Physiological Degradation of Pectin in Papaya Cell Walls: Release of Long Chains Galacturonans Derived from Insoluble Fractions during Postharvest Fruit Ripening.

Papaya (Carica papaya L.) is a fleshy fruit that presents a rapid pulp softening during ripening. However, the timeline on how papaya pectinases act in polysaccharide solubilization and the consequent modification of the cell wall fractions during ripening is still not clear. In this work, the gene expression correlations between, on one hand, 16 enzymes potentially acting during papaya cell wall disassembling and, on the other hand, the monosaccharide composition of cell wall fractions during papaya ripening were evaluated. In order to explain differences in the ripening of papaya samplings, the molecular mass distribution of polysaccharides from water-soluble and oxalate-soluble fractions (WSF and OSF, respectively), as well as the oligosaccharide profiling from the WSF fraction, were evaluated by high performance size exclusion chromatography coupled to a refractive index detector and high performance anion-exchange chromatography coupled to pulse amperometric detection analyses, respectively. Results showed that up-regulated polygalacturonase and ß-galactosidase genes were positively correlated with some monosaccharide profiles. In addition, an overall increase in the retention time of high molecular weight (HMW) and low molecular weight (LMW) polysaccharides in WSF and OSF was shown. The apparent disappearance of one HMW peak of the OSF may result from the conversion of pectin that were crosslinked with calcium into more soluble forms through the action of PGs, which would increase the solubilization of polysaccharides by lowering their molecular weight. Thus, the results allowed us to propose a detailed process of papaya cell wall disassembling that would affect sensorial properties and post-harvesting losses of this commercially important fruit.

The cold storage of green bananas affects the starch degradation during ripening at higher temperature.

The aim of this work was to investigate the starch degradation of bananas stored at low temperature (13°C, cold-stored group) and bananas stored at 19°C (control group) during ripening. The starch granules were isolated during different stages of banana ripening, and their structure was investigated using different techniques. The activities of α-amylase and ß-amylase associated to the starch granules were determined, and their presence was confirmed using immunolocalization assays. The increased molecular mobility likely facilitated the intake and action of α-amylase on the granule surface, where it was the prevalent enzyme in bananas stored at low temperature. The 10 days of storage at low temperature also influenced the sizes and shapes of the granules, with a predominance of rounded granules and pits on the surface along with superior amylose content, the higher amounts of amylopectin A-chains and the subtle increase in the A-type allomorph content.

Activity and expression of banana starch phosphorylases during fruit development and ripening.

Two main forms of starch phosphorylase (EC 2.4.1.1) were identified and purified from banana (Musa acuminata Colla. cv. Nanicão) fruit. One of them, designated phosphorylase I, had a native molecular weight of 155 kDa and subunit of 90 kDa, a high affinity towards branched glucans and an isoelectric point around 5.0. The other, phosphorylase II, eluted at a higher salt concentration from the anion exchanger, had a low affinity towards branched glucans, a native molecular weight of 290 kDa and subunit of 112 kDa. Kinetic studies showed that both forms had typical hyperbolic curves for orthophosphate (Pi) and glucose-1-phosphate, and that they could not react with substrates with a blocked reducing end or alpha-1,6 glucosidic bonds. Antibodies prepared against the purified type-II form and cross-reacting with the type-I form showed that there was an increase in protein content during development and ripening of the fruit. The changes in protein level were parallel to those of phosphorylase activity, in both the phosphorolytic and synthetic directions. Considering the kinetics, indicating that starch phosphorylases are not under allosteric control, it can be argued that protein synthesis makes a contribution to regulating phosphorylase activity in banana fruit and that hormones, like gibberellic acid and indole-3-acetic acid, may play a regulating role. For the first time, starch phosphorylases isoforms were detected as starch-granule-associated proteins by immunostaining of SDS-PAGE gels.