Genotypic Variation In Resistance Research Articles

Aluminum Induces Distinct Changes in the Metabolism of Reactive Oxygen and Nitrogen Species in the Roots of Two Wheat Genotypes with Different Aluminum Resistance.

Aluminum (Al) toxicity in acid soils is a primary factor limiting plant growth and crop yield worldwide. Considerable genotypic variation in resistance to Al toxicity has been observed in many crop species. In wheat (Triticum aestivum L.), Al phytotoxicity is a complex phenomenon involving multiple physiological mechanisms which are yet to be fully characterized. To elucidate the physiological and molecular basis of Al toxicity in wheat, we performed a detailed analysis of reactive oxygen species (ROS) and reactive nitrogen species (RNS) under Al stress in one Al-tolerant (Jian-864) and one Al-sensitive (Yang-5) genotype. We found Al induced a significant reduction in root growth with the magnitude of reduction always being greater in Yang-5 than in Jian-864. These reductions were accompanied by significant differences in changes in antioxidant enzymes and the nitric oxide (NO) metabolism in these two genotypes. In the Al-sensitive genotype Yang-5, Al induced a significant increase in ROS, NO, peroxynitrite (ONOO-) and activities of NADPH oxidase, peroxidase, and S-nitrosoglutathione reductase (GSNOR). A concomitant reduction in glutathione and increase in S-nitrosoglutathione contents was also observed in Yang-5. In contrast, the Al-tolerant genotype Jian-864 showed lower levels of lipid peroxidation, ROS and RNS accumulation, which was likely achieved through the adjustment of its antioxidant defense system to maintain redox state of the cell. These results indicate that Al stress affected redox state and NO metabolism and caused nitro-oxidative stress in wheat. Our findings suggest that these molecules could be useful parameters for evaluating physiological conditions in wheat and other crop species under adverse conditions.

Host-parasite genotypic interactions in the honey bee: the dynamics of diversity.

Parasites are thought to be a major driving force shaping genetic variation in their host, and are suggested to be a significant reason for the maintenance of sexual reproduction. A leading hypothesis for the occurrence of multiple mating (polyandry) in social insects is that the genetic diversity generated within-colonies through this behavior promotes disease resistance. This benefit is likely to be particularly significant when colonies are exposed to multiple species and strains of parasites, but host–parasite genotypic interactions in social insects are little known. We investigated this using honey bees, which are naturally polyandrous and consequently produce genetically diverse colonies containing multiple genotypes (patrilines), and which are also known to host multiple strains of various parasite species. We found that host genotypes differed significantly in their resistance to different strains of the obligate fungal parasite that causes chalkbrood disease, while genotypic variation in resistance to the facultative fungal parasite that causes stonebrood disease was less pronounced. Our results show that genetic variation in disease resistance depends in part on the parasite genotype, as well as species, with the latter most likely relating to differences in parasite life history and host–parasite coevolution. Our results suggest that the selection pressure from genetically diverse parasites might be an important driving force in the evolution of polyandry, a mechanism that generates significant genetic diversity in social insects.

Variation and covariation for Gibberella ear rot resistance and agronomic traits in testcrosses of doubled haploid maize lines

Gibberella ear rot (GER) of maize caused by Fusarium graminearum reduces grain yield and leads to contamination of the grains with deoxynivalenol (DON), a mycotoxin that adversely affects the health of humans and animals. The objectives of this study were to (1) analyze means and genotypic variances for line per se performance (LP) and testcross performance (TP) of doubled haploid (DH) lines for GER severity and DON concentration as well as for some agronomic traits, (2) examine correlations among these traits, (3) validate QTL for resistance detected in previous studies for LP and their effect on TP and (4) investigate the relative efficiency of indirect selection (RE) for LP to improve TP. Testcross progenies of 94 DH lines originating from four flint populations were developed using a susceptible dent tester as pollinator. Artificial inoculations with F. graminearum led to appreciable disease development. Average TP for GER severity and DON concentration were lower than the mean mid-parent values of the tester and DH lines, indicating mid-parent heterosis for resistance. Genotypic variation for resistance was significant for LP and TP. Genotypic correlations between LP and TP were low and resistance QTL for LP had no significant effects on TP. Accordingly, RE for resistance was low, suggesting to allocate resources mostly to the evaluation of testcrosses. Correlations of resistance to GER and DON contamination with grain yield (measured under non-inoculated conditions) were not significant, indicating that selection for resistance and higher grain yield can be carried out simultaneously.

Comparison of screening methods to evaluate the response of St. Augustine grass to Magnaporthe oryzae

The objective of this study was to develop a reliable and high throughput screening method to evaluate the response of St. Augustine grass (Stenotaphrum secundatum) genotypes to the grey leaf spot (GLS) caused by Magnaporthe oryzae infection. Whole plant, detached stolon and detached leaf assays under growth chamber conditions were compared to field conditions on eight commercial and nine advanced breeding lines of St. Augustine grass. Disease was assessed using two variables, lesion size (LS) and overall plant disease severity (SEV). LS and SEV were highly correlated for field and growth chamber screening methods using the whole plant assay (LS r2 = 0·79; SEV r2 = 0·83; P ≤ 0·001), the detached stolon assay (LS r2 = 0·75; SEV r2 = 0·72; P ≤ 0·001), and the detached leaf assay (LS r2 = 0·46; SEV r2 = 0·60; P ≤ 0·001). Genotypic variation for resistance in 17 St. Augustine grass genotypes was identified using all screening methods for LS (P < 0·05) and SEV (P < 0·05). The rank‐sum method was used to classify St. Augustine grass genotypes into highly resistant (HR), resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS) classes based on the rank‐sum values of LS and SEV. Two introduced African polyploids used as parents, and two F1 interploid progeny obtained using an in vitro embryo rescue technique, were classified as highly resistant (HR), or resistant (R), across all screening methods.

Genetic and environmental determinants of silver birch growth and herbivore resistance

Plant growth and herbivore resistance is affected by both environmental and genetic factors. The aim of this study was to examine (1) whether genotype explains variation in herbivore resistance, plant growth and tolerance of feeding within natural silver birch populations, (2) whether there is significant interaction between birch genotype and environment or the study year, and (3) whether there are defence costs in field grown juvenile silver birch saplings. The 22 genotypes studied were assigned insect exposure and insect removal treatments in two different environments. Resistance to insect herbivory was measured as the amount of undamaged leaves of the saplings and tolerance of feeding as a difference in relative height increment of the saplings between insect exposure and removal treatments in relation to the amount of damage (i.e. opposite number of resistance). Our study shows that a local silver birch population has substantial genotypic variation in resistance to insect herbivores and this variation is not affected by the environment or the study year. Genotypic variation was not observed for tolerance of feeding. We also showed that, in contrast to genotypic variation in resistance, the genotypic variation in growth among silver birch genotypes strongly depended on the environment, i.e. silver birch genotypes have a high degree of phenotypic plasticity in their growth. Therefore, defence costs in terms of lower growth rate appear to be highly dependent on the environment. To conclude, our results indicate that silver birch populations have a good potential to adapt to possibly increasing herbivory in boreal forests in warmer future climates.

Identification of QTL for maize resistance to common smut by using recombinant inbred lines developed from the Chinese hybrid Yuyu22

Common smut in maize, caused by Ustilago maydis, reduces grain yield greatly. Agronomic and chemical approaches to control such diseases are often impractical or ineffective. Resistance breeding could be an efficient approach to minimize the losses caused by common smut. In this study, quantitative trait loci (QTL) for resistance to common smut in maize were identified. In 2005, a recombinant inbred line (RIL) population along with the resistant (Zong 3) and susceptible (87-1) parents were planted in Beijing and Zhengzhou. Significant genotypic variation in resistance to common smut was observed at both locations after artificial inoculation by injecting inoculum into the whorl of plants with a modified hog vaccinator. Basing on a genetic map containing 246 polymorphic SSR markers with an average linkage distance of 9.11 cM, resistance QTL were analysed by composite interval mapping. Six additive-effect QTL associated with resistance to common smut were identified on chromosomes 3 (three QTL), 5 (one QTL) and 8 (two QTL), and explained 3.2% to 12.4% of the phenotypic variation. Among the 6 QTL, 4 showed significant QTL x environment (Q x E) interaction effects, which accounted for 1.2% to 2.5% of the phenotypic variation. Nine pairs of epistatic interactions were also detected, involving 18 loci distributed on all chromosomes except 2, 6 and 10, which contributed 0.8% to 3.0% of the observed phenotypic variation. However, no significant epistasis x environment interactions were detected. In total, additive QTL effects and Q x E interactions explained 38.8% and 8.0% of the phenotypic variation, respectively. Epistatic effects contributed 15% of the phenotypic variation. The results showed that besides the additive QTL, both epistasis and Q x E interactions formed an important genetic basis for the resistance to Ustilago maydis in maize.

Effect of time of harvest on the incidence of Fusarium spp. in kernels of silage corn

The effect of time of harvest (at 75, 50 and 25% of milkline) on the incidence of kernel-borne Fusarium spp. was examined in four silage corn (Zea mays) hybrids (MAIZEX Leafy 4, NK BRAND Enerfeast 1, PIONEER 37M81 and MYCOGEN TMF94) in Ottawa, Ontario, in 2001 and 2002. Eleven Fusarium species were isolated over the 2 yr. Fusarium subglutinans was the dominant species recovered from 28.8% of the kernels. Other frequently isolated species included F. oxysporum (2.6%), F. graminearum (2.5%), F. proliferatum (0.3%) and F. sporotrichioides (0.2%). Trace amounts (< 0.1%) of the remaining six species, F. avenaceum, F. crookwellense, F. culmorum, F. equiseti and F. solani, were recovered from the kernels. When the kernels were harvested at 75, 50 and 25% of milkline, the incidence of F. subglutinans increased from 20.9 to 26.7 and to 38.7%, respectively; that of F. graminearum increased from 1.7 to 2.9 and to 3.1%; and for the total of the five main Fusarium species it increased from 28.7 to 32.2 and to 42.3%. Incidence of the other species was not affected by harvesting date. Of the four silage corn hybrids, NK BRAND Enerfeast1 had a significantly lower incidence of Fusarium species in kernels than the other hybrids, indicating a genotypic variation in resistance to kernel-borne infection by Fusarium species.

Both hypersensitive and non-hypersensitive responses are associated with resistance in Salix viminalis against the gall midge Dasineura marginemtorquens

Hypersensitivity responses (HR) play a major role in plant resistance to pathogens. It is often claimed that HR is also important in plant resistance to insects, although there is little unambiguous documentation. Large genotypic variation in resistance against the gall midge Dasineura marginemtorquens is found in Salix viminalis. Variation in larval performance and induced responses within a full-sib S. viminalis family is reported here; 36 sibling plants were completely resistant (larvae died within 48 h after egg hatch, no gall induction), 11 plants were totally susceptible, 25 plants were variable (living and dead larvae present on the same plant). Resistance was associated with HR, but to different degrees; 21 totally resistant genotypes showed typical HR symptoms (many distinct necrotic spots) whereas the remaining 15 genotypes showed no, or very few, such symptoms. Hydrogen peroxide, used as a marker for HR, was induced in genotypes expressing HR symptoms but not in resistant genotypes without symptoms, or in susceptible genotypes. These data suggest that production of hydrogen peroxide, and accompanying cell death, cannot explain larval mortality in the symptomless reaction. Another, as yet unknown, mechanism of resistance may be present. If so, then it is possible that this unknown mechanism also contributes to resistance in plants displaying HR. The apparent complexity observed in this interaction, with both visible and invisible plant responses associated with resistance against an adapted insect species, may have implications for the study of resistance factors in other plant-insect interactions.

A rapid hypersensitive response associated with resistance in the willow Salix viminalis against the gall midge Dasineura marginemtorquens

AbstractGreat genotypic variation in resistance against the gall midge Dasineura marginemtorquens Bremi (Diptera: Cecidomyiidae) exists within its willow host Salix viminalis L. (Salicaceae). In some resistant genotypes larvae die within 40 h after attempting to initiate galls. The present study tested the hypothesis that the hypersensitive response (HR) is involved in incompatible interactions between D. marginemtorquens and S. viminalis. By means of UV/blue light and visible light microscopy, we verified a rapidly (within 12 h after egg hatch) spreading cell death of an HR‐type due to larval attack in resistant willow genotypes. Twelve h after egg hatch, the cell death had spread up to six cell layers in resistant S. viminalis genotypes whereas in susceptible genotypes only up to two cell layers were affected. In the groups of dead cells on the resistant genotypes accumulation of phenolics was observed within 24 h after egg hatch. The rapidity of the cell death induction, the early local accumulation of phenolic compounds, and the strong association of the cell death with larval mortality suggest that the formation of dead cells in resistant willow genotypes should be interpreted as a true HR.

Characterization of quantitative trait loci (QTLs) in cultivated rice contributing to field resistance to sheath blight (Rhizoctonia solani)

Sheath blight, caused by Rhizoctonia solani, is one of the most important diseases of rice. Despite extensive searches of the rice germ plasm, the major gene(s) which give complete resistance to the fungus have not been identified. However, there is much variation in quantitatively inherited resistance to R. solani, and this type of resistance can offer adequate protection against the pathogen under field conditions. Using 255 F4 bulked populations from a cross between the susceptible variety 'Lemont' and the resistant variety 'Teqing', 2 years of field disease evaluation and 113 well-distributed RFLP markers, we identified six quantitative trait loci (QTLs) contributing to resistance to R. solani. These QTLs are located on 6 of the 12 rice chromosomes and collectively explain approximately 60% of the genotypic variation or 47% of the phenotypic variation in the 'Lemont'x'Teqing' cross. One of these resistance QTLs (QSbr4a), which accounted for 6% of the genotypic variation in resistance to R. solani, appeared to be independent of associated morphological traits. The remaining five putative resistance loci (QSbr2a, QSbr3a, QSbr8a, QSbr9a and QSbr12a) all mapped to chromosomal regions also associated with increased plant height, three of which were also associated with QTLs causing later heading. This was consistent with the observation that heading date and plant height accounted for 47% of the genotypic variation in resistance to R. solani in this population. There were also weak associations between resistance to R. solani and leaf width, which were likely due to linkage with a QTL for this trait rather than to a physiological relationship.

Studies on weather damage in mungbean. III. Development of a system for measuring genotypic variation in resistance to weathering

Mungbeans are susceptible, following rainfall, to weather damage which reduces their value for food uses. Cultivars differ in their resistance to weathering, but selection for resistance based on field response has been unsuccessful. Three controlled experimental systems (immersion in water, exposure to simulated rainfall, and exposure to cyclic wetting and drying in a mist chamber) were evaluated for their ability to reproduce the symptoms of weather damage and to differentiate among the responses of cultivars known from field experience to differ in resistance. When seeds were immersed, the susceptible green gram cv. Berken absorbed water faster and had less impermeable seed than the resistant black gram cv. Regur. The green gram cv. Celera showed an intermediate response consistent with its intermediate resistance to weathering. The rainfall simulator produced more realistic conditions for weathering than seed immersion, and symptoms typical of weather damage were produced. However, the responses of cultivars were relatively poorly differentiated and the method showed poor repeatability. The exposure of podded racemes to wetting and drying cycles under controlled conditions of temperature and humidity in a mist chamber provided the most reliable method for simulating weather damage. The degree of damage increased with duration of exposure and the relative resistance of cultivars was consistent with field observation. The extent of weather damage was best measured as reduction in seed viability and as change in the appearance and permeability of the testae of seeds following exposure. The combined use of the mist chamber and these measurement criteria constitute a successful system for the selection and breeding of mungbeans for resistance to pre-harvest weathering.