A temporal genetic analysis of disease resistance genes: resistance to melon vine decline derived from Cucumis melo var. agrestis

Melon vine decline causes severe economic losses to melon crops all over the world. Two fungi. Acremonium cucurbitacearum and Monosporascus cannonballus, have been reported as the main causal agents of root damage, responsible for the decrease in root water and nutrient uptake capacity. However, melon vine decline aetiology is not completely understood, so screening assays must be conducted under natural infestation conditions. In field assays, the erratic effect of uncontrolled environmental factors makes the study of the trait's genetics difficult. The percentage of asymptomatic plants scored at individual moments during the infection process in the field is an imprecise indicator of the resistance level of each genotype. The analysis of disease progress curves does allow for the minimization of the stochastic fluctuations caused by environmental factors at various moments in time. Using this analysis the mode of inheritance of the partial resistance to melon vine decline derived from the accession Pat 81 of Cucumis melo var. agrestis was further studied. Data from the six generations assayed (susceptible and resistant parents. F₁, F₂, and backcrosses BC₁ and BC₂ were fitted to an additive/dominance model without epistatic effects using a scaling test. Monogenic control was proposed as the simplest model to explain the data. Assuming this simple model, the method of analysis also allowed for the characterization of the incomplete penetrance of resistance, which varies as a log probit function of the time. Simple genetic control would simplify the use of Pat 81 in breeding programmes aimed at developing commercial melon varieties with resistance to vine decline.     

Diversity in root architecture and response to P deficiency in seedlings of <Emphasis Type="Italic">Cucumis melo</Emphasis> L.

Breeding for more phosphorus (P)-efficient crops is one strategy to reduce the use of P fertilizers, thus mitigating the environmental and economic impacts of agriculture. Variation in root architecture and the response to P deficiency were studied in C. melo. Forty accessions representing genetic diversity within the species were screened for their root systems in normal and deficient P conditions at the seedling stage. Various parameters of P-uptake and P-use were analyzed in a subset of accessions at 40 days. Significant differences in root architecture were observed, with the taproot systems prevailing among the wild and exotic accessions, and more branched root systems in cultivated stocks. Moreover, differences in the plastic response of roots to P starvation were observed. Variation in different P-use and -uptake traits correlated with the root architecture. Within ssp. melo, the inodorus and flexuosus landraces had larger and more branched roots and more efficient P-uptake, thereby providing a close genepool for breeding. Within ssp. agrestis, conomon and momordica accessions can be sources of interest for the enhancement of variation in root architecture and P-use efficiency into cultivated melons. Therefore, the diversity observed within C. melo species could be useful in breeding P-efficient melon cultivars.     

Potential of Nicotiana glauca (Grah.) as a source of resistance to black root rot Thielaviopsis basicola (Berk. and Broome) Ferr. in tobacco improvement

Nicotiana glauca, a wild relative of Nicotiana tabacum, is an attractive potential source of black root rot (Thielaviopsis basicola)‐resistant germplasm. Moreover, it shows a resistance or tolerance to PVY, TEV, anthracnose and powdery mildew. In this study its potential as a source of resistance to black root rot caused by Th. basicola was investigated. Nicotiana glauca GG (2n = 2x = 24) was crossed as male with two N. tabacum TT (2n = 4x = 48) flue‐cured cultivars: ‘BY103’ and ‘K 326’, both of which are susceptible to black root rot. Amphihaploid F₁ TG (2n = 3x = 36), amphidiploid TTGG (2n = 6x = 72) and sesquidiploid TTG (2n = 5x = 60) hybrids were obtained. The resultant sesquidiploid hybrids were used as maternal components in backcrossing to N. tabacum and a segregating post‐sesquidiploid TTg (2n = 5x = 52–54) offspring was obtained. Amphihaploids exhibited a level of resistance to black root rot characteristic of N. glauca. The expression of resistance varied in the sesquidiploid generation, possibly reflecting cytological instabilities in that progeny. A wide variation in response to black root rot was found for post‐sesquidiploids a clear reflection of extensive chromosome segregation in that hybrid generation.     

Identification of QTL controlling adventitious root formation during flooding conditions in teosinte (Zea mays ssp. huehuetenangensis) seedlings

Adventitious root formation (ARF) at the soil surface is one of the most important adaptations to soil flooding or waterlogging. Quantitative trait loci (QTL) controlling ARF under flooding condition were identified in a 94 F₂ individual population by crossing maize (Zea mays L., B64) × teosinte (Z. mays ssp. huehuetenangensis). A base-map was constructed using 66 SSR and 42 AFLP markers, covering 1,378 cM throughout all ten maize chromosomes. The ARF capacity for seedlings was determined by evaluating the degree of root formation at the soil surface following flooding for 2 weeks. ARF showed continuous variation in the F₂ population. Interval mapping and composite interval mapping analyses revealed that the QTL for ARF was located on chromosome 8 (bin 8.05). Utilising a selective genotyping strategy with an additional 186 F₂ population derived from the same cross combination and 32 AFLP primer combinations, regions on chromosomes 4 (bin 4.07) and 8 (bin 8.03) were found to be associated with ARF. Z. mays ssp. huehuetenangensis contributed all of the QTL detected in this study. Results of the study suggest a potential for transferring waterlogging tolerance to maize from Z. mays ssp. huehuetenangensis.     

Diallel analysis of root-knot nematode resistance based on galling index in upland cotton

The southern root‐knot nematode (RKN) [Meloidogyne incognita (Kofoid and White) Chitwood) RKN] is one of the most destructive pests in the Cotton Belt of the U.S. Developing and employing resistant cultivars is the most economical and efficient method for RKN management. This greenhouse investigation was conducted to understand the quantitative genetic basis of RKN resistance in the major RKN resistance sources using a nine‐parent diallel analysis based on improved RKN evaluation techniques. The selected genotypes consisted of three ‘Aubrun 623 RNR’ (here RNR = R oot‐knot N ematode R esistance)‐derived resistant lines, one moderately resistant cultivar ‘Acala Nem‐X’, and five susceptible cultivars. Comparison between F₁ and their parents in galling index revealed that the RKN resistance is partially dominant. The general combining ability was more important than the specific combining ability for RKN resistance. The estimates for broad‐sense and narrow‐sense heritabilities on galling index were 0.82 and 0.65, respectively, indicating that RKN resistance in the Auburn and Nem‐X resistance sources is largely controlled by genetic and additive effects. The minimum number of genes for RKN resistance was estimated to be two. Therefore, under reliable and uniform RKN inoculation and infestation conditions, single plant selection should be efficient in transferring Auburn RKN resistance into elite genetic backgrounds.     

Powdery mildew resistance in Czech and Slovak barley cultivars

Fifteen powdery mildew resistance genes and the gene MlaN81 derived from ‘Nepal 81’were found in 76 Czech and Slovak spring and winter barley cultivars when tested for reaction to a set of powdery mildew isolates. Nine cultivars (‘Donum’, ‘Expres’, ‘Jubilant’, ‘Orbit’, ‘Primus’, ‘Progres’, ‘Stabil’, ‘Vladan’ and ‘Zlatan’) are composed of lines with different resistance genes. The Mlat gene is present in nine cultivars and was transferred from the Anatolian landrace ‘A‐516′. The resistances derived from ‘KM‐1192’and ‘CI 7672’were identical and designated Ml(Kr). Five winter barley cultivars possess the Ml(Bw) resistance. The winter barley line ‘KM‐2099’carries the mlo gene. The parental cultivar ‘Palestine 10’was also tested in which the genes Mlk1, MlLa were identified. The German cultivar ‘Salome’, a parent of seven cultivars tested, probably carries the gene MlLa in addition to mlo and Mla7. The gene mlo6 may be present in the cultivar ‘Heris’. Most of the results were confirmed by the pedigrees of the cultivars.     

Introgression of clubroot resistance into an elite pak choi inbred line through marker‐assisted introgression breeding

Clubroot is a soilborne disease that severely infects cruciferous species. Pak choi (Brassica rapa subsp. chinensis) is an economically important cruciferous crop cultivated throughout the world. However, no clubroot‐resistant germplasms have been identified in pak choi to date. To improve disease resistance, we used marker‐assisted selection (MAS) to introgress the clubroot resistance (CR) trait from the ‘CCR13685’ Chinese cabbage (B. rapa subsp. pekinensis) inbred line into an elite pak choi inbred line, ‘GHQ11021’. Genetic analysis of F₂ and BC₁ progeny showed that CR of ‘CCR13685’ was controlled by a single dominant gene. We designed nine candidate sequence‐characterized amplified region markers, K‐1 to K‐9, based on two molecular markers linked to the CR gene. We found that K‐3 co‐segregated with CR and an inoculation test confirmed that K‐3 could be used for MAS. Two introgression lines, BC₃‐1‐4 and BC₃‐2‐18, were developed using K‐3 for foreground selection. These lines displayed the same phenotypic properties as ‘GHQ11021’, but were highly resistant to clubroot, indicating that the CR gene of ‘CCR13685’ had been successfully introduced into pak choi.     

Identification of resistant sources and DNA markers linked to genomic region conferring dry root rot resistance in chickpea (Cicer arietinum L.)

A set of 520 chickpea germplasm lines was screened under laboratory conditions using blotter paper technique for reaction to dry root rot caused by Rhizoctonia bataticola (Taub.) Butler. The lines PG06102, BG2094 and IC552137 were identified as resistant for dry root rot. Phenotyping the mapping population consisting of 129 F_2:3 progeny derived from the cross L550 × PG06102 during 2013 winter indicated monogenic inheritance of dry root rot resistance. Fifty‐two of 381 simple sequence repeat (SSR) primers polymorphic between the two parents were used to genotype F₂ resistant and susceptible bulks prepared on the basis of reaction of F_2:3 progeny. Four markers differentiated the resistant and susceptible bulks. All the four polymorphic markers were then assayed on the entire F₂ population. Linkage analysis using 129 F₂ plants revealed that two markers ICCM0299 and ICCM0120b were co‐segregating with resistance to dry root rot. These two markers appeared to have additive effects on resistance and could be potentially utilized in dry root resistance breeding programme.     

Development of cleaved amplified polymorphic sequence (CAPS) markers linked to a green rice leafhopper resistance gene, Grh3(t)

The chromosomal location of the resistance gene for green rice leafhopper (GRLH), an injurious insect for rice, has been determined and RFLP markers closely linked to this gene have been identified. The susceptible japonica rice variety ‘Nipponbare’ was crossed with a resistant japonica rice line ‘Aichi42’, in which green rice leaf hopper resistance had been introduced from an indica variety ‘Rantaj‐emas2’, and the 100 F₂ plants obtained were used for linkage analysis. The green rice leafhopper resistance gene, Grh3(t), was mapped between RFLP markers C288B and C133A on chromosome 6 and co‐segregated with C81. Of the RFLP markers tightly linked to Grh3(t), C81 was converted to a SCAR marker and C133A to a cleaved amplified polymorphic sequence marker that could distinguish the heterozygous genotype to establish an effective marker‐aided selection system for the GRLH resistance gene.     

Variation in Crown and Root Organic Reserves Among Lucerne Genotypes of Different Morphology and Flower Colour

Previous evidence indicates that differences in the concentration of underground organic reserves can drive the survival and growing ability of lucerne under cold and defoliation stresses. In order to provide the selection process with further information on compounds that may influence plant performance under grazing, we assessed variations in cold‐season concentrations of nitrogen and carbon reserves on genotypes that had been identified for morphological features that possibly enhance grazing tolerance. The selected genotypes encompassed distinct morphological patterns (defined as ‘models’) and different taxa within the Medicago sativa complex, as shown by different flower colours. Crown concentrations of reserves were determined on 90 genotypes, whereas root concentrations were measured on a subsample of 15 genotypes. Wide intergenotypic variation was observed for all reserve substances. Comparisons among models and among flower colour classes highlighted the high concentrations of crown carbohydrates and root and crown‐soluble proteins of the model coded as ‘D1’, characterized by prostrate, rhizomatous habit and long dormancy, which largely corresponded to plants with yellow or variegated flowers, typical of ssp. falcata and × varia, respectively. There was a strong ‘flower colour × storage organ’ interaction for sugar concentration, and the results suggested a preferential compartment of sugars in the roots of purple‐flowered genotypes that belonged to the ssp. sativa. A rank correlation analysis indicated a positive relationship between persistence after two years under grazing of half‐sib progenies deriving from 19 genotypes out of the 90 and crown concentrations of carbohydrates of the 19 mother plants.     

Inheritance of resistance to Fusarium head blight in the wheat lines 'CJ 9306' and 'CJ 9403'

Fusarium head blight (FHB or scab) caused by Fusarium graminearum is a worldwide serious disease in wheat. Exploitation and genetic studies of elite resistance sources can speed up the development of resistant cultivars. To characterize the inheritance of host plant resistance in two new lines, ‘CJ 9306’ and ‘CJ 9403’, developed from a recurrent selection programme in China, six generations P₁, P₂, F₁, F₂, B₁ and B₂ of four crosses and 137 F_{6 : 7} recombinant inbred lines (RILs) from one cross were evaluated in the greenhouse for scab resistance using single‐floret inoculation. The data of area under disease progress curve (AUDPC) in F₂, backcross (BC) and RIL populations exhibited mono‐modal distributions without clear‐cut demarcations and skewing towards resistance. An additive–dominance model was well‐fitted, additive effects played a predominating role, and dominance effects were also significant. Continuous distributions with two major peaks and one minor peak for the number or percentage of scabby spikelets (NSS or PSS) in segregating populations implied the existence of major genes or quantitative trait loci (QTL) for resistance. The estimates of broad‐sense and narrow‐sense heritabilities based on the six‐generation experiment were 56–76% and 26–67% respectively. The estimates of broad‐sense heritabilities based on anova with RILs were 89–90%. These two improved lines with excellent scab resistance and good agronomic traits are of interest for wheat breeding and production.     

Evaluation of aluminium tolerance in a spring rye collection by hydroponic screening

Aluminium (A1) tolerance of 63 rye accessions from a world spring rye collection was evaluated using the haematoxylin method and the root growth method. The haematoxylin method is based on the ability of A1‐tolerant seedlings to continue root growth following a short pulse treatment with a high Al concentration, while the root growth method uses the root growth and root tolerance index to judge A1 tolerance. Significantly higher levels of A1 tolerance were found in rye than in the A1‐tolerant bread wheat cultivar ‘Maringa’. Under the assumption that the ability of roots to grow under A1 stress is a combination of root vigour (long roots) and A1 tolerance, a hypothesis allowing for the differentiation of five genotype classes was suggested. This study demonstrated that the haematoxylin method and the root growth parameter identify genotypes with long root growth under A1 stress, but failed to detect A1 tolerance in genotypes with poor root vigour. These genotypes can only be identified using the root tolerance index parameter. However, the haematoxylin method is highly suitable for screening large segregating populations derived from improved germplasm that has been preselected for agronomically preferable traits, including plant vigour.     

Early selection for extreme resistance to potato virus Y and tobacco etch virus in potato using a β-glucuronidase-tagged virus

The Ry_sto gene from Solanum stoloniferum introduced into potato cultivars (Solanum tuberosum L. ssp. tuberosum) confers resistance to potato virus A, potato virus V and potato virus Y (PVY). In addition to PVY, tobacco etch virus (TEV) and a TEV construct that encodes β‐glucuronidase (TEV‐GUS) were inoculated to determine the inheritance of resistance to these viruses in progenies obtained from potato cultivars containing the Ry_sto gene. While cultivars ‘Karlena’ and ‘Delikat’ were susceptible, ‘Bettina’ and clone 927eY were resistant to PVY, TEV and TEV‐GUS, as determined by enzyme linked immunosorbent assay, biotest and GUS assay, respectively. The segregation ratios obtained from the progenies of ‘Bettina’בDelikat’ and 816eY בKarlena’ indicate that resistances to PVY and TEV are governed by one dominant gene or two genes tightly linked in coupling phase. Evidently, Ry_sto confers broad spectrum resistance to potyviruses. TEV resistance could be reliably detected 4 days after inoculation with the TEV‐GUS construct by GUS assay. Therefore, the GUS‐tagged TEV construct can be used for early selection for resistances based on the gene Ry_sto or closely linked genes.     

Genetic and statistical models for estimating genetic parameters of maize seedling resistance to Fusarium graminearum Schwabe root rot

Root lodging is an important problem in corn fields. Fungi recovered from roots include seedling blight and stalk rot pathogens. The objective of this work was to study the inheritance of maize seedling resistance to pathogens causing maize lodging. The Fusarium graminearum strain, 241 Fr1, was isolated from maize lodged plants and identified as the most pathogenic isolate for root rotting. Nine inbred lines of maize and their diallel F₁ crosses plus control genotypes were studied. Seedlings were inoculated at the stage of four-leaves. Disease severity was measured as percentage of the root rotted area. Highly significant differences between inoculated and non-inoculated genotypes were found. Four genetic models and two statistical approaches—the mixed model for the best linear unbiased prediction (BLUP) and the general linear model (GLM)—were used for the analysis. Favorable heterosis of resistance of hybrids over inbreds was the most important effect detected. The general combining ability (GCA) effects were significant for all genetic models and statistical methods studied, and a good agreement existed among the GCA estimates by the different methods. The type of gene action, either additive or dominance, showed a large variation among the parental inbreds and hybrids. Selection of additive effects based exclusively on inbred lines is not sufficient to confer resistance to hybrids, additional selection should be practiced on hybrids to look for favorable dominance effects. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stimulation of Rice (Oryza sativa L.) Seedling Vigour by Smoke-water and Butenolide

The newly identified smoke‐compound butenolide (3‐methyl‐2H‐furo[2,3‐c]pyran‐2‐one) can improve seedling vigour. This study highlights the effects of smoke‐water and butenolide on shoot and root elongation of a local rice variety. Smoke‐water (1 : 500) and butenolide (10^?8 m ) treatments significantly increased shoot length. A low concentration of butenolide (10^?10 m ) promoted maximum root length and seedling weight, which were significantly different from the control. Butenolide‐treated (10^?8 m ) seedlings had a significantly greater number of lateral roots than untreated seedlings. The vigour index of smoke‐water (1 : 500) and butenolide‐treated (10^?8 m ) rice seeds was significantly higher than that of untreated seeds. These findings suggest the possible use of butenolide for promoting vigorous seedlings in rice.     

Towards Enhancement of Early‐Stage Chilling Tolerance and Root Development in Sorghum F1 Hybrids

Sorghum (Sorghum bicolor L. Moench) is regarded a drought‐tolerant alternative to maize as a bioenergy and fodder crop, but its early‐stage chilling sensitivity is obstructing a successful implementation in temperate areas. While several studies have identified quantitative trait loci (QTL) underlying chilling tolerance‐related traits in sorghum lines, little is known about the inheritance of these traits in F₁ hybrids. We have conducted a comprehensive approach to analyse heterosis, combining ability and the relation between line per se and hybrid performance for emergence and early shoot and root development comprising both field trials and controlled environment experiments including chilling tests. To our best knowledge, this is the first study analysing heterosis for sorghum root parameters under chilling. Our results show that most traits are heterotic and that the mid‐parent values are rather poor predictors of hybrid performance. Hybrid breeding programmes should focus on efficient GCA tests and the establishment of genetically diverse pools to maximise heterosis rather than on a too strict selection among lines based on their per se performance. The medium‐to‐high heritabilities estimated for seedling emergence and juvenile biomass suggest that a robust breeding progress for these complex traits is feasible.     

Heritability of resistance to brown spot and root rot of narrow-leafed lupins caused by Pleiochaeta setosa (Kirchn.) Hughes in field experiments

Resistance to brown spot (BS) and Pleiochaeta root rot (PRR) in narrow-leafed lupin (Lupinus angustifolius L.) was assessed on a broad range of breeding lines and cultivars in field trials in Western Australia in 1985 and 1986. Both diseases are caused by Pleiochaeta setosa (Kirchn.) Hughes. Lines were grown in 5m × 1m plots in randomized complete block experiments with various disease pressures associated with cropping history — first, second and third successive lupin crops after cereals at one site in 1985, and first and second lupin crops at two sites in 1986. Best linear unbiased predictors (BLUPs) of defoliation caused by BS across experiments ranged from 64% to 123% of ‘Gungurru’, and BLUPs for PRR lesion severity ranged from 82% to 118% of ‘Gungurru’. For both diseases, genotypic variance was several times greater than genotype × environment variance, but error variance was relatively low for BS and high for PRR. Consequently, broad sense heritability (h²) for BS resistance was high in the seven experiments (range 0.89–0.94) but low for PRR resistance (range 0.00–0.53). There was a moderate correlation between BLUPs for resistance to BS and PRR across experiments (r = 0.36, P < 0.05). Genotypic correlations between resistance to BS and PRR were estimated at 0.57 ± 0.20 and 0.75 ± 0.31 in two experiments in 1985. Breeding progress is likely to be high for BS resistance and there may be slight improvements in PRR resistance associated with BS resistance. However, this field technique is not suitable for rapid breeding progress for PRR resistance.     

Identification of a candidate gene for resistance to root‐knot nematode in a wild peach and screening of its polymorphisms

Root‐knot nematode disease, caused by Meloidogyne species, is an important soil‐borne disease of peach (Prunus persica L.) worldwide. To identify a major locus of genetic resistance to M. incognita, PkMi, in a wild peach species, we reconstructed a linkage group in a BC₁ population of 187 lines using resistance gene analogue markers surrounding the PkMi locus. A resistance gene analogue marker, ppa021062m, co‐segregated with the PkMi locus and was therefore considered a strong candidate for PkMi. Phylogenetic analysis of the deduced protein sequences of ppa021062m, together with the other seven genes for nematode resistance, allowed ppa021062m to be assigned to the Toll/Interleukin1 Receptor‐Nucleotide Binding Site‐Leucine Rich Repeat class, similar to Ma in myrobalan plum (P. cerasifera). Comparative analysis of the candidate gene sequence in four genotypes that had different levels of resistance to root‐knot nematode disease showed that most non‐synonymous SNPs in the genic region were distributed in the TIR and NBS motifs. This study enhances our understanding of the genetic and molecular control of resistance to root‐knot nematode disease in peach.     

Response to selection for root dimension in a Mediterranean natural population of Lolium rigidum Gaud

In the Mediterranean environment, root systems are very important for water utilization. For this reason, the effects of selection for increased root dimensions upon root and shoot characteristics of a natural population of Lolium rigidum Gaud, were analysed. Two cycles of phenotypic recurrent selection were carried out on a population collected in Nurra (north-western Sardinia, Italy). Plants derived after two cycles of selection, together with unselected plants, were grown hydroponically in a greenhouse. Data on root volume, longest root length, number of stems and leaves, leaf length, shoot and root dry weight were collected from the selected material and also from unselected material for use as a control. An analysis of variance was performed and correlation coefficients were calculated. The increased size of the root system did not have a negative effect on the above-ground biomass and there was genetic variation for root traits in the ‘Nurra’ population. The improved population will be utilized for further breeding work to obtain L. rigidum varieties capable of withstanding dry periods during the vegetative phase.     

Inheritance of resistance to the peach root‐knot nematode (Meloidogyne floridensis) in interspecific crosses between peach (Prunus persica) and its wild relative (Prunus kansuensis)

The peach root‐knot nematode, Meloidogyne floridensis (MF), infects majority of available nematode‐resistant peach rootstocks which are mostly derived from peach (Prunus persica) and Chinese wild peach (P. davidiana). Interspecific hybridization of peach with its wild relative, Kansu peach (P. kansuensis), offers potential for broadening the resistance spectrum in standard peach rootstocks. We investigated the inheritance of resistance to MF in segregating populations of peach (‘Okinawa’ or ‘Flordaguard’) × P. kansuensis. A total of 379 individuals from 13 F₂ and BC₁F₁ families were challenged with a pathogenic MF isolate “MFGnv14” and were classified as resistant (R) or susceptible (S) based on root galling intensity. Segregation analyses in F₂ progeny revealed the involvement of a major locus with a dominant or recessive allele determining resistance in progeny segregating 3R:1S and 1R:3S, respectively. Testcrosses with a homozygous‐susceptible peach genotype (‘Flordaguard’ or ‘UFSharp’) confirmed P. kansuensis as a source of new resistance and the heterozygous allelic status of P. kansuensis at the locus conferring resistance to MF. We propose a single‐locus dominant/recessive model for the inheritance of resistance.