Two distinct waxy alleles impact the granule-bound starch synthase in sorghum

Abstract

The granule-bound starch synthase (GBSS) is the enzyme responsible for amylose synthesis in starch granules. Loss of GBSS activity results in starch granules containing mostly amylopectin and little or no amylose, a phenotype described as waxy. Previously, two phenotypic classes of waxy alleles were identified in sorghum (Sorghum bicolor L. Moench) characterized by the absence (waxy a ; wx a ) or presence (waxy b ; wx b ) of the GBSS protein in the endosperm. To characterize these alleles, we examined endosperm architecture using scanning electron microscopy (SEM), assayed GBSS enzymatic activities, and identified DNA lesions associated with the mutations in the GBSS (Sb10g002140) gene. wx a , the allele present in B Tx630 and R Tx2907, contained a large insertion in the third exon, which was consistent with the absence of the GBSS protein previously observed. wx b , the allele present in B 9307 and B TxARG1, contained a missense mutation that resulted in conversion of glutamine 268 to histidine in a conserved domain in starch synthases. In wx b , GBSS activity was less than 25% that of the non-waxy line B Wheatland, and GBSS activity was not detected in wx a . SEM showed that endosperm architecture was very similar in both wx a and wx b alleles, but altered in comparison to non-waxy lines R Tx430 and B Wheatland. Both alleles may have a range of potential applications in grain sorghum because of low amylose content in their starch and the presence or absence of the GBSS protein. PCR based markers were developed for both the wx a and the wx b alleles to aid in molecular breeding of low amylose sorghum.

Molecular characterization and comparative analysis of two waxy alleles in barley

“…Similar aa changes were found in broomcorn millet (Hunt et al, 2010), sorghum (Hamblin et al, 2007;Mclntyre et al, 2008;Sattler et al, 2009), wheat (Yanagisawa et al, 2001;Vanzetti et al, 2010), amaranth grains (Park et al, 2009b), which exhibited the loss of protein activity or no protein. Additionally, a single base mutation in intron 1 in rice was thought to have caused an altered splice site, leading to a reduced level of GBSS I mRNA and protein (Cai et al, 1998;Hirano et al, 1998;Isshiki et al, 1998;Han et al, 2004;Itoch et al, 2003;Yamanaka et al, 2004).…”

“…Similar insertions were also characterized in wheat (Saito et al, 2004;Monari et al, 2005) and sorghum (Sattler et al, 2009), and these insertions can cause changes in GBSS I activity and amylose content in various degrees. However, the 193 bp insertion in barley seems to have no effects on the expression of GBSS I and amylose content because of its existence in both waxy and non-waxy barleys.…”

Increasing the utilisation of sorghum, millets and pseudocereals: Developments in the science of their phenolic phytochemicals, biofortification and protein functionality

“…Sorghum endosperm expresses up to three recessive waxy genes (wxwxwx), with heterowaxy mutants expressing one or two recessive genes (WxWxwx or Wxwxwx) and having intermediate levels of amylose (Sang et al, 2008). The high amylopectin trait is dependent on the absence of activity of the enzyme responsible for amylose synthesis, granule-bound starch synthase (GBSS) (Sattler et al, 2009). In sorghum, there are two classes of waxy mutants, classified according to the actual absence (waxya) or presence (waxyb) of the GBSS protein (Sattler et al, 2009).…”

“…The high amylopectin trait is dependent on the absence of activity of the enzyme responsible for amylose synthesis, granule-bound starch synthase (GBSS) (Sattler et al, 2009). In sorghum, there are two classes of waxy mutants, classified according to the actual absence (waxya) or presence (waxyb) of the GBSS protein (Sattler et al, 2009). DNA lesions were found to be associated with the former and missense mutation, leading to an amino acid substitution, with the latter (Sattler et al, 2009).…”

“…In sorghum, there are two classes of waxy mutants, classified according to the actual absence (waxya) or presence (waxyb) of the GBSS protein (Sattler et al, 2009). DNA lesions were found to be associated with the former and missense mutation, leading to an amino acid substitution, with the latter (Sattler et al, 2009). Interestingly, it has been found that overall the waxy trait does not result in a yield penalty in sorghum hybrids (Rooney et al, 2005).…”