Transcriptome and metabolome analysis of preharvest internal browning in Nane plum fruit caused by high temperatures

The preharvest internal browning of Nane plum fruit, with no visible effects on the appearance of the fruit, has become a serious problem in recent years in its production area in Guangdong Province, China. This study investigated the effects of environmental factors, including temperature, on Nane plum internal browning. Plum orchards at different elevations with different incidences of internal browning were selected. Using fruits with different internal browning incidence levels, the internal browning mechanism was analyzed with transcriptome and metabolome analyses. The results revealed decreased internal browning at high altitudes. Shading treatment significantly reduced internal browning, whereas bagging and insect-proof net-covering treatments significantly increased internal browning. Because bagging and net coverings increase the local ambient temperature, the findings suggest that high temperature is an important factor influencing the internal browning of Nane plum. The metabolome experiments showed that with increased internal browning, the levels of phenolic hydroxyls such as catechol increased, with simultaneous increases in hydrogen peroxide content and oxidase activity. It can be speculated that the oxidation of phenolic hydroxyl substances is the main cause of the preharvest browning of Nane plum. Transcriptome analysis revealed the increased expression of calcium signaling-related and downstream effector genes and indicated an important role of calcium in internal browning, possibly due to its increased content in the fruit. Further, with increasingly serious internal browning, genes related to photosynthesis were down-regulated, while genes related to senescence were up-regulated, thus suggesting the up-regulation of the process of cell senescence during internal browning. In conclusion, heat stress should be eliminated to reduce preharvest internal browning in Nane plum.