Modification of expansin protein abundance in tomato fruit alters softening and cell wall polymer metabolism during ripening

The role of the ripening-specific expansin Exp1 protein in fruit softening and cell wall metabolism was investigated by suppression and overexpression of Exp1 in transgenic tomato plants. Fruit in which Exp1 protein accumulation was suppressed to 3% that of wild-type levels were firmer than controls throughout ripening. Suppression of Exp1 protein also substantially inhibited polyuronide depolymerization late in ripening but did not prevent the breakdown of structurally important hemicelluloses, a major contributor to softening. In contrast, fruit overexpressing high levels of recombinant Exp1 protein were much softer than controls, even in mature green fruit before ripening commenced. This softening was correlated with the precocious and extensive depolymerization of structural hemicelluloses, whereas polyuronide depolymerization was not altered. These data are consistent with there being at least three components to fruit softening and textural changes. One component is a relaxation of the wall directly mediated by Exp1, which indirectly limits part of a second component due to polyuronide depolymerization late in ripening, perhaps by controlling access of a pectinase to its substrate. The third component is caused by depolymerization of hemicelluloses, which occurs independently of or requires only very small amounts of Exp1 protein.

Cooperative disassembly of the cellulose-xyloglucan network of plant cell walls: parallels between cell expansion and fruit ripening.

Modification of the plant primary cell wall is required for both cell expansion and for developmental events, such as fruit softening, where cell size remains static but where wall loosening is an important feature. Recent studies suggest that the cellulose-xyloglucan network is targeted by similar enzymatic activities in both expanding cells and ripening fruit but that unique isoforms are expressed in each process. Disassembly of this structural network probably involves the concerted and synergistic action of suites of these enzyme families, where one family of cell wall modifying proteins might mediate the activity of another, providing the basis for orchestrating ordered cell wall restructuring and turnover.

Temporal sequence of cell wall disassembly in rapidly ripening melon fruit

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.

Tomato fructokinases exhibit differential expression and substrate regulation

Two divergent genes encoding fructokinase, Frk1 and Frk2, have been previously shown to be expressed in tomato (Lycopersicon esculentum L.) and have now been further characterized with regard to their spatial expression and the enzymic properties of the encoded proteins. Frk1 and Frk2 mRNA levels were coordinately induced by exogenous sugar, indicating that both belong to the growing class of sugar-regulated genes. However, in situ hybridization indicated that Frk1 and Frk2 were expressed in a spatially distinct manner, with Frk2 mRNA primarily localized in cells of the fruit pericarp, which store starch, and Frk1 mRNA distributed ubiquitously in pericarp tissue. To evaluate the biochemical characteristics of the products of the Frk1 and Frk2 genes, each cDNA was expressed in a mutant yeast (Saccharomyces cerevisiae) line defective in hexose phosphorylation and unable to grow on glucose or fructose (Fru). Both Frk1 and Frk2 proteins expressed in yeast conferred the ability to grow on Fru and exhibited fructokinase activity in vitro. Although both Frk1 and Frk2 both utilized Fru as a substrate, only Frk2 activity was inhibited at high Fru concentrations. These results indicate that Frk2 can be distinguished from Frk1 by its sensitivity to substrate inhibition and by its temporal and spatial pattern of expression, which suggests that it plays a primary role in plant cells specialized for starch storage.

Regulation of tomato fruit polygalacturonase mRNA accumulation by ethylene: A Re-examination

Polygalacturonase (PG) is the major enzyme responsible for pectin disassembly in ripening fruit. Despite extensive research on the factors regulating PG gene expression in fruit, there is conflicting evidence regarding the role of ethylene in mediating its expression. Transgenic tomato (Lycopersicon esculentum) fruits in which endogenous ethylene production was suppressed by the expression of an antisense 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene were used to re-examine the role of ethylene in regulating the accumulation of PG mRNA, enzyme activity, and protein during fruit ripening. Treatment of transgenic antisense ACC synthase mature green fruit with ethylene at concentrations as low as 0.1 to 1 L/L for 24 h induced PG mRNA accumulation, and this accumulation was higher at concentrations of ethylene up to 100 L/L. Neither PG enzyme activity nor PG protein accumulated during this 24-h period of ethylene treatment, indicating that translation lags at least 24 h behind the accumulation of PG mRNA, even at high ethylene concentrations. When examined at concentrations of 10 L/L, PG mRNA accumulated within 6 h of ethylene treatment, indicating that the PG gene responds rapidly to ethylene. Treatment of transgenic tomato fruit with a low level of ethylene (0.1 L/L) for up to 6 d induced levels of PG mRNA, enzyme activity, and protein after 6 d, which were comparable to levels observed in ripening wild-type fruit. A similar level of internal ethylene (0.15 L/L) was measured in transgenic antisense ACC synthase fruit that were held for 28 d after harvest. In these fruit PG mRNA, enzyme activity, and protein were detected. Collectively, these results suggest that PG mRNA accumulation is ethylene regulated, and that the low threshold levels of ethylene required to promote PG mRNA accumulation may be exceeded, even in transgenic antisense ACC synthase tomato fruit.