The Genetics Of Fusarium Crown Rot Resistance And Its Relationships With Other Traits Of Agronomic Importance In Barley

Barley is a important cereal crop that is grown throughout the world and is ranked fifth in world crop production. Barley is widely use for animal feed and food industry, and is the main raw material for beer production. Barley can be grown in many different climatic regions due to its adaptability to diverse conditions. These climatic conditions include variable growing seasons, temperatures, and precipitation rates. Similar to other cereal crop, the structure of barley genetic variation tends to be strongly reduced in modern elite breeding populations, causing increased vulnerability of modern varieties to diseases and environmental stress. Fusarium crown rot (FCR), caused by various Fusarium species, is a destructive disease of cereal crops in semiarid regions worldwide. Surveys in United States and Australia found that the disease does not only cause yield loss in barley and wheat, FCR-infected plants also produce mycotoxins in grains as well as other tissues. The presence of these compounds in food and feeds can be a serious safety concern. FCR has been reported in all main producing areas worldwide including Europe, South Africa, West Asia, North Africa and China. However, barley varieties with high levels of resistance to this dieaseare are not available. Working toward the breeding of resistant varieties, several sources of FCR resistance had been identified. Two of these identified genotypes were studied in my PhD research project. Genetics of FCR resistance for both of these genotypes were investigated by QTL mapping and possible interactions between FCR resistance and other traits of agronomic importance were assessed by generating and analyzing a series of populations and near isogenic lines (NILs). The feasibility of enhancing FCR resistance by gene pyramiding was also investigated based on QTL identified from the two different sources of resistance. Main results and conclusions derived from my PhD project include:1. Genetics of FCR resistance in the first source of resistance AWCS276(R1) was investigated by generating and analysing two populations of recombinant inbred lines (RILs). A major locus conferring FCR resistance, designated as Qcrs.cpi-4H, was detected in one of the populations (mapping population) and the effects of the QTL was confirmed in the other population. The QTL was mapped to the distal end of chromosome arm4HL and it is effective against both of the Fusarium isolates tested, one F. pseudograminearum and the other F. graminearum. The QTL explains up to45.3%of the phenotypic variance. As distinct from an earlier report which demonstrated co-locations of loci conferring FCR resistance and plant height in barley, a correlation between these two traits was not detected in the mapping population. However, as observed in a screen of random genotypes, an association between FCR resistance and plant growth rate was detected and a QTL controlling the latter was detected near the Qcrs.cpi-4H locus in the mapping population. Existing data indicate that, although growth rate may affect FCR resistance, different genes at this locus are likely involved in controlling these two traits.2. The genetics of FCR resistance in the second source of resistance AWCS079(R2) was investigated by developing and assessing three RIL populations. Two QTL, one located on the long arm of chromosome1H (designated as Qcrs.cpi-1H) and the other on3HL (designated as Qcrs.cpi-3H), were found to be responsible for the FCR resistance of this genotype. Qcrs.cpi-1H is novel as no other FCR loci have been reported on this chromosome arm. Qcrs.cpi-3H co-located with a reduced height (Rht) locus and the effectiveness of the former was significantly affected by the latter. The total phenotypic variance explained by these two QTL was over60%. Significant effects were detected for each of the QTL in each of the three populations assessed.3. The feasibility of enhancing FCR resistance by QTL pyramiding was investigated by targeting the three well-characterized QTL identified from the two sources of resistance, Qcrs.cpi-1H, Qcrs.cpi-3H and Qcrs.cpi-4H. A population consisting265RILs and segregating for these QTL was generated and assessed in two replicated trials. Results from this study showed that, on average, lines with any combination of two of the three QTL gave better resistance than those with a single QTL only, and those lines with all three of the QTL gave the least FCR symptom. These results demonstrated that gene pyramiding can be an effective strategy to further improve FCR resistance in barley breeding programs.4. A recent report based on the study of reduced height (Rht) genes in wheat and barley claimed that, similar to what had been reported in Arabidopsis, DELLA genes likely increased susceptibility to necrotrophs but increased resistance to biotroph. This claim could further restrict options breeders may have in developing elite barley or wheat varieties. I further investigated the possible effects of Rht genes on FCR resistance by developing and assessing15pairs of NILs for the Rht gene uzu which does not produce DELLA proteins. The dwarf isolines all gave better FCR resistance when compared to their respective tall counterparts and the difference in FCR severity between the two isolines for a given pair of NILs was larger in the environment where the difference in height between the isolines was larger. These results further strengthen the argument that plant height affects Fusarium resistance but the effects are unlikely related to DELLA genes. Rather, the effects seem to be due to direct or indirect effects of height difference per se thus caution should be exercised when attempting to exploit any resistant locus which co-locates with an Rht gene.5. A recent study based on a two-row landrace of barley, TX9425, located QTL controlling several traits of spike morphology (SM) in a similar region with FCR resistance on the long arm of chromosome3H. To further characterize this chromosomal region,12pairs of NILs for SM were generated from two populations between TX9425and two different commercial cultivars. A population consisting of1,028lines segregating primarily for the target region was also developed using materials obtained during the production of these NILs. Results from the analyses of the NILs and the NIL-derived population showed that grain density (GD), spike length (SL) and awn length (AL) were likely controlled by a single recessive locus derived from the genotypeTX9425. This locus was mapped to a distance of0.66cM near the SSR marker of GBM1495. Across the12pairs of NILs, the presence of the3HL locus increased GD by53.4%, reduced spike length by38.8%, and reduced awn length by62.7%. In the NIL-derived population, the presence of the3HL locus increased GD by64.6%, reduced spike length by33.7%, and reduced awn length by62.6%. An interesting question arising from this research is why some loci such as the one reported here affect several SM-related traits while others appear to affect one of these traits only. The NILs and the NIL-derived population generated in this study will help answer such a question by providing the germplasm to enable cloning and comparing of the genes responsible for these SM-related traits.