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.
INTRODUCTIONThe phytohormone auxin is central to a myriad of aspects of plant growth and developmental processes. At the cellular level, auxin controls cell division, extension, and differentiation. On a wholeplant level, auxin plays an essential role in processes such as apical dominance, lateral/adventitious root formation, tropisms, fruit set and development, vascular differentiation, and embryogenesis (Friml, 2003). While it is clear that auxin plays a pivotal role in plant growth and development, the molecular effectors by which this hormone exerts its effect are still relatively unknown. For example, in the process of fruit set, the onset of ovary development into fruit and its subsequent development are naturally triggered by signals from successful fertilization. These processes can be initiated in the absence of pollination and fertilization by exogenous auxin or auxin transport inhibitors (Gustafson, 1937;Beyer and Quebedeaux, 1974) or by the ovaryspecific expression of Agrobacterium tumefaciens indoleacetamide monoxygenase (iaaM) or root loci B (rolB) genes, which confer higher auxin production or increased auxin sensitivity, respectively (Ficcadenti et al., 1999;Carmi et al., 2003). The molecular mediators by which auxin impacts this process are still unknown.The recent discovery that the F-box protein Transport Inhibitor Response1 functions as an auxin receptor represents a major breakthrough (Dharmasiri et al., 2005;Kepinski and Leyser, 2005) in understanding ho...
