Analysis of CaCl2 postharvest treatment on strawberry cell wall metabolism and defense responses of the fruit

LANGER, SILVIA; MARINA, MARÍA; CIVELLO, MARCOS; MARTÍNEZ, GUSTAVO A.; VILLARREAL, NATALIA M.

Wageningen

Conferencia; VII International Conference Postharvest Unlimited & XII International Symposium on Postharvest Quality of Ornamental Plants; 2023

The softening of fleshy fruits is largely due to changes in the structure and composition of the primary cell wall, which is a dynamic structure capable of adapting to various biotic and abiotic stimuli. Pectic polymers in strawberry (Fragaria x ananassa, Duch.), can reach up to 60% and they are the main components of the middle lamella. Calcium immersion treatments have been shown to be effective in extending the quality and shelf life of strawberries during storage. However, the molecular and biochemical mechanisms underlying the effects of calcium on the cell wall of these fruits were not clear until recently. In this study, we investigated the influence of CaCl2 treatment on the expression patterns and activities of enzymes highly linked to the metabolism of the strawberry cell wall. Furthermore, we aimed to characterize the impact of the treatments on the defensive responses of the fruit. Ripe strawberries (80-90% of red surface coloration) of cv. Aroma were treated with a 10 g L-1 CaCl2 solution at 25°C for 30 min, while another group was submerged in distilled water at the same temperature and duration (control). Fruits were separated immediately after treatment (initial time) and after storage for 8 days at 4 °C + 2 days at 4 °C (final time). We analyzed the firmness and pectic polymer content of the cell wall, as well as the activities of pectin methyl esterase (PME) and polygalacturonase (PG) enzymes. We also measured the relative expression of key genes involved in the degradation (FaPG1, FaPLA, FaPLB, and FaPLC) and stabilization (FaPME1) of pectins by Real-Time PCR. Our results showed that the calcium treatment inhibited PG hydrolyzing activity while increasing the PME pectin stabilizing activity, corresponding to a lower degree of esterification found in treated fruits compared to controls. The expression of the FaPME1 gene increased, whereas the expressions of FaPG1, FaPLB, and FaPLC decreased at the initial time. Fruits treated with calcium after storage had higher firmness and a higher content of pectins bound by ionic interactions, indicating that Ca2+ ions preserved the structure of pectins through positive regulation of FaPME1 and PME activity and negative regulation of genes involved in the disassembly of the strawberry cell wall. The results related to the fruit defense revealed that the CaCl2 treatment had a positive effect on the total activities of PPO, POD, chitinase, and β-1,3-glucanase enzymes, and also increased the expression of genes responsible for encoding pathogen-related proteins (FaChi2-2, FaChi3, and FaβG2,3). Moreover, the treated fruit showed an inhibition of in vivo growth of Botrytis cinerea, indicating the effectiveness of the CaCl2 treatment in activating fruit defense responses.