From the report «The role of auxin and gibberellin in tomato fruit set», of Maaike de Jong , Celestina Mariani and Wim H. Vriezen. Published at Journal of experimental botany.
Fruit set is defined as the transition of a quiescent ovary to a rapidly growing young fruit. In tomato, as in most angiosperms, compatible pollen has to germinate on the pistil, and forms a pollen tube. This pollen tube then grows through the style and the ovular micropyle to deliver two sperm cells in the embryo sac.
The initiation of tomato fruit growth, fruit set, is very sensitive to environmental conditions. It has been well established that tomato fruit set depends on successful pollination and fertilization, which trigger the fruit developmental programme through the activation of the auxin and gibberellin signalling pathways. However, the exact role of each of these two hormones is still poorly understood, probably because only few of the signalling components involved have been identified so far.
Although the application of either auxin or gibberellin can trigger tomato fruit development independently of pollination and fertilization, there are several indications that each of these hormones has different effects on cell division and cell expansion.
Recent research on fruit set induced by hormone applications has led to new insights into hormone biosynthesis and signalling.
Auxin can promote the biosynthesis of gibberellin, but other growth regulators, such as cytokinin, ethylene, and abscisic acid may also play a role. These hormones together ultimately control the expression of the genes that are actually involved in fruit development, a network which is slowly being unravelled.