Resistant Inbreds Research Articles

Fine mapping a QTL for BYDV-PAV resistance in maize

Barley yellow dwarf (BYD) is one of the economically most important virus diseases of cereals worldwide, causing yield losses up to 80%. The means to control BYD are limited, and the use of genetically resistant cultivars is the most economical and environmentally friendly approach. The objectives of this study were i) to identify the causative gene for BYD virus (BYDV)-PAV resistance in maize, ii) to identify single nucleotide polymorphisms and/or structural variations in the gene sequences, which may cause differing susceptibilities to BYDV-PAV of maize inbreds, and iii) to characterize the effect of BYDV-PAV infection on gene expression of susceptible, tolerant, and resistant maize inbreds. Using two biparental mapping populations, we could reduce a previously published quantitative trait locus for BYDV-PAV resistance in maize to ~ 0.3 Mbp, comprising nine genes. Association mapping and gene expression analysis further reduced the number of candidate genes for BYDV-PAV resistance in maize to two: Zm00001eb428010 and Zm00001eb428020. The predicted functions of these genes suggest that they confer BYDV-PAV resistance either via interfering with virus replication or by inducing reactive oxygen species signaling. The gene sequence of Zm00001eb428010 is affected by a 54 bp deletion in the 5`-UTR and a protein altering variant in BYDV-PAV-resistant maize inbreds but not in BYDV-PAV-susceptible and -tolerant inbreds. This finding suggests that altered abundance and/or properties of the proteins encoded by Zm00001eb428010 may lead to BYDV-PAV resistance.

Evaluation of Inbreds for Resistance to Post-Flowering Stalk Rot Disease in Maize (Zea mays.)

Maize (Zea mays) is the most influential crop in the world, which can grow in temperate as well as tropical conditions. It is a major source for food, livestock, the beverage industry and cosmetics. In recent days maize production decreased drastically due to many diseases. Fusarium stalk rot is one among them. To evaluate the genotypes resistance against PFSR, 40 maize genotypes were subjected against five virulent isolates of Fusarium using the same artificial inoculation method. Varying disease reactions among the inbreds, with some exhibiting premature drying and lodging. After 40 days post-inoculation, internode infection was observed, and disease severity was assessed based on lesion spread within or across internodes. The inbreds were categorized according to their resistance levels, ranging from highly resistant to highly susceptible based on internode infection. Only one inbred showed high resistance to both FUR11 and Raichur isolates. None of the inbreds exhibited high resistance or resistance against the F1 isolate. Eight inbreds showed resistance against multiple isolates. One inbred, DML-1802, displayed high resistance to FUR11 and resistance to Raichur isolate, while another, A8-6, was highly susceptible to all tested isolates, showing infection spreading up to two internodes and observed xylem vessel rotting and hollow stems. Eleven resistant inbreds against Fusarium stalk rot were selected for a breeding program aimed at developing Fusarium stalk rot-resistant hybrids. The identification of variability among Fusarium species causing stalk rot in maize provided valuable insights for future research due to the limited understanding of Fusarium heterogeneity in maize stalk rot.

Disease Reaction Studies for Turcicum Leaf Blight (Exserohilum Turcicum) Disease in Maize (Zea Mays L.) Under Artificial Epiphytotic Conditions

Maize is widely cultivated around the globe under varied climatic conditions. India is one of the major maize producing countries and stands fourth in terms of production. Turcicum leaf blight (TLB) disease caused by the pathogen, Exserohilum turcicum is one of the major diseases of maize causing varying extent of grain yield loss from 15 to 30%. To overcome the losses due to the disease, resistant cultivars needs to be developed. Hence, the identification of resistant sources is important in order to develop TLB resistant hybrids. Present investigation was conducted to identify TLB resistant inbreds and hybrids. Simplified Triple Test Cross (STTC) design was employed to develop 96 hybrids using twenty inbred lines. Further the hybrids and inbred lines along with two checks (Resistant –NAI 137; Susceptible – CM 202) were evaluated for TLB disease reaction under artificial epiphytotic condition during rabi 2019 in simple lattice design with two replications. Scoring was done from 60 to 80 days after sowing based on 1-9 TLB disease scale. Among the inbred lines V-86, MAI-295, V-70, MAI-214, V-85, 40375, V-72, V-39, 40061 were resistant and MAI-288, V-24 were susceptible. Among hybrids, 36 were resistant, 44 were moderately resistant. 14 were moderately susceptible and two hybrids were susceptible to TLB disease. Identified TLB resistant hybrids needs to be further evaluated to test their performance and stability across locations. These resistant sources can be used to transfer TLB disease resistance.

Microbiota of maize kernels as influenced by Aspergillus flavus infection in susceptible and resistant inbreds.

Nearly everything on Earth harbors a microbiome. A microbiome is a community of microbes (bacteria, fungi, and viruses) with potential to form complex networks that involve mutualistic and antagonistic interactions. Resident microbiota on/in an organism are determined by the external environment, both biotic and abiotic, and the intrinsic adaptability of each organism. Although the maize microbiome has been characterized, community changes that result from the application of fungal biocontrol strains, such as non-aflatoxigenic Aspergillus flavus, have not. We silk channel inoculated field-grown maize separately with a non-aflatoxigenic biocontrol strain (K49), a highly toxigenic strain (Tox4), and a combination of both A. flavus strains. Two maize inbreds were treated, A. flavus-susceptible B73 and A. flavus-resistant CML322. We then assessed the impacts of A. flavus introduction on the epibiota and endobiota of their maize kernels. We found that the native microbial communities were significantly affected, irrespective of genotype or sampled tissue. Overall, bacteriomes exhibited greater diversity of genera than mycobiomes. The abundance of certain genera was unchanged by treatment, including genera of bacteria (e.g., Enterobacter, Pantoea) and fungi (e.g., Sarocladium, Meyerozyma) that are known to be beneficial, antagonistic, or both on plant growth and health. Beneficial microbes like Sarocladium that responded well to A. flavus biocontrol strains are expected to enhance biocontrol efficacy, while also displacing/antagonizing harmful microbes.

Maize kernel metabolome involved in resistance to fusarium ear rot and fumonisin contamination.

Fusarium verticillioides poses a threat to worldwide maize production due to its ability to infect maize kernel and synthesize fumonisins that can be accumulated above safety levels for humans and animals. Maize breeding has been proposed as key tool to decrease kernel contamination with fumonisins, but metabolic studies complementary to genomic approaches are necessary to disclose the complexity of maize resistance. An untargeted metabolomic study was proposed using inbreds genetically related but with contrasting levels of resistance in order to uncover pathways implicated in resistance to Fusarium ear rot (FER) and fumonisin contamination in the maize kernel and to look for possible biomarkers. Metabolite determinations were performed in kernels collected at 3 and 10 days after inoculation with F. verticillioides (dat). Discriminant metabolites between resistant and susceptible RILs were rather found at 10 than 3 dat, although metabolite differences at later stages of colonization could be driven by subtle variations at earlier stages of infection. Within this context, differences for membrane lipid homeostasis, methionine metabolism, and indolacetic acid conjugation seemed highly relevant to distinguish between resistant and susceptible inbreds, confirming the polygenic nature of resistance to FER and fumonisin contamination in the maize kernels. Nevertheless, some specific metabolites such as the polyamine spermidine and/or the alkaloid isoquinoline seemed to be promising indirect selection traits to improve resistance to FER and reduce fumonisin accumulation. Therefore, in vitro and in vivo experiments will be necessary to validate the inhibitory effects of these compounds on fumonisins biosynthesis.

Identification of Early and Extra-Early Maturing Tropical Maize Inbred Lines with Multiple Disease Resistance for Enhanced Maize Production and Productivity in Sub-Saharan Africa.

Maize, a staple for millions across sub-Saharan Africa (SSA), faces major biotic constraints affecting production and safety of the crop. These include northern corn leaf blight (NCLB), southern corn leaf blight (SCLB), Curvularia leaf spot (CLS), and aflatoxin contamination by Exserohilum turcicum, Bipolaris maydis, Curvularia lunata, and Aspergillus flavus, respectively. Farmers in SSA would benefit tremendously if high-yielding maize hybrids with multiple disease resistance (MDR) were developed and commercialized. In all, 49 early-maturing (EM; 90 to 95 days to physiological maturity) and 55 extra-early-maturing (EEM, 80 to 85 days to physiological maturity) inbred lines developed by the International Institute of Tropical Agriculture were identified as resistant to NCLB in field evaluations in multiple agroecologies of Nigeria in 2017 and 2018. From each maturity group, the 30 most resistant inbreds were selected for evaluation for resistance to SCLB and CLS using a detached-leaf assay. Additionally, the inbreds were screened for resistance to kernel rot and aflatoxin contamination using a kernel screening assay. In all, 7 EM and 6 EEM maize inbreds were found to be highly resistant to the three foliar pathogens while 10 inbreds were resistant to the foliar pathogens and supported significantly less (P = 0.01) aflatoxin accumulation than other inbreds. Inbreds having MDR should be tested extensively in hybrid combinations and commercialized. Large-scale use of maize hybrids with MDR would (i) increase maize production and productivity and (ii) reduce losses caused by aflatoxin contamination. Overall, planting of EM and EEM maize hybrids with MDR would contribute to food security, reduced aflatoxin exposure, and increased incomes of maize farmers in SSA.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Genetic Analysis of New Maize Hybrids for Yield and Resistance to Aflatoxin Accumulation

Maize (Zea mays L.) is the most important cereal crop in sub-Saharan Africa providing food for many of its inhabitants. However, production is hampered by low yields and aflatoxin contamination, among others. The toxin contaminates maize before harvest and during storage. Ghana lacks regulatory infrastructure for monitoring and detecting aflatoxin in grains prior to market, and most of the local maize varieties have been found to be susceptible to aflatoxin accumulation. Host resistance is envisaged as a key approach in addressing the aflatoxin menace. Thus, this study was conducted to identify new hybrids that combine high yield with resistance to aflatoxin accumulation as well as study the mode of gene controlling tolerance to aflatoxin accumulation. Sixteen aflatoxin resistant inbreds from exotic sources were crossed as males to 10 local germplasm in a North Carolina II design to generate 160 new hybrids. These were planted together with 9 checks using a 13 × 13 alpha lattice with three replications. The new hybrids were evaluated across six environments in two seasons. Five plants each per hybrid were inoculated with a local strain of Aspergillus flavus at a concentration of 9 × 107conidia/ml. Ninety-six out of the 169 crosses were analyzed statistically. Significant effect of environment and genotypes for all traits especially, for aflatoxin accumulation resistance and yield were observed. The general combining ability effect of males for all traits were found significant (P < 0.05) whereas that of the females were not significant for all traits. Inbreds with consistent significant negative GCA effect for aflatoxin reduction were identified (MP715, TZI8, MP719). Furthermore, the underlying genetic control for the aflatoxin accumulation resistance trait was found to be via both GCA and SCA effects. Heritability estimates were moderate, suggesting permissible transfer of traits during selection to create high yielding aflatoxin resistant hybrids for consumers.

Assessing performance of white endosperm testers with varying resistance reactions to Striga (Striga hermonthica) for evaluating resistant maize (Zea mays) inbred lines

AbstractIdentification of testers is crucial for hybrid maize breeding programme. However, limited information is available about ideal testers for characterising the combining ability of Striga resistant maize inbreds. This study was conducted to assess the relative value of three inbred testers with varying resistance reactions to Striga for determining the combining ability of Striga resistant inbreds. Ninety testcrosses involving 30 Striga resistant inbreds and three testers were evaluated under artificial and natural Striga infestation and non‐infested conditions at two locations for 2 years. Lines x tester interaction was significant (p ≤ .05) for most traits, indicating differential ranking of lines by the testers. The GCA effects of testers for most traits were high, highlighting the predominance of additive gene action in controlling the overall performance of testcrosses. The resistant and tolerant testers exhibited desirable GCA effects, broader testcross performance, greater genetic variances and consistent ranking of testcrosses under both growing conditions than the susceptible tester. These testers can be successfully used for identifying superior Striga resistant inbreds to develop high yielding and resistant hybrids for commercialization.

Genetics of resistance to Fusarium stalk rot caused by Fusarium verticilloides in maize (Zea mays L.)

Fusarium stalk rot disease (FSR), incited by Fusarium verticilloides, is becoming an important biotic production constraint in many major maize growing areas causing substantial yield losses. The present investigation was conducted to understand the genetics of resistance to FSR through six generation means and variances, as a first step in addressing the problem. Five crosses were developed by crossing four FSR susceptible inbreds (VL1043, VL108867, VL121096 and VL1218) with two resistant inbreds (CM202 and CM212). Six generations of the five crosses (VL1043 × CM212, VL108867 × CM202, VL121096 × CM212, VL1218 × CM202 and VL1218 × CM212) were evaluated through artificial disease inoculation during post rainy season of 2018 and summer, 2019. The scaling tests and joint scaling tests indicated the inadequacy of additive-dominance model and showed the presence of epistatic gene effects in all the five crosses for FSR resistance. The study further revealed the importance of additive, dominance and additive × additive gene effects in the expression of FSR. The magnitude and direction of the additive genetic effects [a], dominance genetic effects [d], magnitudes of additive genetic variance (2A) and dominance genetic variance (2D) varied with the genetic background of the crosses over seasons. Duplicate gene interaction was evident in the inheritance of FSR resistance. Both, additive and non-additive components were found important thus reciprocal recurrent selection would be more effective in obtaining FSR resistant maize inbred lines.

Physico-chemical bases of resistance in maize against shoot fly, Atherigona naqvii Steyskal during spring season in north India

Host plant resistance is one of the most promising alternatives to insecticides to manage the shoot fly, Atherigona naqvii Steyskal, which is a major biotic constraint to spring sown maize in northern India. In present studies, Nineteen maize inbreds selected from diverse sources were characterized for resistance traits against shoot fly under its natural infestation using moistened fishmeal. Though the antixenosis to shoot fly for oviposition was not observed in the test genotypes. But, the presence of significant variation in the deadhearts incidence revealed the existence of relative amount of resistance among the test genotypes. The Tolerance (measured in the terms of less proportion of deadhearts out of total shoot fly incidence (leaf injury + deadhearts)) and Antibiosis (in terms of larval and pupal weight, duration and survival) were recognized as key mechanisms of this resistance. Tolerance to shoot fly was significantly more in resistant inbreds, SO1SHYQBBB13B, HK12-6-2-4, DMSC 28, CM 143 and Winpop 8 (29.46–36.56%) in comparison to the susceptible genotypes SE 563 and LM 16 (46.31–47.44%). The correlation and stepwise regression analyses revealed that the more seedling vigor, less leaf area, less number of leaves and thin stem had significant impact on the shoot fly deadhearts incidence and provided resistance against this pest. The total soluble sugars (TSS), proteins and amino acids were significantly higher in the susceptible inbreds; whereas, total phenols and tannins; and activities of defensive enzymes viz. phenylalanine ammonia lyase (PAL) and polyphenol oxidase (PPO) were significantly higher in resistant inbreds. Further, the stepwise regression revealed that the larval period, per cent pupation, pupal weight female, pupal weight male and adult emergence of shoot fly were significantly influenced by PAL activity, which explained 71.5, 82.9, 47.0, 29.8 and 39.9% of their total variation, respectively. PPO activity significantly influenced the pupal period (R2 = 59.7%) whereas, the larval weight was significantly influenced by amino acids (R2 = 56.9%), proteins (R2 = 10.2%) and TSS (R2 = 7.7%). So, the correlation and regression analyses revealed that these physico-chemical characters of the maize inbreds can be utilized as reliable markers traits to identify resistant sources to develop shoot fly resistant maize hybrids.

Sugar partitioning and source-sink interaction are key determinants of leaf senescence in maize.

Source-sink communication is one of the key regulators of senescence; however, the mechanisms underlying such regulation are largely unknown. We analysed senescence induced by the lack of grain sink in maize, termed source-sink regulated senescence (SSRS), and compared the associated physiological and metabolic changes with those accompanying natural senescence. Phenotypic characterization of31 diverse field-grown inbreds revealed substantial variation for both SSRS and natural senescence. Partitioning of excess carbohydrates to alternative sinks, mainly internodes and husks, emerged as a critical mechanism underlying both SSRS and stay-green. Time-course analyses of SSRS sensitive (B73) and resistant (PHG35) inbreds confirmed the role of sugar partitioning in SSRS and stay-green. Elevated hemicellulose content in PHG35 internodes highlighted the role of the cell wall as a significant alternative sink. Sugar signalling emerged as an important regulator of SSRS as evident from an increased accumulation of trehalose-6-phosphate and decreased transcript levels of snf1-related protein kinase1, two signalling components associated with senescence, in B73. These findings demonstrate a crucial role of sugar partitioning, signalling, and utilization in SSRS. Available genetic variation for SSRS and a better understanding of the underlying mechanisms would help modify sugar partitioning and senescence to enhance the productivity of maize and related grasses.

Phenotypic and physiological characterization of maize inbred lines resistant and susceptible to maydis leaf blight

Maydis leaf blight (MLB) caused by Cochliobolus heterostrophus is one of the most important fungal diseases of maize. A study was conducted for phenotypical characterization of ten Indian maize inbred lines, which were grouped based on phenotyping and their morpho-physiological attributes against MLB disease. On the basis of phenotyping, they were categorized into five resistant inbreds viz., SC-7-2-1-2-6-1, SC24 (92)-2-3-2-1-1, DMSC-16-2, V-351-1-4 and HKI MBR-139-2, and five susceptible inbreds viz., CM-119, DMSC-28, HKI PC-8, HKI 1040-C2 and V341. The leaf hair density was high in resistant genotypes (249.79/microscopic field, at 100×) and significantly less in susceptible genotypes (99.33/microscopic field at 100×). Stomata count was found less (6.12/microscopic field, at 400×) in resistant genotypes as compared to susceptible genotypes (10.56/microscopic field at 400×). The rate of photosynthesis was high in uninoculated control plant than in the inoculated plants. Under inoculated conditions photosynthetic rate was significantly high in resistant lines than the susceptible inbred lines which were gradually reduced with the advancement of time. The transpiration rate of inoculated plants was also higher than the control plants at 5 days after inoculation (DAI), whereas it was decreased in both inoculated and control plants at 40 DAI. Elevated level of transpiration was detected in susceptible genotypes under inoculated condition. Upon inoculation stomatal conductance was decreased with the passage of time. However, there was a differential behavior in the stomatal conductance recorded in the tested maize inbred lines under control conditions.

Genetic Analysis of Sugarcane mosaic virus Resistance in the Wisconsin Diversity Panel of Maize

Sugarcane mosaic virus (SCMV) and Maize dwarf mosaic virus can cause yield loss in maize (Zea mays L.). Previous studies identified two key genomic regions controlling host resistance: Scmv1 on chromosome 6 and Scmv2 on chromosome 3. In this study a diverse set of 578 inbreds adapted to a northern US Corn Belt environment, was inoculated at the three‐leaf stage with SCMV and rated for presence of virus symptoms at 7, 10, 14, 21, 28, and 35 d after inoculation. Scmv1 was present in all resistant inbreds. A single‐nucleotide polymorphism associated with SCMV resistance was found 5 Mb downstream of the putative location of Scmv1, and resistance at this downstream locus was correlated with a group of inbreds containing the presence allele of a 2.7‐Mb presence–absence variation (PAV) in the region encompassing Scmv1. The Scmv2 region was associated with resistance 35 d after inoculation only among the inbreds containing the presence allele, including Scmv1. Several other loci unlinked to Scmv1 and Scmv2 were associated with resistance in this panel, providing evidence for multigenic modification of the efficacy of Scmv1 and Scmv2. Sixty‐nine inbreds representing haplotype variation in the region surrounding Scmv1 on chromosome 6 were inoculated with SCMV at the three‐ and six‐leaf stage. Symptom development was reduced when inoculation was delayed to the six‐leaf stage, but 24 inbreds became fully symptomatic after 35 d regardless of the growth stage at inoculation. Inbreds with the presence allele of the PAV encompassing Scmv1 were more resistant to SCMV when inoculation was delayed to the six‐leaf stage than those that had the absence allele. Inbreds with the absence allele were susceptible regardless of growth stage at inoculation.

Transcriptome profiling of two maize inbreds with distinct responses to Gibberella ear rot disease to identify candidate resistance genes

BackgroundGibberella ear rot (GER) is one of the most economically important fungal diseases of maize in the temperate zone due to moldy grain contaminated with health threatening mycotoxins. To develop resistant genotypes and control the disease, understanding the host-pathogen interaction is essential.ResultsRNA-Seq-derived transcriptome profiles of fungal- and mock-inoculated developing kernel tissues of two maize inbred lines were used to identify differentially expressed transcripts and propose candidate genes mapping within GER resistance quantitative trait loci (QTL). A total of 1255 transcripts were significantly (P ≤ 0.05) up regulated due to fungal infection in both susceptible and resistant inbreds. A greater number of transcripts were up regulated in the former (1174) than the latter (497) and increased as the infection progressed from 1 to 2 days after inoculation. Focusing on differentially expressed genes located within QTL regions for GER resistance, we identified 81 genes involved in membrane transport, hormone regulation, cell wall modification, cell detoxification, and biosynthesis of pathogenesis related proteins and phytoalexins as candidate genes contributing to resistance. Applying droplet digital PCR, we validated the expression profiles of a subset of these candidate genes from QTL regions contributed by the resistant inbred on chromosomes 1, 2 and 9.ConclusionBy screening global gene expression profiles for differentially expressed genes mapping within resistance QTL regions, we have identified candidate genes for gibberella ear rot resistance on several maize chromosomes which could potentially lead to a better understanding of Fusarium resistance mechanisms.

Identification of a maize (Zea mays) chitinase allele sequence suitable for a role in ear rot fungal resistance

Chitinases are thought to play a role in plant resistance to pathogens, but the extent of this role is unknown. The gene for a maize (Zea mays) chitinase, “chitinase 2”, previously reported to be induced by two ear rot pathogens in one maize inbred, was cloned from mRNA isolated from milk stage kernels of several different inbreds reported to be susceptible or resistant to ear rot pathogens. The chitinase gene sequence of some of the susceptible inbreds had frame shifts which would result in a nonfunctional protein. Other susceptible inbreds, and a few resistant inbreds, were missing regions that would contribute to chitin binding. A putative functional clone from a resistant inbred was expressed in yeast, and produced a protein with chitinase activity against different oligomers of N-acetyl glucosamine and modified chitin. The yeast-produced chitinase was also resistant to degradation by proteases from maize ear rot fungi, and enhanced antifungal activity of miconazole towards Fusarium graminearum. When introduced in maize callus transgenically, the callus expressing the chitinase 2 gene had significantly less growth of the ear rot pathogen, F. graminearium, than callus which did not express the gene. This information suggests susceptibility and resistance to ear rot pathogens is influenced not only by expression levels, but also by the sequence of putative resistance genes. Previous reports of increased expression of putative resistance genes need to be tempered with the realization that they may play no role in resistance if the genes code for nonfunctional or reduced function proteins.

Expression Profiling Coupled with In-silico Mapping Identifies Candidate Genes for Reducing Aflatoxin Accumulation in Maize.

Aflatoxin, produced by Aspergillus flavus, is hazardous to health of humans and livestock. The lack of information about large effect QTL for resistance to aflatoxin accumulation is a major obstacle to employ marker-assisted selection for maize improvement. The understanding of resistance mechanisms of the host plant and the associated genes is necessary for improving resistance to A. flavus infection. A suppression subtraction hybridization (SSH) cDNA library was made using the developing kernels of Mp715 (resistant inbred) and B73 (susceptible inbred) and 480 randomly selected cDNA clones were sequenced to identify differentially expressed genes (DEGs) in response to A. flavus infection and map these clones onto the corn genome by in-silico mapping. A total of 267 unigenes were identified and majority of genes were related to metabolism, stress response, and disease resistance. Based on the reverse northern hybridization experiment, 26 DEGs were selected for semi-quantitative RT-PCR analysis in seven inbreds with variable resistance to aflatoxin accumulation at two time points after A. flavus inoculation. Most of these genes were highly expressed in resistant inbreds. Quantitative RT-PCR analysis validated upregulation of PR-4, DEAD-box RNA helicase, and leucine rich repeat family protein in resistant inbreds. Fifty-six unigenes, which were placed on linkage map through in-silico mapping, overlapped the QTL regions for resistance to aflatoxin accumulation identified in a mapping population derived from the cross between B73 and Mp715. Since majority of these mapped genes were related to disease resistance, stress response, and metabolism, these should be ideal candidates to investigate host pathogen interaction and to reduce aflatoxin accumulation in maize.

Validation of SSR markers for northern corn leaf blight resistance in maize (Zea mays L.)

Marker assisted selection for resistance to Northern Corn Leaf Blight (NCLB) caused by Exserohilum turcicum has been carried out for three years at University of Agricultural Sciences, Dharwad, India. The objectives were to screen resistant and susceptible maize inbreds for E. turcicum to identify polymorphic SSR markers closely-linked to known genes for NCLB resistance and used these polymorphic markers to validate in F2 progenies. Among 36 inbred lines screened under artificial epiphytotic condition, lines DMIT 126 and DMIT 136 showed high resistance (score 1) and high susceptiblity (score 5) for TLB, respectively and were used to develop F2 population. Single marker analysis (SMA) was performed using 248 F2 plants to determine the strength of association between resistance linked markers and disease resistance by calculating simple linear regression coefficient. Two markers Phi 330507 and Umc 1221 showed significant association with resistance to TLB with the R2 (PVE) and LOD score of 13.77 and 8.01%, 22.72 and 13.94, respectively. This result indicated that the validation of two SSR markers linked to TLB resistance was promising in application for molecular-assisted selection for improving TLB resistance in maize breeding programmes.

Susceptibility of Maize to Stalk Rot Caused by Fusarium graminearum Deoxynivalenol and Zearalenone Mutants.

Fusarium graminearum is a destructive pathogen of cereals that can cause stalk rot in maize. Stalk rot results in yield losses due to impaired grain filling, premature senescence, and lodging, which limits production and harvesting of ears. In addition, mycotoxins can make infected tissues unfit for silage. Our objectives were to evaluate the natural variation in stalk rot resistance among maize inbreds, to establish whether deoxynivalenol (DON)- and zearalenone (ZEA)-deficient strains are pathogenic on a panel of diverse inbreds, and to quantify the accumulation of DON in infected stalk tissue. Wild-type F. graminearum and mycotoxin mutants (DON and ZEA) were used to separately inoculate stalks of 9-week-old plants of 20 inbreds in the greenhouse. Plants were evaluated for lesion area at the inoculation point at 0, 2, 14, and 28 days postinoculation and tissues around lesions were sampled to determine the DON content. Regardless of their ability to produce DON or ZEA, all tested F. graminearum strains caused stalk rot; however, significant differences in disease levels were detected. Among the tested inbreds, Mp717 was resistant to all three F. graminearum strains while Mp317 and HP301 were only partially resistant. Accumulation of DON was significantly lower in infected stalks of the resistant and partially resistant inbreds than the susceptible inbreds. Analysis of the 20 inbreds using data from 17 simple-sequence repeats revealed population structure among the individuals; however, there was no association between genetic clustering and stalk rot resistance. These findings are an additional step toward breeding maize inbreds suitable for planting in fields infested with F. graminearum.

Maize peroxidase Px5 has a highly conserved sequence in inbreds resistant to mycotoxin producing fungi which enhances fungal and insect resistance.

Mycotoxin presence in maize causes health and economic issues for humans and animals. Although many studies have investigated expression differences of genes putatively governing resistance to producing fungi, few have confirmed a resistance role, or examined putative resistance gene structure in more than a couple of inbreds. The pericarp expression of maize Px5 has previously been associated with resistance to Aspergillus flavus growth and insects in a set of inbreds. Genes from 14 different inbreds that included ones with resistance and susceptibility to A. flavus, Fusarium proliferatum, F. verticillioides and F. graminearum and/or mycotoxin production were cloned using high fidelity enzymes, and sequenced. The sequence of Px5 from all resistant inbreds was identical, except for a single base change in two inbreds, only one of which affected the amino acid sequence. Conversely, the Px5 sequence from several susceptible inbreds had several base variations, some of which affected amino acid sequence that would potentially alter secondary structure, and thus enzyme function. The sequence of the maize peroxidase Px5 common to inbreds resistant to mycotoxigenic fungi was overexpressed in maize callus. Callus transformants overexpressing the gene caused significant reductions in growth for fall armyworms, corn earworms, and F. graminearum compared to transformant callus with a β-glucuronidase gene. This study demonstrates rarer transcripts of potential resistance genes overlooked by expression screens can be identified by sequence comparisons. A role in pest resistance can be verified by callus expression of the candidate genes, which can thereby justify larger scale transformation and regeneration of transgenic plants expressing the resistance gene for further evaluation.

Variable response of kochia [Kochia scoparia (L.) Schrad.] to auxinic herbicides dicamba and fluroxypyr in Montana

Jha, P., Kumar, V. and Lim, C. A. 2015. Variable response of kochia [Kochia scoparia (L.) Schrad.] to auxinic herbicides dicamba and fluroxypyr in Montana. Can. J. Plant Sci. 95: 965–972. Herbicide-resistant kochia is an increasing concern for growers in the Northern Great Plains of United States and Canada. The objective of the research was to characterize the response of the three putative auxinic herbicide-resistant kochia inbreds (derived from accessions collected from wheat/chemical fallow fields in northern Montana) to dicamba and fluroxypyr relative to a susceptible (SUS) inbred. A dicamba dose-response study indicated that the three putative resistant inbreds (Chot-01, Chot-02, and Chot-03) had R/S ratios of 1.3 to 6.1 based on the visible control response (I50 values), and R/S ratios of 1.5 to 6.8 based on the shoot dry weight response (GR50 values). Dose-response experiments with fluroxypyr determined I50 R/S ratios of 1.4 to 5.7 and GR50 R/S ratios of 1.6 to 4.0 for the three putative resistant inbreds. The selected inbreds showed variable symptomology (phenotype) in response to dicamba and fluroxypyr. Among the three inbreds, Chot-01 exhibited the least epinasty, stem curling/swelling, and chlorosis/necrosis symptoms, and was resistant to dicamba and fluroxypyr. Growers should diversify their weed management tools to manage further spread of auxinic or multiple herbicide-resistant kochia in the region.