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Sugar modulation of α-amylase genes have been described [8, 9]. Indeed, Amy3D and Amy3E are sugarrepressed in rice embryos [8], and sugar repression of Amy3D transcription in anoxic rice aleurones has also been observed [10]. α-Amylase genes are expressed in two different grain tissues, namely the scutellar epithelium and the aleurone layer [11, 12]. The expression pattern of α-amylase genes in these two tissues have been described for Amy1A and Amy3D, showing that both genes are initially expressed in the epithelium and, at a latter stage, in the aleurone [11]. The expression of the GAinduced Amy1A gene largely predominates over that of the Amy3D gene, and overall expression of Amy1A in the aleurone is likely responsible for producing the majority of total α-amylase mRNA [11, 13]. The GA-deficient mutant (Tan-ginbozu), in which one of the steps in the GA-biosynthetic pathway is genetically blocked, allows the evaluation of the importance of GA-induced α-amylases on rice grain germination. The level of the major GA produced during rice grain germination (GA 1) is strongly reduced in the shoots of 10-day-old mutant seedlings [14]. Although no data is available about the GA content in Tan-ginbozu grains during the first days of germination, Mitsunaga and Yamaguchi [15] have shown that α-amylase production is drastically reduced in Tan-ginbozu grains germinating in the absence of exogenous GA, while wild-type rice grains do not require exogenous GA for α-amylase production [16], thus indirectly indicating the absence of enough GA to trigger α-amylase induction in this mutant. The Tan-ginbozu and other GAa-Amylas

Year: 2003
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