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.

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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.

URA 724 est intégrée dans l'UMR6113 ISTO CNRS Université d'OrléansInternational audienceA semiquantitative analysis of the infrared spectra from 650 to 4000 cm-' of six vitrains and the coals associated with three of them is presented. This analysis supplies information on the concentration of several functionalities in the samples. Relationships between infrared data and several parameters (carbon content, vitrinite reflectance, pyrolysis data) are discussed. The results thus obtained contribute to a better understanding of vitrinite structure. They also help to explain the discrepancies between rank parameters obtained from chemical analyses and those determined by petrographic measurements. The comparison between the infrared data of the Cretacic-Albian vitrain from Teruel (Spain) and its associated coal reveals the unusual chemical composition of this vitrain. The infrared analysis of this vitrain and the vitrian from Asturias (Jurassic-Malm) shows unusual features in comparison with “normal” coals. The intensity of the aliphatic peaks relative to the aromatic (A factor) in these vitrains is characteristic of immature samples, closer to those of type I1 kerogens than type 111. This result agrees with the perhydrous character of these samples and helps to explain volatile matter and pyrolysis data. However, for the vitrain from Asturias, the intensity of the bands due to aromatic hydrogen and the ratio between aromatic and aliphatic hydrogen are consistent with a stage of evolution that is higher than the diagenesislcatagenesis transition in agreement with other chemical and petrographic data. In the vitrains from Asturias and Teruel an absorption band at 1500 cm-l is assigned to aromatic C=C bonds. In these vitrains, the band centered at 815 cm-l is the most prominent in the 900-700 cm-l region. These spectral features are unusual in coal spectra, but they seem to be characteristic of perhydrous coals

Light cues from neighboring vegetation rapidly initiate plant shade-avoidance responses. Despite our detailed knowledge of the early steps of this response, the molecular events under prolonged shade are largely unclear. Here we show that persistent neighbor cues reinforce growth responses in addition to promoting auxin-responsive gene expression in and soybean. However, while the elevation of auxin levels is well established as an early event, in, the response to prolonged shade occurs when auxin levels have declined to the prestimulation values. Remarkably, the sustained low activity of phytochrome B under prolonged shade led to () decreased levels of PHYTOCHROME INTERACTING FACTOR 4 (PIF4) in the cotyledons (the organs that supply auxin) along with increased levels in the vascular tissues of the stem, () elevated expression of the PIF4 targets () and , which in turn reduced the expression of the growth-repressive regulator, () reduced abundance of AUXIN RESPONSE FACTOR 6, () reduced expression of and increased abundance of its targets, the auxin receptors, and () elevated auxin signaling as indicated by molecular markers. Mathematical and genetic analyses support the physiological role of this system-level rearrangement. We propose that prolonged shade rewires the connectivity between light and auxin signaling to sustain shade avoidance without enhanced auxin levels.

One of the most striking aspects of plant plasticity is the modulation of development in response to environmental changes. Plant growth and development largely depend on the phytohormone auxin that exerts its function through a partially redundant family of F-box receptors, the TIR1-AFBs. We have previously reported that the Arabidopsis double mutant tir1 afb2 is more tolerant to salt stress than wild-type plants and we hypothesized that down-regulation of auxin signaling might be part of Arabidopsis acclimation to salinity. In this work, we show that NaCl-mediated salt stress induces miR393 expression by enhancing the transcription of AtMIR393A and leads to a concomitant reduction in the levels of the TIR1 and AFB2 receptors. Consequently, NaCl triggers stabilization of Aux/IAA repressors leading to down-regulation of auxin signaling. Further, we report that miR393 is likely involved in repression of lateral root (LR) initiation, emergence and elongation during salinity, since the mir393ab mutant shows reduced inhibition of emergent and mature LR number and length upon NaCl-treatment. Additionally, mir393ab mutant plants have increased levels of reactive oxygen species (ROS) in LRs, and reduced ascorbate peroxidase (APX) enzymatic activity compared with wild-type plants during salinity. Thus, miR393 regulation of the TIR1 and AFB2 receptors could be a critical checkpoint between auxin signaling and specfic redox-associated components in order to coordinate tissue and time-specific growth responses and tolerance during acclimation to salinity in Arabidopsis.

This work analyses a set of perhydrous coals (mainly composed of huminite/vitrinite maceral group) in order to determine the inter-relations between the hydrogen content and the modifications in the coal structure at a molecular level. The study involves the direct solid state characterisation of the coal combined with the analysis of representative fragments of the coal network obtained through flash-pyrolysis. The perhydrous character of the coals is not reflected either in the aliphatic hydrogen concentration (from FTIR data) or by the presence of straight-chain aliphatic moieties in the pyrolysates. This structural study shows that perhydrous coals contain mainly aromatic structures with 1-2 rings and a very small concentration of aromatic rings of large size. In agreement with this, phenol and alkyl phenols are the most prominent degradation products whereas other aromatic compounds (mainly benzene and naphthalene derivatives) are minor and probably evaporative compounds. The major structural elements in the samples studied are simple phenols with a preponderance of substituted para alkyl. The results obtained show that the processes of hydrogen enrichment affect the reactions of aromatisation and condensation. During the natural evolution of the perhydrous coals the transformations of the oxygenated functionalities in the lignin precursor seem to have taken place without the parallel structural reorganisation of the lignin framework responsible for the formation of polycyclic aromatic systems. As a result, the chemical structure of perhydrous vitrinites in coals is substantially modified with respect to that described in 'normal' coals. The results obtained also indicate that the source of hydrogen content and the effect that it has during the subsequent evolution process of the coals, affects the chemical structure of the perhydrous vitrinite and hence its properties and behaviour.

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