Rice Genetics IV - IRRI books - International Rice Research Institute

AromaAromatic rice varieties often receive a premium in many markets. The most prominentexamples are basmati varieties grown in the Punjab region of Pakistan and Indiaand the jasmine rice grown in Thailand. The first aromatic variety in the U.S. wasDelitus, selected in 1911 in Louisiana from the Bertone variety introduced in 1904from France (Jones et al 1941). The aroma was transferred to variety Della (Jodonand Sonnier 1973), which was grown on a modest area for some 20 years. Aroma inDella was controlled by a single recessive gene (Berner and Hoff 1986) that wassubsequently mapped to chromosome 8 by Ahn et al (1992). Pinson (1994) investigatedseveral aromatic varieties, finding that Jasmine 85 and PI 457917 (a semidwarfmutant from Basmati 370) both possess a single recessive gene for aroma and bothare allelic with each other and with the aroma genes in A-301 and Della-X 2 (a shortstaturemutant of Della). Two additional varieties, Amber and Dragon Eyeball 100,both possess two genes for aroma, one of which is allelic to the aroma gene in theother four varieties (Pinson 1994). Several U.S. breeding programs have releasedaromatic varieties in recent years, but various ethnic groups in the U.S. prefer importedjasmine and basmati rice. More than 12% of rice eaten in the U.S. is importedrice (Childs 1999), mostly jasmine and basmati.Pest resistanceThe breeding of disease and insect resistance into rice varieties has been one of themost successful examples of the use of major genes in crop species. These genes havealso become major targets for gene tagging through molecular markers, marker-assistedselection and pyramiding, and positional cloning programs. The developmentof disease- and insect-resistant rice has been reviewed elsewhere (Khush 1984, Khushand Virmani 1985). A brief summary of the more important resistances is providedhere.DiseasesBlast. Rice blast (caused by Magnaporthe grisea) is the most widespread rice diseaseand has been a focus of breeding efforts for decades. The elegant work of Kiyosawa(1981) established the basis for genetic studies of blast resistance genes in japonicarice using Japanese blast strains. Work in the tropics was hampered by the diversity ofthe pathogen races, the apparently high number of resistance genes in indica rice, andinoculation difficulties (Bonman and Mackill 1988). Development of near-isogeniclines improved the situation (Mackill and Bonman 1992). Despite the identificationof many major genes for blast resistance, use of this information has only been commonin temperate areas such as the United States. Katy, for example, which was initiallyreleased in 1989 (Moldenhauer et al 1990), was the only variety with resistanceto all common blast races in the southern U.S. It was initially believed that there wasa single gene, Pi-ta 2 , or a tightly linked group of genes from Tetep controlling thisresistance (Moldenhauer et al 1992). Further research has shown that the resistance inKaty is due to a tightly linked cluster of at least seven genes (Chao et al 1999). In32 Rutger and Mackill