BackgroundThe Protein Disulfide Isomerase (PDI) gene family encodes several PDI and PDI-like proteins containing thioredoxin domains and controlling diversified metabolic functions, including disulfide bond formation and isomerisation during protein folding. Genomic, cDNA and promoter sequences of the three homoeologous wheat genes encoding the "typical" PDI had been cloned and characterized in a previous work. The purpose of present research was the cloning and characterization of the complete set of genes encoding PDI and PDI like proteins in bread wheat (Triticum aestivum cv Chinese Spring) and the comparison of their sequence, structure and expression with homologous genes from other plant species.ResultsEight new non-homoeologous wheat genes were cloned and characterized. The nine PDI and PDI-like sequences of wheat were located in chromosome regions syntenic to those in rice and assigned to eight plant phylogenetic groups. The nine wheat genes differed in their sequences, genomic organization as well as in the domain composition and architecture of their deduced proteins; conversely each of them showed high structural conservation with genes from other plant species in the same phylogenetic group. The extensive quantitative RT-PCR analysis of the nine genes in a set of 23 wheat samples, including tissues and developmental stages, showed their constitutive, even though highly variable expression.ConclusionsThe nine wheat genes showed high diversity, while the members of each phylogenetic group were highly conserved even between taxonomically distant plant species like the moss Physcomitrella patens. Although constitutively expressed the nine wheat genes were characterized by different expression profiles reflecting their different genomic organization, protein domain architecture and probably promoter sequences; the high conservation among species indicated the ancient origin and diversification of the still evolving gene family. The comprehensive structural and expression characterization of the complete set of PDI and PDI-like wheat genes represents a basis for the functional characterization of this gene family in the hexaploid context of bread wheat.

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Protein disulphide isomerase (PDI) catalyses the formation, reduction and isomerization of disulphide bonds in the newly synthesized secretory proteins. Plant PDIs have been shown to be involved in the folding and deposition of seed storage proteins, which makes this enzyme particularly interesting in wheat, as flour quality is strongly affected by composition and structure of seed storage proteins. In hexaploid wheat cultivar (AABBDD) Chinese Spring (CS), the genomic, complementary DNA and promoter sequences of the three homoeologous gene encoding PDI had been isolated and characterized in a previous study revealing high levels of sequence conservation. In this study, we report the isolation and sequencing of a , 700 bp region, comprising , 600 bp of the putative promoter region and 88 bp of the first exon of the typical PDI gene, in five accessions each from Triticum urartu (AA), Aegilops speltoides (BB) and Aegilops tauschii (DD). Sequence analysis indicated large variation among sequences belonging to the different genomes, while close similarity was found within each species and with the corresponding homoeologous PDI sequences of Triticum aestivum cv. CS (AABBDD) resulting in an overall high conservation of the regulatory motifs conferring endosperm-specific expression.

The deduced amino-acid sequences of 17 protein disulphide isomerase (PDI) and PDI-like cDNAs of wheat assigned to nine homoeologous groups were searched for conserved motives by comparison with characterized sequences in different protein databases. The wheat protein sequences encoded by genes of different homoelogous groups showed a high level of structural similarity with the corresponding protein sequences of other species clustering into the same phylogenetic group. The proteins of five groups (I-V) share two thioredoxin-like active domains and show structural similarities with the corresponding proteins of higher eukaryotes, whereas those of the remaining three groups (VI-VIII) contain a single thioredoxin-like active domain. The expression analysis of the nine non-homoeologous wheat genes, which was carried out by quantitative RT-PCR in developing caryopses and in seedlings subjected to temperature stresses, showed their constitutive although highly variable transcription rate. The comprehensive structural and transcriptional characterization of the PDI and PDI-like genes of wheat performed in this study represents a basis for future functional characterization of the PDI gene family in the hexaploid context of bread wheat.