Comprehensive Analysis of the Polygalacturonase Gene Family and Transcriptome Screening for Candidate Genes Associated with Postharvest Softening in Atemoya

Polygalacturonase (PG) is a key enzyme in cell wall metabolism and fruit ripening. Atemoya (Annona cherimola Mill. × A. squamosa L.) is a high-value tropical fruit that undergoes rapid postharvest softening at room temperature. However, the role of the atemoya PG gene family in this process remains unknown. This study determined that storing atemoya at 28 °C significantly reduced fruit firmness and the total pectin content but increased water-soluble pectin (WSP) and PG activity compared to storage at 15 °C. Genome-wide identification of the AaPG gene family in atemoya revealed that 40 AaPG genes were unevenly distributed across seven chromosomes. Nineteen genes were located within six tandem duplication clusters. AaPG proteins exhibited clade-specific differences: Clades B-E contained the polysaccharide lyase family 6 (PL-6) superfamily domain, while Clade A harbored the Aspergillus niger polygalacturonase 1 (Pgu1) domain and lacked several conserved motifs. Expression profiling and reverse transcription quantitative polymerase chain reaction (RT-qPCR) showed that AaPG19, AaPG21, AaPG23 and AaPG24 were specifically induced at 28 °C. Subcellular localization confirmed that these four proteins were located on the plasma membrane. These findings provide insights into the evolution and temperature-dependent regulation of the AaPG family, identifying candidate genes responsible for the rapid softening of atemoya fruit.