Auxin/indole-3-acetic acid (Aux/IAA) proteins are transcriptional regulators 
that mediate many aspects of plant responses to auxin. While functions of most 
Aux/IAAs have been defined mainly by gain-of-function mutant alleles in 
Arabidopsis thaliana, phenotypes associated with loss-of-function mutations have 
been scarce and subtle. We report here that the downregulation of IAA9, a tomato 
(Solanum lycopersicum) gene from a distinct subfamily of Aux/IAA genes, results 
in a pleiotropic phenotype, consistent with its ubiquitous expression pattern. 
IAA9-inhibited lines have simple leaves instead of wild-type compound leaves, 
and fruit development is triggered before fertilization, giving rise to 
parthenocarpy. This indicates that IAA9 is a key mediator of leaf morphogenesis 
and fruit set. In addition, antisense plants displayed auxin-related growth 
alterations, including enhanced hypocotyl/stem elongation, increased leaf 
vascularization, and reduced apical dominance. Auxin dose-response assays 
revealed that IAA9 downregulated lines were hypersensitive to auxin, although 
the only early auxin-responsive gene that was found to be upregulated in the 
antisense lines was IAA3. The activity of the IAA3 promoter was stimulated in 
the IAA9 antisense genetic background, indicating that IAA9 acts in planta as a 
transcriptional repressor of auxin signaling. While no mutation in any member of 
subfamily IV has been reported to date, the phenotypes associated with the 
downregulation of IAA9 reveal distinct and novel roles for members of the 
Aux/IAA gene family.