Changes in the metabolome and cell wall composition
Kiwifruits (Actinidia deliciosa) are steadily gaining popularity with consumers. Usually, fully developed fruits of the perennial vine are harvested unripe in the countries of origin – most commonly Italy, New Zealand and Chile. Thus fruits can withstand the long transport by ship and attain its maturity at the destination. Well-defined maturity indices like soluble solids content (Brix) and fruit firmness should ensure uniform ripening and good storability. Important changes during ripening concern especially sugars and sugar derivatives as well as organic acids, but also cell wall composition and flavor. Reaching the quality expected by the consumer in terms of flavor and fruit firmness is challenging: some fruits remain firm and unripe even weeks after the purchase, while other fruits reach eating ripeness quite fast. It is still unclear why kiwifruits ripen so differently. It is well known that the volatile plant hormone ethylene, which is produced by the kiwifruit itself as well as other fruit, plays a major role in bringing kiwifruit to eating ripeness. However, the effects of ethylene on kiwifruit metabolism have not yet been elucidated completely.
The project at the Department of Safety and Quality of Fruits and Vegetables of the Max Rubner-Institut aimed to characterize the metabolic changes during kiwifruit ripening. Therefore, kiwifruits were ripened in a climate chamber under controlled conditions and classified into different ripening stages according to their fruit firmness. By a final ethylene treatment, overripe kiwifruits were produced. The kiwifruit metabolome at the different ripening stages was analyzed at the MRI using comprehensive two-dimensional gas chromatography (GC×GC-MS) and GC-MS and, in collaboration with the Karlsruhe Institute of Technology (KIT, workgroup of Prof. Burkhard Luy, Institute of Organic Chemistry, Chair for Bioanalytic), additionally by NMR. Furthermore, postharvest cell wall polysaccharide modifications were determined using different chromatographic approaches (Workgroup of Prof. Mirko Bunzel, Institute of Applied Biosciences, Chair for Food Chemistry and Phytochemistry).
With the help of the analytical methods used, the changes in the metabolite profile of the kiwifruit during postharvest ripening could be described in detail. While the firmness of the fruit continuously decreased during ripening, both expected and unexpected processes took place at the molecular level. The increase in the content of sugars and the degradation of organic acids are typical post-harvest processes that were also observed during the ripening of the kiwifruit. However, the acids and sugars are substance classes that comprise many different members. The use of untargeted metabolomics methods highlighted that not all members of a substance class behave in the same way, but that each substance is built up or broken down according to its role in the fruit's metabolism as required. For example, the well-known monosaccharides glucose and fructose were, as expected, released by the degradation of storage carbohydrates to increase the sweetness of the fruit. The sucrose contents remained constant, presumably because an equilibrium was established between the formation of sucrose by starch degradation and the cleavage of sucrose into glucose and fructose. The marked increase in the disaccharide trehalose must also be understood in the context of sucrose metabolism, as the metabolite trehalose-6-phosphate exerts an important regulatory role here. In comparison, galactinol - an intermediate of the synthesis of storage carbohydrates - was almost completely degraded in the middle maturation stage, as the polymeric reserve substances were increasingly broken down into their building blocks in this phase. The wood sugar xylose was increasingly released from structural carbohydrates of the cell walls in the middle phase of ripening, but subsequently metabolised again. Furthermore, the concentrations of malic acid and succinic acid decreased, while the contents of citric acid, another prominent fruit acid, remained constant. In contrast, free galacturonic acid could only be detected in the fruit during the final phase of ripening, as it was released to a significant extent from the side chains of the pectin polymers only in the overripe stage. Further, in this study, additional fruits were examined that did not reach the edible state due to a too short cold stimulus before ripening, but retained a medium fruit flesh firmness while suffering an increasing water loss. It could be shown that at least some metabolic pathways were regulated differently in these fruits, so that in total not all processes necessary to reach full ripeness could take place.
The results were published in the scientific journal Postharvest Biology and Technology.
C. Mack, D. Wefers, P. Schuster, C. H. Weinert, B. Egert, S. Bliedung, et al., Untargeted multi-platform analysis of the metabolome and the non-starch polysaccharides of kiwifruit during postharvest ripening. Postharvest Biology and Technology 2017 Vol. 125 Pages 65-76