Molecular Analysis of the Processes of Surface Brown Spot (SBS) Formation in Pear Fruit (Pyrus bretschneideri Rehd. cv. Dangshansuli) by De Novo Transcriptome Assembly

Browning disorder, which usually occurs post-harvest in pears subjected to long-term storage, can cause browning of the pear flesh and/or core. In 2011, investigators in China found a novel type of brown spot (designated as surface brown spot, SBS) in pre-harvest 'Dangshansuli' pears (Pyrus bretschneideri Rehd.). SBS has a large impact on the exterior quality of the pears. Interestingly, the brown coloration was only found on the peel and not the flesh or the core. In this paper, de novo transcriptome analysis of the exocarp of pears with SBS using Illumina sequencing showed that SBS up-regulated the expression of genes related to oxidative phosphorylation, phenolic compound synthesis and polyphenoloxidase (PPO), and SBS was associated with inhibition of primary and secondary metabolism genes. Ca(2+)-sensor proteins might be involved in the signal transduction that occurs during the process of SBS formation, and this signaling is likely to be regulated by H2O2, abscisic acid (ABA) and gibberellic acid (GA3). Phytohormone and mineral element analyses confirmed that GA3, ABA, H2O2 and Ca(2+) contribute to SBS formation. In addition to the seasonal characteristics, low levels of O2 and Ca(2+) in the fruit are potential causes of the browning response due to exposure to oxidative stress, oxidative-reductive imbalance and the accumulation of reactive oxygen species (ROS), which affected the membrane integrity. Disruption of the membranes allows for PPO and phenolic compounds to come into contact, and the phenolic compounds are oxidized to form the browning pigments.