The mutant su1‐ref allele is of interest in the commercial sweet maize (Zea mays L.) industry. It was the only starch synthesis mutation used for sweet maize until the 1960s and is still used today, especially in combinations with other loci. The wild‐type Su1 allele produces a starch debranching‐type isoamylase, ISA1, required for normal amylopectin and starch granule crystal formation in maize endosperm. Five naturally occurring mutant alleles have been identified at the su1 locus, but they have not been characterized for seedling emergence, field traits, mature kernel composition, or ISA1 enzyme activity. These five alleles were backcrossed to A619 and A632, two modern field maize inbreds. The su1 inbreds and wild‐type checks were grown in El Monte, Chile, in 2009 to 2010 and Madison, WI, in 2010 to 2011 for seed production to evaluate seedling emergence, field traits, and mature kernel composition. In a greenhouse in Madison, WI, in 2011 to 2012, the su1 inbreds were grown and kernels were harvested at 20 d after pollination (DAP) for enzymatic analysis. Significant differences were identified among the su1 inbreds, where su1‐ne was consistently poorer for emergence and field traits and the su1‐pu allele was consistently better than the other su1 mutant alleles for the evaluated traits. Differences were observed in the presence of ISA1 enzyme; however, no functional ISA1 enzyme activity was observed. Further research is necessary to elucidate the differences for appearance, carbohydrate composition, and seed germination and emergence traits in lines with no functional ISA1 enzyme activity.
Evaluation of endosperm carbohydrates kernels of pseudostarchy, su1‐ref, and wild type inbreds will give us an understanding of pseudostarchy composition relative to wild type and should inform us on the underlying biochemistry of starch synthesis. A set of seven maize inbreds, fixed for the su1‐ref allele, were developed from two divergently selected maize populations. Three of these inbreds (pse) have a pseudostarchy kernel phenotype and four (esu) have a typical sugary kernel appearance. These seven inbreds were grown in two environments each with two field replications with the inbreds Ia453 su1‐ref and Ia453 Su1 (wild type). Polysaccharide composition was evaluated at 14, 21, 28, 35, and 50 (mature seed) days after pollination (DAP). Amylose, amylopectin, and sugar concentrations were measured on mature seed tissue. No major carbohydrate differences were identified among the pse inbreds and the Ia453 Su1 inbred. At all stages of development, the pse inbreds Wpse2 and Wpse3 contained water‐soluble polysaccharide (WSP) concentration similar to that of Ia453 Su1. The proportion of amylopectin in total starch in pse inbreds was not different from Ia453 Su1 indicating that amylopectin is being produced at wild type levels in pse inbreds. Pseudostarchy inbreds cannot be distinguished from Su1 maize inbreds based on carbohydrate concentrations during endosperm development. These results indicate that the biochemical mechanisms producing wild type starch without a wild type Su1 allele are activated early in development and sufficient debranching takes place to allow the formation of normal crystalline starch granules.
The su1‐ref allele, the only mutant used in sweet maize (Zea mays L.) until the 1960s, continues to be used in sweet maize improvement programs. Wild‐type Su1 encodes the starch debranching enzyme (DBE) isoamylase1 (ISA1) that is required for amylopectin production. Loss of Su1 function also leads to a reduction in activity of another DBE enzyme, pullulanase. Five naturally occurring su1 mutations have been identified and characterized as having different carbohydrate compositions. The alleles have not been evaluated for isoamylase and pullulanase DBE activity or for mature kernel composition of hybrid allele combinations. The five su1 alleles were backcrossed into two field corn lines, A619 and A632. The resulting inbreds were crossed in a factorial design. The hybrids were grown in 2015 and 2016 in Madison, WI, for an evaluation of mature kernel composition. In 2016, the su1 inbreds were grown and immature tissue was harvested 21 d after pollination for enzymatic analysis. Significant differences were identified among the su1 hybrids for starch and water‐soluble polysaccharides (WSP). Hybrids containing the su1‐p allele consistently accumulated the greatest amount of starch and the lowest amount of WSP, and those with the su1‐ne allele had the lowest amount of starch and greatest WSP content. No significant differences were observed among the su1 inbreds for isoamylase or pullulanase activity. Little to no isoamylase activity was observed, whereas pullulanase activity was reduced among the su1 mutants compared with the wild type, Ia453‐Su1. Our data indicated that differences in starch and WSP observed among the su1 alleles was not due to isoamylase or pullulanase DBE activity.
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