SummaryTomato (Solanum lycopersicum L.) fruit-set and growth depend on gibberellins (GAs). Auxins, another kind of hormone, can also induce parthenocarpic fruit growth in tomato, although their possible interaction with GAs is unknown. We showed that fruit development induced by the auxins indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid (2,4-D) were significantly reduced by the simultaneous application of inhibitors of GA biosynthesis, and that this effect was reversed by the application of GA 3 . This suggested that the effect of auxin was mediated by GA. Parthenocarpic fruits induced by 2,4-D had higher levels of the active GA 1 , its precursors and metabolites, than unpollinated non-treated ovaries, but similar levels as those found in pollinated ovaries. Application experiments of radioactive-labelled GAs to unpollinated ovaries showed than 2,4-D altered GA metabolism (both biosynthesis and catabolism) in vivo. Transcript levels of genes encoding copalyldiphosphate synthase (SlCPS), SlGA20ox1, SlGA20ox2 and SlGA20ox3, and SlGA3ox1 were higher in unpollinated ovaries treated with 2,4-D. In contrast, transcript levels of SlGA2ox2 (out of the five SlGA2ox genes known to encode this kind of GA-inactivating enzyme) were lower in ovaries treated with 2,4-D. Our results support the idea that auxins induce fruit-set and growth in tomato, at least partially, by enhancing GA biosynthesis (GA 20-oxidase, GA 3-oxidase and CPS), and probably by decreasing GA inactivation (GA2ox2) activity, thereby leading to higher levels of GA 1 . The expression of diverse Aux/indole-3-acetic acid (IAA) and auxin response factors, which may be involved in this effect of auxin, was also altered in 2,4-D-induced ovaries.

The role of gibberellins (GAs) in tomato (Solanum lycopersicum) fruit development was investigated. Two different inhibitors of GA biosynthesis (LAB 198999 and paclobutrazol) decreased fruit growth and fruit set, an effect reversed by GA 3 application. LAB 198999 reduced GA 1 and GA 8 content, but increased that of their precursors GA 53 , GA 44 , GA 19 , and GA 20 in pollinated fruits. This supports the hypothesis that GA 1 is the active GA for tomato fruit growth. Unpollinated ovaries developed parthenocarpically in response to GA 3 . GA 1 5 GA 4 . GA 20 , but not to GA 19 , suggesting that GA 20-oxidase activity was limiting in unpollinated ovaries. This was confirmed by analyzing the effect of pollination on transcript levels of SlCPS, SlGA20ox1, -2, and -3, and SlGA3ox1 and -2, encoding enzymes of GA biosynthesis. Pollination increased transcript content of SlGA20ox1, -2, and -3, and SlCPS, but not of SlGA3ox1 and -2. To investigate whether pollination also altered GA inactivation, full-length cDNA clones of genes encoding enzymes catalyzing GA 2-oxidases (SlGA2ox1, -2, -3, -4, and -5) were isolated and characterized. Transcript levels of these genes did not decrease early after pollination (5-d-old fruits), but transcript content reduction of all of them, mainly of SlGA2ox2, was found later (from 10 d after anthesis). We conclude that pollination mediates fruit set by activating GA biosynthesis mainly through up-regulation of GA20ox. Finally, the phylogenetic reconstruction of the GA2ox family clearly showed the existence of three gene subfamilies, and the phylogenetic position of SlGA2ox1, -2, -3, -4, and -5 was established.

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