of resistance to CMD and CBB. Both sets were crossed in a half-diallel design. The first set of seven progenitors and their 21 F1 progenies were planted in a randomised complete block design (RCBD) with three replications in two different locations for two seasons in the forest ecology. The second set of five progenitors and their 10 F1 progenies were planted in a RCBD with three replications in two locations in the coastal savannah ecological zone of Ghana. Both experiments were evaluated for CMD and CBB resistance, fresh root yield, dry root yield, root number, harvest index, dry matter content, plant height at maturity and height at first branching, levels of branching and plant vigour. Results of the combined analysis of variance revealed that the environment effect was significant for all the traits. General combining ability and specific combining ability effects were significant for most of the traits. Narrow sense heritability was significant for plant vigour, root number, CMD and CBB in both the zones. CMD and root number also had a predictability ratio of close to one, indicating the importance of additive gene effects.

Identification of ta-siRNAs and Cis-nat-siRNAs in Cassava and Their Roles in Response to Cassava Bacterial Blight

Trans-acting small interfering RNAs (ta-siRNAs) and natural cis-antisense siRNAs (cis-nat-siRNAs) are recently discovered small RNAs (sRNAs) involved in post-transcriptional gene silencing. ta-siRNAs are transcribed from genomic loci and require processing by microRNAs (miRNAs). cis-nat-siRNAs are derived from antisense RNAs produced by the simultaneous transcription of overlapping antisense genes. Their roles in many plant processes, including pathogen response, are mostly unknown. In this work, we employed a bioinformatic approach to identify ta-siRNAs and cis-nat-siRNAs in cassava from two sRNA libraries, one constructed from healthy cassava plants and one from plants inoculated with the bacterium Xanthomonas axonopodis pv. manihotis (Xam). A total of 54 possible ta-siRNA loci were identified in cassava, including a homolog of TAS3, the best studied plant ta-siRNA. Fifteen of these loci were induced, while 39 were repressed in response to Xam infection. In addition, 15 possible cis-natural antisense transcript (cis-NAT) loci producing siRNAs were identified from overlapping antisense regions in the genome, and were found to be differentially expressed upon Xam infection. Roles of sRNAs were predicted by sequence complementarity and our results showed that many sRNAs identified in this work might be directed against various transcription factors. This work represents a significant step toward understanding the roles of sRNAs in the immune response of cassava.

TALE1 from Xanthomonas axonopodis pv. manihotis acts as a transcriptional activator in plant cells and is important for pathogenicity in cassava plants

Many plant-pathogenic bacteria suppress pathogen-associated molecular pattern (PAMP)-triggered immunity by injecting effector proteins into the host cytoplasm during infection through the type III secretion system (TTSS). This type III secretome plays an important role in bacterial pathogenicity in susceptible hosts. Xanthomonas axonopodis pv. manihotis (Xam), the causal agent of cassava bacterial blight, injects several effector proteins into the host cell, including TALE1(Xam) . This protein is a member of the Transcriptional Activator-Like effector (TALE) protein family, formerly known as the AvrBs3/PthA family. TALE1(Xam) has 13.5 tandem repeats of 34 amino acids each, as well as two nuclear localization signals and an acidic activation domain at the C-terminus. In this work, we demonstrate the importance of TALE1(Xam) in the pathogenicity of Xam. We use versions of the gene that lack different domains in the protein in structure-function studies to show that the eukaryotic domains at the 3' end are critical for pathogenicity. In addition, we demonstrate that, similar to the characterized TALE proteins from other Xanthomonas species, TALE1(Xam) acts as a transcriptional activator in plant cells. This is the first report of the identification of a TALE in Xam, and contributes to our understanding of the pathogenicity mechanisms employed by this bacterium to colonize and cause disease in cassava.