Temporal sequence of cell wall disassembly in rapidly ripening melon fruit

Abstract

The Charentais variety of melon (Cucumis melo cv Reticulatus F1 Alpha) was observed to undergo very rapid ripening, with the transition from the preripe to overripe stage occurring within 24 to 48 h. During this time, the flesh first softened and then exhibited substantial disintegration, suggesting that Charentais may represent a useful model system to examine the temporal sequence of changes in cell wall composition that typically take place in softening fruit. The total amount of pectin in the cell wall showed little reduction during ripening but its solubility changed substantially. Initial changes in pectin solubility coincided with a loss of galactose from tightly bound pectins, but preceded the expression of polygalacturonase (PG) mRNAs, suggesting early, PG-independent modification of pectin structure. Depolymerization of polyuronides occurred predominantly in the later ripening stages, and after the appearance of PG mRNAs, suggesting the existence of PG-dependent pectin degradation in later stages. Depolymerization of hemicelluloses was observed throughout ripening, and degradation of a tightly bound xyloglucan fraction was detected at the early onset of softening. Thus, metabolism of xyloglucan that may be closely associated with cellulose microfibrils may contribute to the initial stages of fruit softening. A model is presented of the temporal sequence of cell wall changes during cell wall disassembly in ripening Charentais melon.

Figure 1

Sequential chemical-extraction protocol used for the preparation and isolation of Charentais melon fruit cell wall fractions.

Figure 2

Flesh firmness (black bars) and internal ethylene concentration (white bars) of Charentais melon fruit at defined developmental stages. kgf, Kilograms force.

Figure 3

Gel-filtration profiles of water-soluble polysaccharides derived from six developmental stages of Charentais melon fruit fractionated on Sepharose CL-4B. Column fractions (2.0 mL) were assayed for UA content (•) using the m-hydroxybiphenyl method (Blumenkrantz and Asboe-Hansen, 1973) or for TS (○) using the phenol-sulfuric acid method (Dubois et al., 1956). Dextran molecular-mass markers (kD) used as a calibration scale are shown at the top. i through iv, Subfractions described in the text; V_o, void volume; V_t, total volume.

Figure 4

Gel-filtration profiles of CDTA-soluble polysaccharides derived from six developmental stages of Charentais melon fruit. Details are as described for Figure 3.

Figure 5

Gel-filtration profiles of Na₂CO₃-soluble polysaccharides derived from six developmental stages of Charentais melon fruit. Details are as described for Figure 3.

Figure 6

Gel-filtration profiles of 4% KOH-soluble polysaccharides derived from six developmental stages of Charentais melon fruit and fractionated on Sepharose CL-6B. Column fractions (2.0 mL) were assayed for xyloglucan (•) as described in Maclachlan and Brady (1994) or for TS (○) using the phenol-sulfuric acid method (Dubois et al., 1956). Details are as described for Figure 3.

Figure 7

Gel-filtration profiles of 24% KOH-soluble polysaccharides derived from six developmental stages of Charentais melon fruit. The superimposed profiles from the xyloglucan assay of R1 and R2 polymers are shown in the inset, top right. Details are as described for Figures 3 and 6.

Figure 8

Model of the temporal sequence of cell wall changes, pectinase activity, and PG mRNA expression in ripening Charentais melon fruit at defined developmental stages. XG, Xyloglucan.