PYRAMIDING OF BACTERIAL BLIGHT RESISTANCE AND SEMIDWARFING GENES IN TYPE 3 BASMATI

Abstract

Traditional Basmati varieties grown in India are low yielding, tall and lodge even under low nitrogen fertilizer dose. In addition to lodging, all the traditional varieties are highly susceptible to bacterial blight (BB) caused by X. oryzae pv. oryzae (Xoo), thus making it a high risk crop for fanners. Deployment of resistant varieties is the only viable alternative for managing this disease. Several BB resistant genes have been identified from cultivated germplasm and introgressed from wild species which are being deployed for management of the disease. The present investigation was, therefore, undertaken for pyramiding two highly effective BB resistance genes namely, xal3 and Xa21 and one semidwarfing gene sd-1 into Type 3 through marker assisted selection along with regions of Type 3 that span amylose and aroma loci during backcross breeding. A total of 7 Xoo isolates of 7 pathotype groups obtained from Punjab Agricultural University, Ludhiana were used for screening Type 3, Basmati 370, Tarori basmati, Basmati 386 and some non-basmati rice cultivars along with differential lines for various BB resistance genes and it was found that none of the basmati cultivars had any BB resistance gene. For pyramiding of the above genes Oryza sativa cultivar PR106-P2 was taken as the donor and Type 3 as the recurrent parent. This donor variety with three bacterial blight resistance genes Xa21, xal3, and xa5 and semidwarfing gene sd-1 was developed through marker assisted selection (MAS) at PAU, Ludhiana. Molecular markers pTA248, RG136, 'h', waxy &RM190, RM42 linked toXa21, xal3, sd-1, intermediate amylose content, fragrance genes, respectively were found to be polymorphic between the recipient Type 3 and donor parent PR106-P2. sd-1 marker (Ellis and Speilmeyer, 2002) linked to semidwarfing gene was not found to be polymorphic between the parents. Therefore, the marker 'h' linked to semidwarfism (Monna et ah, 2002) was used in the study. Fi plants obtained from the cross Type 3 / PR106-P2 were crossed with Type 3 to get BCiFi seeds. BCiFi plants heterozygous for all the three genes Xa21, xal3 and sd-1 were selected using linked molecular markers pTA248, RG136 and 'h' respectively and backcrossed with Type 3 to get BC2Fi seeds. All the lines analyzed for the linked molecular markers were also screened with mixture of Xool to Xoo7 isolates and the lesion length was recorded. 16 BC2F3 progenies with all the three genes mostly in homozygous condition were selected on the basis of linked molecular markers and similarity to Type 3 phenotypically. BC2F3 progenies were evaluated at IIT Roorkee and SVBPUA&T, Meerut against the Xoo isolates prevalent in northern India. The result indicated that the combination ofXa21 + xal3 was more effective against the pathogen than the individual genes. Marker assisted background selection (MABS) was carried in the same 16 BC2F3 progenies. 209 SSR markers from various linkage maps of rice were tested for polymorphism between the parents. 95/209 markers found polymorphic between the parents were used for background profiling of the selected lines and a dendrogram was constructed using ISSR and SSR markers. These lines showed background recovery from 81.57% to 92.10%. Among the 16 BC2F3 progenies 38-5-2 and 38-5-36 closely clustered along with recurrent parent Type 3. Even with very vigorous selection for recurrent parent phenotype higher recovery of the recurrent parent could not be obtained due to linkage drag along the target genes. So instead of doing background selection at the end, as was done during this study, it should be initiated in BCiFi generation with attempts to recover recurrent parent markers at one end of the target gene and then at the other end of the target gene on the carrier chromosome in BC2F]. BC2F3 was selfed to get BC2F4 progenies. Among the bulked 21 BC2F4 progenies, 38-5-36-1 and 38-5-36-2 were very close to Type 3 in respect of aroma and even superior in respect of elongation after cooking. Preliminary testing of BC1F4 progenies homozygous for xal3, Xa21 and sd-1 in replicated trial for yield and yield contributing traits showed significant increase in yield per plot over the recurrent parent Type 3 under disease free condition. This could be attributed to their significantly reduced plant height, increased number of tillers per plant, number of branches per panicle, number of grains per panicle probably as a result of sd-1 and its pleiotrophic effect on various yield contributing traits. Pyramiding of two or more BB resistance genes seems a viable approach for developing durable BB resistant varieties. It was possible to pyramid a dominant (Xa21) and a two recessive (xa!3 and sd-1) genes in popular variety Type 3 through MAS. Utmost, care was taken to select only the photosensitive progenies with all the three genes so that the exquisite quality characteristic of Type 3, a traditional basmati variety, could be maintained to meet consumer preferences and export quality. The release and cultivation of such lines suited for late sowing could save valuable underground water in the rice growing areas.