Starch Modification: A Model for Wheat MAS Breeding
Abstract In the past two decades, three types of starch mutants, waxy (Wx), high amylose (HA) and sweet type wheat (SW) have been developed in Japan. Each of these lines was obtained by identifying “partial“ mutants with mutations in one or two homeoelogous genes derived from the A, B or D-genomes, followed by crossing to produce fully null mutants. The Wx line lacks the three granule-bound starch synthase I (GBSSI) proteins responsible for amylose synthesis, and HA lacks the three starch synthase IIa (SSIIa) enzymes that are involved in amylopectin synthesis. SW lacks all active GBSSI and SSIIa enzymes. “Partial“ null mutants have also been used to obtain more subtle modifications in starch quality. For example, the GBSSI-B1 single null lines produce starch which is slightly reduced in amylose. This suggested that the identification of other desirable lines using MAS would allow us to further fine-tune starch characteristics. Sixty-four homozygous lines differing in GBSSI and SSIIa composition can be selected from progeny of crosses between HA and Wx. Co-dominant markers for all GBSSI and SSIIa genes enabled us to select all 64 lines quickly and effectively. The modulation of starch characteristics serves as a model that demonstrates the utility of mutation identification in combination with MAS in hexaploid wheat. The availability of genome sequence information, combined with new methods of mutation detection, reduces the amount of work involved in finding single mutations in wheat, and one can easily see the potential for expanding this methodology to other traits.
Starch is one of major components of cereal seed, and starch composition greatly influences the quality of cereal-based food products. Starch is composed of amylose, a linear chain of glucosyl units joined by α-1,4 linkages, and amylopectin,
which is a highly branched molecule with α-1,6 linked side chains. The key enzyme
in amylose synthesis is granule bound starch synthase (GBSSI, alternatively called
waxy protein), and lines lacking GBSSI have amylose-free starch. In contrast, many enzymes are involved in amylopectin synthesis, including several types of starch synthases, as well as branching and debranching enzymes. Starch synthase IIa (SSIIa), which elongates the outside branches of the amylopectin molecules, appears to play a particularly important role in amylopectin synthesis.
Null mutations of the genes encoding the enzymes involved in starch synthesis can result in changes in starch structure and properties, which may affect the processing and cooking properties of starch. In diploid cereals such as maize and rice, many starch mutants have been identified and used extensively in both basic and applied research (Boyer and Hannah 2001; Vandeputte and Delcour 2004). However, although starch mutants of common wheat could potentially provide interesting new materials for the food industry, none of the typical starch mutants identified in maize and rice have been available until recently. In the past two decades, three types of starch mutants were developed in Japan; waxy (Wx, 1995), high amylose (HA, 1999) and sweet type wheat (SW, 2006).