The auxin Sl-IAA17 transcriptional repressor controls fruit size via the regulation of endoreduplication-related cell expansion.

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.

Genome-wide identification, phylogenetic analysis, expression profiling, and protein-protein interaction properties of TOPLESS gene family members in tomato.

Members of the TOPLESS gene family emerged recently as key players in gene repression in several mechanisms, especially in auxin perception. The TOPLESS genes constitute, in 'higher-plant' genomes, a small multigenic family comprising four to 11 members. In this study, this family was investigated in tomato, a model plant for Solanaceae species and fleshy fruits. Six open reading frames predicted to encode topless-like proteins (SlTPLs) containing the canonical domains (LisH, CTLH, and two WD40 repeats) were identified in the tomato genome. Nuclear localization was confirmed for all members of the SlTPL family with the exception SlTPL6, which localized at the cytoplasm and was excluded from the nucleus. SlTPL genes displayed distinctive expression patterns in different tomato organs, with SlTPL1 showing the highest levels of transcript accumulation in all tissues tested except in ripening fruit where SlTPL3 and SlTPL4 were the most prominently expressed. To gain insight into the specificity of the different TOPLESS paralogues, a protein-protein interaction map between TOPLESS and auxin/indole-3-acetic acid (Aux/IAA) proteins was built using a yeast two-hybrid approach. The PPI map enabled the distinction of two patterns: TOPLESS isoforms interacting with the majority of Aux/IAA, and isoforms with limited capacity for interaction with these protein partners. Interestingly, evolutionary analyses of the TOPLESS gene family revealed that the highly expressed isoforms (SlTPL1, SlTPL3, and SlTPL4) corresponded to the three TPL-related genes undergoing the strongest purifying selection, while the selection was much weaker for SlTPL6, which was expressed at a low level and encoded a protein lacking the capacity to interact with Aux/IAAs.

Sl-IAA27 gene expression is induced during arbuscular mycorrhizal symbiosis in tomato and in Medicago truncatula.

Aux/IAA genes play a pivotal role in auxin transcriptional regulation. Their functions were mainly studied in Arabidopsis through analysis of gain-of-function mutants. In the tomato, the Solanaceae reference species, different studies on Sl-IAA down-regulated lines showed specific role for Sl-IAA genes. Our recent work revealed that the Sl-IAA 27 gene displays a distinct behavior compared with most Aux/IAA genes, being down-regulated by auxin. Interestingly, the silencing of Sl-IAA27 leads to altered chlorophyll accumulation in leaves, reduced fertilization, altered fruit development and altered root formation. Here we report that IAA27 could be a key auxin signaling gene involved in AM in tomato and also in Medicago model plant. Indeed both Sl-IAA27 and its closest homolog in Medicago truncatula, Mt-IAA27, are overexpressed in mycorrhized roots. These data are in line with the putative role of auxin in arbuscular mycorrhization.

Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato.

The phytohormone auxin is known to regulate several aspects of plant development, and Aux/IAA transcription factors play a pivotal role in auxin signaling. To extend our understanding of the multiple functions of Aux/IAAs further, the present study describes the functional characterization of Sl-IAA27, a member of the tomato Aux/IAA gene family. Sl-IAA27 displays a distinct behavior compared with most Aux/IAA genes regarding the regulation of its expression by auxin, and the Sl-IAA27-encoded protein harbors a unique motif of unknown function also present in Sl-IAA9 and remarkably conserved in monocot and dicot species. Tomato transgenic plants underexpressing the Sl-IAA27 gene revealed multiple phenotypes related to vegetative and reproductive growth. Silencing of Sl-IAA27 results in higher auxin sensitivity, altered root development and reduced Chl content in leaves. Both ovule and pollen display a dramatic loss of fertility in Sl-IAA27 down-regulated lines, and the internal anatomy of the flower and the fruit are modified, with an enlarged placenta in smaller fruits. In line with the reduced Chl content in Sl-IAA27 RNA interference (RNAi) leaves, genes involved in Chl synthesis display lower expression at the level of transcript accumulation. Even though Sl-IAA27 is closely related to Sl-IAA9 in terms of sequence homology and the encoded proteins share common structural features, the data indicate that the two genes regulate tomato fruit initiation and development in a distinct manner.

Genome-wide identification, functional analysis and expression profiling of the Aux/IAA gene family in tomato.

Auxin is a central hormone that exerts pleiotropic effects on plant growth including the development of roots, shoots, flowers and fruit. The perception and signaling of the plant hormone auxin rely on the cooperative action of several components, among which auxin/indole-3-acetic acid (Aux/IAA) proteins play a pivotal role. In this study, we identified and comprehensively analyzed the entire Aux/IAA gene family in tomato (Solanum lycopersicum), a reference species for Solanaceae plants, and the model plant for fleshy fruit development. Functional characterization using a dedicated single cell system revealed that tomato Aux/IAA proteins function as active repressors of auxin-dependent gene transcription, with, however, different Aux/IAA members displaying varying levels of repression. Phylogenetic analysis indicated that the Aux/IAA gene family is slightly contracted in tomato compared with Arabidopsis, with a lower representation of non-canonical proteins. Sl-IAA genes display distinctive expression pattern in different tomato organs and tissues, and some of them display differential responses to auxin and ethylene, suggesting that Aux/IAAs may play a role in linking both hormone signaling pathways. The data presented here shed more light on Sl-IAA genes and provides new leads towards the elucidation of their function during plant development and in mediating hormone cross-talk.