Summary Previous studies demonstrated that auxin and nitric oxide (NO) are plant growth regulators that coordinate several plant physiological responses determining root architecture. Nonetheless, the way in which these factors interact to affect these growth and developmental processes is not well understood. The Arabidopsis thaliana F-box proteins TIR1/AFBs are auxin receptors that mediate degradation of Aux/IAA repressors to induce auxin-regulated responses. A broad spectrum of NO-mediated protein modifications are known in eukaryotic cells. Here, we provide evidence that NO donors increase auxin-dependent gene expression while NO depletion blocks Aux/IAA protein degradation. NO also enhances TIR1-Aux/IAA interaction as evidenced by pull-down and two hybrid assays. In addition, we provide evidence for NO-mediated modulation of auxin signaling through S-nitrosylation of the TIR1 auxin receptor. S-nitrosylation of cysteine is a redox-based post-translational modification that contributes to the complexity of the cellular proteome. We show that TIR1 C140 is a critical residue for TIR1-AuxIAA interaction and TIR1 function. These results suggest that TIR1 S-nitrosylation enhances TIR1-Aux/IAA interaction facilitating Aux/IAA degradation, and subsequently promoting activation of gene expression. Our findings underline the importance of NO in phytohormone signaling pathways.

Auxin regulates gene expression through direct physical interaction with TIR1/AFB receptor proteins during different processes of growth and development in plants. Here we report the contribution of auxin signaling pathway to the adaptative response against abiotic stress in Arabidopsis. Phenotypic characterization of tir1/afb auxin receptor mutants indicates a differential participation of each member under abiotic stress. In particular, tir1 afb2 and tir1 afb3 mutants resulted more tolerant to oxidative stress. In addition, tir1 afb2 showed increased tolerance against salinity measured as chlorophyll content, germination rate and root elongation compared with wild-type plants. Furthermore, tir1 afb2 displayed a reduced accumulation of hydrogen peroxide and superoxide anion, as well as enhanced antioxidant enzymes activities under stress. A higher level of ascorbic acid was detected in tir1 afb2 compared with wild-type plants. Thus, adaptation to salinity in Arabidopsis may be mediated in part by an auxin/redox interaction.