Grain aroma is the most attractive characteristic of high quality rice increasingly demanded not only by the Asian market but also widely recognized in Europe and all over the world. Cooked rice fragrance is composed of more than one hundred volatile compounds such as hydrocarbons, alcohols, aldehydes, ketones, acids, esters, phenols, pyridines, pyrazines, and other compounds (Yajima et al., 1978; Maga, 1984; Takashi et al., 1980; Paule and Power, 1989). The “popcorn-like” aromatic compound, 2-acetyl-1-pyrroline (2AP), was discovered as the major potent flavor component of all aromatic rice, crust of bread wheat and rye bread (Buttery et al., 1982, 1983). 2-acetyl-1-pyrroline is chiefly responsible for the characteristic fragrance of many aromatic rice varieties (Tanchotikul and Hsieh, 1991). Surprisingly, this rice fragrance has also been isolated and identified from pandan leaves (Buttery et al., 1983), bread flowers (Vallaris Glabra Ktze.) (Wongpornchai et al., 2003), wet millet (Seitz et al., 1993), popcorn (Schieberle, 1991), Bacillus ceres (Romanczyk et al., 1995), fungi (Nagsuk et al., 2004), and aromatic vegetable soybean (Glycine max) (Fushimi and Masuda, 2001). 2-acetyl-1-pyrroline is present in all parts of the aromatic rice plant (stems, leaves, grains) except roots (Lorieux et al., 1996). While this fragrance is present in aromatic grains, it is not present in all grains.
The aromatic compound 2-acetyl-1-pyrroline has a pyrroline ring similar to the amino acid proline (FIG. 1). The first evidence linking the amino acid proline as the precursor synthesizing 2-acetyl-1-pyrroline was found in experiments in cell and callus culture (Suprasanna et al., 1998; Suprasanna et al., 2002). That conclusion was supported by experiments using isotopic labeling showing that the precursor of the grain 2-acetyl-1-pyrroline is most likely the amino acid proline in That Hom Mali (THM) Rice (Yoshihashi et al., 2002) and probably in other aromatic rice. However, the exact biosynthetic pathway of 2-acetyl-1-pyrroline is yet to be elucidated. Thus, there is a need to identify genes involved in 2-acetyl-1-pyrroline synthesis and provide a method to increase 2-acetyl-1-pyrroline levels in plants and fungi to increase aroma.