The SPINDLY (SPY) gene is a crucial component of the gibberellin signal transduction pathway in Arabidopsis thaliana (L.) Heynh. In this study, the cloning of the SPINDLY-orthologous gene (LeSpy) from tomato (Lycopersicon esculentum Mill.) and its characterization are reported. SPY and LeSpy show high sequence similarity along their entire lengths, which is reflected in the conservation of exon-intron structure and of all sequence motives previously identified. To analyse the relationship between the Arabidopsis spindly and the tomato procera mutant, which show similar phenotypes, the LeSpy gene was characterized in wild-type and procera tomato plants. These analyses as well as mapping of LeSpy revealed that LeSpy and Procera are different genes.

We investigated the role of gibberellins (GAs) in the phenotype of parthenocarpic fruit (pat), a recessive mutation conferring parthenocarpy in tomato (Solanum lycopersicum L.). Novel phenotypes that parallel those reported in plants repeatedly treated with gibberellic acid or having a GA-constitutive response indicate that the pat mutant probably expresses high levels of GA. The retained sensitivity to the GA-biosynthesis inhibitor paclobutrazol reveals that this condition is dependent on GA biosynthesis. Expression analysis of genes encoding key enzymes involved in GA biosynthesis shows that in normal tomato ovaries, the GA20ox1 transcript is in low copy number before anthesis and only pollination and fertilization increase its transcription levels and, thus, GA biosynthesis. In the unpollinated ovaries of the pat mutant, this mechanism is de-regulated and GA20ox1 is constitutively expressed, indicating that a high GA concentration could play a part in the parthenocarpic phenotype. The levels of endogenous GAs measured in the floral organs of the pat mutant support such a hypothesis. Collectively, the data indicate that transcriptional regulation of GA20ox1 mediates pollination-induced fruit set in tomato and that parthenocarpy in pat results from the mis-regulation of this mechanism. As genes involved in the control of GA synthesis (LeT6, LeT12 and LeCUC2) and response (SPY) are also altered in the pat ovary, it is suggested that the pat mutation affects a regulatory gene located upstream of the control of fruit set exerted by GAs.

Plant hormones are considered to be important mediators of the fruit developmental signal after pollination. The role of phytohormones in tomato (Solanum lycopersicum) fruit set was investigated here. Transcriptome analysis of ovaries was performed using two complementary approaches: cDNA-amplified fragment length polymorphism (AFLP) and microarray analysis. The gene expression profiles obtained suggest that, in addition to auxin and gibberellin, ethylene and abscisic acid (ABA) are involved in regulating fruit set. Before fruit development, many genes involved in biotic and abiotic responses are active in the ovary. In addition, genes involved in ethylene and ABA biosynthesis were strongly expressed, suggesting relatively high ethylene and ABA concentrations before fruit set. Induction of fruit development, either by pollination or by gibberellin application, attenuated expression of all ethylene and ABA biosynthesis and response genes within 24 h. It is proposed that the function of ABA and ethylene in fruit set might be antagonistic to that of auxin and gibberellin in order to keep the ovary in a temporally protected and dormant state; either to protect the ovary tissue or to prevent fruit development before pollination and fertilization occur.

The initiation of tomato fruit growth, fruit set, is very sensitive to environmental conditions. Therefore, an understanding of the mechanisms that regulate this process can facilitate the production of this agriculturally valuable fruit crop. Over the years, 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. Recent research on fruit set induced by hormone applications has led to new insights into hormone biosynthesis and signalling. The aim of this review is to consolidate the current knowledge on the role of auxin and gibberellin in tomato fruit set.