Regulation of Wx gene expression to low amylose in wheat endosperm

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Amylose synthesis in wheat endosperm is mainly controlled by the granule-bound starch synthase of about 60kDa or so-called waxy (Wx) protein. The Wx proteins are the product of the Wx genes at a triplicate set of single-copy homoeoloci located on chromosomes 7A (Wx-Al), 4A (Wx-Bl) and 7D (Wx-Dl). Using 'Chinese Spring' and its aneuploid lines including nullisomic-tetrasomics, tetrasomics, ditelosomics and deletion stocks, and single-chromosome substitution lines for these chromosomes, the effects of varying the dosage of whole chromosomes and chromosome arms, as well as the effects of null alleles, upon amylose synthesis were investigated. Nullisomic 4A and deletion of chromosome segments carrying the Wx-Bl gene reduced the amylose content by more than 3%. A reasonable agreement was found in the substitution lines. This confirms that the absence of the Wx-Bl gene or substitution of this gene by its null allele has the most striking consequence on decreasing amylose synthesis. The removal of chromosomes carrying either the Wx-Al or Wx-Dl gene reduces amylose content by less than 2%. A similar reduction was revealed by substitution of these two genes by the null alleles. Double dosages of chromosomes 7A,4A and 7D did not increase amylose content, while the tetrasomic chromosomes produced more of the respective Wx proteins. This suggests that a certain level of the Wx gene activity or the Wx proteins led to a maximum amount of amylose.