Molecular mapping of aluminum: Tolerance in rice

Rice is the world's single most important food crop and is a primary food source for about half of the world's population. More than 90%of the world's rice is grown and consumed in Asia, where 60% of the earth's population lives. Rice accounts for 35 to 60% of the calories consumed by 3.5 billion Asians. The demand for rice is increasing tremendously as populations in rice-growing countries continue to increase at an alarming rate. There is a remarkably genetic diversity in cultivated rice, but a series of biotic and abiotic stresses continues to limit the rice productivity. Thus, there is an urgent need to identify and exploit sources of tolerance genes to various stresses, and subsequently broaden the rice gene pool. Wild species of Oryza are an important reservoir of useful genes and can be exploited both to broaden the existing narrow genetic base and enrich the existing varieties with desired agronomic traits. Aluminum (Al)toxicity is the most important factor limiting crop production in acid soils, which account for 40% of the arable land of the world. Oryza rufipogon is a progenitor of cultivated rice and possesses many valuable genes for biotic and abiotic stress tolerance. The narrow genetic base of the breeding materials used, the complex nature of the trait involved, and the lack of an easy and efficient screening method for Al toxicity in the field are all causes of the slow progress in rice breeding for Al tolerance. With the advent of molecular techniques, it is now possible to identify genotypes that have a high level of Al tolerance at the early growth stage without the masking effect of the environment.

The present study was carried out to identify and map quantitative trait loci (QTLs) controlling Al tolerance in rice using molecular techniques. A population of 171 F6 recombinant inbred lines (RILs), O. sativa (IR64), the Al susceptible parent, and O. rufipogon, the Al tolerance parent, were evaluated for Al tolerance using nutrient solution with and without 40 ppm of active Al^+3. The solutions were renewed daily to maintain the solution pH at 4.0. The experiment was designed in a randomized complete block design with four replications. After fourteen days of exposure in the solutions, with and without Al, the root length and root dry weight were measured. All the traits showed significant difference between parents and among progenies. Most of the traits did not fit normal distribution, but showed transgressive segregation.