New sources of resistance to <Emphasis Type="Italic">Cucumber mosaic virus</Emphasis> in <Emphasis Type="Italic">Capsicum annuum</Emphasis>

Background

Cucumber mosaic virus (CMV) is the most serious virus disease affecting chilli (Capsicum annuum L.) worldwide and the absence of natural resistance makes management of CMV outbreaks difficult. The characterization of improved sources of resistance to CMV in chilli would facilitate the development of commercially acceptable chilli varieties with adequate levels of CMV resistance. A total of 30 chilli genotypes were evaluated for their reaction to CMV in field and artificial inoculated conditions during 2010-2011 and 2011-2012. Large differences were observed among genotypes for disease incidence, severity indexes, and yield losses. Based on observed data, genotype CA23 (Noakhali) was identified as resistant, while CA12 (Comilla-2) was categorized as moderately resistant to CMV both in natural and inoculated conditions. Enzyme-linked immunosorbent assay absorbance values of samples taken from CMV-infected leaves corresponded well with visible viral symptoms for these genotypes. The identified C. annuum CA23 and CA12 genotypes represent previously undescribed and potentially useful sources of CMV resistance.

     

Characterization of genes <Emphasis Type="Italic">Rpp2</Emphasis>, <Emphasis Type="Italic">Rpp4</Emphasis>, and <Emphasis Type="Italic">Rpp5</Emphasis> for resistance to soybean rust

Asian rust, caused by the fungus Phakopsora pachyrhizi, is the most severe disease currently threatening soybean crops in Brazil. The development of resistant cultivars is a top priority. Genetic characterization of resistance genes is important for estimating the improvement when these genes are introduced into soybean plants and for planning breeding strategies against this disease. Here, we infected an F₂ population of 140 plants derived from a cross between ‘An-76’, a line carrying two resistance genes (Rpp2 and Rpp4), and ‘Kinoshita’, a cultivar carrying Rpp5, with a Brazilian rust population. We scored six characters of rust resistance (lesion color [LC], frequency of lesions having uredinia [%LU], number of uredinia per lesion [NoU], frequency of open uredinia [%OU], sporulation level [SL], and incubation period [IP]) to identify the genetic contributions of the three genes to these characters. Furthermore, we selected genotypes carrying these three loci in homozygosis by marker-assisted selection and evaluated their genetic effect in comparison with their ancestors, An-76, PI230970, PI459025, Kinoshita and BRS184. All three genes contributed to the phenotypes of these characters in F₂ population and when pyramided, they significantly contributed to increase the resistance in comparison to their ancestors. Rpp2, previously reported as being defeated by the same rust population, showed a large contribution to resistance, and its resistance allele seemed to be recessive. Rpp5 had the largest contribution among the three genes, especially to SL and NoU. Only Rpp5 showed a significant contribution to LC. No QTLs for IP were detected in the regions of the three genes. We consider that these genes could contribute differently to resistance to soybean rust, and that genetic background plays an important role in Rpp2 activity. All three loci together worked additively to increase resistance when they were pyramided in a single genotype indicating that the pyramiding strategy is one good breeding strategy to increase soybean rust resistance.     

Molecular mapping of <Emphasis Type="Italic">Pc68</Emphasis>, a crown rust resistance gene in <Emphasis Type="Italic">Avena</Emphasis><Emphasis Type="Italic">sativa</Emphasis>

Crown rust, which is caused by Puccinia coronata f. sp. avenae, P. Syd. & Syd., is the most destructive disease of cultivated oats (Avena sativa L.) throughout the world. Resistance to the disease that is based on a single gene is often short-lived because of the extremely great genetic diversity of P. coronata, which suggests that there is a need to develop oat cultivars with several resistance genes. This study aimed to identify amplified fragment length polymorphism AFLP markers that are linked to the major resistance gene, Pc68, and to amplify the F₆ genetic map from Pc68/5*Starter × UFRGS8. Seventy-eight markers with normal segregation were discovered and distributed in 12 linkage groups. The map covered 409.4 cM of the Avena sativa genome. Two AFLP markers were linked in repulsion to Pc68: U8PM22 and U8PM25, which flank the gene at 18.60 and 18.83 centiMorgans (cM), respectively. The marker U8PM25 is located in the linkage group 4_12 in the Kanota × Ogle reference oat population. These markers should be useful for transferring Pc68 to genotypes with good agronomic characteristics and for pyramiding crown rust resistance genes.     

Genetics of resistance to <Emphasis Type="Italic">Cucumber mosaic virus</Emphasis> in <Emphasis Type="Italic">Cucumis sativus</Emphasis> var. <Emphasis Type="Italic">hardwickii</Emphasis> R. Alef

The genetics of resistance to Cucumber mosaic virus (CMV) in Cucumis sativus var. hardwickii R. Alef, the wild progenitor of cultivated cucumber was assessed by challenge inoculation and by natural infection of CMV. Among the 31 genotypes of C. sativus var. hardwickii collected from 21 locations in India the lowest mean percent disease intensity (PDI) was recorded in IC-277048 (6.33%) while the highest PDI was observed in IC-331631 (75.33%). All the four cultivated varieties (DC-1, DC-2, CHC-1 and CHC-2) showed very high PDI and susceptible disease reaction. Based on mean PDI, 8 genotypes were categorized as resistant, 13 as moderately resistant, 9 as moderately susceptible and one as susceptible. A chi-square test of frequency distribution based on mean PDI in F₂ progenies of six resistant × susceptible crosses revealed monogenic recessive Mendelian ratio 1(R):3(S) to be the best fit. This monogenic recessive model was further confirmed by 1(R):1(S) ratio as the best fit for back cross with resistant parent and no fit for either 3:1 or 1:1 in the back cross with the susceptible parent. The results revealed that CMV resistance in C. sativus var. hardwickii was controlled by a single recessive gene. Considering the cross compatibility between C. sativus var. hardwickii and cultivated cucumber, the resistance trait can be easily transferred to cultivated species through simple backcross breeding.     

Increased resistance to <Emphasis Type="Italic">Ustilago zeae</Emphasis> and <Emphasis Type="Italic">Fusarium verticilliodes</Emphasis> in maize inbred lines bred for <Emphasis Type="Italic">Fusarium graminearum</Emphasis> resistance

Preliminary field observations in our maize breeding nurseries indicated that breeding for improved resistance to gibberella ear rot (Fusarium graminearum) in maize may indirectly select for resistance to another ear disease, common smut (Ustilago zeae). To investigate this, we compared the disease severity ratings obtained on 189 maize inbreds, eight of which included our inbreds developed with selection for gibberella ear rot resistance after field inoculation and breeding for 8–10 years. No correlation was found between disease severities for the 189 inbreds but the eight gibberella-resistant lines were consistently more resistant to smut. To further examine this relationship and to determine if these eight inbreds would be useful for developing inbreds with either common smut or fusarium ear rot (F. verticilliodes) resistance, we conducted a Griffing’s diallel analysis on six inbreds of maize, four with high levels of gibberella ear rot resistance representing all of the pedigree groups in our eight gibberella lines, and two with very low levels. Our most gibberella ear rot resistant inbreds, CO433 and CO441, had the lowest disease ratings for all three diseases, the consistently largest general combining ability effects and several significant specific combining ability effects. It was concluded that some inbreds bred specifically for gibberella ear rot would also be useful in breeding for resistance to common smut and fusarium ear rot.     

Genetic analysis of resistance to <Emphasis Type="Italic">soil-borne wheat mosaic virus</Emphasis> derived from <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

Genetic Analysis of Resistance to Soil-Borne Wheat Mosaic Virus Derived from Aegilops tauschii. Euphytica. Soil-Borne Wheat Mosaic Virus (SBWMV), vectored by the soil inhabiting organism Polymyxa graminis, causes damage to wheat (Triticum aestivum) yields in most of the wheat growing regions of the world. In localized fields, the entire crop may be lost to the virus. Although many winter wheat cultivars contain resistance to SBWMV, the inheritance of resistance is poorly understood. A linkage analysis of a segregating recombinant inbred line population from the cross KS96WGRC40 × Wichita identified a gene of major effect conferring resistance to SBWMV in the germplasm KS96WGRC40. The SBWMV resistance gene within KS96WGRC40 was derived from accession TA2397 of Aegilops taushcii and is located on the long arm of chromosome 5D, flanked by microsatellite markers Xcfd10 and Xbarc144. The relationship of this locus with a previously identified QTL for SBWMV resistance and the Sbm1 gene conferring resistance to soil-borne cereal mosaic virus is not known, but suggests that a gene on 5DL conferring resistance to both viruses may be present in T. aestivum, as well as the D-genome donor Ae. tauschii.     

Resistance to <Emphasis Type="Italic">Cucumber green mottle mosaic virus</Emphasis> in <Emphasis Type="Italic">Cucumis sativus</Emphasis>

Cucumber green mottle mosaic virus (CGMMV) is a severe threat for cucumber production worldwide. At present, there are no cultivars available in the market which show an effective resistance or tolerance to CGMMV infection, only wild Cucumis species were reported as resistant. Germplasm accessions of Cucumis sativus, as well as C. anguria and C. metuliferus, were mechanically infected with the European and Asian strains of CGMMV and screened for resistance, by scoring symptom severity, and conventional RT-PCR. The viral loads of both CGMMV strains were determined in a selected number of genotypes using quantitative RT-PCR. Severe symptoms were found following inoculation in C. metuliferus and in 44 C. sativus accessions, including C. sativus var. hardwickii. Ten C. sativus accessions, including C. sativus var. sikkimensis, showed intermediate symptoms and only 2 C. sativus accessions showed mild symptoms. C. anguria was resistant to both strains of CGMMV because no symptoms were expressed and the virus was not detected in systemic leaves. High amounts of virus were found in plants showing severe symptoms, whereas low viral amounts found in those with mild symptoms. In addition, the viral amounts detected in plants which showed intermediate symptoms at 23 and 33 dpi, were significantly higher in plants inoculated with the Asian CGMMV strain than those with the European strain. This difference was statistically significant. Also, the amounts of virus detected over time in plants did not change significantly. Finally, the two newly identified partially resistant C. sativus accessions may well be candidates for breeding programs and reduce the losses produced by CGMMV with resistant commercial cultivars.     

Genome-wide SNP-based association mapping of resistance to <Emphasis Type="Italic">Phytophthora sojae</Emphasis> in soybean (<Emphasis Type="Italic">Glycine max</Emphasis> (L.) Merr.)

Phytophthora root rot (PRR) is among the most important soybean (Glycine max (L.) Merr.) diseases worldwide, and the host displays complex genetic resistance. A genome-wide association study was performed on 337 accessions from the Yangtze-Huai soybean breeding germplasm to identify resistance regions associated with PRR resistance using 60,862 high-quality single nucleotide polymorphisms markers. Twenty-six significant SNP-trait associations were detected on chromosomes 01 using a mixed linear model with the Q matrix and K matrix as covariates. In addition, twenty-six SNPs belonged to three adjacent haplotype blocks according to a linkage disequilibrium blocks analysis, and no previous studies have reported resistance loci in this 441 kb region. The real-time RT-PCR analysis of the possible candidate genes showed that two genes (Glyma01g32800 and Glyma01g32855) are likely involved in PRR resistance. Markers associated with resistance can contribute to marker-assisted selection in breeding programs. Analyses of candidate genes can lay a foundation for exploring the mechanism of P. sojae resistance.     

Fine mapping of a <Emphasis Type="Italic">Xantha</Emphasis> mutation in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The recessive mutation of the XANTHA gene (XNT) transforms seedlings and plants into a yellow color, visually distinguishable from normal (green) rice. Thus, it has been introduced into male sterile lines as a distinct marker for rapidly testing and efficiently increasing varietal purity in seed and paddy production of hybrid rice. To identify closely linked markers and eventually isolate the XNT gene, two mapping populations were developed by crossing the xantha mutant line Huangyu B (indica) with two wild type japonica varieties; a total of 1,720 mutant type F₂ individuals were analyzed for fine mapping using polymorphic InDel markers and high dense microsatellite markers. The XNT gene was mapped on chromosome 11, within in a fragment of ~100 kb, where 13 genes are annotated. The NP_001067671.1 gene within the delimited region is likely to be a candidate XNT gene, since it encodes ATP-dependent chloroplast protease ATP-binding subunit clp A. However, no sequence differences were observed between the mutant and its parent. Bioinformatics analysis demonstrated that four chlorophyll deficient mutations that were previously mapped on the same chromosome are located outside the XNT region, indicating XNT is a new gene. The results provide useful DNA markers not only for marker assisted selection of the xantha trait but also its eventual cloning.     

Genetic analysis reveals a dominant <Emphasis Type="Italic">S</Emphasis> locus and an <Emphasis Type="Italic">S</Emphasis> suppressor locus in natural self-compatible <Emphasis Type="Italic">Brassica napus</Emphasis>

A self-incompatible (SI) line, S-1300, and its maintainer 97-wen135, a self-compatible (SC) line, were used to study the inheritance of maintenance for self-incompatibility in B. napus. The ratio of SI plants to SC plants from S-1300 × 97-wen135 F₂ and (S-1300 × 97-wen135) × 97-wen135 was 346:260 and 249:232, fitting the expected ratio of 9:7 and 1:1, respectively. Based on these observations, here we propose a genetic model in which two independent loci, S locus and S suppressor locus (sp), are predicted to control the inheritance of maintenance for self-incompatibility in B. napus. The genotypes of S-1300 and 97-wen135 are S ₁₃₀₀ S ₁₃₀₀ sp ₁₃₀₀ sp ₁₃₀₀ and S ₁₃₅ S ₁₃₅ sp ₁₃₅ sp ₁₃₅ , respectively. S ₁₃₅ is dominant to S ₁₃₀₀ , but coexistence of sp ₁₃₀₀ and sp ₁₃₅ fails to suppress S locus. Both S ₁₃₀₀ and S ₁₃₅ can be suppressed by sp ₁₃₅ , while sp ₁₃₀₀ can suppress S ₁₃₅ but not S ₁₃₀₀ . The model contains two characteristics: that a dominant S locus exists in self-compatible B. napus, and that co-suppression will occur when sp loci are heterozygous. The model has been validated by the segregation of S phenotypes in the (S-1300 × 97-wen135) × S-1300, the progenies of SC S-1300 × 97-wen135 F₂ plants and DH population developed from S-1300 × 97-wen135 F₁. This is the first study to report co-suppression of S suppressor loci in B. napus. The genetic model will be very useful for developing molecular markers linked to maintenance for self-incompatibility and for dissecting the mechanism of SI/SC in B. napus.     

Dominant gene for common bean resistance to common bacterial blight caused by <Emphasis Type="Italic">Xanthomonas</Emphasis><Emphasis Type="Italic">axonopodis</Emphasis> pv. <Emphasis Type="Italic">phaseoli</Emphasis>

The common bacterial blight pathogen [Xanthomonas axonopodis pv. phaseoli (Xap)] is a limiting factor for common bean (Phaseolus vulgaris L.) production worldwide and resistance to the pathogen in most commercial cultivars is inadequate. Variability in virulence of the bacterial pathogen has been observed in strains isolated from Puerto Rico and Central America. A few common bean lines show a differential reaction when inoculated with different Xap strains, indicating the presence of pathogenic races. In order to study the inheritance of resistance to common bacterial blight in common bean, a breeding line that showed a differential foliar reaction to Xap strains was selected and was crossed with a susceptible parent. The inheritance of resistance to one of the selected Xap races was determined by analysis of segregation patterns in the F₁, F₂, F₃ and F₄ generations from the cross between the resistant parent PR0313-58 and the susceptible parent ‘Rosada Nativa’. The F₁, F₂ and F₃ generations were tested under greenhouse conditions. Resistant and susceptible F_3:4 sister lines were tested in the field. The statistical analysis of all generations followed the model for a dominant resistance gene. The resistant phenotype was found to co-segregate with the SCAR SAP6 marker, located on LG 10. These results fit the hypothesis that resistance is controlled by a single dominant gene. The symbol proposed for the resistance gene is Xap-1 and for the bacterial race, XapV1.     

Search in <Emphasis Type="Italic">Solanum</Emphasis> (section <Emphasis Type="Italic">Lycopersicon</Emphasis>) germplasm for sources of broad-spectrum resistance to four <Emphasis Type="Italic">Tospovirus</Emphasis> species

The genus Tospovirus was considered as monotypic with Tomato spotted wilt virus (TSWV) being the only assigned species. However, extensive studies with worldwide isolates revealed that this genus comprises a number of species with distinct virulence profiles. The Neotropical South America is one center of Tospovirus diversity with many endemic species. Groundnut ringspot virus (GRSV), TSWV, Tomato chlorotic spot virus (TCSV), and Chrysanthemum stem necrosis virus (CSNV) are the predominant tomato-infecting species in Brazil. Sources of resistance were found in Solanum (section Lycopersicon) mainly against TSWV isolates from distinct continents, but there is an overall lack of information about resistance to other viral species. One-hundred and five Solanum (section Lycopersicon: Solanaceae) accessions were initially evaluated for their reaction against a GRSV isolate by analysis of symptom expression and systemic virus accumulation using DAS-ELISA. A subgroup comprising the most resistant accessions was re-evaluated in a second assay with TSWV, TCSV, and GRSV isolates and in a third assay with a CSNV isolate. Seven S. peruvianum accessions displayed a broad-spectrum resistance to all viral species with all plants being free of symptoms and systemic infection. Sources of resistance were also found in tomato cultivars with the Sw-5 gene and also in accessions of S. pimpinellifolium, S. chilense, S. arcanum, S. habrochaites, S. corneliomuelleri, and S. lycopersicum. The introgression/incorporation of these genetic factors into cultivated tomato varieties might allow the development of genetic materials with broad-spectrum resistance, as well as with improved levels of phenotypic expression.     

Papaya ringspot virus resistance in <Emphasis Type="Italic">Carica papaya</Emphasis> via introgression from <Emphasis Type="Italic">Vasconcellea quercifolia</Emphasis>

We report the first successful production of PRSV-P resistant backcross (BC) papaya plants following intergeneric hybridisation between C. papaya and a Vasconcellea species after 50 years of reports on unsuccessful attempts. This follows our previous reports of PRSV-P resistant F₁ hybrids developed by intergeneric hybridisation between C. papaya and V. quercifolia. One PRSV-P resistant BC 1 (BC₁) plant was produced after 114,839 seeds were dissected from 940 fruits. The seeds yielded 1,011 embryos and 733 germinated in vitro from which 700 developed into plantlets that were screened in a glasshouse and in the field under high disease pressure and exposure to inoculation by viruliferous aphids. From the PRSV-P resistant backcross 1 (BC₁) male plant, 1465 plants [137 BC₂, 546 SbC₂ (BC2 sib-crosses), 147 BC₃, 379 SbC₃ and 256 BC₄] were grown from seed and inoculated with PRSV-P and virus resistant BC₃ and BC₄ plants were selected from these generations. Presence or absence of virus was confirmed by ELISA serological tests. BC plants generally developed mild symptoms of PRSV-P after periods ranging from 5 to 18 months in the field but many showed the ability to produce new growth free of symptoms. All control plants developed severe symptoms after 3 months in the field. Some BC₃ and BC₄ plants were free from viral infection after 18 months in the field. Subsequently they developed very mild symptoms on their leaves and a few ringspots on their fruit. They continued to grow vigorously and produce fruit for 3 years under high disease pressure provided by the infected controls and other susceptible plants. Good quality marketable fruit were produced on these plants. Application of these results should lead to restoration of the papaya industry in virus-infested regions of the Philippines and worldwide.     

A survey of soybean germplasm for resistance to <Emphasis Type="Italic">Phytophthora sojae</Emphasis>

Phytophthora root and stem rot of soybean caused by Phytophthora sojae is a destructive disease affecting soybean production regions throughout the world. The utilization of resistant cultivars is the most economical and environmentally safe method for controlling this disease. This work aims to screen the effective sources of special and partial resistance for the development of resistant cultivars. A total of 611 soybean germplasm lines from three ecological regions were evaluated for their responses to three P. sojae strains, namely, PNJ1, PNJ3, and PNJ4, using the hypocotyl inoculation technique. The soybean germplasm lines elicited eight different reaction types with three strains. Among these, 106 were resistant and 253 were susceptible to the three strains. A total of 123 soybean germplasm lines identified as susceptible to the three strains by the hypocotyl inoculation method were evaluated for partial resistance to PNJ1 using the slant board assay. Thirty-nine cultivars displayed high levels of partial resistance to PNJ1. The results of this study can be utilized to plant appropriately resistant cultivars in infected fields and to provide good breeding materials.     

Sources of resistance to <Emphasis Type="Italic">Phytophthora</Emphasis> root rot within the genus <Emphasis Type="Italic">Beta</Emphasis>

Phytophthora root rot caused by Phytophthora drechsleri Tucker is one of the most devastating sugar beet diseases in tropical areas. To identify genetic resources resistant to this disease, an aggressive isolate of P. drechsleri was selected. Then, a screening method was optimized based on the standard scoring scales of 1–9 (1: no symptoms, 9: complete plant death). Finally, 19 sugar beet lines, three cultivars, and 14 accessions of the wild species Beta vulgaris subsp. maritima, B. macrocarpa, B. procumbens, and B. webbiana were evaluated for resistance to the most aggressive isolate of P. drechsleri by using the optimized method (inoculum included 20 g of rice seed together with superficial wound creation). The isolates of P. drechsleri had significant variation in aggressiveness, and Kv10 was the most aggressive isolate on the susceptible variety Rasoul. The lines O.T.201-15, SP85303-0 (resistant check), and S2-24.P.107 had the lowest disease index with scores of 3.09, 3.13, and 3.27 respectively; they were categorized into the resistant group. The interaction between isolates and genotypes was not significant, which indicated the same response of each genotype to different isolates. Investigating the resistance of different generations of sugar beet revealed that progeny selection would be an effective method for increasing the resistance level of breeding materials to P. drechsleri. Among the wild species, the accession 9402 belonging to B. macrocarpa and the accession 7234 of B. vulgaris subsp. maritima had the lowest disease index (2.29 and 2.60, respectively) and were categorized into the resistant group.     

Fine mapping of <Emphasis Type="Italic">Ren3</Emphasis> reveals two loci mediating hypersensitive response against <Emphasis Type="Italic">Erysiphe necator</Emphasis> in grapevine

Grapevine (Vitis vinifera L.) is economically very important for the production of wine, table grapes and raisins. However, grapevine is threatened by a brought range of pathogens. A destructive disease worldwide is powdery mildew caused by the ascomycete Erysiphe necator. In the grapevine cultivar `Regent’ a resistance locus against E. necator, Ren3, was previously reported. It spans an interval of approximately seven Mb on chromosome 15. We attempted to delimit this interval to facilitate its further molecular analysis. New simple sequence repeat markers targeted to the Ren3 region were designed. They were applied for fine mapping in the cross populations of ‘Regent’ × ‘Lemberger’ and ‘Regent’ × ‘Cabernet Sauvignon’ that segregate for E. necator resistance. Complementarily we scored E. necator infection levels of ‘Regent’ × ‘Lemberger’ progeny at different time points over the course of the vegetation period in 2015 and 2016. Subsequent QTL analysis revealed a maximum LOD value that shifted during the season from marker GF15-10 located at 2.2 Mb to marker GF15-53 located at 3.5 Mb and to marker ScORA7* located at 9.4 Mb on chromosome 15 (positions according to the grapevine reference genome of PN40024). To investigate the Ren3-encoded resistance mechanism we performed detached leaf infection assays for microscopic studies. These revealed that Ren3 carrying individuals react with a hypersensitive response. Results of detached leaf assays on recombinants in the Ren3 locus indicate that not only one, but two distinct genetic regions on chromosome 15 mediate hypersensitive response against E. necator.     

Ureide essay to assess N<Subscript>2</Subscript>-fixation abilities of soybean (<Emphasis Type="Italic">Glycine max</Emphasis>) genotypes under different <Emphasis Type="Italic">Bradyrhizobium</Emphasis> strains

The high protein content of soybean (Glycine max) seeds results in high nitrogen demand, causing a huge nitrogen uptake during plant growth. As a legume crop, soybean can fix atmospheric N through symbiotic associations with Bradyrhizobia and perform well in African nitrogen poor soils. This study aimed at establishing the ability of promiscuous soybean genotypes to fix nitrogen and devise the relationship between nodule scores and amount of nitrogen fixed. Twelve soybean genotypes were inoculated with Bradyrhizobium japonicum Strain USDA 110 (specific) and Bradyrhizobium sp. Strain USDA 3456 (native) and raised in pots in a greenhouse. At the R3.5 growth stage, nodules were scored and xylem sap was extracted, which xylem sap was used to carry out ureide, amino-N, and nitrates assays. The relative abundance of ureide was used to devise the proportion of nitrogen fixed by each genotype. The proportion of nitrogen derived from atmospheric N2 (Ndfa) ranged from 47.9 to 78.8% under USDA 3456 and from 36.7 to 78.7% under USDA 110. A strong correlation was found between nodule scores, especially nodules’ effectiveness, and Ndfa. The genotypes Wondersoya (78.8%), Maksoy 2N (78.4%), Namsoy 3 (78.3%), and Maksoy 3N (75.7%) had high nitrogen-fixing ability in response to USDA 3456. Promiscuous soybean genotypes can fix nitrogen equally under both native and specific Bradyrhizobium types. Nodules’ effectiveness can be a good predictor of biological nitrogen fixation. This study highlighted that crop improvement to boost soybean production in Africa should target promiscuous varieties for better yield with less inputs.     

Microsporogenesis of <Emphasis Type="Italic">Rps</Emphasis>8/<Emphasis Type="Italic">rps</Emphasis>8 heterozygous soybean lines

Phytophthora root and stem rot caused by Phytophthora sojae, is one of the most damaging diseases of soybean, for which management is principally done by planting resistant cultivars with race specific resistance which are conferred by Rps (Resistance to Phytophthora sojae) genes. The Rps8 locus, identified in the South Korean landrace PI 399073, is located in a 2.23 Mbp region on soybean chromosome 13. In eight cv. Williams (rps8/rps8) × PI 399073 (Rps8/Rps8) populations, this region exhibited strong segregation distortion. In a cross between the South Korean lines PI 399073 (Rps8/Rps8) and PI 408211B (multiple Rps genes) this region segregated in a Mendelian fashion. In this study, microsporogenesis was evaluated to identify meiotic abnormalities that may be associated with the segregation distortion of the Rps8 region. Pollen was collected from greenhouse-grown plants of the parental genotypes: Williams, PI 399073, and PI 408211B; as well as selected Rps8/rps8 RILs from Williams × PI 399073 BC₄F_2:3 and PI 399073 × PI 408211B F_4:5 populations. There were no differences for pollen viability among the genotypes. However, for PI 399073, a mix of dyads, triads, tetrads and pentads was observed. A high frequency of meiotic abnormalities including fragments, laggards, multinucleated microspores; and microcytes containing DNA was also observed in Rps8/rps8 Williams × PI 399073 BC₄F_2:3 RILs. These meiotic abnormalities may contribute to the high degree of segregation distortion present in the Williams × PI 399073 populations.     

Wild <Emphasis Type="Italic">Coffea arabica</Emphasis> resistant to <Emphasis Type="Italic">Meloidogyne paranaensis</Emphasis> and genetic parameters for resistance

Arabica coffee production is based on highly productive cultivars; however, these cultivars are susceptible to infestation by several biotic agents, including root-knot nematodes. Collections of wild Coffea arabica germplasm represent an important source of genetic variability for resistant cultivar development. In this study, 1046 plants derived from 71 wild coffee trees were evaluated with respect to Meloidogyne paranaensis resistance. In addition to information on plants reactions, we also evaluated the genetic parameters related to resistance. Progenies from the five most promising plants were also evaluated regarding resistance to M. incognita and M. exigua. The yield potential of selected plants was estimated through analysis of data for fruits harvested in 4 different years. Forty-seven plants were considered resistant based on reproduction factor values. The estimated heritability was high for all analyzed variables leading to substantial selection gain, mainly at the progeny mean level. On the basis of heritabilities and genetic correlations, we conclude that selection could be performed based on values of the gall and egg mass index. However, higher genetic gain could be obtained based on nematode count variables. A second experiment confirmed the reactions of the selected five plants to M. paranaensis, and multiple resistance was detected in three of them. The resistant accessions also have yield potential.     

Measurement of the field response of <Emphasis Type="Italic">Musa</Emphasis> genotypes to <Emphasis Type="Italic">Radopholus Similis</Emphasis> and <Emphasis Type="Italic">Helicotylenchus Multicinctus</Emphasis> and the implications for nematode resistance breeding

Crop growth and damage parameters (plant growth and yield, root damage and nematode population densities), believed to be associated with resistance of Musa genotypes to nematodes under field conditions, were evaluated in a field trial of 24 Musa genotypes inoculated at planting with a combination of Radopholus similis and Helicotylenchus multicinctus with the objective to identify parameters with strong association with nematode resistance and high heritability. Correlation and path analysis of the association between plant growth, yield, root damage and nematode population densities showed a strong negative association between percentage dead roots, percentage root necrosis, R. similis and H. multicinctus population densities and yield. The strongest negative association was observed between percentage dead roots and yield. Broad-sense genotype heritability estimates demonstrated that heritability estimates for percentage dead roots, number of large lesions and nematode population density were most affected by inoculation with nematodes. These results indicate therefore that effective selection for nematode resistance under field conditions could be obtained by using an index, that includes percentage dead roots, the number of large lesions, and nematode population density.