iTRAQ labelling was performed following the manufacturer's instructions (Applied Biosystems). One hundred microgram peptide for each sample was labelled with iTRAQ regent. The −72 h PG sample (100 μg) were labelled with reagents 113 and 114. The 0 h PG samples were labelled with reagents 115, 116 and 117. The 72 h PG samples were labelled with reagents 118, 119 and 121. The labeling reaction was performed at room temperature for 1 h, then pooled and dried by vacuum centrifugation.