Zongxiang Chen

Mapping quantitative trait loci controlling sheath blight resistance in two rice cultivars (Oryza sativa L.)

Abstract Rice sheath blight, caused by Rhizoctonia solani Kühn, is one of the three major diseases of rice. The present study was conducted with an F2 clonal population of Jasmine 85/Lemont. The F2 population, including 128 clonal families, was inoculated by short toothpicks incubated with a strain, RH-9 of the fungus. Based on field disease evaluations in 2 years and a genetic map with 118 evenly distributed molecular markers, we identified six quantitative trait loci (QTLs) contributing to sheath blight resistance. These QTLs, qSB-2, qSB-3, qSB-7, qSB-9-1, qSB-9-2 and qSB-11, were located on chromosomes 2, 3, 7, 9 and 11, respectively. The respective alleles of qSB-2, qSB-3, qSB-7, and qSB-9-2 from Jasmine 85 could explain 21.2%, 26.5%, 22.2% and 10.1% of the total phenotypic variation, respectively; while the alleles of qSB-9-1 and qSB-11 from Lemont could explain 9.8% and 31.2% of the total phenotypic variation. Of these qSB-2 and qSB-11 could be detected in both years, while remaining loci were detected only in a single year. Furthermore, four QTLs (qHD-2, qHD-3, qHD-5 and qHD-7) controlling heading date and three QTLs (qPH-3, qPH-4 and qPH-11) controlling plant height were also identified. Though rice sheath blight resistance may be influenced by morphological traits, such as heading date and plant height, in the present study most detected resistance loci were not linked to the loci for heading date or plant height.

Prospect of the QTL-qSB-9Tq utilized in molecular breeding program of japonica rice against sheath blight

The major QTL-qSB-9(Tq) conferring partial resistance to rice (Oryza sativa L.) sheath blight (Rhizoctonia solani Kühn) has been verified on chromosome 9 of the indica rice cultivar, Teqing. In this study, the prospect of this QTL utilized in molecular breeding program of japonica rice for sheath blight resistance was investigated. Most of the japonica rice cultivars showed lower level of sheath blight resistance than the indica rice cultivars. At the corresponding site of qSB-9(Tq), nine typical japonica rice cultivars from different ecological regions or countries proved to possess the susceptible allele(s). Introgression of qSB-9(Tq) into these cultivars enhanced their resistance level by decreasing sheath blight score of 1.0 (0.5-1.3), which indicated that qSB-9(Tq) had a large potential in strengthening the resistance of japonica rice to sheath blight. The use of the three molecular markers, which were polymorphic between Teqing and many japonica rice cultivars, promotes the application of qSB-9(Tq) in a concrete molecular breeding program.

Tagging major quantitative trait loci for sheath blight resistance in a rice variety, Jasmine 85

This study was conducted with a clonal F2 population of rice from a cross between Jasmine 85, a resistant variety, and Lemont, a susceptible cultivar. The rice plants belonging to each F2 clone were divided into two plots, which were put in two replicates, respectively. Clonal parents were tested as controls. The plants were inoculated by short toothpicks incubated with RH-9, a virulent isolate of the pathogenic fungus,Rhizoctonia solani, which causes rice sheath blight. The extreme resistant and susceptible clonal lines were selected for construction of resistant and susceptible DNA pools, respectively. A total of 94 polymorphic markers evenly distributed on 12 rice chromosomes were used for bulked segregant analysis, three positive ones were found polymorphic between the two DNA pools, and three major QTLs for sheath blight resistance, Rh-2, Rh-3 and Rh-7, were identified. The three major QTLs were located on chromosomes 2, 3 and 7, and could explain 14.4%, 26.1% and 22.2% of the phenotypic variation.

Fine mapping of an incomplete recessive gene for leaf rolling in rice (Oryza sativa L.)

Genetic analysis and fine mapping of genes controlling leaf rolling were conducted using two backcrossed generations (BC4F2, BC4F3) derived from a cross between QMX, a non-rolled leaf cultivar as a recurrent parent, and JZB, a rolled leaf NIL of ZB as a donor parent. Results indicated that leaf rolling was mainly controlled by an incompletely recessive major gene, namelyrl(t), and at the same time, affected by quantitative trait loci (QTLs) and/or the environment. A genetic linkage map was constructed using MAPMAKER/EXP3.0 with eight polymorphic markers on chromosome 2, which were screened by BAS method from 500 SSR markers and 15 newly developed insertion/deletion (InDel) markers. The position ofrl(t) was estimated with composite interval mapping (CIM) method using WinQTLcart2.5. Generl(t) was mapped between markers InDel 112 and RM3763, and 1.0 cM away from InDel 112 using 241 plants in BC4F2 population. To fine maprl(t), one BC4F3 line with 855 plants was generated from one semi-rolled leaf plant in BC4F2. Four new polymorphic InDel markers were developed, including InDel 112.6 and InDel 113 located between markers InDel112 and RM3763. Based on the information of recombination offered by 191 rolled leaf plants and 185 non-rolled leaf plants from the BC4F3 line, we mappedrl(t) to a 137-kb region between markers InDel 112.6 and InDel 113. Homologous gene analysis suggested thatrl(t) was probably related to the process of leaf development regulated by microRNA.

Evaluation of the effect of qSB-9Tq involved in quantitative resistance to rice sheath blight using near-isogenic lines.

Resistance to rice sheath blight disease that causes significant yield losses worldwide is controlled by multiple quantitative trait loci (QTL). The accurate resistance effect of qSB-9Tq, a major QTL that derived from the indica cultivar Teqing, was investigated by using a backcross population of Teqing and Lemont (the latter as the recurrent parent) and a set of near-isogenic lines (NIL) developed in this study. A strong pathogenic Rhizoctonia solani line, RH-9, was used for inoculation. Two different experimental designs were adopted. One was a completely randomized experiment in which plants that exhibited three different genotypes at qSB-9Tq locus were distinguished by detecting the marker genotypes flanking qSB-9Tq in the BC6F2 segregation population. The other was a randomized block design experiment with three replicates using the three NILs of qSB-9Tq. The combined results of the two experiments suggested that qSB-9Tq was a dominant resistance QTL and could reduce disease rating by approximately 1...

InDel and SNP Markers and Their Applications in Map-based Cloning of Rice Genes

High-density markers are necessary for map-based cloning of rice genes, but the currently available markers are not satisfactory enough. InDel (insertion-deletion length polymorphism) and SNP (single nucleotide polymorphism) are the new generation of molecular markers and can basically meet the need of fine mapping. InDel and SNP markers can be developed through bioinformatics. These markers are valuable markers with the characters of low cost, high specificity and stability. This article introduced the methods for designing InDel and SNP markers, taking the mapping of a rice rolled leaf gene as an example. In addition, some key factors in improving the design efficiency were also discussed.

Overexpression of OsOSM1 Enhances Resistance to Rice Sheath Blight

Sheath blight (SB), caused by Rhizoctonia solani, is one of the most destructive rice diseases worldwide. It has been difficult to generate SB-resistant varieties through conventional breeding because of the quantitative nature of rice resistance to SB. In this study, we found that overexpression of the OsOSM1 gene, encoding an osmotin protein belonging to the pathogenesis-related protein 5 family, is able to improve rice resistance to SB in field tests. Although there are two osmotin genes in rice, OsOSM1 is the one mainly expressed in leaf sheath at the booting stage, coinciding with the critical stage of SB development in the field. In addition, OsOSM1 expression is strongly induced by R. solani in SB-resistant rice variety YSBR1 but not in susceptible varieties, suggesting its involvement in SB resistance. Overexpression of OsOSM1 (OsOSM1ox) in susceptible variety Xudao 3 significantly increases resistance to SB in transgenic rice. The OsOSM1 mRNA levels in different transgenic lines are found to be positively correlated with their SB resistance levels. Intriguingly, although extremely high levels of OsOSM1 were detrimental to rice development, appropriately elevated levels of OsSOM1 were obtained that enhanced rice SB resistance without affecting rice development or grain yield. The OsSOM1 protein is localized on plasma membrane. OsOSM1 is upregulated by jasmonic acid (JA); furthermore, JA-responsive marker genes are induced in OsOSM1ox lines. These results suggest that the activation of JA signaling pathway may account for the increased resistance in transgenic OsOSM1ox lines. Taken together, our results demonstrate that OsOSM1 plays an important role in defense against rice SB disease and provides a new target for engineering resistance to SB.

Evaluation of Resistance of a Novel Rice Line YSBR1 to Sheath Blight

The novel rice (Oryza sativa L.) line YSBR1 is an intersubspecies hybrid with high level of partial resistance to sheath blight (SB). For the purpose of assessing the resistant level of YSBR1 using multiple approaches, the disease score of YSBR1 was investigated after artificial inoculation in the field and greenhouse in 2006 and 2007. The inhibition rate SB-toxin to the embryo root was also used as a criterion of SB resistance. Compared with other rice genotypes, YSBR1 had significantly higher resistance to SB and the resistance was stable and reliable. The average SB disease score of YSBR1 was 2.39±0.23 and felt in “resistant” grade in the “0–9” rating system when the susceptible control Lemont showed a mean disease score of 7.55±0.26. Correlation analysis showed that the SB-toxin inhibition rate was significantly correlated with disease score from the field test. The SB-toxin inhibition rate of YSBR1 was 57.55%, which was similar to that of Jasmine 85 but significantly lower than that of other tested varieties and Lemont (97.39%). In addition to the high resistance to SB, YSBR1 is also in good performance of plant type and height, growth duration, and grain yield, indicating its prosperous application in variety improvement of SB resistance in rice breeding programs.

A New Rice Cultivar Wulingjing 1 Resistant to Rice Stripe Virus Developed by Marker Assisted Selection

Abstract Rice stripe disease is one of the most serious diseases in rice (Oryza sativa L.) production area in Jiangsu Province, China since 2002. A highly susceptible rice cultivar with excellent food-tasted quality, Wuyujing 3, was improved through the molecular marker-assisted selection in a backcross of Zhendao 88 (with resistance gene Stvb-i). In each backcross generation, the progenies with Stvb-i were selected based on its closely flanking markers. In the advanced backcross generations, marker genotyping of the target gene and visual selection for the phenotype of the recurrent parent were conducted in the selected progeny plants during backcrossing. As a result, a novel cultivar, Wulingjing 1, was fleetingly bred in the BC3F4 generation, whose resistance to the disease was significantly enhanced and the main agronomic traits and characteristics, especially the excellent food-tasted quality were consistent with that of the recurrent parent. In the resistance tests in multiple locations organized by the Seed Management Department of Jiangsu Province, the diseased plant ratio of Wulingjing 1 was only 4.4%, which was significantly lower than that of the recurrent parent (53.2%).

Rice Pyramiding Breeding Using Sheath Blight Resistance QTL qSB-9 TQ and Stripe Disease Resistance Gene Stv-b i : Rice Pyramiding Breeding Using Sheath Blight Resistance QTL qSB-9 TQ and Stripe Disease Resistance Gene Stv-b i

以携带抗纹枯病QTLqSB-9TQ的籼稻品种特青和携带抗条纹叶枯病基因Stv-bi的粳稻品种镇稻88为优良等位基因供体亲本，江苏省推广的粳稻品种武育粳3号和武粳15为受体亲本，分别杂交并连续回交。在回交及自交分离世代，利用开发的覆盖目标基因区间的双侧分子标记对目标基因进行辅助选择。至回交BC4F1世代，同一遗传背景2个回交方向的中选单株间聚合杂交，获得2个目标基因位点均纯合的聚合F3株系。条纹叶枯病抗性鉴定和纹枯病抗性接种鉴定结果表明，聚合株系对条纹叶枯病均表现抗病；以0-9级评级标准评价，聚合株系的纹枯病较相应的轮回亲本分别低1．1～1．6级和0．8～1．4级。结合回交低世代抗性鉴定结果分析，自行开发的分子标记对目标基因的辅助选择是有效的。讨论了抗纹枯病育种及分子标记辅助选择聚合育种的相关问题。