26. Genetic markers associated with field resistance to leaf and neck blast across locations in rice (Oryza sativa L.)

Department of Genetics and Plant Breeding, University of Agricultural Sciences, GKVK, Bangalore-560065 India

     Blast disease caused by Pyricularia grisea Sacc. is the most serious fungal disease of rice (Oryza sativa L.). One hundred and fourteen doubled haploid (DH) lines of a cross between 1R64, an indica variety adapted to irrigated conditions and Azucena, an upland aromatic japonica variety (Guiderdoni et al. 1992) were used for molecular mapping of QTL associated with leaf and neck blast resistance. The RFLP map and molecular data generated by Huang et al. (1994) were used for QTL mapping. The DH population, along with their parents and susceptible checks IR50, HR12 and CO39 were screened for blast at IRRI, Philippines in 1997 and at three locations in South India (1995) in Uniform Blast Nursery. Leaf blast was scored using 0-5 scale (Mackill and Bonman, 1992) at regular intervals from seventh day after inoculation to transplanting date. Neck blast was scored prior to harvest (Anonymous, 1996) in the main field. Three traits, i.e., percentage diseased leaf area (DLA%), number of susceptible lesions (LSN) and susceptible lesion size (LSI) were scored in all the four selected locations.
     The phenotypic evaluation for blast indicated IR64 to posses high level of leaf blast resistance across four locations. The male parent Azucena was moderately susceptible in all the locations. Nursery evaluation at IRRI, Philippines showed thirty two partially resistant DH lines whereas nineteen exhibited resistance in single spore inoculation using P06-06 isolate. Nine doubled haploid lines and IR64 showed resistant lesions in the presence of high level of inoculum in nursery at Ponnampet (1995), South India. Ghesquiere et al. (1996) in their study with the same population identified few resistant lines in France using six diverse strains of Magnaporthe grisea. Sanger et al. (1997) observed resistant lesions on IR64 in highly favorable blast disease environment of the Northern Hill zone of Chhatisgarh, Madhya Pradesh, India.

     Interval mapping of QTL using the computer package MAPMAKER/QTL (Lincoln et al. 1992) with threshold LOD »= 2.00, identified a total of twenty five QTL for leaf blast and two QTL for neck blast resistance. Among them, fifteen QTL were detected at blast nursery, IRRI and ten at Ponnampet. Between the two QTL identified for neck blast resistance, Bangalore and Mudigere contributed one QTL each (Table 1). Seventeen QTL were identified for DLA (qDLA), six QTL for LSN (qLSN) and two QTL for LSI (qLSI). Eight QTL had major effect (LOD »= 3.00) for leaf blast resistance (Table 1, Fig. 1). Some of the QTL were expressed in more than one location and are presented in Table 2, and multiple effect of chromosomal segment was hypothesized for these QTL. These results insinuate that pleiotropism rather than close linkage of different QTL could be the major reason why QTL for different blast related traits were frequently detected in the same intervals across locations. QTL on chro.# 9 controlling neck blast resistance (qNBL) in Bangalore (LOD = 2.03, 8.5% variation) overlapped leaf blast resistance QTL flanked by RG358-RZ12 (LOD = 2.76, 6.00% variation) of CO39/Moroberekan recombinant inbred population screened at Ponnampet. The minor QTL for lesion size on chro.#11 brack-eted by RG103-Npb186 flanking markers (LOD = 2.02, 9.50% variation) partially overlapped major gene Pi-7(t) (Wang et al., 1994) linked to RG1O3A-RG16 markers in Moroberekan, a resistant japonica variety. The presence of QTL in one location and itŐs absence in another indicated the differences in the pathogenic races. While, the major QTL identified across wide geographic locations and that too on similar chromosomal location indicates the stability of the QTL for blast resistance. Such markers associated with QTL which are common across locations can be used for selecting blast resistant lines. Our study will give reliable data on stability of the molecular markers to be used in Marker-assisted selection, map based cloning and developing of isogenic lines for blast disease resistance.


Anonymous, 1996. Standard evaluation system for rice (4th edition). IRRI, P.O.Box 933, Los Banos, Philippines. pp17-l9.

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McCouch, S.R., Y.G. Cho, M. Yano, E. Paul, M. Blinstrub, H. Morishima and T. Kinoshita, 1997. Report on QTL nomenclature, RON 14: 11-13.

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