Colleen G. Santino

Developmental and transgenic analysis of two tomato fruit enhanced genes

Tomato fruit development is characterized by distinct developmental stages: fruit set, periods of rapid cell division and cell expansion, and the period where processes associated with ripening are dominant. During each of these stages, different aspects of cellular metabolism are favored. Accompanying these developmental changes are dramatic differences in gene expression, with a subset of genes being expressed early and a subset being expressed later in development. We have isolated and characterized several sequences from tomato that are expressed primarily in immature green fruit. Two of these genes (Tfm7 and Tfm5) have been characterized more extensively and their sequence indicates that they encode proteins corresponding to a proline-rich protein (PRP) and a glycine-rich protein (GRP). RNA blot analysis indicates that the transcripts from these genes are present at the earliest stages of fruit development, and continue to be expressed throughout the growth period of the fruit. Expression analysis during development indicates that the gene encoding the PRP may be down-regulated by ethylene. As a means to understanding the functional significance and the transcriptional contribution of these tissue-limited proteins during development, we constructed promoter-reporter gene fusions to identify which cell types express each of these sequences. GUS protein produced in transgenic plants by both promoter-reporter gene constructs was detected in most tissues of the fruit including the pericarp, columella, and placental tissues of young immature fruit through the mature green stage. However, only one of the promoter sequences conferred expression in the fruit locular tissue.