Expression and evolutionary analysis of soluble inorganic pyrophosphatase gene family in pear and four other Rosaceae species

In plants, soluble inorganic pyrophosphatase (PPA) play an indispensable role in inorganic pyrophosphate hydrolysis. However, the expression and the evolutionary history of PPA gene family remain unclear in pear and Rosaceae species. In this study, thirty-nine PPA genes were identified from five Rosaceae species including pear (Pyrus bretschneideri), apple (Malus domestica), peach (Prunus persica), strawberry (Fragaria vesca) and Japanese apricot/mei (Prunus mume). Based on the structural characteristics of the genes and phylogenetic analysis, members of PPA gene family were classified into two main subfamilies (A and B). Whole-genome duplication (WGD) and dispersed gene duplication were the main force, which drove the expansion of PPA gene family in pear and other Rosaceae species. Purifying selection played a dominant role in the evolution of PPA genes. Subcellular localization analysis demonstrated that pear PPA genes located in the cytosol, chloroplasts or near the nuclear envelope. PPA genes showed multiple expression patterns in different pear tissues. Several members of PPA gene family presented particularly higher expression level in only one of the six tissues, such as PbrPPA2, PbrPPA5 and PbrPPA8. Expression analysis showed that five PPA genes were involved in the growth of pear pollen tube, meanwhile four PPA genes could respond to excess of PPi in pear pollen tube. Overall, we established a comprehensive landscape in the functional characterization and evolution of PPA gene family in pear and Rosaceae species, and provided gene expression data for exploring the functional roles of PPA genes in the growth of pear pollen tube.