What Rot! Strawberry Proteomics and the Art of Staying Fresh

It’s summer and strawberries are on the menu – Wimbledon, Pimms, strawberry lemonade and all that jazz – but how do producers keep these succulent fruits in tiptop condition? Understanding more about how strawberries and other soft fruits mature during storage might lead to better environmental conditions that could preserve crop quality for consumer acceptance. Many producers harvest optimally then place the crops into suitable storage conditions to reduce stress and delay senescence. However, little is known about changes in cellular pathways in the fruits themselves during storage.

Li et al. (2015) examined changes in the strawberry proteome to uncover fruit responses to these storage conditions, in order to learn more about the regulatory mechanisms at work as the fruit ripens postharvest¹. The researchers studied changes in undamaged, healthy fruits of strawberry species, Fragaria ananassa, Duch. cv. ‘Akihime’, following nine days of storage in controlled atmosphere (CA: 2% oxygen, 12% carbon dioxide) at low temperature. They compared results with those obtained after storage in air at either low (LT) or room temperature (RT). The research team also noted physical characteristics of the strawberries, measuring firmness, acidity, volatile production and total acidity that would influence consumer preferences.

Li et al. extracted protein content from the fruits then digested the preparations using trypsin. They then examined the peptide digests by nano high performance liquid chromatography tandem mass spectrometry (nano HPLC-MS/MS) using an LTQ XL mass spectrometer (Thermo Scientific), searching spectral data against NCBI database Viridiplantae for protein matches. The researchers used a normalized spectral count-based label free quantitative approach to determine changes in protein abundance between days 0 (day of harvest) and after nine days, performing three technical replicates and three biological replicates for each storage condition.

Physical characteristics showed that CA storage maintained fruit firmness and acidity levels better than LT or RT environments. CA also maintained production of the volatile compounds in the fruits that contribute to aroma.

Noting that identifying the complete strawberry proteome itself is difficult since the full genome has yet to be determined, the researchers counted a total of 454 proteins in fruit from across the three storage conditions. During CA and LT storage conditions, total protein abundance reduced, suggesting that a reduction in enzyme activity is part of the senescence response to oxidative stress during storage.

Using the pre-storage strawberry proteomes as a baseline, Li et al. used hierarchical clustering analysis to group sets of proteins associated with the response to storage that might give insight into pathways associated with oxidative processes taking place in the fruits. Their results showed three distinct clusters of the 73 proteins that changed in abundance in all three storage conditions. Cluster 1 contained proteins that either increased or decreased with storage; cluster 2 contained proteins that decreased; and proteins in cluster 3 increased with storage.

When the research team examined the types of proteins involved in the response to the different storage conditions they found involvement in several pathways including carbohydrate and energy metabolism, volatile compound synthesis, and protein processing, among others. Discussing these pathways with reference to strawberry fruit physical traits, the researchers found that their results could explain alterations in fruit quality observed with storage.

In conclusion, the team suggest that the data collected could help producers and food scientists understand more about the post-harvest responses in strawberries. Work like this could lead to improvements in handling to maintain fruit quality with optimized conditions that support strawberry cellular responses to the post-harvest stress of storage.

Reference

1. Li. L. et al. (2015) “Label-free quantitative proteomics to investigate strawberry fruit proteome changes under controlled atmosphere and low temperature storage“, Journal of Proteomics 120 (pp. 44-57)