Mechanical and optical assessment of the ripening of tomato fruit with reduced polygalacturonase activity

Abstract

The ripening characteristics of modified tomatoes (Lycopersicon esculentum Mill cv Ailsa Craig), which express antisense RNA to polygalacturonase (PG) and thus have very low activity of this enzyme, were compared with control fruit. Previous studies of these fruits showed that although PG activity was reduced to approximately 1% of that in untransformed tomatoes, this reduction had no effect on softening. Further detailed mechanical assessments have now been performed which revealed small, but significant differences in the fruit ripening characteristics between control and antisense fruits. Compression along the polar axis of the PG antisense fruit was significantly reduced relative to the control. Deformation at failure (h), 'modulus' (M), coefficient of compression ( K J , and coefficient of shear (K,) values from probe tests along the equatorial axis also indicated that the antisense fruit was firmer. Although these significant differences in texture properties were observed between the antisense fruit and the control, they were quite limited in extent, as compared to the normal evolution observed during ripening. Scanning electron microscope studies on the subexocarpic region of the pericarp showed that cell wall separation was reduced in the ripe antisense tomatoes, and this may explain the change in mechanical properties. Optical measurements made with both Hunter Color Difference (Hunter Lab Ltd, Fairfax, VA, USA) and Micromatch 2000 (ICS Texicon Ltd, Altrincham, UK) spectrophotometer systems showed that the 'a' (green/red) component of colour of the antisense tomatoes was increased relative to that of the control samples. The effect of PG on softening and of cell wall breakdown on mechanical and optical properties is also considered.

Antisense Suppression of Deoxyhypusine Synthase in Tomato Delays Fruit Softening and Alters Growth and Development

“…Indeed, the polygalacturonase activity of transgenic fruit was only 33% of that for corresponding wild-type fruit at 7 d postbreaker when both types of fruit were still red-firm, and 20% of the activity for wild-type fruit at 21 d postbreaker when the transgenic fruit were still red-firm and the wildtype fruit were soft and extensively senescent. This finding that reduced polygalacturonase activity correlates with enhanced structural integrity of tomato fruit during postharvest storage has been noted previously in studies of transgenic fruit with suppressed polygalacturonase activity (Kramer et al, 1992;Langley et al, 1994;Brummell and Harpster, 2001). Of particular interest is the finding that suppression of DHS had no effect on fruit ripening.…”

“…Experiments with transgenic tomato fruit in which polygalacturonase mRNA and protein levels were suppressed have demonstrated, though, that the softened texture of a ripe fruit does not result directly from polygalacturonase-mediated modifications to the pectin network (Brummell and Harpster, 2001). Down-regulation of polygalacturonase does, however, delay fruit senescence and enhance resistance to postharvest pathogens (Langley et al, 1994). Pectin methylesterase activity also increases in tomato fruit during the early ripening stages (Harriman et al, 1991), but although its suppression in transgenic fruit resulted in reduced pectin depolymerization, there was again no effect on firmness during ripening (Tieman and Handa, 1994).…”

Network Inference Analysis Identifies an APRR2-Like Gene Linked to Pigment Accumulation in Tomato and Pepper Fruits

“…Following the recording of the Hunter color scale values of L, a, and b, tomato color index (TCI) was calculated: TCI = (2,000a)/(L 3 (a2 + b2)0.5) (Langley et al, 1994). Results were means 6 SE of three determinations for each fruit of the three replicates.…”

Biochemistry of fruit softening: an overview

“…However, it substantially affects the textural properties (increased viscosity) of pastes, the integrity of stored fruits, and resistance to postharvest pathogens (Kramer et al, 1992;Langley et al, 1994). Antisense suppression of PG activity to 0.5 % of wild-type levels (Sheehy et al, 1988;Smith et al, 1988) resulted in only a modest reduction of polyuronide depolymerization (Brummell and Labavitch, 1997) and a very small increase in firmness later in ripening (Kramer et al, 1992;Langley et al, 1994). Furthermore, expression of a chimeric PG transgene in a nonsoftening mutant tomato fruit that normally lacks this activity resulted in polyuronide solubilization but did not restore softening (Giovannoni et al, 1989).…”