Abstract
Background Phytosulfokine (PSK) functions as a plant peptide growth factor that plays important and diverse roles in plant development and stress responses. Nevertheless, PSKs have not been systematically analyzed in wheat.
Results A genome-wide comparative analysis of PSK genes in wheat was conducted and 15 TaPSKs were identifed and divided into four subgroups in the wheat genome based on sequence similarity. The examination of motif compositions of TaPSK genes revealed the presence of the YIYTQ signature motif in the C-terminus of all TaPSK polypeptide precursors, with a highly conserved feature across diferent species. Exogenous application of TaPSK peptide promoted root growth in wheat. Quantitative RT-PCR analysis revealed that the TaPSKs exhibited preferential or tissue specifc expression patterns in wheat. In particular, three homologs of the TaPSK1 genes were specifcally expressed in grains, with the strongest expression observed in the developing grains at 15 days after anthesis. Compared with wild type, transgenic rice lines overexpressing TaPSK1-A exhibited larger grain size and higher thousand-grain weight. The promoter region and genomic sequence of the wheat TaPSK1-A gene were cloned. Sequence polymorphism showed that fve single-nucleotide polymorphisms (SNPs) were identifed in the promoter region of TaPSK1-A. A Kompetitive Allele-Specifc PCR (KASP) marker was developed for TaPSK1-A based on –806 bp SNP (C/T transition), and two haplotypes, TaPSK1-A-HapI and TaPSK1-A-HapII were detected in 260 wheat accessions collected from diferent regions. The expression of TaPSK1-A, promoter activity, and thousand-grain weight (TGW) in the TaPSK1-A-HapII haplotype were higher than those in the TaPSK1-A-HapI haplotype. Furthermore, yeast one-hybrid assays revealed  the binding of TaNF-YB1 and TaERF39 to the promoter regions of the TaPSK1-A gene, and TaMADS29 could bind to the promoters of TaPSK1-B and TaPSK1-D genes.
Conclusions Comparative genome-wide analysis of TaPSK peptide family revealed that the TaPSK1 gene is involved  in wheat grain development, and the developed TaPSK1-A-KASP marker could be utilized for marker-assisted selection breeding of wheat.