Biostimulant Properties of Protein Hydrolysates: Recent Advances and Future Challenges.

Over the past decade, plant biostimulants have been increasingly used in agriculture as environment-friendly tools that improve the sustainability and resilience of crop production systems under environmental stresses. Protein hydrolysates (PHs) are a main category of biostimulants produced by chemical or enzymatic hydrolysis of proteins from animal or plant sources. Mostly composed of amino acids and peptides, PHs have a beneficial effect on multiple physiological processes, including photosynthetic activity, nutrient assimilation and translocation, and also quality parameters. They also seem to have hormone-like activities. Moreover, PHs enhance tolerance to abiotic stresses, notably through the stimulation of protective processes such as cell antioxidant activity and osmotic adjustment. Knowledge on their mode of action, however, is still piecemeal. The aims of this review are as follows: (i) Giving a comprehensive overview of current findings about the hypothetical mechanisms of action of PHs; (ii) Emphasizing the knowledge gaps that deserve to be urgently addressed with a view to efficiently improve the benefits of biostimulants for different plant crops in the context of climate change.

Leafamine®, a Free Amino Acid-Rich Biostimulant, Promotes Growth Performance of Deficit-Irrigated Lettuce.

Water deficit causes substantial yield losses that climate change is going to make even more problematic. Sustainable agricultural practices are increasingly developed to improve plant tolerance to abiotic stresses. One innovative solution amongst others is the integration of plant biostimulants in agriculture. In this work, we investigate for the first time the effects of the biostimulant -Leafamine®-a protein hydrolysate on greenhouse lettuce (Lactuca sativa L.) grown under well-watered and water-deficit conditions. We examined the physiological and metabolomic water deficit responses of lettuce treated with Leafamine® (0.585 g/pot) or not. Root application of Leafamine® increased the shoot fresh biomass of both well-watered (+40%) and deficit-irrigated (+20%) lettuce plants because the projected leaf area increased. Our results also indicate that Leafamine® application could adjust the nitrogen metabolism by enhancing the total nitrogen content, amino acid (proline) contents and the total protein level in lettuce leaves, irrespective of the water condition. Osmolytes such as soluble sugars and polyols, also increased in Leafamine®-treated lettuce. Our findings suggest that the protective effect of Leafamine is a widespread change in plant metabolism and could involve ABA, putrescine and raffinose.

Auxin-mediated nitrate signalling by NRT1.1 participates in the adaptive response of Arabidopsis root architecture to the spatial heterogeneity of nitrate availability.

To optimize their nitrogen nutrition, plants are able to direct root growth in nitrate-rich patches. This depends in Arabidopsis on the NRT1.1 nitrate transporter/sensor. NRT1.1 was shown to display on homogenous medium, an auxin transport activity that lowers auxin accumulation in lateral roots and inhibits their growth at low nitrate. Using a split-root system, we explored the hypothesis that preferential lateral root growth in the nitrate-rich side involves the NRT1.1-dependent repression of lateral root growth in the low nitrate side. Data show that NRT1.1 acts locally to modulate both auxin levels and meristematic activity in response to the low nitrate concentration directly experienced by lateral roots leading to a repression of their growth. A stimulatory role of NRT1.1 in the high nitrate side, which does not rely on changes in auxin levels, is also observed. Altogether, our data suggest that NRT1.1 allows preferential root colonization of nitrate-rich patches by both preventing root growth in response to low nitrate, through modulation of auxin traffic, and stimulating root growth in response to high nitrate, through a yet uncharacterized mechanism. In addition, transcriptional regulation of NRT1.1 affects both mechanisms allowing plants to modulate the effect of nitrate on root branching.

Potential denitrification and nitrous oxide production in the sediments of the Seine River Drainage Network (France).

To investigate bottom sediment denitrification at the scale of the Seine drainage network, a semi-potential denitrification assay was used in which river sediments (and riparian soils) were incubated for a few hours under anaerobic conditions with non limiting nitrate concentrations. This method allowed the nitrous oxide (N(2)O) concentration in the headspace, as well as the nitrate, nitrite, and ammonium concentrations to be determined during incubation. The rates at which nitrate decreased and N(2)O increased were then used to assess the potential denitrification activity and associated N(2)O production in the Seine River Basin. We observed a longitudinal pattern characterized by a significant increase of the potential rate of denitrification from upstream sectors to large downstream rivers (orders 7-8), from approximately 3.3 to 9.1 microg NO(3)(-)-N g(-1) h(-1), respectively, while the N(2)O production rates was the highest both in headwaters and in large order rivers (0.14 and 0.09 N(2)O-N g(-1) h(-1), respectively) and significantly lower in the intermediate sectors (0.01 and 0.03 N(2)O-N g(-1) h(-1)). Consequently, the ratio N(2)O production:NO(3) reduction was found to reach 5% in headstreams, whereas it averaged 1.2% in the rest of the drainage network, an intermediate percentage being found for the riparian soils. Finally, the ignition loss of sediments, together with other redundant variables (particulate organic carbon content: g C 100 g(-1) dry weight [dw], moisture: g water 100 g(-1) dw, sediment size <50 mum: g material size <50 mum 100 g(-1) dw) were found to control these activities. However, the biodegradability of organic matter must be measured to better understand the factor controlling denitrification and its associated N(2)O production.

Enhanced thrombosis in atherosclerosis-prone mice is associated with increased arterial expression of plasminogen activator inhibitor-1.

OBJECTIVE: This study was undertaken to investigate the origin and pathophysiological importance of plasminogen activator inhibitor (PAI-1) in atherosclerosis. METHODS AND RESULTS: We used the ferric chloride model to induce carotid artery injury in apolipoprotein E knockout (apoE-/-) and wild-type (WT) mice. ApoE-/- mice fed high-fat diet for 4 months developed severe hypercholesterolemia and had significantly elevated plasma PAI-1 levels (2.3+/-0.3 versus 0.6+/-0.1 ng/mL in WT mice; P<0.05). These mice exhibited a prothrombotic phenotype with shortened times to thrombotic arterial occlusion (8.6 versus 11.5 minutes; P<0.001) and reduced recanalization rates (12% versus 51%; P<0.0001) compared with WT mice. In situ hybridization, reverse transcriptase-polymerase chain reaction, and immunohistochemistry showed a significantly upregulated PAI-1 expression in P-selectin-positive (activated) endothelial cells lining normal-appearing arterial segments and within the advanced atherosclerotic lesions of apoE-/- mice. No significant upregulation of PAI-1 expression was found in the other organs studied, and only trace amounts of PAI-1 mRNA were detected in murine platelets. Importantly, deletion of the PAI-1 gene reversed the prothrombotic tendency and reduced neointimal growth after injury in apoE-/- mice despite the persistence of excessive hypercholesterolemia. CONCLUSIONS: These results suggest that increased vascular expression of PAI-1 may contribute to the elevated circulating levels of the inhibitor and be responsible, at least in part, for the prothrombotic phenotype in apoE-/- mice.

Nitrous oxide production kinetics during nitrate reduction in river sediments.

A significant amount of nitrogen entering river basins is denitrified in riparian zones. The aim of this study was to evaluate the influence of nitrate and carbon concentrations on the kinetic parameters of nitrate reduction as well as nitrous oxide emissions in river sediments in a tributary of the Marne (the Seine basin, France). In order to determine these rates, we used flow-through reactors (FTRs) and slurry incubations; flow-through reactors allow determination of rates on intact sediment slices under controlled conditions compared to sediment homogenization in the often used slurry technique. Maximum nitrate reduction rates (R(m)) ranged between 3.0 and 7.1microg Ng(-1)h(-1), and affinity constant (K(m)) ranged from 7.4 to 30.7mg N-NO(3)(-)L(-1). These values were higher in slurry incubations with an R(m) of 37.9microg Ng(-1)h(-1) and a K(m) of 104mg N-NO(3)(-)L(-1). Nitrous oxide production rates did not follow Michaelis-Menten kinetics, and we deduced a rate constant with an average of 0.7 and 5.4ng Ng(-1)h(-1) for FTR and slurry experiments respectively. The addition of carbon (as acetate) showed that carbon was not limiting nitrate reduction rates in these sediments. Similar rates were obtained for FTR and slurries with carbon addition, confirming the hypothesis that homogenization increases rates due to release of and increasing access to carbon in slurries. Nitrous oxide production rates in FTR with carbon additions were low and represented less than 0.01% of the nitrate reduction rates and were even negligible in slurries. Maximum nitrate reduction rates revealed seasonality with high potential rates in fall and winter and low rates in late spring and summer. Under optimal conditions (anoxia, non-limiting nitrate and carbon), nitrous oxide emission rates were low, but significant (0.01% of the nitrate reduction rates).

Nitrate-regulated auxin transport by NRT1.1 defines a mechanism for nutrient sensing in plants.

Nitrate is both a nitrogen source for higher plants and a signal molecule regulating their development. In Arabidopsis, the NRT1.1 nitrate transporter is crucial for nitrate signaling governing root growth, and has been proposed to act as a nitrate sensor. However, the sensing mechanism is unknown. Herein we show that NRT1.1 not only transports nitrate but also facilitates uptake of the phytohormone auxin. Moreover, nitrate inhibits NRT1.1-dependent auxin uptake, suggesting that transduction of nitrate signal by NRT1.1 is associated with a modification of auxin transport. Among other effects, auxin stimulates lateral root development. Mutation of NRT1.1 enhances both auxin accumulation in lateral roots and growth of these roots at low, but not high, nitrate concentration. Thus, we propose that NRT1.1 represses lateral root growth at low nitrate availability by promoting basipetal auxin transport out of these roots. This defines a mechanism connecting nutrient and hormone signaling during organ development.

The Arabidopsis NRT1.1 transporter participates in the signaling pathway triggering root colonization of nitrate-rich patches.

Localized proliferation of lateral roots in NO(3)(-)-rich patches is a striking example of the nutrient-induced plasticity of root development. In Arabidopsis, NO(3)(-) stimulation of lateral root elongation is apparently under the control of a NO(3)(-)-signaling pathway involving the ANR1 transcription factor. ANR1 is thought to transduce the NO(3)(-) signal internally, but the upstream NO(3)(-) sensing system is unknown. Here, we show that mutants of the NRT1.1 nitrate transporter display a strongly decreased root colonization of NO(3)(-)-rich patches, resulting from reduced lateral root elongation. This phenotype is not due to lower specific NO(3)(-) uptake activity in the mutants and is not suppressed when the NO(3)(-)-rich patch is supplemented with an alternative N source but is associated with dramatically decreased ANR1 expression. These results show that NRT1.1 promotes localized root proliferation independently of any nutritional effect and indicate a role in the ANR1-dependent NO(3)(-) signaling pathway, either as a NO(3)(-) sensor or as a facilitator of NO(3)(-) influx into NO(3)(-)-sensing cells. Consistent with this model, the NRT1.1 and ANR1 promoters both directed reporter gene expression in root primordia and root tips. The inability of NRT1.1-deficient mutants to promote increased lateral root proliferation in the NO(3)(-)-rich zone impairs the efficient acquisition of NO(3)(-) and leads to slower plant growth. We conclude that NRT1.1, which is localized at the forefront of soil exploration by the roots, is a key component of the NO(3)(-)-sensing system that enables the plant to detect and exploit NO(3)(-)-rich soil patches.