(QTLs) is to identify loci of similar location and effect in multiple populations and/or environments. The literature contains two prior 1995;Marchetti etal., 1996;McClung etal.,1997). These publications reporting the location of QTLs affecting resistance to techniques are based on phenotypic evaluation of resissheath blight (SB) disease in rice (Oryza sativa L.), but lack of agree- tance in inoculated field plots. Disease response is not ment between QTLs in the studies left all 12 unconfirmed, limiting consistent, however, even within inoculated research the potential of marker-assisted selection of this trait with worldwide plots. This inconsistency is experienced as differences importance.Theearlierlinkageanalyseswereimpreciseduetohetero- between replications as well as in year-to-year and locazygosity, segregation, and limited plot size and replication. We evalu- tion-to-location differences. The development of SB is atedareplicatedsetofpure-breedingrecombinantinbredlines(RILs) sensitive to surrounding humidity and temperature, to increase reliability of the quantitative disease data and in turn which are themselves affected by plant density, tiller improve accuracy of the QTL mapping. The RILs were F2:10 descennumber,leaf angleandlength, lodging,and waterdepth. dants from an early-generation ‘Lemont’ ‘Teqing’ population wherein SB resistance QTLs (SB-QTLs) were first identified. The As these plant traits and/or water depth vary within and present study confirmed the location and effect of six SB-QTLs, con- between plots, disease severity also varies. The nongefirmed the existence but not the specific location of another, and netic macro- and micro-environmental factors affecting identified eight new loci. Three of the confirmed QTLs were also disease severity are so numerous and of strong impact found to be independent from undesirable plant height and maturity that one generally observes a range of disease severity effects.Thisresearchdemonstrated theimportanceofusingreplicated even within plots containing genetically pure materials, phenotypic data for reliably establishing the identity and effect of such as released cultivars. Thus, the disease index develputative QTLs for complex traits such as SB resistance. Further oped by Marchetti and Bollich (1991) was designed to marker development to facilitate marker-assisted selection of these account for differences in disease severity observed bethree SB-QTLs is warranted. tween plants within single plots. On a larger scale, with southern U.S. coastal weather patterns, very late and sometimes very early maturing

Confirming QTLs and Finding Additional Loci Conditioning Sheath Blight Resistance in Rice Using Recombinant Inbred Lines

Rice sheath blight, caused by Rhizoctonia solani Kuhn, 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.

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

Sheath blight caused by Rhizoctonia solani Kuhn is one of the important diseases of rice, resulting in heavy yield loss in rice every year. No rice line resistant to sheath blight has been identified till date. However, in some rice lines a high degree of resistance to R. solani has been observed. An indica rice line, Tetep, is a well documented source of durable and broad spectrum resistance to rice blast as well as quantitative resistance to sheath blight. The present study identified genetic loci for quantitative resistance to sheath blight in rice line Tetep. A mapping population consisting of 127 recombinant inbred lines derived from a cross between rice cultivars HP2216 (susceptible) and Tetep (resistant to sheath blight) was evaluated for sheath blight resistance and other agronomic traits for 4 years across three locations. Based on sheath blight phenotypes and genetic map with 126 evenly distributed molecular markers, a quantitative trait loci (QTLs) contributing to sheath blight resistance was identified on long arm of chromosome 11. Two QTL mapping approaches i.e., single marker analysis and composite interval mapping in multi environments were used to identify QTLs for sheath blight resistance and agronomical traits. The QTL qSBR11-1 for sheath blight resistance was identified between the marker interval RM1233 (26.45 Mb) to sbq33 (28.35 Mb) on chromosome 11. This region was further narrowed down to marker interval K39516 to sbq33 (~0.85 Mb) and a total of 154 genes were predicted including 11 tandem repeats of chitinase genes which may be responsible for sheath blight resistance in rice line Tetep. A set of 96 varieties and a F2 population were used for validation of markers linked to the QTL region. The results indicate that there is very high genetic variation among varieties at this locus, which can serve as a starting point for allele mining of sheath blight resistance.

Identification of major quantitative trait loci qSBR11-1 for sheath blight resistance in rice

Rhizoctonia solani is a widely distributed phytopathogen that causes banded leaf and sheath blight in maize and sheath blight in rice. Here, we identified an F-box protein (ZmFBL41) that confers resistance to banded leaf and sheath blight through a genome-wide association study in maize. Rice overexpressing ZmFBL41 showed elevated susceptibility to R. solani. Two amino acid substitutions in this allele prevent its interaction with ZmCAD, which encodes the final enzyme in the monolignol biosynthetic pathway, resulting in the inhibition of ZmCAD degradation and, consequently, the accumulation of lignin and restriction of lesion expansion. Knocking out the ZmCAD-homologous gene OsCAD8B in rice enhanced susceptibility to R. solani. The results reveal a susceptibility mechanism in which R. solani targets the host proteasome to modify the secondary metabolism of the plant cell wall for its invasion. More importantly, it provides an opportunity to generate R. solani-resistant varieties of different plant species.

Natural variation in ZmFBL41 confers banded leaf and sheath blight resistance in maize

Sheath blight of rice, caused by Rhizoctonia solani, is one of the most important rice diseases worldwide; however, no rice cultivar has been found to be completely resistant to this fungus To facilitate detailed analysis of sheath blight resistance at genetic, molecular, biochemical, and functional genomic levels, new methods were developed for effective and uniform infection and accurate evaluation of the disease The efficiency of R solani infection was tested on two resistant (Tetep and Jasmine 85) and two susceptible (Chucheongbyeo, Junambyeo) cultivars using three different inoculum types (agar block, liquid cultured mycelia ball, and mycelia suspension) By covering the inoculated sheaths with aluminum foil to maintain humidity, 100% infection rate was achieved in this study Liquid cultured mycelia balls caused significantly longer lesions (54 cm) than other types of inoculum, including agar block (24 cm) and mycelia suspension (16 cm) An improved method for evaluating sheath blight disease was selected by comparing two methods for evaluating disease severity among three partially resistant cultivars and five susceptible cultivars inoculated with liquid cultured mycelia balls In addition, a new formula was developed to calculate the disease susceptibility index Lesion length and the susceptibility index generally were correlated in each leaf, but there were discrepancies between the two evaluation methods due to differences in plant architecture among the cultivars The susceptibility index calculated using the new formula was the most accurate method for evaluating sheath blight disease across all cultivars The effect of heading date and panicle number also was evaluated in relation to sheath blight resistance Cultivars with late heading dates generally were more resistant to sheath blight than those with early heading dates

https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-92-1-0025

X. Y. Sha

Papers

Resistance of some rice varieties to sheath blight ShB