The Expression of the Maize Sucrose-Phosphate Synthase Gene in Lycopersicon esculentum: A Focus on the Effects of Breeding Transgenic Tomato Lines to a Wild-type Cultivar

Abstract

Sucrose-phosphate synthase (SPS) is a photosynthetic enzyme which regulates sucrose synthesis by catalyzing the conversion of fructose 6- phosphate and UDPglucose to sucrose phosphate. Because SPS is involved in sucrose production, it is thought to be one of the key enzymes in the interface between photosynthesis and starch-sucrose partitioning. In the hopes of increasing fruit yield by causing increased partitioning of photosynthate product to sucrose, Calgene, Inc. inserted a maize SPS gene into a tomato cultivar. However, two studies of tomato plants containing the maize SPS gene fail to clearly indicate the influence of increased SPS expression on fruit yield. Because these studies were conducted under different growth conditions using different generations of transgenic tomatoes, the conflicting results may be caused either by environmental or by genetic factors. In order to increase the genetic uniformity between the plants studied, the transgenic tomato plants were back-crossed to the wild-type cultivar which lacked the maize gene insert. This study explored the effects of backcrossing on the expression of the maize SPS gene by growing the progeny of back-crossed tomato plants both in the lab and in field plots. SPS activity was measured at two stages of the transgenic plants' life cycle using Vmax enzymatic assays. SPS levels in wild-type plants were also measured in order to determine the levels of endogenous tomato SPS activity. These measurements were then compared to the activity levels in the transgenic plant lines. Assuming that the maize SPS gene is stably inherited, we proposed that the gene expression should correspond to expected genotypic ratios. In three of the five transgenic lines studied, significantly more plants exhibited wildtype (low) levels of SPS activity than expected. In addition, all of the transgenic lines showed a developmental shift toward wild-type SPS activity levels. Earlier studies of maize SPS gene expression in tomato plants indicated that the gene was both stably inherited and expressed in predictable activity ranges. Although traditional thought maintained that back-crossing does not alter expression patterns, the data from this research indicates that backcrossing to a wild-type cu1tivar may either disrupt the stability of the maize gene inheritance or enable the tomatoes to regulate maize SPS gene expression.