Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2012
Publication Date: 4/17/2013
Publication URL: http://handle.nal.usda.gov/10113/58828
Citation: Giroux, M., Kim, K., Hogg, A., Martin, J., Beecher, B.S. 2013. The puroindoline b-2 variants are expressed at low levels relative to the puroindoline D1 genes in wheat seeds. Crop Science. 53:833-841. Interpretive Summary: Wheat grain varies texture, or kernel hardness. Kernel texture affects the properties of the resulting flour and is therefore very important for end-use quality. This is the reason why wheat is divided into ‘soft’ and ‘hard’ market classes. Wheat grain texture is controlled by two genes, called puroindoline-a and puroindoline-b. The products of these two genes act together to make the kernel soft. Hard wheat lacks a functional copy of either puroindoline-a or puroindoline-b. These genes are of considerable interest in wheat quality research due to their enormous impact on wheat quality and thus marketability. Recently a new set of genes very similar to puroindoline-b has been discovered. The amount of product produced from these pinb-2 genes was determined and compared to that produced by the puroindoline-a and b genes. Pinb-2 levels ranged from undectable to 7.6% of puroindoline-b levels. Given these relatively low levels of each of the Pinb-2 genes, it is unlikely that they substantially contribute to grain hardness variation.
Technical Abstract: Grain hardness in wheat is largely controlled by the Hardness locus. This locus contains the Puroindoline a and b genes, which were thought to exist as single copy genes on chromosome 5D. In fact, four additional copies of Pinb have been reported, termed Pinb-2v-1 – Pinb-2v4, which map to the group 7 chromosomes. Since PINA and PINB cooperatively bind to starch granules and PINB limits grain softness, we set out to determine Pinb-2 gene variant expression levels relative to Pinb. The tested genotypes included the soft wheat variety Alpowa, Chinese Spring and two Chinese Spring group 7 nulli-tetra lines. Deletion of chromosome 7A (and Pinb-2v4) or 7D (and Pinb-2v1) did not impact either the level of total or starch bound puroindolines indicating that the expression level of these Pinb-2 genes is not sufficient to modify grain hardness. To accurately measure transcript levels of Pina, Pinb, and each of the four Pinb-2 variants RNA-seq was utilized using RNA extracted from developing seeds of each genotype. Pinb transcript levels ranged from 1.24 fold that of Pina in Alpowa to 1.60 fold higher in Chinese Spring. Relative to Pinb, Pinb-2v1 expression levels averaged 1 % while Pinb-2v2 levels ranged up to 7.6 %. Pinb-2v3 transcripts were detectable only in Alpowa at 1.0 % of Pinb-D1a while Pinb-2v4 transcripts were undetectable in all genotypes. Given the relative low expression levels of each of the Pinb-2 genes, it is unlikely that they substantially contribute to grain hardness variation.