Comparative analysis of polygalacturonase in the fruit of strawberry cultivars

The role of polygalacturonase (PG) in the development, ripening, and softening of fruit from two strawberry cultivars with different flesh firmness and softening characteristics was compared. Changes in PG activity and gene expression during development, ripening and softening were measured. The PG genes from each cultivar were cloned and analyzed, and were classified with other PG genes using phylogenetic analysis. In Toyonoka fruit, PG activity increased gradually, reaching a peak during the pink stage, and remained at this level during post-harvest softening. Changes in PG gene expression were consistent with PG activity in these softer fruits. In the firmer Sweet Charlie fruits, PG activity was detected during the initial development stage, reaching a peak at the white stage, thereafter decreasing gradually with ripening and remaining at this lower level throughout softening. Changes in PG gene expression and PG activity were not consistent in these fruit. For both Toyonoka and Sweet Charlie PG genes (FaTPG and FaSCPG, respectively), the open reading frame was 1218 bp, encoding 405 amino acids. Five different nucleotide sites were observed between the two sequences, leading to two amino acid sequence mutations. FaTPG, FaSCPG, and PG genes from the Fragaria vesca genome were classified into three clades using phylogenetic analysis. The clade containing PG genes involved in fruit softening had functional similarity but there were no functional differences between FaTPG and FaSCPG. Differences in PG activity, gene sequence, and gene expression may have led to different roles of PG during ripening and softening in strawberries with different textures.