Resistant Inbred Lines Research Articles

Quantitative Trait loci (QTL) Identification for Resistance to Mal de Rio Cuarto Virus (MRCV) in Maize Based on Segregate Population

Mal de Rio Cuarto virus (MRCV) is one of the most devastating pathogens in maize (Zea mays L.) causing se- rious yield loss in susceptible cultivars in Argentina. An effective solution to control the virus is to use resistant genes to improve the behavior of susceptible genotypes. The goals of this work are to identify simple sequence repeats (SSR) linked to quantitative trait loci (QTL), for resistance to MRCV, and to validate QTL found in previous research with dif- ferent genetic backgrounds. Two hundred and eight F2:3 families derived from a cross between a susceptible inbred line, B73, and a resistant inbred line, LP116, were evaluated across three environments in the disease-endemic region. Disease incidence (INC), disease severity (SEV) and disease severity index (DSI) were employed to evaluate F2:3 families. Based on this segregant population, ninety-eight simple sequence repeat (SSR) markers selected from 173 markers were used. The means of all measured traits followed near-normal distribution; therefore these traits may be considered as quantita- tively inherited characters. For INC, SEV and DSI all estimates showed a moderate heritability ranging from 0,33 to 0,72. Some quantitative trait loci (QTL), each explaining around 10% of phenotypic variance were found on chromosome 1, 6, 8 and 10. The identification of QTL conferring resistance to MRCV may contribute to breeding programs seeking to pro- tect the crop through improved genetic resistance.

Generation Means Analysis for Estimation of Genetic Parameters for Striga hermonthica Resistance in Maize (Zea mays L.)

Striga hermonthica is a parasitic weed species that causes considerable yield loss in susceptible maize cultivars in western Kenya. A generation means analysis was conducted to estimate gene effects influencing resistance/tolerance to Striga hermonthica in maize cultivars adapted to western Kenya. A resistant inbred line developed by Maseno University, MSMP1/P2 was crossed to Striga susceptible inbred line 5057. Experimental units comprised of resistant parent P1, susceptible parent P2, their F1 and F2 generations, and BC1P1 and BC1P2 generations. The six generations were evaluated under Striga sick plots at Nyahera during the short rains of 2012 and long rains of 2013. Field evaluation for entries were grown in a randomized complete block design with three replicates. The results showed significant differences among generations for emerged Striga, Striga damage rating and maize growth including Plant and Ear height. Most of the additive, dominance, additive x additive, additive x dominance and dominance x dominance effects were significant indicating the importance of the additive, dominance and epistatic modes of gene actions in controlling emerged Striga, Striga damage rating, plant and ear height. Most components conferring Striga tolerance behaved in a qualitative manner. Dominance effects had higher mean values over additive effects. Significant dominant genetic effects (d) for Striga emergence and Striga damage rating suggested preponderance of non-additive genetic effects in the inheritance of Striga tolerance. The results also reveal the involvement of duplicate epistasis where the dominance estimate and dominance x dominance interaction had opposite signs. The presence of duplicate type of gene interaction confirms the importance of dominance gene effects.

Registration of NY195, NY212, NY215, and NY266 Anthracnose Stalk Rot Resistant Inbred Lines of Maize

Registration of NY195, NY212, NY215, and NY266 Anthracnose Stalk Rot Resistant Inbred Lines of Maize

Evaluation of ecological stability and plasticity of inbred lines of winter rye

The results of an ecological and genetic study of 50 lines of inbred winter rye plants are presented. The study was performed under a wide range of the weather factors, which occurred for years of the investigation (2004–2013). The following eight traits were studied: height of plants, number of grains in an ear, weight of 1000 grains, weight of the grains of an ear, falling number, viscosity of water extract, and content of protein and starch in grain. To each line and trait, factor of ecological variation (CVecol), factor of phenotypic stability (SF) (according to D. Lewis, 1954), and ecological plasticity (bi) (according to S.A. Eberhart and W.A. Russell, 1966) were calculated. The range of ecologic variation (CVecol) of the majority of the traits was approximately similar to the range of interlinear variation (CVgen). The slightly varying traits (SF = 1.1–1.32) had maximum buffer capacity, and high varying traits (SF = 2.02–3.17) had rather low buffer capacity. There was a positive correlation (r = 0.47–0.97) of all traits between CVecol, SF, and bi. It was shown that high ecological response of a trait (bi) does not provide high phenotypic stability of the trait. It was concluded that the lines that possess a low CVecol and a high SF of the traits are more important for selection than the lines having high ecological plasticity (bi > 1). In the years of unfavorable weather conditions, rather high adaptability can be provided by ecologically resistant inbred lines.

Transcriptome analysis of maize leaf systemic symptom infected by Bipolaris zeicola.

Bipolaris zeicola is a fungal pathogen that causes Northern corn leaf spot (NCLS), which is a serious foliar disease in maize and one of the most significant pathogens affecting global food security. Here, we report a genome-wide transcriptional profile analysis using next-generation sequencing (NGS) of maize leaf development after inoculation with B. zeicola. We performed High-Throughput Digital Gene Expression analysis to identify differentially expressed genes (DEGs) in resistant inbred Mo17 lines after infection with B. zeicola at four successive disease development stages—CP (contact period), PP (penetration period), IP (incubation period), and DP (disease period); the expression of the genes was compared with those in a CK (mock-treatment) control. In addition, a sensitive maize line (Zheng58) was used for the comparisons with the Mo17. Among all tested genes, 466 differentially expressed genes were identified in all libraries, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of these genes suggested that they are involved in many biological processes related to systemic symptom development, such as plant hormone signal transduction, starch and sucrose metabolism, phenylpropanoid biosynthesis and photosynthesis. Our systematic analysis provides comprehensive transcriptomic information regarding systemic symptom development in fungal-infected plants. This information will help in furthering our understanding of the detailed mechanisms of plant responses to fungal infection.

Evaluation of quality protein maize inbred lines for resistance to maize weevil Sitophilus zeamais (Coleoptera: curculionidae) and other important agronomic traits

Searching new sources of resistance to the maize weevil are critical in a successful breeding program to address grain damage by postharvest pests. This study was undertaken to evaluate resistance in 28 quality protein maize (QPM) inbred lines against Sitophilus zeamais infestation and their important agronomic attributes, and consequently their value for use in breeding programs to develop resistant QPM hybrids. Susceptibility index (SI), which considers the progeny number in the F1 generation and the time the insects take to mature from egg to adult, was used to measure the susceptibility. Out of the 28 inbred lines tested, one inbred line (CML-142) was resistant, three (CML-144/144-7-b (F2)-4-2-1-1-1-1-1, POOL 15QPFS-693-B-2-B-#-B-B-B-# and CML-149) were moderately resistant and the remaining 24 inbred lines were categorized as susceptible to highly susceptible. Importantly, those inbred lines that are resistant and moderately resistant had better yield and other agronomic performances as compared to the susceptible ones. These resistant and moderately resistant inbred lines showed considerable reduction in grain damage and losses for S. zeamais, suggesting that they contained genes that confer resistance to the pest. Correlations between kernel size, breadth and texture with some important susceptibility parameters were found to be significant, indicating possible influence of these factors together with others in determining the susceptibility of the QPM inbred lines to S. zeamais. Identification of QPM lines resistant to S. zeamais in the present study offers an opportunity to breed for resistance to maize weevil in developing QPM hybrid with enhanced resistance to S. zeamais.

Inhibition of the spread of endophytic Sporisorium reilianum renders maize resistance to head smut

Head smut, caused by the fungal pathogen Sporisorium reilianum, poses a grave threat to maize (Zea mays) production worldwide. Here we report cytological and molecular evidence for maize resistance to head smut. During early stages of root infection, S. reilianum mycelium was capable of penetrating the root epidermis of both resistant (Ji1037) and susceptible (HZ4) inbred lines. S. reilianum hyphae were observed in the root–stem junction at 6days after inoculation. In an attempt to monitor hyphal spread within the maize plant, a highly specific and sensitive real-time PCR method was established to estimate the hyphal content in infected maize tissues. During the upward growth of endophytic S. reilianum, the extent of hyphal spread was markedly different between Ji1037 and HZ4. Very little or no pathogen was detected in aerial parts of Ji1037, whereas large amounts of pathogen accumulated in aboveground tissues, particularly inflorescences, of HZ4. Thus, maize resistance to S. reilianum was achieved mainly by inhibition of endophytic hyphal growth rather than by prevention of early-root penetration by the pathogen.

A recombinant inbred line population of tomato and its genetic map constructed based on a solanum lycopersicum x s. pimpinellifolium cross

A recombinant inbred line (RIL) population of tomato was developed from a cross between a tomato breeding line (NC 84173) and an accession (LA 0722) of the tomato wild species S. pimpinellifolium. NC 84173 is a horticulturally-superior, multiple-disease resistant inbred line that has been used as a parent in production of several commercial tomato hybrid cultivars. LA 0722 is a self-compatible, inbred accession, which was previously identified as a genetic source for fruit quality, disease resistance, and abiotic stress tolerance. The RIL population is in the F9 generation and consists of 145 lines. A genetic linkage map of the population was developed with 191 molecular markers, including 129 RFLPs and 62 RGAs. The genetic map spans 1505 cM of the 12 tomato chromosomes with an average inter-marker distance of 7.9 cM. The RFLP markers were chosen from the high-density map of tomato, previously developed based on a S. lycopersicum  S. pennellii F2 population. The RGA markers were derived using degenerate oligonucleotide primers designed based on conserved leucine-rich repeat (LRR),

Genetic detection of clubroot resistance loci in a new population of Brassica rapa

Clubroot is one of the most serious diseases affecting Brassica crop production worldwide. In order to identify the location of clubroot resistance genes in Chinese cabbage, we constructed a linkage map for an F2 population derived from a cross between a resistant turnip inbred line, 'Siloga' (B. rapa ssp. rapifera), and a susceptible Chinese cabbage inbred line, 'BJN3' (B. rapa ssp. pekinensis). The newly developed genetic map included 207 markers and covered 1151.7 cM. In combination with clubroot resistance tests from the field, the data allowed us to identify three quantitative trait loci of clubroot resistance, using a composite interval mapping method. Clubroot resistance genes QS_B1.1, QS_B3.1, and QS_B8.1 were located on chromosomes A1, A3, and A8, respectively, of B. rapa. Their contribution rates were 8.18%, 70.55%, and 7.28%, respectively. QS_B1.1 was a novel locus of clubroot resistance, independent of any published clubroot-resistance loci. QS_B3.1 was first detected in 'Siloga,' mapped to the previous CRb and CRa region, whereas QS_B8.1 was closely linked to Crr1b. Epistatic interactions with additive effects were detected between QS_B3.1 and QS_B8.1.

A New Race of Verticillium dahliae Causing Leaf Mottle of Sunflower in Europe.

Sunflower (Helianthus annuus L.) plants with symptoms of interveinal chlorosis were observed in the summer of 2013 in one field in Cadiz (Spain) where the performance of 30 hybrids was assessed. Symptoms affected 80% of the hybrids with incidence as high as 90%. Chlorosis and yellowing near the leaf margin were visible at floral initiation, and they progressed from the lower to upper leaves. Mottled leaves were observed near the top of the plants. On severely affected leaves, chlorotic patches enlarged, coalesced, and large areas of the leaves became necrotic and dried. Cross sections of the lower stem showed a brown discoloration of the vascular system. The fungus that was consistently isolated from stem and petiole tissues of sunflower plants was morphologically identified as Verticillium dahliae Kleb. (Vd) (5) and molecularly confirmed by PCR amplification of the 526-bp band (4). The race of the isolates was determined in a greenhouse experiment at 18 to 28°C from February to April 2014. Isolates 1-13 and 2-13 of Vd, obtained from two of the hybrids in 2013, one of them being Transol, were independently inoculated to 1-month-old plants of each of three sunflower genotypes: the susceptible hybrid Transol and the inbred lines HA89 (carrying the V1 gene for resistance to Vd) (2) and HAR5 (resistant to other diseases but with unknown reaction to Vd). Plants were inoculated by immersing roots in a suspension of 106 conidia per ml for 30 min. Inoculated plants were individually transplanted to 1-liter pots filled with sand/silt. Roots of the control treatments were immersed in water. Six replications (pots) were established for each isolate × genotype combination, according to a complete randomized 3 × 3 factorial design. Five weeks after inoculation, symptoms developed in 100% of the plants in the three sunflower genotypes. Severity of symptoms (SS) in each plant was assessed as percentage of foliar tissue affected. Significantly higher SS occurred on inoculated plants as compared to non-inoculated plants, which did not develop symptoms. Mean disease severity on inoculated plants was 80% (averaged across isolates and genotypes). A significant effect of genotypes was obtained. Mean SS averaged across isolates were 98, 73, and 69% for HAR5, HA89, and Transol, respectively. When stem tissues from the three sunflower genotypes were sampled and incubated on potato dextrose agar at 25°C, the mycelial growth of Vd was confirmed for the inoculated plants but not for the control plants. Isolates of Vd infecting the resistant inbred line HA89 have only been identified in Argentina (1) and the United States (3). To our knowledge, this is the first report of a race overcoming the V1 gene in HA89 in Europe. This poses a risk to commercial sunflower breeding programs in European countries.

High-resolution mapping and characterization of qRgls2, a major quantitative trait locus involved in maize resistance to gray leaf spot.

BackgroundGray leaf spot (GLS) caused by Cercospora zeae-maydis (Czm) or Cercospora zeina (Cz) is a devastating maize disease and results in substantial yield reductions worldwide. GLS resistance is a quantitatively inherited trait. The development and cultivation of GLS-resistant maize hybrids are the most cost-effective and efficient ways to control this disease.ResultsWe previously detected a major GLS resistance QTL, qRgls2, in bin 5.03–04, which spans the whole centromere of chromosome 5 encompassing a physical distance of ~110-Mb. With advanced backcross populations derived from the cross between the resistant Y32 and susceptible Q11 inbred lines, a sequential recombinant-derived progeny testing strategy was adapted to fine map qRgls2. We narrowed the region of qRgls2 from an initial ~110-Mb to an interval of ~1-Mb, flanked by the markers G346 and DD11. qRgls2 showed predominantly additive genetic effects and significantly increased the resistance percentage by 20.6 to 24.6% across multiple generations. A total of 15 genes were predicted in the mapped region according to the 5b.60 annotation of the maize B73 genome v2. Two pieces of the mapped qRgls2 region shared collinearity with two distant segments on maize chromosome 4.ConclusionsqRgls2, a major QTL involved in GLS resistance, was mapped to a ~1-Mb region close to the centromere of chromosome 5. There are 15 predicted genes in the mapped region. It is assumed that qRgls2 could be widely used to improve maize resistance to GLS.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0230-6) contains supplementary material, which is available to authorized users.

Aflatoxin Resistance in Maize: What Have We Learned Lately?

Aflatoxin contamination of maize grain is a huge economic and health problem, causing death and increased disease burden in much of the developing world and income loss in the developed world. Despite the gravity of the problem, deployable solutions are still being sought. In the past 15 years, much progress has been made in creating resistant maize inbred lines; mapping of genetic factors associated with resistance; and identifying possible resistance mechanisms. This review highlights this progress, most of which has occurred since the last time a review was published on this topic. Many of the needs highlighted in the last reviews have been addressed, and several solutions, taken together, can now greatly reduce the aflatoxin problem in maize grain. Continued research will soon lead to further solutions, which promise to further reduce and even eliminate the problem completely.

Toward elucidation of genetic and functional genetic mechanisms in corn host resistance to Aspergillus flavus infection and aflatoxin contamination.

Aflatoxins are carcinogenic mycotoxins produced by some species in the Aspergillus genus, such as A. flavus and A. parasiticus. Contamination of aflatoxins in corn profusely happens at pre-harvest stage when heat and drought field conditions favor A. flavus colonization. Commercial corn hybrids are generally susceptible to A. flavus infection. An ideal strategy for preventing aflatoxin contamination is through the enhancement of corn host resistance to Aspergillus infection and aflatoxin production. Constant efforts have been made by corn breeders to develop resistant corn genotypes. Significantly low levels of aflatoxin accumulation have been determined in certain resistant corn inbred lines. A number of reports of quantitative trait loci have provided compelling evidence supporting the quantitative trait genetic basis of corn host resistance to aflatoxin accumulation. Important findings have also been obtained from the investigation on candidate resistance genes through transcriptomics approach. Elucidation of molecular mechanisms will provide in-depth understanding of the host–pathogen interactions and hence facilitate the breeding of corn with resistance to A. flavus infection and aflatoxin accumulation.

Identification of candidate genes for fusarium yellows resistance in Chinese cabbage by differential expression analysis

Fusarium yellows caused by Fusarium oxysporum f. sp. conglutinans is an important disease of Brassica worldwide. To identify a resistance (R) gene against Fusarium yellows in Chinese cabbage (Brassica rapa var. pekinensis), we analyzed differential expression at the whole genome level between resistant and susceptible inbred lines using RNA sequencing. Four hundred and eighteen genes were significantly differentially expressed, and these were enriched for genes involved in response to stress or stimulus. Seven dominant DNA markers at putative R-genes were identified. Presence and absence of the sequence of the putative R-genes, Bra012688 and Bra012689, correlated with the resistance of six inbred lines and susceptibility of four inbred lines, respectively. In F(2) populations derived from crosses between resistant and susceptible inbred lines, presence of Bra012688 and Bra012689 cosegregated with resistance, suggesting that Bra012688 and Bra012689 are good candidates for fusarium yellows resistance in Chinese cabbage.

Identification and validation of breeder-friendly DNA markers for Pl arg gene in sunflower

Downy mildew is a fungal disease of sunflower that can lead to severe yield losses. The damage caused by the pathogen can be controlled by growing resistant sunflower varieties. Gene Pl arg was introgressed into cultivated sunflower from the wild species Helianthus argophyllus and provides resistance against all known downy mildew races. In this study, we used a mapping population from the cross-RHA 419/RHA-N-49. We identified a new co-segregating simple sequence repeat marker ORS675 and confirmed the co-segregation of markers ORS716 and ORS662 with Pl arg gene. The markers were validated on two registered resistant inbred lines RHA 443 and RHA 464, as well as on twenty inbred lines RH 1–20 obtained through methods of classical breeding. Molecular marker ORS716 was assessed for usefulness in selecting resistant progeny in 12 BC populations. Markers were found to be valuable for molecular breeding in diverse genetic backgrounds and enabled transfer of the resistance gene in different sunflower genotypes.

Responses of highly resistant and susceptible maize to vascular puncture inoculation with Maize dwarf mosaic virus

Mechanisms for genetic resistance to diseases caused by potyviruses, especially Maize dwarf mosaic virus (MDMV), in maize (Zea mays L.) remain unclear. Previous studies indicated that MDMV can replicate in and spread to groups of cells in maize leaves of the virus resistant inbred line Pa405, but systemic movement of the virus did not occur. In this study, the responses of Pa405 and the virus-susceptible inbred line Oh28 were compared in germinating maize embryos. Four days post inoculation (dpi), MDMV was detected immunocytochemically in 45% of resistant and 19% of susceptible shoots, respectively. Systemic movement of MDMV out of inoculated leaves occurred between 2 and 4 dpi in Oh28 seedlings. Microarray analysis was used to examine the effects of MDMV inoculation on gene expression in shoots of resistant and susceptible lines at 4 dpi. Approximately 15% of transcripts were differentially expressed between the resistant and susceptible lines. Of the transcripts with >10 fold greater expression in one line, more than 70% were up-regulated in the resistant line. Substantially fewer genes were differentially expressed in MDMV- and mock-inoculated shoots, with 21 and 30 genes being differentially regulated in the susceptible and resistant lines, respectively. For a subset of genes, differential expression was validated using quantitative real-time PCR. There was no clustering of differentially expressed transcripts in the previously identified QTLs important for MDMV resistance. These results provide a short list of virus responsive transcripts to begin to explain the resistance response in Pa405 versus the susceptible response in Oh28.

Genome-wide association study of resistance to rough dwarf disease in maize

Maize rough dwarf disease (MRDD), caused by the Maize rough dwarf virus (MRDV) is highly prevalent across summer at maize-growing areas in China. To investigate the genetic architecture underlying this viral disease, a set of 236 Chinese maize inbred lines was evaluated for resistance to MRDD in 2010 and 2011. Based on 41,101 single-nucleotide polymorphism (SNP) markers with minor allele frequencies (MAF) greater than 5 %, a genome-wide association study (GWAS) was employed to identify genomic loci for resistance to MRDD. A total of 73 SNPs were found to be associated with resistance to MRDD at a significance threshold of -log10 (P) > 4 controlling the false discovery rate (FDR) at α = 0.1. Fourteen of these SNPs were detected in both of the two environments tested. A total of 48 SNPs were identified in linkage disequilibrium (LD) blocks containing candidate resistance genes, including protein kinase genes. Using the pedigree information and whole-genome SNP analysis of five highly resistant inbred lines derived from the hybrid ‘P78599’, nine derivative fragments harbouring SNPs associated with MRDD resistance were detected. One 81.57 Mb fragment in particular located in bin 8.03, which contained six SNPs associated with MRDD resistance, and included the major quantitative trait loci (QTL) that had been identified in the previous study. These results suggest that the SNPs and fragments associated with MRDD resistance, especially those in bin 8.03, could be used for fine mapping of resistance genes and developing resistant varieties in maize.

Genetic Parameters and Correlation between Tomato Late Blight Field Resistance and Fruit Quality

The objective of this study was to estimate genetic parameters and quantify the correlations between late blight (LB) resistance and fruit quality traits in 220 families F2:3, under tropical field conditions. The families were arranged in randomized blocks in field trials, with six resistant inbred lines used as control, in plots of five plants. It evaluated the severity in the middle of the epidemy, final severity, area under the curve of progress of late blight, pH, total soluble solids, titratable acidity and flavor. There is genetic variability for epidemiological variables and total soluble solids. Heritabilities, obtained by parent-offspring regression, were high and indicated the possibility of satisfactory genetics gains and success in the introgression of resistance alleles. The correlations were negative, of high to intermediate magnitude, among the variables area under the curve of progress of late blight, total soluble solids and total titratable acidity.

Identification of Resistance to Maize rayado fino virus in Maize Inbred Lines.

Maize rayado fino virus (MRFV) causes one of the most important virus diseases of maize in America. Severe yield losses, ranging from 10 to 50% in landraces to nearly 100% in contemporary cultivars, have been reported. Resistance has been reported in maize populations, but few resistant inbred lines have been identified. Maize inbred lines representing the range of diversity in the cultivated types and selected lines known to be resistant to other viruses were evaluated to identify novel sources of resistance to MRFV. The virus was transmitted to maize seedlings using the vector Dalbulus maidis, and disease incidence and severity were evaluated beginning 7 days postinoculation. Most of the 36 lines tested were susceptible to MRFV, with mean disease incidence ranging from 21 to 96%, and severity from 1.0 to 4.3 (using a 0 to 5 severity scale). A few genotypes, including CML333 and Ki11, showed intermediate levels of resistance, with 14 and 10% incidence, respectively. Novel sources of resistance, with incidence of less than 5% and severity ratings of 0.4 or less, included the inbred lines Oh1VI, CML287, and Cuba. In Oh1VI, resistance appeared to be dominant, and segregation of resistance in F2 plants was consistent with one or two resistance genes. The discovery of novel sources of resistance in maize inbred lines will facilitate the identification of virus resistance genes and their incorporation into breeding programs.

Difference between resistant and susceptible maize to systematic colonization as revealed by DsRed-labeled Fusarium verticillioides

Fusarium verticillioides was labeled with DsRed via Agrobacterium tumefaciens-mediated transformation to examine differences in colonization and reactions of resistant and susceptible inbred lines of maize (Zea mays L.). The extent of systemic colonization of F. verticillioides in roots from maize lines either resistant or susceptible to the fungus was studied by visualizing the red fluorescence produced by the fungus expressing DsRed. The difference in quantities of colony forming units (CFU) in roots and basal stems, production of fumonisin B1, and pH of root were determined. Although F. verticillioides colonized both resistant and susceptible lines, differences were observed in the pattern and extent of fungal colonization in the two types of maize lines. The fungus colonized the susceptible lines producing mosaic patterns by filling the individual root cells with hyphae. Such a pattern of colonization was rarely observed in resistant lines, which were less colonized by the fungus than the susceptible lines in terms of CFUs. The production of mycotoxin fumonisin B1 in roots from different lines was closely correlated with the amount of F. verticillioides colonization, rather than the pH or amylopectin concentrations in the root. The findings from this study contribute to a better understanding of the defense mechanism in resistant maize lines to F. verticillioides.