Nevertheless, GB level accumulated in plants could
be altered by any limiting factor in the complex
biosynthetic pathway. Either BADH or CMO protein
activity is crucial for the ultimate GB-synthesizing
ability. Indeed, rice is the only cereal crop that does not
accumulate GB (Rathinasabapathi et al., 1993). Although
the rice genome contains both CMO and
BADH homologs required for GB synthesis, their activities
of coding proteins have not yet been characterized
experimentally. In addition to the extensively
defective RNA processing of rice BADH homologs
found in this study, recent transgenic experiments
suggest a functional defect in both CMO and BADH
homologs in rice. Kishitani et al. (2000) reported the
transgenic rice plants constitutively expressing a precise
barley BADH1 converted high levels of exogenously
applied betaine aldehyde to GB more efficiently than
did wild-type plants. The lower conversion efficiency
in the wild-type plants probably results from the
limitation of precise native BADH proteins found in
this study. More recently, Shirasawa et al. (2006) demonstrated
transgenic rice plants harboring a single