Transcriptional regulation of auxin metabolic-enzyme genes during tomato fruit development

Auxin plays crucial roles in many stages of fruit development. To understand the mechanism of regulation exerted by auxin during fruit development, we analyzed the variations in the levels of indole-3-acetic acid (IAA) and its precursors, as well as the variations in the expression of the genes involved in IAA metabolism in developing tomato (Solanum lycopersicum 'Ailsa Craig') fruits. The concentrations of IAA and its precursor indole-3-pyruvic acid (IPyA) increased in the ovary three days after anthesis. The transcript levels of the auxin biosynthetic-enzyme genes SlTAR2, ToFZY1, ToFZY2, and ToFZY5 also increased during this stage. These results suggest that the de novo IAA biosynthesis in the pollinated ovaries is controlled through the IPyA pathway, and is involved in fruit set and early fruit growth. Contrarily, the transcripts of the auxin inactivating-enzyme genes SlGH3-9 and SlGH3-15 were accumulated in the unpollinated ovaries and decreased after pollination. This suggests that the auxin inactivating-enzymes might be involved in the control of IAA levels in the unpollinated ovaries. Further, we found that application of synthetic auxin 2,4-dichlorophenoxyacetic acid on the unpollinated ovaries increased the transcript levels of SlTAR2, ToFZY1, ToFZY2, and ToFZY5 and decreased those of SlGH3-9 and SlGH3-15. These results suggest that elevated auxin signaling via pollination or exogenous auxin treatment might promote further auxin accumulation in the ovaries.