LETTER TO THE EDITOR

Genetic Engineering Of Food Texture It would seem a reasonable use of this vehicle to note an important event in

the annals of food texture research. Two laboratories, one in the U.S. and one in England, have succeeded through the use of genetic engineering in producing tomatoes having reduced polygalacturonase (PG) activity.

Tomato cotyledons were transformed by co-cultivation with a bacteria carrying a PG antisense binary plasmid. Transformed plants were selected and the fruit harvested. This transgenic fruit now contained a PG gene in reverse orientation, leading to production of a mRNA complementary to the normal PC mRNA (hence ‘antisense’). It is thought that this complementary mRNA binds to normal mRNA and prevents its translation. Hence, lower PG levels are expressed ( 7 s 90% reduction) and the fruit softens more slowly.

The PG enzyme hydrolyzes pectins in the middle lamellae of cell walls, leading to the softening that accompanies ripening. Advantages claimed for the new tomato include fruit that can be vine-ripened and then shipped without refrig- eration since softening and bruising are retarded and lower temperature proc- essing, resulting in a higher proportion of solids in the final product. Reduction in PG levels would be expected to abate softening, however initial tests showed no differences in the texture of transgenic and control fruit when measured by compressibility. Yet, whole fruit compressibility does not measure softening of pericarp tissue per se , where PG activity is localized, but overall fruit turgor instead. Unpublished results from our laboratory suggest that puncture or pericarp compression tests are better suited to textural differences resulting from differ- ential PG activity.

This occurrence is worthy of comment because it is thought to represent the first successful (assuming field tests substantiate laboratory results) application of genetic engineering to a problem in food texture. Previous work, using tra- ditional plant breeding techniques, have led to a number of tomato mutations deficient in PG activity. All of these, however, exhibit overlapping effects on ripening that include impaired color development. A major breakthrough asso- ciated with the genetic engineering approach is specificity-the new tomato synthesizes normal levels of lycopene.

Thus, another tool has been added to the workchcst of those researchers whose goal is modifying and optimizing food texture-perhaps the most powerful and yet mosl controversial method ever made available to scientists. Future devel- opments in this field are eagerly awaited since problems that have proved in- tractable in the past may well yield to this technique.

REFERENCES

ROBERTS, L. 1988. Genetic engineers build a better tomato. Science 241, 1290.

SHEEHY, R. E., KRAMER, M. and HIATT, W. R. 1988. Reduction of po- lygalacturonase activity in tomato fruit by antisense RNA. €'roc. Natl. Acad. Sci. 85, 8805.

SMITH, C.J.S., WATSON, C. F., RAY, J., BIRD, C. R., MORRIS, P. C., SCHUCH, W. and GRIERSON, D. 1988. Antisense RNA inhibition of pol ygalacturonase gene expression in transgenic tomatoes. Nature 334, 724.

D. W. STANLEY, R. L. JACKMAN and A. G. MARANGONI Department of Food Science, University of Guelph