Corneti S

Demontis A

Giuliani S

Maccaferri M

Mantovani P

Massi A

Sanguineti MC

Stefanelli S

Tuberosa R

English

Sydney eScholarship

 1A major genetic factor for durable leaf rust resistance in durum wheat maps in the distal region of chromosome arm 7BL  Maccaferri M1, Mantovani P1, Giuliani S1, Sanguineti MC1, Demontis A2, Massi A2, Corneti S1, Stefanelli S1,  Tuberosa R1 1 DISTA, University of Bologna, Bologna, Italy; 2 Società Produttori Sementi Bologna Spa, Argelato (BO), Italy  ABSTRACT The genetic basis of leaf rust (Puccinia triticina Eriks.) resistance was studied using 176 RILs from Colosseo × Lloyd and 62 advanced lines derived from multiple crosses. RILs were tested under field conditions with a mixture of leaf rust isolates and at the seedling stage with single isolates. In the field experiment, the percentage of infected leaf area was evaluated through the disease developmental cycle and the area under disease progress curve (AUDPC) was calculated. A major QTL (QLr.ubo-7B.2) for leaf rust resistance at both adult (field conditions) and seedling stages was identified on the distal region of chr. 7BL. In the field, the QTL showed an R2 of 72.9% and a peak LOD score of 44.5 for AUDPC. The presence of this major QTL was validated by a linkage disequilibrium-based test using field data of advanced lines from multiple crosses. The association results confirmed the QTL location between Xbarc340.2 and Xgwm344.2, with the corresponding AUDPC R2 values ranging from ca. 10 to ca. 35% depending on the year. QLr.ubo-7B.2 maps in a gene-dense region (7BL10-0.78-1.00) known to carry several genes/QTLs in wheat and barley for resistance to rusts and other major cereal fungal diseases, including Lr14a and Lr19, two major candidates for this gene. INTRODUCTION Although leaf rust is a threat to durum wheat (Triticum turgidum L. var. durum), genetic and molecular mapping studies aimed at characterizing leaf rust resistance genes in durum wheat (Herrera-Foessel et al., 2005, 2007a; Martinez et al., 2007) have been undertaken only recently. Obtaining cultivars (cvs.) with durable resistance (see Johnson, 1984) is a major target in wheat breeding. Thus, we targeted the genetics of the resistance to Puccinia triticina conferred by the durum wheat cv. Creso (released in Italy in 1974 and derived from CIMMYT’s and Italian materials). Creso represents an important source of durum wheat durable resistance under field conditions that has remained effective since 1975 in cultivation environments characterised by recurrent leaf rust epidemics (Pasquini and Casulli, 1993; Martinez et al., 2007). The objectives of this study were: (i) to map the genetic determinants of the durable resistance to leaf rust from the cv. Creso and its resistant derivatives, and (ii) to identify SSRs linked to the resistance gene to enhance the efficiency of selection for resistant genotypes. MATERIALS AND METHODS A population of 176 recombinant inbred lines (RILs, F6:8) was produced by Società Produttori Sementi Bologna (PSB, Bologna, Italy) from the cross between the Italian cv. Colosseo (C) and the North American cv. Lloyd (L). Pedigree of Colosseo (Mexa’s mutant × Creso) indicates a direct lineage with Creso, the main source of leaf rust resistance. Lloyd (Cando × Edmore, NDSU) is a susceptible cv. under field conditions in Italy. A panel of 62 F6:8 lines from the breeding programs of PSB and Sementi Samoggia Srl (Crevalcore, Italy), with diverse pedigrees involving Creso or its resistant derivatives Colosseo and Plinio, was used to validate the major gene for leaf rust resistance inherited from Creso. Artificially inoculated field trials (three reps) were set up in Argelato, Bologna (Po Valley), Italy. The C × L RILs were evaluated in 2006, while the panel of advanced lines, together with Creso, Colosseo, Plinio and four susceptible checks, were evaluated in 2006 and 2007. Inoculation was carried out with a mixture of 16 leaf rust isolates collected in Italy. Reaction to leaf rust was recorded by visually estimating the percentage of infected leaf area (infected leaf area index, LRS) according to the modified Cobb scale (Peterson et al., 1948). Two to three visual scores were recorded. The area under the disease progress curve (AUDPC) was then calculated. The seedling leaf rust response of the parents and RILs was evaluated using a selection of isolates and the infection type (IT) responses were evaluated using the decimal 0-9 McNeal’ scale (McNeal et al., 1971). A genetic map based on a total of 554 SSR and DArT markers was obtained. The map (2022 cM) included 17 main linkage groups and two additional small groups.  QTL analysis was carried out on a map with 213 selected and evenly spaced markers (inter-marker interval ≥ 5 cM). The panel of diverse advanced lines related to Creso was molecularly characterized with: i) a set of 43 highly polymorphic and evenly distributed SSRs (used to evaluate the familial relationships among and within the lineages); ii) 14 SSRs selected to map in the 7BL chromosome region found to harbour the major leaf rust resistance QTL in the C × L population.  Composite interval mapping (CIM; Zeng 1994) was used to search for QTLs using LRS and AUDPC values from field data and the seedling IT values scored with a single isolate representative of the isolates virulent to Lloyd and avirulent to Colosseo. CIM analysis was  2 performed in Windows QTL-Cartographer version 2.5 (http://statgen.ncsu.edu/qtlcart/WQTLCart.htm). Association analysis for the SSR markers at the 7BL chr. region was performed using a mixed linear model (MLM) as implemented in the software TASSEL (http://www2.maizegenetics.net/). Only alleles with more than six counts in the advanced line set were considered in the association test.  RESULTS AND DISCUSSION Major and minor QTLs for leaf rust resistance The frequency distribution of the RILs tested under open field in 2006 for LRS and AUDPC indicated the presence of a major gene/QTL accounting for most of the phenotypic variation. The heritability ranged from 72 to 82% for LRS (recorded at three stages) and was equal to 83% for the AUDPC values. Colosseo showed a substantially resistant response throughout the disease cycle, while Lloyd confirmed its susceptible response, with LRS values up to 46.9% in the latest infection stage. Heading date (HD), thousand kernel weight (TKW) and test weight (TW) also showed high heritability values, ranging from 81 to 86%. The QTL analysis carried out using all the RILs evidenced the presence of only one QTL. This major QTL (QLr.ubo-7B.2), with the allele for leaf rust resistance contributed by Colosseo, was mapped in the distal region of the 7BL chr. arm (wheat deletion bin 7BL10-0.78-1.00), within a 5 cM interval (LOD - 2 supporting interval) flanked by microsatellite markers Xbarc340.2 and Xgwm146 in the upper part and by Xgwm344.2 in the distal part of the region. The QTL was detectable across the complete infection cycle, with R2 values for LRS ranging from 49.8% at the early stage of disease development (i.e. kernel milk stage, Zadoks 75) to 76.9% in the late phase (i.e. end of grain-filling, Zadoks 80). Using AUDPC, a very high QTL R2 value (72.9%) was detected. The additive effects (computed as half of the phenotypic difference between the means of the two RIL groups homozygous for the Lloyd and the Colosseo allele, respectively, at the QTL peak position) ranged from 6.6 to 18.4% for LRS at the early and late stage of the disease cycle, respectively. The effect of the QTL was also large on both kernel weight and test weight, with a gain equal to 1.8 g per 1,000 kernels and 0.8 kg hl-1 in favour of the RILs carrying the resistance allele from Colosseo, respectively. Although a total of four QTLs were detected for HD, none of them was located in the chr. region harbouring QLr.ubo-7B.2 (data not shown). A total of 29 markers (1 EST-SSR, 5 SSRs and 23 DArTs) were mapped in the QLr.ubo-7B.2 region within a 50 cM-wide interval; based on these markers, the QTL peak was clearly positioned in the distal region of the linkage group. Table 1 reports the results of the simple regression analysis using LRS (late disease stage) and AUDPC for those markers mapped in the QTL region. The LOD peak region, near to Xgwm344.2, included several DArT markers. A slight decrease in significance was observed at the end of the linkage group.  Three additional minor QTLs were identified by means of selective CIM analysis carried out on a subset of 76 lines with the molecular haplotype homogeneous to Lloyd at the QLr.ubo-7B.2 significance (LOD 3.0) chr. interval. At two of the three QTLs, the resistance allele was contributed by Lloyd (QLr.ubo-2A in the distal region of chr. arm 2AL and QLr.ubo-3A in the proximal region of chr. 3AS), while at the third QTL (QLr.ubo-7B.1 in the proximal region of chr. 7BS) the resistance allele was contributed by Colosseo. Each of the three QTLs showed an additive effect equal to ca. 5% for LRS and ca. 50 units for AUDPC. The resistance of Colosseo was also expressed at the seedling stage. Colosseo showed an average IT value = 3 (resistance) with 14 out of the 16 Italian leaf rust isolates tested. Two isolates were clearly virulent to Colosseo/Creso (IT response = 8, similar to that showed by Lloyd). One isolate avirulent to Colosseo was used to characterize the complete set of RILs at the seedling stage. Heritability of the IT score was equal to 91.9%. Similarly to what was found in the open field experiment, the resistance at the seedling stage was controlled by a major QTL. This QTL, with a LOD peak value equal to 32.8, explained the 58.9% of the phenotypic variation; the QTL significance interval and LOD peak position were coincident with the QLr.ubo-7B.2 ones.  Validation  of QLr.ubo-7B.2 on leaf rust resistance in a panel of lines related to Creso. The effects of QLr.ubo-7B.2 were investigated in an association study based on a 2-year field data of a panel of 62 Creso-related lines. In the advanced lines, the LD among the set of loci mapping in the 7BL region showed highly significant P values and high D’ values (> 0.3-0.5) over distances of 10 cM or more. Thus, the number of markers was adequate to validate the results obtained with the C × L RIL population.  The results of the association analysis over two years agreed with those obtained from the C × L population. Out of 14 SSRs spanning the 7BL distal chr. region, only those located within the QLr.ubo-7B.2 significance interval were associated to leaf rust response. In particular, Xwmc526.1, Xwmc526.2, Xbarc340.2, Xgwm146 and Xgwm344.2 showed a significant association to both LRS and AUDPC, with the latest three markers showing the highest significance association level. At these markers, the alleles most probably inherited from Creso or its resistant derivatives were associated to the lower leaf rust infection level. Our results indicate the presence of a major QTL for leaf rust resistance at the seedling and adult plant stages in the deletion bin 7BL10-0.78-1.00, which is one of the regions of the wheat chr. group 7 with the highest density of EST loci. The distal region near to the telomeric end of chr. group 7 of wheat is known to be rich in resistance genes, resistance gene analogs (RGAs) and defence-response genes (Dilbirligi et al., 2004).  Up to now, three important leaf rust resistance genes have been mapped in the distal regions of chr. group 7: Lr19, introgressed from a short terminal 7EL segment of  3Lophopyrum ponticum (Sharma and Knott, 1966), and the two closely linked genes Lr14a and Lr14b (distal portion of chr. 7BL; Dyck and Samborski, 1970). Moreover, the mapping location (Herrera-Foessel et al., 2007b) of Lr14a, one of the few designated resistance genes that originated from Triticum turgidum, is coincident with that of QLr.ubo-7B.2. The availability of precise genetic stocks for the above mentioned genes/QTL in a homogeneous genetic background could facilitate testing for allelism and gene postulation studies.    Table 1. Results of the simple linear regression analysis for all the SSR and DArT markers mapped in the 7BL chr. region harbouring the major QTL for leaf rust resistance (QLr.ubo-7B.2). The Adult plant leaf rust response data (infected leaf area index, LRS, and area under the disease progress curve, AUDPC) of the 176 C × L RILs have been used. The P-values are based on the F test and the b coefficients are the slope coefficients of the linear regression equation.  Markers Chr. position LRS-late (%) AUDPC (units)  (cM) P LOD b P LOD b Xbarc340.2 209.8 **** 23.11 14.37 **** 20.95 96.63 Xgwm146 212.7 **** 30.22 15.71 **** 28.09 107.39 372675/wPt-0504 215.4 **** 45.26 17.54 **** 41.30 120.19 wPt-6869/wPt-7219 215.7 **** 46.79 17.69 **** 43.01 121.42 Xgwm344.2 217.2 **** 49.64 17.96 **** 45.34 123.13 wPt-4038/wPt-1085 218.5 **** 50.23 18.06 **** 45.55 123.72 378059 218.8 **** 41.66 18.11 **** 38.43 117.65 wPt-4259  219.1 **** 49.64 18.04 **** 45.34 123.13 304264/312227/312780 220.0 **** 49.41 17.17 **** 44.72 122.89 304724 221.4 **** 34.07 16.22 **** 30.85 110.97   ACKNOWLEDGEMENTS  The financial contributions of the EU (BioExploit Food CT 2005-513959) and of the Emilia-Romagna Region (PRRIITT Project, Misura 3.4-A CEREALAB) are gratefully acknowledged.  REFERENCES  Dilbirligi M, Erayman M, Sanghu D, Sidhu D, Gill KS (2004). Identification of wheat chromosomal regions containing expressed resistance genes. Genetics 166: 461-481. Dyck PL and Samborski DJ (1970). The genetics of two alleles for leaf rust resistance at the Lr14a locus in wheat. Can J Genet Cytol 12: 689-694. Herrera-Foessel SA, Singh RP, Huerta-Espino J, Yuen J, Djurle A (2005). New genes for leaf rust resistance in CIMMYT durum wheats. Plant Disease 89: 809-814 Herrera-Foessel SA, Singh RP, Huerta-Espino J, William M, Rosewarne G, Djurle A, Yuen J (2007a). Identification and mapping of Lr3 and a linked leaf rust resistance gene in durum wheat. Crop Sci 47: 1459-1466. Herrera-Foessel SA, Singh RP, Huerta-Espino J,  William HM, Garcia V, Djurle A, Yuen J. (2007b). Identification and molecular characterization of leaf rust resistance gene Lr14a in durum wheat. Plant Disease 92: 469-473. Johnson R (1984). A critical analysis of durable resistance. Annu Rev Phytopathol 22: 309-330. Martinez F, Sillero JC, Rubiales D. (2007). Resistance to leaf rust in cultivars of bread wheat and durum wheat grown in Spain. Plant Breed 126: 13-18. McNeal FM, Conzak CF, Smith EP, Tade WS and Russell TS (1971). A uniform system for recording and processing cereal research data. USDA Agric. Res. Surv. Washington DC, ARC, 34-121. Pasquini M, Casulli F (1993). Resistenza “durevole” a Puccinia recondita f. sp. tritici ed Erysiphe graminis  f.sp. tritici in frumenti duri italiani. Phytopath Medit 32: 135-142. Peterson RF, Campbell AB, and Hannah AE (1948). A diagrammatic scale for estimating rust intensity of leaves and stem of cereals. Can. J. Res. Sect. C 26: 496-500. Sharma D, Knott DR (1966). The transfer of leaf-rust resistance from Agropyron to Triticum by irradiation. Can J Genet Cytol 8: 137-143. Zeng ZB (1994). Precision mapping of Quantitative Trait Loci. Genetics 136: 1457-1468.   

A major genetic factor for durable leaf rust resistance in durum wheat maps in the distal region of chromosome arm 7BL

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A major genetic factor for durable leaf rust resistance in durum wheat maps in the distal region of chromosome arm 7BL

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