Kiwifruit (Actinidia chinensis), known as ‘the king of vitamin C’, has often been classified into climacteric fruit, although it behaves a different pattern of ripening physiology with the typical climacteric fruit, such as tomato or banana. Fruit with good storability is relatively rare in A. chinensis germplasm and most other species of Actinidia have a rapidly softening fruit and limit their shelf life. To maximize the storage and shelf life of fruit is of great importance to kiwifruit industry and thus understanding the ripening mechanism of kiwifruit is the first step.  

Previous studies indicate that cell wall disassembly contributes to softening and ripening of kiwifruit, and pectin degradation plays an important role in cell wall breakdown. As we know, it is PG (polygalacturonase), an important hydrolytic enzyme, that is responsible for pectin degradation. To date, there are several previous reports indicating PG enzyme is involved in kiwifruit softening process, but PG protein is encoded by a multigene family and the real PG genes responsible for pectin degradation and fruit softening are still not fully identified.  

Researchers from Key Laboratory of Plant Germplasm Enhancement and Specialty Agriculture of Wuhan Botanical Garden utilized the recently released and manually annotated kiwifruit Red5 genome database to perform the genome-wide analysis of PG family members and the correlation of PG enzyme activities, gene expressions with fruit softening pattern in order to identify the real PG genes involved in kiwifruit softening.  

The research obtained a total of 51 PG family members in kiwifruit genome, which were unevenly distributed on 20 out of 29 chromosomes. Two members, AcPG4 and AcPG18 were found to be phylogenetically closed to other known PG genes involved in fruit softening. They investigated the expression profiles of cell wall modifying enzymes, including PG, PL (pectate and pectin lyases) and PE (pectinesterase) during kiwifruit ‘Donghong’ softening under ambient temperature.  

The results showed that PG and PE activities largely correlated with the change of pectin content and firmness, and their further correlation analysis with PG gene expression patterns facilitated us to identify three AcPG genes (AcPG4, AcPG18 and AcPG8, especially the former two) related with pectin degradation and fruit softening.  

Finally, the findings not only benefit the functional characterization of kiwifruit PG genes and also provide a subset of potential PG candidate genes for further genetic manipulation to create new cultivars with good storability. 

This research was funded by the Strategic Priority Research Program of Chinese Academy of Sciences, the Technological Innovation Project of Hubei Province (Key Program), and the National Key Research and Development Program of China. The results have been published in Plants entitled “Genome-Wide Identification and Expression Analysis of Polygalacturonase Gene Family in Kiwifruit (Actinidia chinensis) during Fruit Softening”. 

Phylogenetic relationship of PG genes from kiwifruit genome with other known PG genes involved in fruit softening and ripening (Image by HUANG Wenjun )