A physiological and molecular study of the effects of nickel deficiency and phenylphosphorodiamidate (PPD) application on urea metabolism in oilseed rape (Brassica napus L.)

Background and aims

Urea is the major nitrogen (N) form supplied as fertilizer in agriculture. However, urease, a nickel-dependent enzyme, allows plants to use external or internally generated urea as a nitrogen source. Since a urease inhibitor is frequently applied in conjunction with urea fertilizer, the N-metabolism of plants may be affected. The aim of this study was to determine physiological and molecular effects of nickel deficiency and a urease inhibitor on urea uptake and assimilation in oilseed rape.

Methods

Plants were grown on hydroponic solution with urea as the sole N source under three treatments: plants treated with nickel (+Ni) as a control, without nickel (?Ni) and with nickel and phenylphosphorodiamidate (+Ni+PPD). Urea transport and assimilation were investigated.

Results

The results show that Ni-deficiency or PPD supply led to reduced growth and reduced ¹⁵N-uptake from urea. This effect was more pronounced in PPD-treated plants, which accumulated high amounts of urea and ammonium. Thus, Ni-deficiency or addition of PPD, limit the availability of N and decreased shoot and root amino acid content. The up-regulation of BnDUR3 in roots indicated that this gene is a component of the stress response to nitrogen-deficiency. A general decline of glutamine synthetase (GS) activity and activation of glutamate dehydrogenase (GDH) and increases in its expression level were observed in control plants. At the same time, in (?N) or (+Ni+PPD) treated plants, no increases in GS or GDH activities and expression level were found.

Conclusions

Overall results showed that plants require Ni as a nutrient (while most widely used nutrient solutions are devoid of Ni), whether they are grown with or without a urea supply, and that urease inhibitors may have deleterious effects at least in hydroponic grown oilseed rape.     

Communities of Endophytic Sebacinales Associated with Roots of Herbaceous Plants in Agricultural and Grassland Ecosystems Are Dominated by Serendipita herbamans sp. nov

Endophytic fungi are known to be commonly associated with herbaceous plants, however, there are few studies focusing on their occurrence and distribution in plant roots from ecosystems with different land uses. To explore the phylogenetic diversity and community structure of Sebacinales endophytes from agricultural and grassland habitats under different land uses, we analysed the roots of herbaceous plants using strain isolation, polymerase chain reaction (PCR), transmission electron microscopy (TEM) and co-cultivation experiments. A new sebacinoid strain named Serendipita herbamans belonging to Sebacinales group B was isolated from the roots of Bistorta vivipara, which is characterized by colourless monilioid cells (chlamydospores) that become yellow with age. This species was very common and widely distributed in association with a broad spectrum of herbaceous plant families in diverse habitats, independent of land use type. Ultrastructurally, the presence of S. herbamans was detected in the cortical cells of Plantago media, Potentilla anserina and Triticum aestivum. In addition, 13 few frequent molecular operational taxonomic units (MOTUs) or species were found across agricultural and grassland habitats, which did not exhibit a distinctive phylogenetic structure. Laboratory-based assays indicate that S. herbamans has the ability to colonize fine roots and stimulate plant growth. Although endophytic Sebacinales are widely distributed across agricultural and grassland habitats, TEM and nested PCR analyses reinforce the observation that these microorganisms are present in low quantity in plant roots, with no evidence of host specificity.     

Nitrate supply induces changes in polyamine content and ethylene production in wheat plants grown with ammonium

In order to explore the mechanisms of nitrate's beneficial effect on ammonium-grown plants, we investigated the effects of nitrate on free and conjugated polyamine plant content and ethylene biosynthesis in wheat (Triticum aestivum L.) plants grown with ammonium nutrition. Two different doses of nitrate (100 microM and 5 mM) were supplied to ammonium-fed plants, in order to determine whether the effects of nitrate require significant doses (nutritional character), or can be promoted by very low doses (pseudo-hormonal character). Our results showed that nitrate's effects on putrescine, spermidine and spermine contents of ammonium-grown plants tended to follow the pattern associated with strict nitrate nutrition. Both low (100 microM) and high (5 mM) nitrate doses caused a rapid and significant increase in free spermidine content in roots and shoots, which was well correlated with reduced root ethylene production. In shoots, this increase in free spermidine was correlated with changes in the conjugation pattern, while in roots these changes appear to be due to alternative mechanisms. On the other hand, no clear relationship between the supply of a lower dose of nitrate (100 microM) and a reduction of free putrescine content was observed. With higher doses of nitrate (5 mM) we observed a reduction of free putrescine content that was well correlated with increases in its conjugated forms. In conclusion, nitrate's effects on putrescine, spermidine and spermine contents of ammonium-fed plants tended to follow the pattern associated with strict nitrate nutrition, corroborating its beneficial effect.     

Effect of <Emphasis Type="Italic">N</Emphasis>-(<Emphasis Type="Italic">n</Emphasis>-butyl) thiophosphoric triamide on urea metabolism and the assimilation of ammonium by <Emphasis Type="Italic">Triticum aestivum</Emphasis> L.

The use of urea as an N fertilizer has increased to such an extent that it is now the most widely used fertilizer in the world. However, N losses as a result of ammonia volatilization lead to a decrease in its efficiency, therefore different methods have been developed over the years to reduce these losses. One of the most recent involves the use of urea combined with urease inhibitors, such as N-(n-butyl) thiophosphoric triamide (NBPT), in an attempt to delay the hydrolysis of urea in the soil. The aim of this study was to perform an in-depth analysis of the effect that NBPT use has on plant growth and N metabolism. Wheat plants were cultivated in a greenhouse experiment lasting 4 weeks and fertilized with urea and NBPT at different concentrations (0, 0.012, 0.062, 0.125%). Each treatment was replicated six times. A non-fertilized control was also cultivated. Several parameters related with N metabolism were analysed at the end of growth period. NBPT use was found to have visible effects, such as a transitory yellowing of the leaf tips, at the end of the first week of treatment. At a metabolic level, plants treated with the inhibitor were found to have more urea in their tissues and a lower amino acid content, lower glutamine synthetase activity, and lower urease and glutamine synthetase content at the end of the study period, whereas their urease activity seemed to have recovered by this stage.     

Nitrogen assimilation and transpiration: key processes conditioning responsiveness of wheat to elevated [CO2] and temperature

Although climate scenarios have predicted an increase in [CO₂] and temperature conditions, to date few experiments have focused on the interaction of [CO₂] and temperature effects in wheat development. Recent evidence suggests that photosynthetic acclimation is linked to the photorespiration and N assimilation inhibition of plants exposed to elevated CO₂. The main goal of this study was to analyze the effect of interacting [CO₂] and temperature on leaf photorespiration, C/N metabolism and N transport in wheat plants exposed to elevated [CO₂] and temperature conditions. For this purpose, wheat plants were exposed to elevated [CO₂] (400 vs 700 µmol mol^?1) and temperature (ambient vs ambient + 4°C) in CO₂ gradient greenhouses during the entire life cycle. Although at the agronomic level, elevated temperature had no effect on plant biomass, physiological analyses revealed that combined elevated [CO₂] and temperature negatively affected photosynthetic performance. The limited energy levels resulting from the reduced respiratory and photorespiration rates of such plants were apparently inadequate to sustain nitrate reductase activity. Inhibited N assimilation was associated with a strong reduction in amino acid content, conditioned leaf soluble protein content and constrained leaf N status. Therefore, the plant response to elevated [CO₂] and elevated temperature resulted in photosynthetic acclimation. The reduction in transpiration rates induced limitations in nutrient transport in leaves of plants exposed to elevated [CO₂] and temperature, led to mineral depletion and therefore contributed to the inhibition of photosynthetic activity.     

Two Biostimulants Derived from Algae or Humic Acid Induce Similar Responses in the Mineral Content and Gene Expression of Winter Oilseed Rape (Brassica napus L.)

Different strategies, known as crop biofortification, can be used to increase micronutrient concentrations in harvested parts to reduce nutrient deficiencies in the human diet. Apart from fertilization and genetic selection, a more environmentally friendly, less expensive, and more immediate solution could rely on the use of biostimulants derived from natural materials. Two biostimulants, AZAL5 and HA7, which are derived from seaweed and black peat, respectively, have been previously described as promoting growth of Brassica napus and having a substantial effect on gene expression. They were further studied to evaluate their effects on N and S and a wide range of other nutrients (that is, K, Ca, P, Mg, Fe, Na, Mn, B, Si, Cu, and Zn). Providing these two biostimulants in the nutrient solution did not change the mineral supply significantly, but they mostly stimulated root growth and macronutrient uptake (N, S, K, and P) at a level similar to growth. Both biostimulants also stimulate chloroplast division. More surprisingly, they also increased Mg, Mn, Na, and Cu plant concentrations and root-to-shoot translocation of Fe and Zn. These observations were associated with an increased expression of a Cu transporter (COPT2) and NRAMP3, a gene putatively involved in Fe and Zn translocation. Overall, this study showed that specific nutrient balance and transport were stimulated by both biostimulants more significantly than growth, offering new perspectives for biofortification strategies.     

Divergence Times and Phylogenetic Patterns of Sebacinales,a Highly Diverse and Widespread Fungal Lineage

Patterns of geographic distribution and composition of fungal communities are still poorly understood. Widespread occurrence in terrestrial ecosystems and the unique richness of interactions of Sebacinales with plants make them a target group to study evolutionary events in the light of nutritional lifestyle. We inferred diversity patterns, phylogenetic structures and divergence times of Sebacinales with respect to their nutritional lifestyles by integrating data from fossil-calibrated phylogenetic analyses. Relaxed molecular clock analyses indicated that Sebacinales originated late Permian within Basidiomycota, and their split into Sebacinaceae and Serendipitaceae nom. prov. likely occurred during the late Jurassic and the early Cretaceous, coinciding with major diversifications of land plants. In Sebacinaceae, diversification of species with ectomycorrhizal lifestyle presumably started during the Paleocene. Lineage radiations of the core group of ericoid and cavendishioid mycorrhizal Sebacinales started probably in the Eocene, coinciding with diversification events of their hosts. The diversification of Sebacinales with jungermannioid interactions started during the Oligocene, and occurred much later than the diversification of their hosts. Sebacinales communities associated either with ectomycorrhizal plants, achlorophyllous orchids, ericoid and cavendishioid Ericaceae or liverworts were phylogenetically clustered and globally distributed. Major Sebacinales lineage diversifications started after the continents had drifted apart. We also briefly discuss dispersal patterns of extant Sebacinales.