A Major Locus on Wheat Chromosome 7B Associated With Late-Maturity α-Amylase Encodes a Putative ent-Copalyl Diphosphate Synthase

Abstract

Many wheat varieties have the potential to develop unacceptably high levels of α-amylase in the grains if exposed to a cool temperature shock or simply cool temperature during the early to middle stages of grain filling. This phenomenon is referred to as late maturity α-amylase (LMA). The enzyme persists in the grain until harvest and may result in wheat with a low Falling Number that does not meet receival and export specifications. Resistance to LMA is therefore a valuable target for wheat breeders and wheat industries in general. Genetic evidence implicating a locus on the long arm of chromosome 7B in variation in LMA phenotype was confirmed in this investigation. Through intensive fine-mapping an ent-copalyl diphosphate synthase (CPS), hitherto named LMA-1, was identified as the likely candidate gene associated with variation in LMA phenotype. Single Nucleotide Polymorphisms (SNPs) within the LMA-1 coding sequence of Chinese Spring, Maringa and Halberd result in either prematurely terminated or functionally altered proteins that are associated with useful levels of resistance to LMA. LMA-1 transcripts detected in de-embryonated grain tissue from around 15 days after anthesis, several days before the synthesis of α-amylase, were low in the resistant varieties Chinese Spring and Maringa compared with LMA susceptible genotype Spica. This was associated with a dramatic reduction in the concentrations of intermediates in the gibberellin biosynthesis pathway such as GA19, evidence that LMA-1 was functioning as CPS in the gibberellin biosynthesis pathway. A survey of a large collection of Australian and international wheat varieties distinguished 9 major haplotypes at the LMA-1 locus. Generally, within classes, there was notable variation for LMA phenotype and evidence for genotypes whose resistance is presumed to be due to genetic loci located elsewhere on the wheat genome. Further investigation is required to characterize the sequence of steps between LMA-1 and α-amylase synthesis as well as to gain a better understanding of the role and potential impact of other genetic loci. Diagnostic markers for sources of resistance and SNP variation reported in this study should assist breeders to deploy resistance associated with LMA-1 variants in breeding programs.

Late-maturity α-amylase (LMA): exploring the underlying mechanisms and end-use quality effects in wheat

“…At this time, the major QTL located on the long arm of 7B explains 31-42% of LMA phenotypic variance (Mrva and Mares 2001a;Derkx et al 2021). The 7B QTL has been confirmed in nine publications and identified in germplasm from genetically diverse breeding programs (Mrva and Mares 2001a;Mrva et al 2009;Emebiri et al 2010;Mohler et al 2014;Zhang et al 2014;Martinez et al 2018;Börner et al 2018;Derkx et al 2021). Fifteen minor QTL have also been identified.…”

“…Studies have demonstrated that LMA is a multi-genic trait, with associated QTL across all three genomes that appear to be independently effective and additive in their contribution to the LMA phenotype (Mrva and Mares 2002;Mares and Mrva 2008;Derkx et al 2021). At this time, the major QTL located on the long arm of 7B explains 31-42% of LMA phenotypic variance (Mrva and Mares 2001a;Derkx et al 2021).…”

“…Studies have demonstrated that LMA is a multi-genic trait, with associated QTL across all three genomes that appear to be independently effective and additive in their contribution to the LMA phenotype (Mrva and Mares 2002;Mares and Mrva 2008;Derkx et al 2021). At this time, the major QTL located on the long arm of 7B explains 31-42% of LMA phenotypic variance (Mrva and Mares 2001a;Derkx et al 2021). The 7B QTL has been confirmed in nine publications and identified in germplasm from genetically diverse breeding programs (Mrva and Mares 2001a;Mrva et al 2009;Emebiri et al 2010;Mohler et al 2014;Zhang et al 2014;Martinez et al 2018;Börner et al 2018;Derkx et al 2021).…”

“…The 7B QTL is the first to be mapped to a single candidate gene. Designated LMA-1, the proposed gene encodes a putative ent-copalyl diphosphate synthase (CPS), which in other plant species is involved in gibberellin (GA) biosynthesis (Derkx et al 2021). When gene sequences were examined, LMA-susceptible varieties carried a fully functional protein encoding gene sequence.…”

“…In recent years, studies focused on the underlying genetic, biochemical, and physiological mechanisms that lead to LMA have increased (Kondhare et al 2012(Kondhare et al , 2013(Kondhare et al , 2014Barrero et al 2013Barrero et al , 2020Cheng et al 2014;Derkx and Mares 2020;Derkx et al 2021;Liu et al 2021). Collectively, LMA studies have shown that this condition is a consequence of genotype, environment, and grain developmental stage (Mares and Mrva 2014;Derkx and Mares 2020;Derkx et al 2021). In addition, the level of expression of pre-maturity, high pI α-amylase is variable and hard to predict.…”