Cocoyam Root Research Articles

Nutritional evaluation, microstructure, and storage stability of wheat (Triticum aestivum) and cocoyam (Colocasia esculenta) flour blends at different ratios

AbstractThe flour blend between wheat and cocoyam flours at different proportions (70:30, 50:50, and 40:60) was tested for physicochemical, functional, and nutritional quality. When compared to other blend ratios, the flour blend at 70:30 (wheat and cocoyam flour) had significantly (p < 0.05) higher protein, fat, and crude fiber content (50:50 and 40:60). The flour blends at 50:50 and 60:40 wheat to cocoyam flour ratio had considerably higher water absorption, bulk density, and swelling power than wheat and cocoyam flour blends at 70:30. When compared to other blend ratios, the wheat and cocoyam flour blend at 70:30 exhibited a significantly lower pasting viscosity (50:50 and 60:40). The pasted samples' microstructure revealed a honeycomb structure that increased as the quantity of cocoyam flour added to wheat flour increased. The wheat and cocoyam flour mixes competed favorably with wheat flour in terms of amino acid profile, and the fortified flours had the lowest peroxide values. The blends of wheat and cocoyam flour in proportions of 70:30 and 40:60 can be useful in the culinary and food industries.Practical applicationsThe problem of high cost of importation of wheat is one of the major challenges of African country and cocoyam root is underutilized in some African countries. Production of flour from cocoyam root serve as alternative use of cocoyam root which improve the utilization of cocoyam root. Compositing cocoyam flour with wheat flour may be a way to go in reducing wheat importation. Cocoyam flour added to wheat flour increase the functional properties and the pasting properties of the flour blend. The amino acids of the flour blends also compete favorably with amino acid profile of wheat flour alone. This shows that the flour blends from wheat and cocoyam flour can serve as new raw material in making bread, doughnut, and other confectionaries in food industry. This may reduce the price of wheat flour in the market and reduce the cost of living for low‐income earners.

Characterization of endophytic Streptomyces strains from roots of cocoyam (Xanthosoma sagittifolium L. Schott) in the South West Region of Cameroon, their in vitro plant growth promoting abilities and biocontrol efficacy against Pythium myriotylum

The focus of this study was to isolate, evaluate and characterize endophytic actinobacteria from cocoyam roots collected in the localities around Mount Cameroon for their potential antagonistic activities against Pythium myriotylum (the major pathogen that limits growth and productivity of this crop in Cameroon) and their ability to promote plant growth. A total of 171 actinobacteria were isolated from healthy cocoyams and were screened for their antagonistic activities against P. myriotylum, Phytophthora megakarya, Fusarium oxysporum and Colletotrichum gloeosporioides. Four isolates (PERM1, PERM2, PERM4 and PERM5) were selected based on of their strongest antagonistic activity in vitro against P. myriotylum. These four isolates were subsequently identified using phenotypic and molecular tests, characterized for extracellular hydrolytic enzymes, plant growth-promoting (PGP) traits, and their effects were evaluated on tissue culture-derived cocoyam plantlets. The sequencing of the 16S rRNA genes and alignment with sequences retrieved through the EzTaxon program indicated that PERM2, PERM4 and PERM5 have low similarity percentages with S. samsunensis EU077190 (92.4% sequence similarity), S. roietensis KX394336 (81% sequence similarity), S. flavoviridis AB184842 (91% sequence similarity) respectively. The comparison of some physiological and biochemical properties of PERM2,PERM4 and PERM5 with the most closely related Streptomyces species showed that these strains differ phenotypically. PERM2, PERM4 and PERM5 might represent new species. All selected Streptomyces were able to produce siderophores, indole-3-acetic acid (IAA), aminocyclopropane-1-carboxylate (ACC) deaminase, and solubilized tricalcium phosphate. With 20.30 μg/ml of IAA, the PERM2 isolate significantly increased the length of roots as compared to the positive control. These results suggest the possibility of using endophytic Streptomyces strains, especially isolate PERM2 in the protection of cocoyams against root rot disease and the promotion of root growth.

Cyclic lipopeptide-producing Pseudomonas koreensis group strains dominate the cocoyam rhizosphere of a Pythium root rot suppressive soil contrasting with P. putida prominence in conducive soils.

Pseudomonas isolates from tropical environments have been underexplored and may form an untapped reservoir of interesting secondary metabolites. In this study, we compared Pseudomonas and cyclic lipopeptide (CLP) diversity in the rhizosphere of a cocoyam root rot disease (CRRD) suppressive soil in Boteva, Cameroon with those from four conducive soils in Cameroon and Nigeria. Compared with other soils, Boteva andosols were characterized by high silt, organic matter, nitrogen and calcium. Besides, the cocoyam rhizosphere at Boteva was characterized by strains belonging mainly to the P. koreensis and P. putida (sub)groups, with representations in the P. fluorescens, P. chlororaphis, P. jessenii and P. asplenii (sub)groups. In contrast, P. putida isolates were prominent in conducive soils. Regarding CLP diversity, Boteva was characterized by strains producing 11 different CLP types with cocoyamide A producers, belonging to the P. koreensis group, being the most abundant. However, putisolvin III-V producers were the most dominant in the rhizosphere of conducive soils in both Cameroon and Nigeria. Furthermore, we elucidated the chemical structure of putisolvin derivatives-putisolvin III-V, and described its biosynthetic gene cluster. We show that high Pseudomonas and metabolic diversity may be driven by microbial competition, which likely contributes to soil suppressiveness to CRRD.

Evaluation of antagonistic activities against Pythium myriotylum and plant growth promoting traits of Streptomyces isolated from Cocoyam (Xanthosoma sagittifolium (L.) Schott) rhizosphere

The focus of this study was to isolate, evaluate and characterize cocoyam rhizospheric actinobacteria collected from Mount Cameroon for their potential antagonistic activities against Pythium myriotylum, the causative agent of cocoyam root rot disease, and/or their ability to promote plant growth. Actinobacteria were isolated from soil samples by using serial dilution method. Dual culture assay was used to screen the isolates against fungal phytopathogens including P. myriotylum. The selected isolates were characterized morphologically, physiologically, biochemically and genetically. Furthermore, these isolates were subjected to in vitro production of various plant growth promoting (PGP) traits. Among the 287 isolates distributed into 12 distinct groups, PFK4, PFBOT7 and PFEL2 had high antagonistic effects against P. myriotylum and they were regarded as promising candidates for further study of their biocontrol ability. The identification of actinobacterial isolates was done using universal PCR of partial 16S rRNA sequences combined with bioinformatics and phylogenetic procedures. The results revealed that all these isolates belong to the genus Streptomyces spp. The isolate PFK4 showed 99% similarity with Streptomyces albulus while PFBOT7 showed 99% similarity with Streptomyces albus and PFEL2 showed 98% similarity with Streptomyces gandoceansis based on their 16S rRNA gene sequences. In this study for the first time, we report S. gandocaensis strain for the biological control of Pythium myriotylum. The role of these isolates in Plant Growth Promotion (PGP) by in vitro production of PGP traits such as phosphate solubilisation, siderophore, indole-3-acetic acid (IAA), ACC deaminase and cell wall degrading enzymes production were also evaluated and showed as potential PGP agents. Concurrent production of PGP substances coupled with broad spectrum of antifungal and antibacterial activities of these three strains suggest their potential use as promising biocontrol and PGP tools for the development of an alternative control method for cocoyam cultivation.

Fluorescent Pseudomonas and cyclic lipopeptide diversity in the rhizosphere of cocoyam (Xanthosoma sagittifolium).

Cocoyam (Xanthosoma sagittifolium (L.)), an important tuber crop in the tropics, is severely affected by the cocoyam root rot disease (CRRD) caused by Pythium myriotylum. The white cocoyam genotype is very susceptible while the red cocoyam has some field tolerance to CRRD. Fluorescent Pseudomonas isolates obtained from the rhizosphere of healthy red and white cocoyams from three different fields in Cameroon were taxonomically characterized. The cocoyam rhizosphere was enriched with P. fluorescens complex and P. putida isolates independent of the plant genotype. LC-MS and NMR analyses revealed that 50% of the Pseudomonas isolates produced cyclic lipopeptides (CLPs) including entolysin, lokisin, WLIP, putisolvin and xantholysin together with eight novel CLPs. In general, CLP types were linked to specific taxonomic groups within the fluorescent pseudomonads. Representative CLP-producing bacteria showed effective control against CRRD while purified CLPs caused hyphal branching or hyphal leakage in P. myriotylum. The structure of cocoyamide A, a CLP which is predominantly produced by P. koreensis group isolates within the P. fluorescens complex is described. Compared with the white cocoyam, the red cocoyam rhizosphere appeared to support a more diverse CLP spectrum. It remains to be investigated whether this contributes to the field tolerance displayed by the red cocoyam.

Phenazines and cyclic lipopeptides produced by Pseudomonas sp. CMR12a are involved in the biological control of Pythium myriotylum on cocoyam (Xanthosoma sagittifolium)

Root rot disease caused by Pythium myriotylum is the most devastating disease of the tropical tuber crop cocoyam (Xanthosoma sagittifolium) with production losses of up to 90%. This study was conducted in order to determine the role of phenazines (PCN), and two cyclic lipopeptides namely sessilins and orfamides, produced by Pseudomonas sp. CMR12a, in the suppression of the Cocoyam Root Rot Disease (CRRD). Previously generated biosynthetic mutants of CMR12a that were deficient in either one, two or all three metabolites were used in in vitro, microscopic and plant experiments. Using CMR12a and its biosynthetic mutants, plant experiments revealed that mutants, which produced sessilins, orfamides or phenazines alone or at least any two of the metabolites, could successfully suppress the cocoyam root rot disease caused by P. myriotylum albeit in varying capacities. In vitro assays using filter sterilized bacterial supernatants showed that besides the WT strain, all mutants except the null mutant, which makes none of the metabolites, could successfully inhibit P. myriotylum. Following contact between the mycelium of P. myriotylum and purified/semi-purified metabolites, microscopic analysis showed that a concentration of 1 nM and higher of crude sessilins resulted in cell damage including vacuolization and lysis whereas amounts of 25 nM PCN and higher could produce a similar phenotype including mycelial bloating. Treatments with orfamide at 10 nM and higher resulted in mycelial lysis. Interestingly, during plant experiments, the null mutant was able to give some level of disease suppression suggesting that CMR12a produces other compound(s) which could be antagonistic towards our target pathogen. In summary, phenazines, sessilins and orfamides, produced by Pseudomonas sp. CMR12a, can play independent and additive roles in the suppression of Pythium-mediated CRRD.

Characteristics of Xanthosoma sagittifolium roots during cooking, using physicochemical analysis, uniaxial compression, multispectral imaging and low field NMR spectroscopy.

To effectively promote the industrial utilization of cocoyam (Xanthosoma sagittifolium) roots for enhanced food sustainability and security, there is a need to study their molecular, mechanical and physicochemical properties in detail. The physicochemical and textural characteristics of the red and white varieties of cocoyam roots were thus analysed by low field nuclear magnetic resonance relaxometry, multispectral imaging, uniaxial compression testing, and relevant physicochemical analysis in the current study. Both varieties had similar dry matter content, as well as physical and mechanical properties. However, up to four fast-interacting water populations were observed in the roots, dependent on the root variety and their degree of gelatinization during cooking. Changes in the relaxation parameters indicated weak gelatinization of starch at approximately 80°C in both varieties. However, shorter relaxation times and a higher proportion of restricted water in the white variety indicated that this variety was slightly more sensitive towards gelatinization. A strong negative correlation existed between dry matter and all multispectral wavelengths>800nm, suggesting the potential use of that spectral region for rapid analysis of dry matter and water content of the roots. The small, but significant differences in the structural and gelatinization characteristics of the two varieties indicated that they may not be equally suited for further processing, e.g. to flours or starches. Processors thus need to choose their raw materials wisely dependent on the aimed product characteristics. However, the spectroscopic methods applied in the study were shown to be effective in assessing important quality attributes during cooking of the roots.

Quantification of soil loss due to white cocoyam (Colocasia esculentus) and red cocoyam (Xanthosoma sagittifolium) harvesting in traditional farming system

Soil loss due to crop harvesting (SLCH) refers to nutrients, colloids and other soil particles exported from the field in the process of harvesting of root and tuber crops such as cocoyam, yam, potato, carrot and cassava. During harvest, soil adhering to the crop, loose soil, clods and stones are transported from the field to the market place or storage. Consequently, the depth of the top soil is reduced. Only few studies, however, tried to estimate SLCH values for cocoyam but there is no available information on the influence of cocoyam species on SLCH. This study was conducted on basement complex rock derived soil (Typic Kandiustalf) in Southwest Nigeria between 2011 and 2012 to compare the quantity of soil loss due to harvesting of white and red cocoyam. Traditional hand rubbing was compared with hand rubbing+washing. The energy dissipated and cost analysis for harvesting of both cocoyam species were compared. The soil sticking to the harvested tubers was hand rubbed thoroughly before washing to determine SLCH. Mean weight of the tubers after washing of white and red cocoyam was 2.0tha−1 and 1.3tha−1 respectively. Averagely, soil loss value per unit of crop mass (SLCHspec) of white and red cocoyam was 14.6kg−1ha−1harvest−1yr−1 and 15.1kg−1ha−1harvest−1yr−1 respectively. Soil loss ratio (hand rubbing/washing) was 3:1 for white cocoyam and ≈3:1 for red cocoyam in 2011. Corresponding ratio for 2012 was ≈6:1 for white and 5:1 for red cocoyam. The energy dissipated in harvesting white cocoyam (124.7Jmound−1) was higher than red cocoyam (121.5Jmound−1) due to higher white cocoyam root density. The cost analysis of cocoyam production using conventional (unwashed tubers) and the innovative approach (hand rubbing followed by washing of tubers) suggests that hand rubbing after harvest offers the cheapest alternative. Differences in SLCHspec could be due to skin roughness of red cocoyam leading to higher soil loss per unit yield. Morphology was another factor that explains the variation in the soil loss at cocoyam harvesting. The amount of soil loss due to cocoyam harvesting justifies the need to conduct further investigations on this process of soil erosion under mechanised agriculture.

Induction of Resistance in Cocoyam (Xanthosoma sagittifolium) to Pythium myriotylum by Corm Treatments with Benzothiadiazole and its Effect on Vegetative Growth

Root rot disease of cocoyam ( Xanthosoma sagittifolium ) caused by Pythium myriotylum (Oomycete) is a major constraint in cocoyam production in many countries in the world. Benzothiadiazole (BTH), a systemic acquired resistance inducer, was used in two concentrations (15 and 50 mg/L) as corm soaking treatment for 24 h duration to test for its effect on root rot disease resistance and vegetative growth of cocoyam under greenhouse conditions. The fungicide Metalm 72 WP (metalaxyl + cuprous oxide) was used as standard chemical control. Corm treatment with BTH significantly reduced cocoyam root rot disease (CRRD) severity compared with the pathogen-inoculated control. This was expressed by a disease index percentage of 37 and 20 for 15 and 50 mg/L BTH respectively after 28 days post-inoculation as opposed to the control with a disease index of 60. BTH at 50 mg/L was found to be more effective than the fungicide in inducing significant levels of resistance against P. myriotylum . In the absence of the pathogen, BTH and the fungicide applied as

Pathogenicity of Fungi Associated With the Cocoyam Root Rot Blight Complex in South-Eastern Nigeria

Disease surveys were carried out in major cocoyam (Xanthosoma sagittifolium (L) Schott) growing communities of Igbariam, Lowa and Umudike in the humid tropics of South-eastern Nigeria in 2009 and 2010 cropping seasons. The highest mean disease incidence of 46.7% and severity of 2.85 occurred in Lowa while the lowest figures of 33.3% and 2.33, respectively were recorded in Umudike. Isolations of associated `fungi made from infected cocoyam root tissues and rhizosphere soils yielded eight different fungal isolates. These were Aspergillus niger Thom and Raper, Botryodiplodia theobromae Pat., Fusarium solani (Mart.) Sacc., Pythium myriotylum Dresch., Rhizoctonia solani Kuhn, Rhizopus nigricans Ehhrenb., Trichoderma harzianum Rifai and Sclerotium rolfsii Sacc. The results showed that P. myriotylum was the most frequently isolated organism in infected cocoyam roots in the three locations with percentage frequency of occurrence of 82–90% and 60–72% abundance in infected soils. Pathogencity tests on susceptible cocoyam variety – Xanthosoma sagittifolium cv ede ocha (NXS001), using all fungal isolates, showed that only P. myriotylum was pathogenic on the test crop. The results of this investigation showed that the cocoyam root rot disease was incited by a single organism viz: Pythium myriotylum and not by a complex of organisms as suggested by earlier workers.Keywords: Cocoyam root rot blight complex; cocoyam root rot disease; Pythium myriotylum, Cocoyam, Xanthosoma sagittifolium

A PCR-RFLP assay for identification and detection of Pythium myriotylum, causal agent of the cocoyam root rot disease

Development of a PCR-RFLP assay that could reliably distinguish strains of Pythium myriotylum that are pathogenic to cocoyam from nonpathogens, as well as in planta detection of the pathogen. Sequences of the internal transcribed spacer regions of nuclear ribosomal DNA (rDNA-ITS) containing ITS1 and ITS2 of P. myriotylum isolates from cocoyam and other hosts were aligned and a restriction map was generated. rDNA-ITS alignment report revealed a new single nucleotide polymorphism (SNP; thymine/cytosine) downstream to previously published SNP (guanine/adenine) between isolates of P. myriotylum that are pathogenic to cocoyam and nonpathogenic strains. This new SNP is within the restriction site of the endonuclease AarI. Based on this SNP, a PCR-RFLP assay was developed for specific detection of P. myriotylum. The PCR amplicons of all isolates of P. myriotylum that infect cocoyam were cleaved by AarI, resulting to two bands (600/400 bp); but those from other hosts showed a single band (1000 bp), confirming the presence and specificity of the AarI restriction site. Also, the assay was effective in in planta detection of the pathogen on infected cocoyam roots without prior isolation of a pure culture. A PCR-RFLP method was developed that differentiates isolates of P. myriotylum that are pathogenic to cocoyam from nonpathogens as well as from other fungi commonly found in the cocoyam rhizosphere. Early and rapid detection of the pathogen could be of great importance in certifying planting materials as disease-free, enhancing sustainable management practices and limiting economic losses.

N-Acylhomoserine lactone quorum-sensing signalling in antagonistic phenazine-producing Pseudomonas isolates from the red cocoyam rhizosphere

Forty fluorescent Pseudomonas strains isolated from white and red cocoyam roots were tested for their ability to synthesize N-acyl-l-homoserine lactones (acyl-HSLs). Remarkably, only isolates from the red cocoyam rhizosphere that were antagonistic against the cocoyam root rot pathogen Pythium myriotylum and synthesized phenazine antibiotics produced acyl-HSLs. This supports the assumption that acyl-HSL production is related to the antagonistic activity of the strains. After detection, the signal molecules were identified through TLC-overlay and liquid chromatography-multiple MS (LC-MS/MS) analysis. In our representative strain, Pseudomonas CMR12a, production of the signal molecules could be assigned to two quorum-sensing (QS) systems. The first one is the QS system for phenazine production, PhzI/PhzR, which seemed to be well conserved, since it was genetically organized in the same way as in the well-described phenazine-producing Pseudomonas strains Pseudomonas fluorescens 2-79, Pseudomonas chlororaphis PCL1391 and Pseudomonas aureofaciens 30-84. The newly characterized genes cmrI and cmrR make up the second QS system of CMR12a, under the control of the uncommon N-3-hydroxy-dodecanoyl-homoserine lactone (3-OH-C12-HSL) and with low similarity to other Pseudomonas QS systems. No clear function could yet be assigned to the CmrI/CmrR system, although it contributes to the biocontrol capability of CMR12a. Both the PhzI/PhzR and CmrI/CmrR systems are controlled by the GacS/GacA two-component regulatory system.

A PCR-RFLP assay for identification and detection of Pythium myriotylum, causal agent of the cocoyam root rot disease

A PCR-RFLP assay for identification and detection of Pythium myriotylum, causal agent of the cocoyam root rot disease

Efecto de la aplicación de abonos orgánicos en la supresión de <i>Pythium myriotylum</i> en plantas de tiquisque (<i>Xanthosoma sagittifolium</i>)

Los abonos orgánicos ejercen efecto supresivo sobre patógenos de planta; su capacidad supresora varía de acuerdo al tipo de abono y al sistema planta-patógeno. Se evaluó el efecto supresor de diferentes abonos orgánicos en el sistema tiquisque-Pythium myriotylum. Se determinó la madurez, estabilidad, y actividad microbiana de los diferentes abonos, así como el efecto del tipo de compostaje (compost vs vermicompost), el material de origen (estiércol vs broza de café) y el grado de madurez, sobre la supresión del patógeno. El efecto supresivo sobre el desarrollo de la enfermedad en plantas de tiquisque se estableció por medio de una escala visual de síntomas a los 3, 6 y 9 días del transplante. Los abonos presentaron relaciones C/N entre 6 y 15 y ninguno afectó la germinación o la longitud de las raíces de plántulas de pepino. Los abonos maduros clasificaron como estables y los inmaduros, con excepción del compost de broza, como inestables. La menor incidencia de pudrición de raíces por P. myriotylum se obtuvo con el uso de vermicompost a base de estiércol maduro, mientras que la menor severidad de la enfermedad ocurrió cuando el suelo se enmendó con el compost y el vermicompost maduro a base de estiércol. En términos generales los abonos obtenidos a partir de estiércol fueron más supresivos, y presentaron una mayor actividad microbiana, que los producidos a base de broza de café. Se concluye que el tipo de compostaje, el origen y el grado de madurez tienen influencia sobre la capacidad supresora.

Variation of Pythium-induced cocoyam root rot severity in response to soil type

In Cameroon, andosols are suspected to be suppressive to cocoyam ( Xanthosoma sagittifolium) root rot disease (CRRD) caused by the Oomycete pathogen Pythium myriotylum. To determine factors involved in disease suppressiveness, andosols were studied in comparison to ferralsols known to be disease-conducive. Soil samples were collected from six sites of which three were in andosols around Mount Cameroon (Boteva, Njonji, and Ekona) and the three others in ferralsols (Bakoa, Lapkwang, and Nko’o canane). Greenhouse plant experiments were used to assess soil suppressiveness. Soils were artificially infested with two levels of P. myriotylum inoculum (100 and 300 mycelia strands g −1 soil) prior to planting cocoyam. Disease severity was significantly higher in ferralsols than in andosols. Andosols partly lost their suppressiveness as a result of autoclaving and could recover suppressiveness following recolonisation by their original microflora. Soil microbial groups implicated in the disease suppression were investigated by assessing the effect of fungicide, bactericide, and pasteurisation on andosol suppressiveness. Andosols suppressiveness was significantly reduced following pasteurisation and treatment with fungicide and bactericide. The possible influence of microbial biomass on andosol suppressiveness was investigated by comparing microbial populations of suppressive andosols to those in andosols that had lost suppressiveness. A comparative analysis of suppressive and conducive soil properties was performed to identify soil variables, which may contribute to soil suppressiveness. Soil chemical analysis results showed that organic matter content was higher in andosols than in ferralsols. In addition, the content of mineral nutrients such as Ca, K, Mg and N, was higher in andosols than in ferralsols. These soil variables negatively correlated with disease severity. By contrast, sand and clay, which were higher in ferralsols than in andosols, were positively related to disease severity. This study has confirmed the suppressive nature of andosols from Mount Cameroon to CRRD. The results suggest that high organic matter content is likely mediating P. myriotylum suppression in andosols by improving soil structure, increasing soil nutrient content and microbial biomass, and sustaining microbial activity.

Peroxidase Activity, Isozyme Patterns and Electrolyte Leakage in Roots of Cocoyam Infected with Pythium myriotylum

AbstractKnowledge of host defence response in cocoyam against Pythium myriotylum is essential in understanding disease resistance and the development of effective control measures for the cocoyam root rot disease. Four cocoyam accessions were assessed for root rot disease reaction by inoculating the roots of intact plants in the greenhouse and detached roots in the laboratory with P. myriotylum isolate ‘Mamu’ from Cameroon. Electrolyte leakage in detached infected roots and peroxidase activities and isozyme expressions in crude protein extracts collected from healthy roots of inoculated and non‐inoculated plants at 0, 2, 4, 6 and 8 days post inoculation (dpi) were used independently to assess resistance/tolerance. Based on the results, cocoyam accessions RO1054 and RO3015 were rated tolerant/moderately resistant while accessions RO2063 and ‘Local White’ (LW) were rated susceptible to P. myriotylum isolate ‘Mamu’. The time‐course spectrophotometric analysis of peroxidase activity showed an increase in activity in the roots of the tolerant RO1054, 8 days after inoculation. One peroxidase isoform (Rf 0.66) was induced only in roots of the susceptible inoculated accessions ‘Local White’ and R02063 at 4 and 6 dpi. One acidic peroxidase isozyme (pI 4.5) and one basic isozyme (pI 7.4) were detected only in roots of inoculated plants. Our results suggest that electrolyte leakage in infected roots can be used to assess tolerance/susceptibility in cocoyam and that tolerance to the root rot disease in cocoyam may be associated in part with an increased generation of peroxidase activity in the roots.

Characterization of CMR5c and CMR12a, novel fluorescentPseudomonasstrains from the cocoyam rhizosphere with biocontrol activity

To screen for novel antagonistic Pseudomonas strains producing both phenazines and biosurfactants that are as effective as Pseudomonas aeruginosa PNA1 in the biocontrol of cocoyam root rot caused by Pythium myriotylum. Forty pseudomonads were isolated from the rhizosphere of healthy white and red cocoyam plants appearing in natural, heavily infested fields in Cameroon. In vitro tests demonstrated that Py. myriotylum antagonists could be retrieved from the red cocoyam rhizosphere. Except for one isolate, all antagonistic isolates produced phenazines. Results from whole-cell protein profiling showed that the antagonistic isolates are different from other isolated pseudomonads, while BOX-PCR revealed high genomic similarity among them. 16S rDNA sequencing of two representative strains within this group of antagonists confirmed their relatively low similarity with validly described Pseudomonas species. These antagonists are thus provisionally labelled as unidentified Pseudomonas strains. Among the antagonists, Pseudomonas CMR5c and CMR12a were selected because of their combined production of phenazines and biosurfactants. For strain CMR5c also, production of pyrrolnitrin and pyoluteorin was demonstrated. Both CMR5c and CMR12a showed excellent in vivo biocontrol activity against Py. myriotylum to a similar level as Ps. aeruginosa PNA1. Pseudomonas CMR5c and CMR12a were identified as novel and promising biocontrol agents of Py. myriotylum on cocoyam, producing an arsenal of antagonistic metabolites. Present study reports the identification of two newly isolated fluorescent Pseudomonas strains that can replace the opportunistic human pathogen Ps. aeruginosa PNA1 in the biocontrol of cocoyam root rot and could be taken into account for the suppression of many plant pathogens.

Loss of pectin is an early event during infection of cocoyam roots by Pythium myriotylum

Cocoyam (Xanthosoma sagittifolium) is an important tuber crop in most tropical zones of Africa and America. In Cameroon, its cultivation is hampered by a soil-borne fungus Pythium myriotylum which is responsible for root rot disease. The mechanism of root colonisation by the fungus has yet to be elucidated. In this study, using microscopical and immunocytochemical methods, we provide a new evidence regarding the mode of action of the fungus and we describe the reaction of the plant to the early stages of fungal invasion. We show that the fungal attack begins with the colonisation of the peripheral and epidermal cells of the root apex. These cells are rapidly lost upon infection, while cortical and stele cells are not. Labelling with the cationic gold, which binds to negatively charged wall polymers such as pectins, is absent in cortical cells and in the interfacial zone of the infected roots while it is abundant in the cell walls of stele cells. A similar pattern of labelling is also found when using the anti-pectin monoclonal antibody JIM5, but not with anti-xyloglucan antibodies. This suggests that early during infection, the fungus causes a significant loss of pectin probably via degradation by hydrolytic enzymes that diffuse and act away from the site of attack. Additional support for pectin loss is the demonstration, via sugar analysis, that a significant decrease in galacturonic acid content occurred in infected root cell walls. In addition, we demonstrate that one of the early reactions of X. sagittifolium to the fungal invasion is the formation of wall appositions that are rich in callose and cellulose.

Peroxidase and Pectin Methylesterase Activities in Cocoyam (Xanthosoma sagittifolium L. Schott) Roots upon Pythium myriotylum Inoculation

AbstractCocoyam (Xanthosoma sagittifolium) is an important source of energy food for humid and semihumid tropic populations and consists of three cvs distinguished by the colour of their flesh, their productivity and their root rot disease scores: one sensitive (white), one intermediate (red) and one resistant (yellow). Changes in peroxidase (POD; EC 1.11.1.7) and pectin methylesterase (PME; EC 3.1.1.11) activities from roots of the three cvs were examined by isoelectrofocusing (IEF), polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulphate (SDS)‐PAGE upon inoculation with Pythium myriotylum. The evaluation of total POD activity shows the existence of a correlation between the sensitivity to P. myriotylum and POD activity. After 2 days of inoculation, POD activity of yellow cv. increased by 41.5%, while in the white and the red cvs the increase was only 13.4% and 7.5% respectively. In the soluble fractions of the resistant cv., the patterns of POD isozymes showed three new bands (Rf = 0.25; 0.38 and 0.85) and in ionically bound fraction, a new acidic (Rf = 0.2) specifically in inoculated root. The study of substrate specificity of POD with chlorogenic acid (C‐POD), ferulic acid (F‐POD) and indole acetic acid (IAA‐POD) showed an induction of C‐POD and F‐POD and a decrease in IAA‐POD in cocoyam roots after inoculation with P. myriotylum. Constitutively, ionically bound C‐POD and F‐POD activities were higher in roots of the yellow (11.6 and 11.7 EU/g Fw) than in those of the red (5.3 and 7.0 EU/g Fw) and the white (2.3 and 4.2 EU/g Fw) cvs. In addition, isoelectric‐focusing analysis of PME revealed an alteration of their profile correlated with inoculation. After inoculation, the appearance of a strong (pI: 9.7) and low (pI: 7.6) PME activities in the roots of yellow cv. were detected. Our findings suggest that POD and PME might be involved in the defence mechanism of cocoyam against P. myriotylum.

Variations in the Phenolic Contents of Cocoyam Clones in Correlation to Resistance to Pythium Myriotylum

Phenolic compounds from leaves and roots of infected and healthy cocoyam clones resistant (RO1075), tolerant (RO1043), and susceptible (RO1157) to Pythium myriotylum were quantified and tested for their in vitro fungitoxicity on the causal agent of the cocoyam root rot disease. All clones, infected or not, have phenolic compounds showing fungitoxic activity. The phenolic content of the tolerant and susceptible clones is less than that observed in the resistant one meanwhile in the resistant clone RO1075, a large increase in phenolic content is observed particularly in the roots during attack by pathogen.