Auxin-induced fruit-set in tomato is mediated at least in part by gibberellins

Abstract

Tomato (Solanum lycopersicum L.) fruit-set and growth depend on gibberellins (GA). Auxins, another kind of hormone, can also induce parthenocarpic fruit growth in tomato, although their possible interaction with GA 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 simultaneous application of inhibitors of GA biosynthesis (Paclobutrazol and LAB 198999), and that this effect was reversed by applied GA3. This suggested that the effect of auxin was mediated by GA. Parthenocarpic fruits induced by 2,4-D had higher contents of the active GA1, its precursors and metabolite, than unpollinated non-treated ovaries, but similar to pollinated ovaries. Application experiments of radioactive-labelled GAs to unpollinated ovaries showed than 2,4-D altered in vivo GA metabolism (both biosynthesis and catabolism). Transcript levels of genes encoding copalyldiphosphate synthase (SlCPS), SlGA20ox1, -2 and -3, 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 enzymes) were lower in 2,4-D treated ovaries. 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 decreasing GA inactivation (GA2ox) activities, leading to higher GA1 content. The expression of diverse Aux/IAA and auxin response factors, which may be involved in this effect of auxin, was also altered in 2,4-D-induced ovaries.