Molecular Characterization of Slow Leaf-Rusting Resistance in Wheat

been widely used to characterize foliar disease resistance (Jeger and Viljanen-Rollinson, 2001) because it Slow leaf-rusting resistance in wheat (Triticum aestivum L) is gainreflects both severity and rate of disease development ing acceptance as a breeding objective because of its durability in comparison with race-specific resistance. CI 13227 was previously (Wilcoxson et al., 1975). IR and ID were also considered reported to provide the highest level of slow leaf-rusting resistance. to be important factors of disease epidemics (Parlevliet, The objective of this study was to characterize the slow leaf-rusting 1979). resistance conferred by CI 13227 using molecular markers. A populaGenetic studies indicated that slow leaf-rusting resistion of recombinant inbred lines (RILs) derived from CI 13227/Suwon tance is under polygenic control with moderately high 92 was evaluated for final severity (FS), area under disease progress heritability (Bjarko and Line, 1988a; Das et al., 1992). curve (AUDPC), infection rate (IR), and infection duration (ID) of Additive gene effects are predominant for slow leafleaf rust. Four hundred fifty-nine amplified fragment length polymorrusting, but additive additive interactions have also phism (AFLP) markers and 28 simple sequence repeat (SSR) markers been detected (Bjarko and Line, 1988b; Das et al., 1992). were analyzed in the population. Two quantitative trait loci (QTL), Therefore, slow-rusting resistance should be amenable designated as QLr.osu-2B and QLr.osu-7BL, were consistently associated with AUDPC, FS, and IR of leaf rust, caused by Puccinia to selection for improving resistance to leaf rust in wintriticina (previously P. recondita Rob. Ex Desm. f. sp. tritici). The ter wheat. percentages of phenotypic variance explained by each QTL varied Two genes associated with slow leaf-rusting resistance with experiments and traits, ranging from 13.4 to 18.8% for AUDPC, have been identified, Lr34 (Dyck, 1977) and Lr46 (Singh 12.5 to 20.8% for FS, and 12.9 to 16.1% for IR. The third QTL for et al., 1998). The Lr34 gene has been widely used in leaf rust ID, designated as QLrid.osu-2DS, was located on chromowheat breeding programs because of its durable resissome 2DS and explained 26.4 and 21.47% of the phenotypic variance tance to leaf rust, its association with Yr18, a stripe rust in 1994 and 1995, respectively. Both the QTL and correlation analysis resistance gene, and its association with tolerance to indicate reasonable progress in leaf-rusting resistance by selecting for Barley yellow dwarf virus infection (McIntosh, 1992; final severity. SSR markers closely associated with QLr.osu-2B or Singh, 1993). The combination of Lr34 with other genes, QLr.osu-7BL have potential to be used in marker-assisted selection (MAS) for durable leaf rust resistant cultivars. such as Lr12 and/or Lr13, provided durable leaf rust resistant cultivars worldwide (Roelfs, 1988), so not surprisingly, several attempts have been made to tag Lr34 with molecular markers. L rust is one of the major wheat diseases worldNelson et al. (1997a) found two loci associated with wide. The short-lived nature of race-specific leaf leaf rust resistance: one on 7DS, the expected position rust resistance genes greatly compromises the efforts of Lr34, and another on 2BS. Both loci cumulatively of breeders who use them, almost routinely, to breed explained 45% of the phenotypic variance. William et resistant cultivars. Alternatively, a more durable form al. (1997) identified three RAPD markers associated of resistance is attributed to slow leaf-rusting, for which with leaf rust resistance using bulked segregant analysis certain genotypes have been identified and character(BSA). Two of them were located on 7BL and the third ized (Caldwell et al., 1970; Kuhn et al., 1978; Shaner one hybridized to chromosome 1BS and 1DS. Faris et and Finney, 1980; Singh et al., 1998; Messmer et al., al. (1999) also found that a chromosome region on 7BL 2000). Methods used to assess slow leaf-rusting resiscontributed to leaf rust resistance under natural infectance include the severity measured either once at the tion. Messmer et al. (2000) detected six QTL for leaf peak of disease expression or several times during the rust resistance, and one major QTL on 7BL from the course of disease in a growing season. The AUDPC has highly resistant parent Forno explained 35% of the phenotypic variance. Forno showed leaf tip necrosis. BeX.-Y. Xu and B.F. Carver, Dep. of Plant and Soil Sciences, Oklahoma cause Lr34 was reported to be closely linked to the State Univ., Stillwater, OK 74078; G. Bai, USDA-ARS, Plant Science leaf tip necrosis gene, Ltn (Singh, 1992), the major rust and Entomology Research Unit and Dep. of Agronomy, Kansas State resistance gene in Forno is likely Lr34. Schnursch et al. Univ., Manhattan, KS 66506; G.E. Shaner, Dep. of Botany and Plant Pathology, Purdue Univ., West Lafayette, IN 47907; Robert M. Hun(2004) detected eight QTL for leaf rust resistance with ger, Dep. of Entomology and Plant Pathology, Oklahoma State Univ., two having major effects: one on 7DS and another on Stillwater, OK 74078. A portion of this research was funded by the 1BS of Forno. Suenaga et al. (2003) identified a microsaOklahoma Wheat Research Foundation and the Oklahoma Agric. tellite marker close to Lr34. Exp. Stn. Mention of trade names or commercial products in this In addition, another gene (Lr46) for slow leaf-rusting article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. resistance was identified on chromosome 1B of the Department of Agriculture. Received 29 June 2004. Genomics, Molecular Genetics & Biotechnology. *Corresponding author (gbai@bear. Abbreviations: AFLP, amplified fragment length polymorphism; agron.ksu.edu). AUDPC, area under disease progress curve; FS, final severity; ID, infection duration; IR, infection rate; MAS, marker-assisted selection; Published in Crop Sci. 45:758–765 (2005). © Crop Science Society of America QTL, quantitative trait loci; RILs, recombinant inbred lines; SSRs, simple sequence repeats. 677 S. Segoe Rd., Madison, WI 53711 USA 758 Published online February 23, 2005