Isolation of a Polygalacturonase Gene from Citrus sinensis Fruit and Its Expression Relative to Fruit Mastication Trait, Fruit Development, and Calcium or Boron Treatments

Abstract

Polygalacturonase (PG) plays a significant role in pectin solubilization and depolymerization in vivo and is involved in numerous plant developmental processes. In this study, a polygalacturonase gene (CitPG, EF185420) was isolated from Citrus sinensis. It is 1,686 bp with a putative open reading frame of 1,338 bp encoding 445 amino acids. Phylogenetic analysis showed that CitPG belongs to clade B which includes PGs expressed in fruits or dehiscence zones. CitPG expression profiles during fruit ripening were compared between two pairs of varieties which differ in their mastication traits: C. sinensis cv. Fengjiewancheng (FJWC) vs. C. sinensis cv. Fengjie 72-1(FJ72-1) and Citrus reticulata cv. Kinokuni (NM) vs. C. reticulata cv. Miguang (MG), in which the mastication trait of “FJWC” and “NM” are inferior to their respective partner cultivars. Results indicated that the CitPG transcript level was significantly lower in the inferior mastication variety at the late ripening stage or at all ripening stages. Calcium or boron treatment resulted in a significant reduction in CitPG transcription level in fruit pulp at harvest time (235 days after anthesis). This was accompanied by an increase in protopectin content. In addition, the mRNA level of CitPG in the fruit peel and pulp increased during the fruit's rapid cell enlargement stage and was inversely correlated with protopectin levels. These results suggest that CitPG is associated with the favorable enhancement of the fruit mastication trait and is involved in dissolving the protopectin during cell enlargement.