Toward characterizing high and low dry matter kiwifruit through large-scale transcriptomic analysis
Dry matter content (DMC) is an important indicator of fruit quality and long-term storage capacity of kiwifruit. In this study, the post-harvest physiological, metabolomic and transcriptomic influence of slightly varied DMC status on Hayward kiwifruit ripening under several conditions was investigated. Kiwifruit with distinct DMC levels was treated with 1-methylcyclopropene (1-MCP) and systematically monitored for physiological changes at various intervals during cold storage (0C). Following 90- and 120-days of cold storage, high DMC kiwifruit generally exhibited superior physiological characteristics, such as increased pericarp and placenta firmness, soluble solid content (SSC), titratable acidity (TA), starch content and compared to low DMC kiwifruit, regardless of treatment. At harvest, global gene expression analysis revealed minor differences between groups, with beta-amylase being the highest up-regulated gene in high DMC kiwifruit. Moreover, the gene expression patterns between DMC groups became more distinct after 90- and 120-days of cold storage. Interestingly, the placenta tissue displayed almost 4 times more differentially expressed genes (DEGs) and transcription factors (TFs) compared to the pericarp, highlighting its greater importance in ripening responses following 1-MCP treatment. Overall, high DMC kiwifruit displayed an enhanced up-regulation of Chalcone synthase, which corresponds to flavonoid biosynthesis. The induction of flavonoid biosynthesis suggests an enhanced response against low temperatures in high DMC kiwifruit, which may partly explain their greater quality. On the other hand, in low DMC kiwifruit, an up-regulation of Diacylglycerol acyltransferases were more evident, which may be an indication of attempts to mitigate cold stress, senescence, or cell wall modifications through lipid metabolism. Furthermore, the application of 1-MCP seems as a promising method to delay senescence and to prolong the storage capacity of low DMC kiwifruit, positively affecting firmness, while preserving it at comparable levels as the untreated high DMC kiwifruit. In conclusion, our results emphasize, through a wide metabolomic and transcriptomic analysis, the importance of unravelling the DMC kiwifruit biology, which could help breeding programs improve kiwifruit quality traits.