Molecular Characterization and Expression Profiling of the Protein Disulfide Isomerase Gene Family in Brachypodium distachyon L

Protein disulfide isomerases (PDI) are involved in catalyzing protein disulfide bonding and isomerization in the endoplasmic reticulum and functions as a chaperone to inhibit the aggregation of misfolded proteins. Brachypodium distachyon is a widely used model plant for temperate grass species such as wheat and barley. In this work, we report the first molecular characterization, phylogenies, and expression profiles of PDI and PDI-like (PDIL) genes in B. distachyon in different tissues under various abiotic stresses. Eleven PDI and PDIL genes in the B. distachyon genome by in silico identification were evenly distributed across all five chromosomes. The plant PDI family has three conserved motifs that are involved in catalyzing protein disulfide bonding and isomerization, but a different exon/intron structural organization showed a high degree of structural differentiation. Two pairs of genes (BdPDIL4-1 and BdPDIL4-2; BdPDIL7-1 and BdPDIL7-2) contained segmental duplications, indicating each pair originated from one progenitor. Promoter analysis showed that Brachypodium PDI family members contained important cis-acting regulatory elements involved in seed storage protein synthesis and diverse stress response. All Brachypodium PDI genes investigated were ubiquitously expressed in different organs, but differentiation in expression levels among different genes and organs was clear. BdPDIL1-1 and BdPDIL5-1 were expressed abundantly in developing grains, suggesting that they have important roles in synthesis and accumulation of seed storage proteins. Diverse treatments (drought, salt, ABA, and H2O2) induced up- and down-regulated expression of Brachypodium PDI genes in seedling leaves. Interestingly, BdPDIL1-1 displayed significantly up-regulated expression following all abiotic stress treatments, indicating that it could be involved in multiple stress responses. Our results provide new insights into the structural and functional characteristics of the plant PDI gene family.