NO (nitric oxide) production from sunflower plants (Helianthus annuus L.), detached spinach leaves (Spinacia oleracea L.), desalted spinach leaf extracts or commercial maize (Zea mays L.) leaf nitrate reductase (NR, EC 1.6.6.1) was continuously followed as NO emission into the gas phase by chemiluminescence detection, and its response to post-translational NR modulation was examined in vitro and in vivo. NR (purified or in crude extracts) in vitro produced NO at saturating NADH and nitrite concentrations at about 1% of its nitrate reduction capacity. The K(m) for nitrite was relatively high (100 microM) compared to nitrite concentrations in illuminated leaves (10 microM). NO production was competitively inhibited by physiological nitrate concentrations (K(i)=50 microM). Importantly, inactivation of NR in crude extracts by protein phosphorylation with MgATP in the presence of a protein phosphatase inhibitor also inhibited NO production. Nitrate-fertilized plants or leaves emitted NO into purified air. The NO emission was lower in the dark than in the light, but was generally only a small fraction of the total NR activity in the tissue (about 0.01-0.1%). In order to check for a modulation of NO production in vivo, NR was artificially activated by treatments such as anoxia, feeding uncouplers or AICAR (a cell permeant 5'-AMP analogue). Under all these conditions, leaves were accumulating nitrite to concentrations exceeding those in normal illuminated leaves up to 100-fold, and NO production was drastically increased especially in the dark. NO production by leaf extracts or intact leaves was unaffected by nitric oxide synthase inhibitors. It is concluded that in non-elicited leaves NO is produced in variable quantities by NR depending on the total NR activity, the NR activation state and the cytosolic nitrite and nitrate concentration.

SummaryQuantitative data on nitric oxide (NO) production by plants, and knowledge of participating reactions and rate limiting factors are still rare. We quantified NO emission from tobacco (Nicotiana tabacum) wild-type leaves, from nitrate reductase (NR)-or nitrite reductase (NiR)-deficient leaves, from WT-or from NR-deficient cell suspensions and from mitochondria purified from leaves or cells, by following NO emission through chemiluminescence detection. In all systems, NO emission was exclusively due to the reduction of nitrite to NO, and the nitrite concentration was an important rate limiting factor. Using inhibitors and purified mitochondria, mitochondrial electron transport was identified as a major source for reduction of nitrite to NO, in addition to NR. NiR and xanthine dehydrogenase appeared to be not involved. At equal respiratory activity, mitochondria from suspension cells had a much higher capacity to produce NO than leaf mitochondria. NO emission in vivo by NiR-mutant leaves (which was not nitrite limited) was proportional to photosynthesis (high in light þCO 2 , low in light )CO 2 , or in the dark). With most systems including mitochondrial preparations, NO emission was low in air (and darkness for leaves), but high under anoxia (nitrogen). In contrast, NO emission by purified NR was not much different in air and nitrogen. The low aerobic NO emission of darkened leaves and cell suspensions was not due to low cytosolic NADH, and appeared only partly affected by oxygendependent NO scavenging. The relative contribution of NR and mitochondria to nitrite-dependent NO production is estimated.

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Under drought, CO2 assimilation rates decrease already at small leaf water deficits. At least part of the inhibition is attributed to non‐stomata1 effects at the chloroplast level, with electron transport and phosphorylation being main targets of inhibition. These findings are questioned by direct measurements of photosynthetic capacity with systems that are not Limited by stomata, e.g. leaf slices in solution or leaves at ex‐ternal CO2 concentrations exceeding 5%. Here, photosynthesis was rather insensitive to dehydration down to 50–70% relative water content, and different plant species re‐sponded in a very similar way. More severe dehydration affected not only pboto‐synthesis, but also dark CO2 fixation and presumably also photorespiration. Rever‐sible and unspecific inhibition is thought to be mediated mainly by increased concen‐trations of solutes in dehydrated cells. Inhibition of photorespiration might favour photoinhibition when long‐term water stress is coupled with full sunlight. Photo‐inhibition, together with general senescence phenomena might be involved in long‐term effects of water stress under natural drought conditions. This offers an explanation for the conflicting results of short‐term water stress experiments and studies carried out under field conditions.

One of the great challenges in the near future will be the sustainable production of sufficient amounts of safe food worldwide. A combination of adverse demographic factors and climatological perturbations is expected to impact on food systems globally (Vermeulen et al., 2012). To ensure food security in the coming years, multidisciplinary approaches are needed, and useful leads are likely to emerge from advances in plant biotechnology. However, improving plants' performance under restrictive growth conditions will require a deep understanding of the molecular processes that underlie their extraordinary physiological plasticity. Much research in plant biology in recent decades has focused on phenomenological descriptions of changes in gene expression at the mRNA level. However, in eukaryotes, shifts in transcript levels do not always correlate with equivalent changes in protein levels, since a variety of post-transcriptional events may uncouple transcription from translation (Vogel and Marcotte, 2012). This is particularly relevant in plants, given the intrinsically complex properties of their translational apparatus, the presence of additional genetic systems in mitochondria and chloroplasts, and the occurrence of environment-dependent variation of cytosolic ribosome composition (Hummel et al., 2012). Recent advances in next-generation-sequencing, combined with breakthrough technologies like ribosome footprint profiling, may help to close the gap between transcriptional and translational studies and further elucidate plant responses to environmental factors. In this Research Topic, we present reviews and original research articles that extend our knowledge of post-transcriptional and translational mechanisms that regulate the production of plant proteins which ultimately execute the cellular functions required for adaptation to environmental challenges. Many of the contributions highlight the importance of translational regulation. Among the panoply of translational regulators, the TOR kinase emerges as a key regulatory element. Dobrenel et al. have used transgenic plants deficient in TOR activity to reveal its role in regulating levels of chloroplast ribosomal proteins, possibly by recognizing a sequence motif present in their mRNAs. This work also demonstrated that the TOR pathway is involved in phosphorylation of the ribosomal

Summary• Chilling triggers rapid molecular responses that permit the maintenance of plant cell homeostasis and plant adaptation. Recent data showed that nitric oxide (NO) is involved in plant acclimation and tolerance to cold. The participation of NO in the early transduction of the cold signal in Arabidopsis thaliana was investigated.• The production of NO after a short exposure to cold was assessed using the NOsensitive fluorescent probe 4, 5-diamino fluoresceine diacetate and chemiluminescence. Pharmacological and genetic approaches were used to analyze NO sources and NO-mediated changes in cold-regulated gene expression, phosphatidic acid (PtdOH) synthesis and sphingolipid phosphorylation.• NO production was detected after 1-4 h of chilling. It was impaired in the nia1nia2 nitrate reductase mutant. Moreover, NO accumulation was not observed in H7 plants overexpressing the A. thaliana nonsymbiotic hemoglobin Arabidopsis haemoglobin 1 (AHb1). Cold-regulated gene expression was affected in nia1nia2 and H7 plants. The synthesis of PtdOH upon chilling was not modified by NO depletion. By contrast, the formation of phytosphingosine phosphate and ceramide phosphate, two phosphorylated sphingolipids that are transiently synthesized upon chilling, was negatively regulated by NO.• Taken together, these data suggest a new function for NO as an intermediate in gene regulation and lipid-based signaling during cold transduction.

Populus euphratica Olivier is known to exist in saline and arid environments. In this study we investigated the physiological mechanisms enabling this species to cope with stress caused by salinity. Acclimation to increasing Na 1 concentrations required adjustments of the osmotic pressure of leaves, which were achieved by accumulation of Na 1 and compensatory decreases in calcium and soluble carbohydrates. The counterbalance of Na 1 /Ca 21 was also observed in mature leaves from field-grown P. euphratica trees exposed to an environmental gradient of increasing salinity. X-ray microanalysis showed that a primary strategy to protect the cytosol against sodium toxicity was apoplastic but not vacuolar salt accumulation. The ability to cope with salinity also included maintenance of cytosolic potassium concentrations and development of leaf succulence due to an increase in cell number and cell volume leading to sodium dilution. Decreases in apoplastic and vacuolar Ca 21 combined with suppression of calcineurin B-like protein transcripts suggest that Na 1 adaptation required suppression of calcium-related signaling pathways. Significant increases in galactinol synthase and alternative oxidase after salt shock and salt adaptation point to shifts in carbohydrate metabolism and suppression of reactive oxygen species in mitochondria under salt stress.

Summary1 Although most theories on plant defence assume that costs will result from the production and maintenance of defensive traits, studies on the costs of induced defence against pathogens are comparatively rare. 2 We focus on ®tness costs resulting from the chemical induction of systemic acquired resistance (SAR), a rather unspeci®c form of defence, which can be induced by and is eective against a broad spectrum of bacteria, fungi and viruses.3 We used a model system in which we treated wheat plants that were protected against fungi by`traditional' fungicides with BION 1 (a benzothiadiazole which induces pathogen resistance). Treated plants were therefore compelled to invest in defence without gaining any pro®t from the induction. 4 Treated plants achieved lower biomass than untreated controls, and developed fewer shoots and ears and therefore produced fewer seeds. The eects were most pronounced in plants that suered from a shortage of nitrogen, and were observed only when pathogen resistance was induced during lateral shoot production. Later treatment revealed no signi®cant eects. 5 We discuss whether the dierences between treated and control plants can be interpreted as a consequence of allocation costs. Such costs could result from metabolic competition between processes involved in plant growth and the synthesis of defence-related compounds.