Late-maturity α-amylase expression in wheat is influenced by genotype, temperature and stage of grain development.

MAIN CONCLUSION: Late-maturity α-amylase (LMA) expression in wheat grains can be induced by either a cool temperature shock close to physiological maturity or continuous cool maximum temperatures during grain development. Late-maturity α-amylase (LMA) is a genetic trait in wheat (Triticum aestivum L.) involving the production of α-amylase during grain development, which can result in an unacceptably low Falling Number (FN) in mature grain and consequent grain downgrading. Comparison of the FN test, an α-amylase activity assay and a high pI α-amylase-specific ELISA on the same meal samples gave equivalent results; ELISA was used for further experiments because of its isoform specificity. A cool temperature shock during the middle stages of grain development is known to induce LMA and is used for phenotypic screening. It was determined that a cool temperature treatment of seven days was required to reliably induce LMA. Glasshouse studies performed in summer and winter demonstrated that temperature affected the timing of sensitivity to cool-shock by altering the rate and duration of grain development, but that the sensitive grain developmental stage was unchanged at 35-45% moisture content. Wheat varieties with Rht-B1b or Rht-D1b dwarfing genes responded to a cool-shock only from mid grain filling until physiological maturity, whilst genotypes with Rht8c or without a dwarfing gene expressed LMA in response to a cool-shock during a wider developmental range. A continuous cool maximum temperature regimen (23 °C/15 °C day/night) during grain development also resulted in LMA expression and showed a stronger association with field expression than the cool-shock treatment. These results clarify how genotype, temperature and grain developmental stage determine LMA expression, and allow for the improvement of LMA phenotypic screening methods.

Identification of differentially senescing mutants of wheat and impacts on yield, biomass and nitrogen partitioning.

Increasing photosynthetic capacity by extending canopy longevity during grain filling using slow senescing stay-green genotypes is a possible means to improve yield in wheat. Ethyl methanesulfonate (EMS) mutated wheat lines (Triticum aestivum L. cv. Paragon) were screened for fast and slow canopy senescence to investigate the impact on yield and nitrogen partitioning. Stay-green and fast-senescing lines with similar anthesis dates were characterised in detail. Delayed senescence was only apparent at higher nitrogen supply with low nitrogen supply enhancing the rate of senescence in all lines. In the stay-green line 3 (SG3), on a whole plant basis, tiller and seed number increased whilst thousand grain weight (TGW) decreased; although a greater N uptake was observed in the main tiller, yield was not affected. In fast-senescing line 2 (FS2), yield decreased, principally as a result of decreased TGW. Analysis of N-partitioning in the main stem indicated that although the slow-senescing line had lower biomass and consequently less nitrogen in all plant parts, the proportion of biomass and nitrogen in the flag leaf was greater at anthesis compared to the other lines; this contributed to the grain N and yield of the slow-senescing line at maturity in both the main tiller and in the whole plant. A field trial confirmed senescence patterns of the two lines, and the negative impact on yield for FS2 and a positive impact for SG3 at low N only. The lack of increased yield in the slow-senescing line was likely due to decreased biomass and additionally a possible sink limitation.

Overgrowth (Della) mutants of wheat: development, growth and yield of intragenic suppressors of the Rht-B1c dwarfing gene.

A suppressor screen using the dwarf Rht-B1c Della mutant of wheat (Triticum aestivum L.) led to the isolation of overgrowth mutants, which retained the original dwarfing gene but grew at a faster rate because of a new mutation elsewhere in that gene. Forty-six alleles were identified, which included amino acid substitutions, premature stop codons, and splice site alterations. The sites of amino acid substitution were primarily localised around conserved motifs in the DELLA protein, and these mutants showed a wide range in their extent of growth recovery (dwarf, semidwarf, tall). Detailed growth comparisons were made on a wide height range of backcrossed overgrowth alleles, comparing stem and spike growth, leaf size, tillering, phenological development, coleoptile length, grain dormancy and grain yield. There were large and reproducible differences between alleles for some traits, whereas others were largely unaffected or varied with growth conditions. Some of the overgrowth alleles offer promise as alternatives to the Rht-B1b and Rht-D1b dwarfing genes, allowing a wider range of height control, improved grain dormancy and equivalent grain yield. The collection of mutants will also be valuable as a resource to study the effect of height on different physiological or agronomic traits, and in elucidating DELLA protein function.