SUMMARYHydrogen peroxide (H 2 O 2 ) and nitric oxide (·NO) are key reactive species in signal transduction pathways leading to activation of plant defense against biotic or abiotic stress. Here, we investigated the effect of pretreating citrus plants (Citrus aurantium L.) with either of these two molecules on plant acclimation to salinity and show that both pre-treatments strongly reduced the detrimental phenotypical and physiological effects accompanying this stress. A proteomic analysis disclosed 85 leaf proteins that underwent significant quantitative variations in plants directly exposed to salt stress. A large part of these changes was not observed with salt-stressed plants pre-treated with either H 2 O 2 or sodium nitroprusside (SNP; a ·NO-releasing chemical). We also identified several proteins undergoing changes either in their oxidation (carbonylation; 40 proteins) and/or S-nitrosylation (49 proteins) status in response to salinity stress. Both H 2 O 2 and SNP pre-treatments before salinity stress alleviated salinity-induced protein carbonylation and shifted the accumulation levels of leaf S-nitrosylated proteins to those of unstressed control plants. Altogether, the results indicate an overlap between H 2 O 2 -and ·NO-signaling pathways in acclimation to salinity and suggest that the oxidation and S-nitrosylation patterns of leaf proteins are specific molecular signatures of citrus plant vigour under stressful conditions.

Dormancy is an adaptive trait that enables seed germination to coincide with favorable environmental conditions. It has been clearly demonstrated that dormancy is induced by abscisic acid (ABA) during seed development on the mother plant. After seed dispersal, germination is preceded by a decline in ABA in imbibed seeds, which results from ABA catabolism through 8′-hydroxylation. The hormonal balance between ABA and gibberellins (GAs) has been shown to act as an integrator of environmental cues to maintain dormancy or activate germination. The interplay of ABA with other endogenous signals is however less documented. In numerous species, ethylene counteracts ABA signaling pathways and induces germination. In Brassicaceae seeds, ethylene prevents the inhibitory effects of ABA on endosperm cap weakening, thereby facilitating endosperm rupture and radicle emergence. Moreover, enhanced seed dormancy in Arabidopsis ethylene-insensitive mutants results from greater ABA sensitivity. Conversely, ABA limits ethylene action by down-regulating its biosynthesis. Nitric oxide (NO) has been proposed as a common actor in the ABA and ethylene crosstalk in seed. Indeed, convergent evidence indicates that NO is produced rapidly after seed imbibition and promotes germination by inducing the expression of the ABA 8′-hydroxylase gene, CYP707A2, and stimulating ethylene production. The role of NO and other nitrogen-containing compounds, such as nitrate, in seed dormancy breakage and germination stimulation has been reported in several species. This review will describe our current knowledge of ABA crosstalk with ethylene and NO, both volatile compounds that have been shown to counteract ABA action in seeds and to improve dormancy release and germination.

During seed germination, the transition from a quiescent metabolic state in a dry mature seed to a proliferative metabolic state in a vigorous seedling is crucial for plant propagation as well as for optimizing crop yield. This work provides a detailed description of the dynamics of protein synthesis during the time course of germination, demonstrating that mRNA translation is both sequential and selective during this process. The complete inhibition of the germination process in the presence of the translation inhibitor cycloheximide established that mRNA translation is critical for Arabidopsis seed germination. However, the dynamics of protein turnover and the selectivity of protein synthesis (mRNA translation) during Arabidopsis seed germination have not been addressed yet. Based on our detailed knowledge of the Arabidopsis seed proteome, we have deepened our understanding of seed mRNA translation during germination by combining twodimensional gel-based proteomics with dynamic radiolabeled proteomics using a radiolabeled amino acid precursor, namely [35 S]-methionine, in order to highlight de novo protein synthesis, stability, and turnover. Our data confirm that during early imbibition, the Arabidopsis translatome keeps reflecting an embryonic maturation program until a certain developmental checkpoint. Furthermore, by dividing the seed germination time lapse into discrete time windows, we highlight precise and specific patterns of protein synthesis. These data refine and deepen our knowledge of the three classical phases of seed germination based on seed water uptake during imbibition and reveal that selective mRNA translation is a key feature of seed germination. Beyond the quantitative control of translational activity, both the selectivity of mRNA translation and protein turnover appear as specific regulatory systems, critical for timing the molecular events leading to successful germination and seedling establishment. Molecular & Cellular Proteomics 13: 10.1074/mcp.M113.032227, 252-268, 2014.Seed germination is a vital stage in the plant life cycle during which embryo cells experience a programmed transition from a quiescent to a highly active metabolic state. Accordingly, seed quality in terms of germination vigor is of paramount importance for both ecological aspects and practical applications, as it influences the level, timing, and uniformity of seedling emergence in a wide range of environmental conditions (1, 2). In efforts to decipher the complex molecular mechanisms that control seed germination, genetic, genomic, and postgenomic analyses have been developed on the model plant Arabidopsis thaliana (3-6). Germination is classically described as a sequential time course divided into three major phases of seed water uptake (7). Phase I is characterized by rapid seed imbibition, which is crucial for the transition from the quiescent metabolic state of the dry seed to the high metabolic activity of the hydrated seed. Phase II corresponds to a period during which the seed imbibition level remains const...