Abstract
The cytoplasmic male sterile line system comprising CRMS 32A and its maintainer line CRMS 32B is a popular choice for the development of new hybrids in India as CRMS 32A, having Kalinga 1 cytoplasm (other than WA), is a viable alternative to WA cytoplasm. However, both lines are susceptible to bacterial blight (BB), a major disease on rice. As enhancement of host plant resistance is the most effective and economical strategy to control this disease, four resistance genes (Xa4, xa5, xa13, and Xa21) were transferred from a BB pyramid line of IR64, into the A and B lines using a marker-assisted backcrossing (MAB) breeding strategy. During the transfer of genes into CRMS 32B, foreground selection was applied using markers associated with the genes, and plants having resistance alleles of the donor, are selected. Selection for morphological and quality traits was practiced to select plants similar to the recurrent parent. The four gene and three gene pyramid lines exhibited high levels of resistance against the BB pathogen when challenged with eight virulent isolates. Using genome wide based SSR markers for background selection, pyramids having >95% of the recurrent parent genome were identified. With CRMS 32B gene pyramid as donor, the four resistance genes were transferred into the A line through repeated backcrosses and the A line pyramids also exhibited high level of resistance against BB. Through a combination of selection at phenotypic and molecular levels, four BB resistance genes were successfully introduced into two parental lines (CRMS 32 B and A) of Rajalaxmi, an elite popular hybrid. The pyramided B lines did exhibit high levels of resistance against BB. Selection for morphological and quality traits and background selection hastened the recovery of the recurrent parent genome in the recombinants. Through repeated backcrosses, all the four resistance genes were transferred to CRMS 32A and test crosses suggest that the maintenance ability of the improved CRMS 32B lines is intact. These improved maintainer and CMS lines can directly be used in hybrid rice breeding and the new hybrids can play an important role in sustainable rice production in India.
Highlights
Rice is the most important cereal crop in India in terms of area, production and consumption and occupies a prominent place in Indian agriculture
This study reports the successful improvement of resistance to Bacterial blight (BB) in CRMS 32B and CRMS 32A lines by pyramiding Xa4, xa5, xa13, and Xa21, four resistance genes, in these lines, using marker-assisted selection (MAS)
In the bioassays conducted against eight isolates of Xanthomonas oryzae pv. oryzae (Xoo), the donor (CRMAS 2231–2237) showed a high level of resistance (5 cm)
Summary
Rice is the most important cereal crop in India in terms of area, production and consumption and occupies a prominent place in Indian agriculture. Like normal inbred rice varieties, hybrid rice is affected by several biotic stresses (Khush and Jena, 2009). Oryzae (Xoo), is a devastating disease in the rice growing countries of Asia and causes significant grain yield losses, especially in hybrid rice, worldwide (Chen et al, 2000), because most of the parental lines are susceptible to virulent and prevalent races of Xoo under field conditions (Perez et al, 2008). Infection at maximum tillering stage results in blighting of leaves, which eventually causes significant yield losses in severely infected fields ranging from 20 to 30%, but this can reach as high as 80% (Mew et al, 1993; Srinivasan and Gnanamanickam, 2005; Noh et al, 2007). Due to the ineffectiveness of chemicals and antibiotics in controlling the disease, genetic enhancement of host plant resistance through incorporation of major resistance (R) genes is a viable practical option against BB (Khush et al, 1989) as it is the most efficient, economical, and an environment friendly approach
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