Abstract
Bacterial leaf blight caused by Xanthomonas oryzae pv oryzae (Xoo) and blast caused by Magnaporthe oryzae are major diseases responsible for significant yield loss in rice production across all rice growing regions. Host plant resistance has been advocated as a sustainable means of guarding against the diseases. This experiment was conducted with the aim to introgress multiple resistance genes against bacterial leaf blight and blast diseases through marker-assisted backcross breeding. Two dominant (Xa4 and Xa21) and two recessive (xa5 and xa13) Xoo resistance genes were introgressed into a high yielding Malaysian rice variety Putra-1 with genetic background of three blast resistance (Piz, Pi2 and Pi9) genes. Eight polymorphic tightly linked functional and SSR markers were used for foreground selection of target genes. Seventy nine polymorphic SSR markers were used in background selection. The plants were challenged at initial stage of breeding and challenged again at BC2F2 with the most virulent Malaysian pathotypes of Xoo (P7.7) and Magnaporthe oryzae (P7.2) to test their resistance. Results obtained from foreground marker segregation analysis at BC1F1 and BC2F1 showed that the marker polymorphism both fitted into the Mendel’s single gene segregation ratio of 1:1 for both Xoo and blast resistance. At BC2F2, results indicated that foreground marker polymorphism fitted into the expected Mendelian ratio of 1:2:1 for blast resistance only. Marker-assisted background selection revealed high percentage of recurrent parent genome recovery (95.9%). It was concluded that the inheritance of blast resistance in the introgressed lines was mainly due to single gene action while the inheritance of Xoo resistance was substantially due to single nuclear gene action. The incorporation of four bacterial leaf blight and three blast resistance genes (Xa4 + xa5 + xa13 + Xa21; Pi9 + Pi2 + Piz) in the newly developed lines would provide for broad spectrum and durable resistance against the two major diseases studied.
Highlights
Rice production is a very important activity all around the world
The parental polymorphism was identified between the two parents in all four target resistance genes for Xanthomonas oryzae pv oryzae (Xoo) using Xa21FR and pTA248 (Xa21), Xa13prom, RM21 and RM13, MP (Xa4); and three blast resistance genes using RM8225 (Piz) and RM6836 (Pi9, Pi2 and Piz) (Table 1)
Four bacterial leaf blight (Xa21FR, Xa13prom, pTA248 and RM13) and two blast (RM8225 and RM6865) polymorphic foreground markers were used for the foreground selection
Summary
Rice production is a very important activity all around the world. Many African and Asian countries rely on rice as a source of income and daily calories [1,2,3]. Bacteria leaf blight which is caused by the pathogen Xanthomonas oryzae pv. Oryzae (Xoo) is one of the most serious diseases responsible for significant yield reduction in rice. The disease constrains the photosynthetic area of the crop thereby causing partial grain filling and this leads to poor yield [4]. Use of resistant varieties of rice is the best approach to guard against the disease as chemical control is not effective. 42 Xoo resistance genes have been identified in rice and some of them have been incorporated into new varieties [5]. The disease is favoured by temperatures between 25 to 34 ◦ C, with relative humidity more than 70% and most rice growing regions in Malaysia are affected by the disease [6]
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