TIR1/AFB proteins are a class of auxin receptors with key roles in plant development and biotic and abiotic stress responses; several have been identified as targets of the auxin-mimicking herbicide picloram. In this study, we identified five putative TIR1/AFB gene family members in the important vegetable crop Solanum melongena (eggplant) and characterized them using bioinformatics tools and gene expression analyses. Phylogenetic analysis of the TIR1/AFBs classified them into three subgroups based on their Arabidopsis and Solanum lycopersicum homologs. AFB6 homologs were present only in S. melongena and S. lycopersicum, whereas AFB2/3 homologs were found only in Arabidopsis. One pair of S. melongena TIR1 homologs were located in syntenic regions in the genome and appeared to have arisen by segmental duplication. Promoter analysis revealed 898 cis-elements in the TIR1/AFB promoters, 125 of which were related to hormones, stress, light, or growth responses, but only SmAFB5 had a cis-acting regulatory element involved in auxin responsiveness (AuxRR-core). RNA sequencing and expression profiling showed that the TIR1/AFB genes were differentially expressed at different growth stages and in response to light, temperature, and drought. Only SmTIR1A expression was significantly induced by picloram treatment and different growth stages. TIR1/AFB expression is regulated by microRNAs (miRNAs) in other plant species, and we identified 6 or 29 miRNAs that potentially targeted the five TIR1/AFB genes on the basis of comparisons with S. lycopersicum and S. tuberosum miRNAs, respectively. Three-dimensional protein structure predictions revealed that all the TIR1/AFB proteins were very similar in structure, differing only in the numbers of alpha helices and in one angle linking an α helix and a β sheet. For measuring the function of TIR1/AFB genes in response to drought, SmAFB5 was selected, and knockdown by virus-induced gene silence (VIGS) 35S::SmAFB5 lines showed resistance to drought compared to controls. These analyses provide insight into the potential functions of TIR1/AFBs during growth and in response to stress; they highlight differences among the SmTIR1/AFBs that may be useful for eggplant breeding.

Auxin/indole acetic acid (Aux/IAA) proteins repress the expression of auxin-responsive genes to mediate environmental abiotic stresses. Distinct auxinic herbicides are differentially involved in the auxin signaling pathway, affecting the functional role of specific Aux/IAA members. However, the role of Aux/IAA in modulating the sensitivity of eggplant (Solanum melongena L.) to the auxinic herbicide picloram remains largely unexplored. This study aims to identify putative Aux/IAA gene family members in eggplant and to characterized their functional roles in response to auxinic herbicide picloram (HRAC, group 4), offering potential molecular targets for the development of herbicide-resistant eggplant. We conducted the genome-wide identification of Aux/IAA gene family members in eggplant and analyzed their expression profiles in response to picloram. We systematically characterized the functional role of SmIAA15 in response to the herbicide picloram and uncovered the molecular mechanisms underlying SmIAA15-mediated sensitivity of eggplant to picloram using a combination of molecular and biochemical approaches. We identified twenty-three putative Aux/IAA gene family members in eggplant. Under picloram treatment, SmIAA15 was significantly induced and localized to the nucleus. Overexpression of SmIAA15 in tobacco notably decreased resistance against picloram, while virus-induced gene silencing of SmIAA15 in eggplant enhanced resistance to picloram. KEGG analysis of transcriptome data from SmIAA15-silenced lines revealed that the enhanced resistance to picloram was associated with the glutathione metabolism pathway, as evidenced by increased glutathione S-transferases (GST) enzyme activity and upregulation of GST-related genes in SmIAA15-silenced lines. Consistently, stable transgenic tomato lines overexpressing SmIAA15 displayed a picloram-induced phenotype with outwardly curved leaves, and decreased GST activity compared to the non-transgenic controls. Collectively, our findings demonstrate that SmIAA15 acts as a negative regulator in response to picloram by regulating the glutathione S-transferase enzyme activity, potentially contributing to the molecular breeding of picloram-resistant eggplant.