Fruit-set and Early Fruit Growth in Tomato are Associated with Increases in Indoleacetic Acid, Cytokinin, and Bioactive Gibberellin Contents

Fruit-set, defined as the activation of a developmental program which converts the ovary into a developing fruit, depends on the crosstalk among plant hormones. Here we show that in pollinated ovaries fruit-set is associated with an increase in indole-3-acetic acid (IAA) content and high transcript levels of ToFZY, a gene involved in the tryptophan-dependent auxin biosynthesis pathway. In unpollinated ovaries developed parthenocarpically in response to the synthetic auxin chlorophenoxyacetic acid (4-CPA), ToFZY mRNA levels and IAA content slightly increase. The most likely sequence of events after fertilization involves auxin-mediated activation of gibberellin (GA) synthesis. Fertilization events not only strongly increase SlGA20ox1 and SlGA20ox3 message levels but also increase SlGA2ox2, SlGA2ox4, and SlGA2ox5 mRNA levels, suggesting a concerted regulation to modulate the level of bioactive GAs, GA(1) and GA(3). 4-chlorophenoxyacetic acid was found to mimic the fertilization events in the stimulation of SlGA20ox1 and SlGA20ox3 mRNA levels, which were also enhanced and increased earlier, but in contrast with pollinated ovaries, SlGA2ox2, SlGA2ox4, and SlGA2ox5 mRNA levels were repressed leading to higher levels of bioactive GAs. We have also analyzed the content of abscisic acid (ABA) and its metabolites dihydrophaseic acid, phaseic acid, and ABA-glucosyl ester and the level of cytokinins (CKs) (free bases and their corresponding ribosides and ribotides) in pollinated and auxin-treated tomato fruits. We show that ABA levels decrease whereas the levels of free CKs increase immediately after pollination or auxin treatment.