BackgroundTomato fruit ripening is controlled by ethylene and is characterized by a shift in color from green to red, a strong accumulation of lycopene, and a decrease in β-xanthophylls and chlorophylls. The role of other hormones, such as auxin, has been less studied. Auxin is retarding the fruit ripening. In tomato, there is no study of the carotenoid content and related transcript after treatment with auxin.ResultsWe followed the effects of application of various hormone-like substances to “Mature-Green” fruits. Application of an ethylene precursor (ACC) or of an auxin antagonist (PCIB) to tomato fruits accelerated the color shift, the accumulation of lycopene, α-, β-, and δ-carotenes and the disappearance of β-xanthophylls and chlorophyll b. By contrast, application of auxin (IAA) delayed the color shift, the lycopene accumulation and the decrease of chlorophyll a. Combined application of IAA + ACC led to an intermediate phenotype. The levels of transcripts coding for carotenoid biosynthesis enzymes, for the ripening regulator Rin, for chlorophyllase, and the levels of ethylene and abscisic acid (ABA) were monitored in the treated fruits. Correlation network analyses suggest that ABA, may also be a key regulator of several responses to auxin and ethylene treatments.ConclusionsThe results suggest that IAA retards tomato ripening by affecting a set of (i) key regulators, such as Rin, ethylene and ABA, and (ii) key effectors, such as genes for lycopene and β-xanthophyll biosynthesis and for chlorophyll degradation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0495-4) contains supplementary material, which is available to authorized users.

Auxin is known to regulate cell division and cell elongation, thus controlling plant growth and development. Part of the auxin signaling pathway depends on the fine-tuned degradation of the auxin/indole acetic acid (Aux/IAA) transcriptional repressors. Recent evidence indicates that Aux/IAA proteins play a role in fruit development in tomato (Solanum lycopersicum Mill.), a model species for fleshy fruit development. We report here on the functional characterization of Sl-IAA17 during tomato fruit development. Silencing of Sl-IAA17 by an RNA interference (RNAi) strategy resulted in the production of larger fruit than the wild type. Histological analyses of the fruit organ and tissues demonstrated that this phenotype was associated with a thicker pericarp, rather than larger locules and/or a larger number of seeds. Microscopic analysis demonstrated that the higher pericarp thickness in Sl-IAA17 RNAi fruits was not due to a larger number of cells, but to the increase in cell size. Finally, we observed that the cell expansion in the transgenic fruits is tightly coupled with higher ploidy levels than in the wild type, suggesting a stimulation of the endoreduplication process. In conclusion, this work provides new insights into the function of the Aux/IAA pathway in fleshy fruit development, especially fruit size and cell size determination in tomato.