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Background: In viticulture, iron (Fe) chlorosis is a common abiotic stress that impairs plant development and leads to yield and quality losses. Under low availability of the metal, the applied N form (nitrate and ammonium) can play a role in promoting or mitigating Fe deficiency stresses. However, the processes involved are not clear in grapevine. Therefore, the aim of this study was to investigate the response of two grapevine rootstocks to the interaction between N forms and Fe uptake. This process was evaluated in a hydroponic experiment using two ungrafted grapevine rootstocks Fercal (Vitis berlandieri x V. vinifera) tolerant to deficiency induced Fe chlorosis and Couderc 3309 (V. riparia x V. rupestris) susceptible to deficiency induced Fe chlorosis. Results: The results could differentiate Fe deficiency effects, N-forms effects, and rootstock effects. Interveinal chlorosis of young leaves appeared earlier on 3309 C from the second week of treatment with NO3−/NH4+ (1:0)/-Fe, while Fercal leaves showed less severe symptoms after four weeks of treatment, corresponding to decreased chlorophyll concentrations lowered by 75% in 3309 C and 57% in Fercal. Ferric chelate reductase (FCR) activity was by trend enhanced under Fe deficiency in Fercal with both N combinations, whereas 3309 C showed an increase in FCR activity under Fe deficiency only with NO3−/NH4+ (1:1) treatment. With the transcriptome analysis, Gene Ontology (GO) revealed multiple biological processes and molecular functions that were significantly regulated in grapevine rootstocks under Fe-deficient conditions, with more genes regulated in Fercal responses, especially when both forms of N were supplied. Furthermore, the expression of genes involved in the auxin and abscisic acid metabolic pathways was markedly increased by the equal supply of both forms of N under Fe deficiency conditions. In addition, changes in the expression of genes related to Fe uptake, regulation, and transport reflected the different responses of the two grapevine rootstocks to different N forms. Conclusions: Results show a clear contribution of N forms to the response of the two grapevine rootstocks under Fe deficiency, highlighting the importance of providing both N forms (nitrate and ammonium) in an appropriate ratio in order to ease the rootstock responses to Fe deficiency. [ABSTRACT FROM AUTHOR]

A fully mechanistic dynamical model for plant nitrate uptake is presented. Based on physiological and regulatory pathways and based on physical laws, we form a dynamic system mathematically described by seven differential equations. The model evidences the presence of a short‐term positive feedback on the high‐affinity nitrate uptake, triggered by the presence of nitrate around the roots, which induces its intaking. In the long run, this positive feedback is overridden by two long‐term negative feedback loops which drastically reduces the nitrate uptake capacity. These two negative feedbacks are due to the generation of ammonium and amino acids, respectively, and inhibit the synthesis and the activity of high‐affinity nitrate transporters. This model faithfully predicts the typical spiking behavior of the nitrate uptake, in which an initial strong increase of nitrate absorption capacity is followed by a drop, which regulates the absorption down to the initial value. The model outcome was compared with experimental data and they fit quite nicely. The model predicts that after the initial exposure of the roots with nitrate, the absorption of the anion strongly increases and that, on the contrary, the intensity of the absorption is limited in presence of ammonium around the roots. [ABSTRACT FROM AUTHOR]

The low bioavailability of nutrients, especially nitrogen (N) and phosphorus (P), is one of the most limiting factors for crop production. In this study, under N- and P-free nutrient solution (-N-P), nodulating white lupin plants developed some nodules and analogous cluster root structures characterized by different morphological, physiological, and molecular responses than those observed upon single nutrient deficiency (strong acidification of external media, a better nutritional status than -N+P and +N-P plants). The multi-elemental analysis highlighted that the concentrations of nutrients in white lupin plants were mainly affected by P availability. Gene-expression analyses provided evidence of interconnections between N and P nutritional pathways that are active to promote N and P balance in plants. The root exudome was mainly characterized by N availability in nutrient solution, and, in particular, the absence of N and P in the nutrient solution triggered a high release of phenolic compounds, nucleosides monophosphate and saponines by roots. These morphological, physiological, and molecular responses result from a close interplay between N and P nutritional pathways. They contribute to the good development of nodulating white lupin plants when grown on N- and P-free media. This study provides evidence that limited N and P availability in the nutrient solution can promote white lupin-Bradyrhizobium symbiosis, which is favourable for the sustainability of legume production. [ABSTRACT FROM AUTHOR]

The simultaneous presence of different N‐forms in the rhizosphere leads to beneficial effects on nitrogen (N) nutrition in plants. Although widely used as fertilizers, the occurrence of cross connection between urea and ammonium nutrition has been scarcely studied in plants. Maize fed with a mixture of urea and ammonium displayed a better N‐uptake efficiency than ammonium‐ or urea‐fed plants (Buoso et al., Plant Physiol Biochem, 2021a; 162: 613–623). Through multiomic approaches, we provide the molecular characterization of maize response to urea and ammonium nutrition. Several transporters and enzymes involved in N‐nutrition were upregulated by all three N‐treatments (urea, ammonium, or urea and ammonium). Already after 1 day of treatment, the availability of different N‐forms induced specific transcriptomic and metabolomic responses. The combination of urea and ammonium induced a prompt assimilation of N, characterized by high levels of some amino acids in shoots. Moreover, ZmAMT1.1a, ZmGLN1;2, ZmGLN1;5, ZmGOT1, and ZmGOT3, as well transcripts involved in glycolysis‐TCA cycle were induced in roots by urea and ammonium mixture. Depending on N‐form, even changes in the composition of phytohormones were observed in maize. This study paves the way to formulate guidelines for the optimization of N fertilization to improve N‐use efficiency in maize and therefore limit N‐losses in the environment. [ABSTRACT FROM AUTHOR]

Nitrogen (N) as well as Phosphorus (P) are key nutrients determining crop productivity. Legumes have developed strategies to overcome nutrient limitation by, for example, forming a symbiotic relationship with N-fixing rhizobia and the release of P-mobilizing exudates and are thus able to grow without supply of N or P fertilizers. The legume-rhizobial symbiosis starts with root release of isoflavonoids that act as signaling molecules perceived by compatible bacteria. Subsequently, bacteria release nod factors, which induce signaling cascades allowing the formation of functional N-fixing nodules. We report here the identification and functional characterization of a plasma membrane-localized MATE-type transporter (LaMATE2) involved in the release of genistein from white lupin roots. The LaMATE2 expression in the root is upregulated under N deficiency as well as low phosphate availability, two nutritional deficiencies that induce the release of this isoflavonoid. LaMATE2 silencing reduced genistein efflux and even more the formation of symbiotic nodules, supporting the crucial role of LaMATE2 in isoflavonoid release and nodulation. Furthermore, silencing of LaMATE2 limited the P-solubilization activity of lupin root exudates. Transport assays in yeast vesicles demonstrated that LaMATE2 acts as a proton-driven isoflavonoid transporter. [ABSTRACT FROM AUTHOR]

Despite the wide use of urea and ammonium as N-fertilizers, no information is available about the proper ratio useful to maximize the efficiency of their acquisition by crops. Ionomic analyses of maize seedlings fed with five different mixes of urea and ammonium indicated that after 7 days of treatment, the elemental composition of plant tissues was more influenced by ammonium in the nutrient solution than by urea. Within 24 h, similar high affinity influx rates of ammonium were measured in ammonium-treated seedlings, independently from the amount of the cation present in the nutrient solution (from 0.5 to 2.0 mM N), and it was confirmed by the similar accumulation of 15N derived from ammonium source. After 7 days, some changes in ammonium acquisition occurred among treatments, with the highest ammonium uptake efficiency when the urea-to-ammonium ratio was 3:1. Gene expression analyses of enzymes and transporters involved in N nutrition highlight a preferential induction of the cytosolic N-assimilatory pathway (via GS, ASNS) when both urea and ammonium were supplied in conjunction, this response might explain the higher N-acquisition efficiency when both sources are applied. In conclusion, this study provides new insights on plant responses to mixes of N sources that maximize the N-uptake efficiency by crops and thus could allow to adapt agronomic practices in order to limit the economic and environmental impact of N-fertilization. • The simultaneous presence of urea and ammonium in the nutrient solution promotes the acquisition of ammonium. • The multielement composition of maize seedlings is more influenced by ammonium in the external media than by urea. • This study provides useful indication about the proper ratio urea to ammonium useful to maximize the uptake efficiency of ammonium in maize. [ABSTRACT FROM AUTHOR]

Abstract Background The mechanisms by which nitrate is transported into the roots have been characterized both at physiological and molecular levels. It has been demonstrated that nitrate is taken up in an energy-dependent way by a four-component uptake machinery involving high- and low- affinity transport systems. In contrast very little is known about the physiology of nitrate transport towards different plant tissues and in particular at the leaf level. Results The mechanism of nitrate uptake in leaves of cucumber (Cucumis sativus L. cv. Chinese long) plants was studied and compared with that of the root. Net nitrate uptake by roots of nitrate-depleted cucumber plants proved to be substrate-inducible and biphasic showing a saturable kinetics with a clear linear non saturable component at an anion concentration higher than 2 mM. Nitrate uptake by leaf discs of cucumber plants showed some similarities with that operating in the roots (e.g. electrogenic H+ dependence via involvement of proton pump, a certain degree of induction). However, it did not exhibit typical biphasic kinetics and was characterized by a higher Km with values out of the range usually recorded in roots of several different plant species. The quantity and activity of plasma membrane (PM) H+-ATPase of the vesicles isolated from leaf tissues of nitrate-treated plants for 12 h (peak of nitrate foliar uptake rate) increased with respect to that observed in the vesicles isolated from N-deprived control plants, thus suggesting an involvement of this enzyme in the leaf nitrate uptake process similar to that described in roots. Molecular analyses suggest the involvement of a specific isoform of PM H+-ATPase (CsHA1) and NRT2 transporter (CsNRT2) in root nitrate uptake. At the leaf level, nitrate treatment modulated the expression of CsHA2, highlighting a main putative role of this isogene in the process. Conclusions Obtained results provide for the first time evidence that a saturable and substrate-inducible nitrate uptake mechanism operates in cucumber leaves. Its activity appears to be related to that of PM H+-ATPase activity and in particular to the induction of CsHA2 isoform. However the question about the molecular entity responsible for the transport of nitrate into leaf cells therefore still remains unresolved.

Abstract Background Plants react to iron deficiency stress adopting different kind of adaptive responses. Tomato, a Strategy I plant, improves iron uptake through acidification of rhizosphere, reduction of Fe3+ to Fe2+ and transport of Fe2+ into the cells. Large-scale transcriptional analyses of roots under iron deficiency are only available for a very limited number of plant species with particular emphasis for Arabidopsis thaliana. Regarding tomato, an interesting model species for Strategy I plants and an economically important crop, physiological responses to Fe-deficiency have been thoroughly described and molecular analyses have provided evidence for genes involved in iron uptake mechanisms and their regulation. However, no detailed transcriptome analysis has been described so far. Results A genome-wide transcriptional analysis, performed with a chip that allows to monitor the expression of more than 25,000 tomato transcripts, identified 97 differentially expressed transcripts by comparing roots of Fe-deficient and Fe-sufficient tomato plants. These transcripts are related to the physiological responses of tomato roots to the nutrient stress resulting in an improved iron uptake, including regulatory aspects, translocation, root morphological modification and adaptation in primary metabolic pathways, such as glycolysis and TCA cycle. Other genes play a role in flavonoid biosynthesis and hormonal metabolism. Conclusions The transcriptional characterization confirmed the presence of the previously described mechanisms to adapt to iron starvation in tomato, but also allowed to identify other genes potentially playing a role in this process, thus opening new research perspectives to improve the knowledge on the tomato root response to the nutrient deficiency.

Conversion of conventional farming (CF) to organic farming (OF) is claimed to allow a sustainable management of soil resources, but information on changes induced on dissolved organic matter (DOM) are scarce. Among DOM components, dissolved humic substances (DHS) were shown to possess stimulatory effects on plant growth. DHS were isolated from CF and OF soil leacheates collected from soil monolith columns: first in November (bare soils) and then in April and June (bare and planted soils). DHS caused an enhancement of nitrate uptake rates in maize roots and modulated several genes involved in nitrogen acquisition. The DHS from OF soil exerted a stronger biostimulant action on the nitrate uptake system, but the first assimilatory step of nitrate was mainly activated by DHS derived from CF soil. To validate the physiological response of plants to DHS exposure, real-time RT-PCR analyses were performed on those genes most involved in nitrate acquisition, such as ZmNRT2.1 , ZmNRT2.2 , ZmMHA2 (coding for two high-affinity nitrate transporters and a PM H+-proton pump), ZmNADH:NR , ZmNADPH:NR , and ZmNiR (coding for nitrate reductases and nitrite reductase). All tested DHS fractions induced the upregulation of nitrate reductase (NR), and in particular the OF2 DHS stimulated the expression of both tested transcripts encoding for two NR isoforms. Characteristics of DHS varied during the experiment in both OF and CF soils: a decrease of high molecular weight fractions in the OF soil, a general increase in the carboxylic groups content, as well as diverse structural modifications in OF vs. CF soils were observed. These changes were accelerated in planted soils. Similarity of chemical properties of DHS with the more easily obtainable water-soluble humic substance extracted from peat (WEHS) and the correspondence of their biostimulant actions confirm the validity of studies which employ WEHS as an easily available source of DHS to investigate biostimulant actions on agricultural crops. [ABSTRACT FROM AUTHOR]

Improvement of plant iron nutrition as a consequence of metal complexation by humic substances (HS) extracted from different sources has been widely reported. The presence of humified fractions of the organic matter in soil sediments and solutions would contribute, depending on the solubility and the molecular size of HS, to build up a reservoir of Fe available for plants which exude metal ligands and to provide Fe-HS complexes directly usable by plant Fe uptake mechanisms. It has also been shown that HS can promote the physiological mechanisms involved in Fe acquisition acting at the transcriptional and post-transcriptional level. Furthermore, the distribution and allocation of Fe within the plant could be modified when plants were supplied with water soluble Fe-HS complexes as compared with other natural or synthetic chelates. These effects are in line with previous observations showing that treatments with HS were able to induce changes in root morphology and modulate plant membrane activities related to nutrient acquisition, pathways of primary and secondary metabolism, hormonal and reactive oxygen balance. The multifaceted action of HS indicates that soluble Fe-HS complexes, either naturally present in the soil or exogenously supplied to the plants, can promote Fe acquisition in a complex way by providing a readily available iron form in the rhizosphere and by directly affecting plant physiology. Furthermore, the possibility to use Fe-HS of different sources, size and solubility may be considered as an environmental-friendly tool for Fe fertilization of crops. [ABSTRACT FROM AUTHOR]

Humic substances are known to positively influence plant growth and nutrition. In particular, the water-extractable fraction of humic substances (WEHS) has been shown to enhance nitrate acquisition, increasing the activity of high affinity nitrate uptake system. However, molecular bases of this physiological response are not clarified so far. Thus, in the present work, the physiological effect of WEHS on nitrate acquisition in maize roots was correlated with changes in the root transcriptomic profile. Results confirmed that WEHS caused a faster induction of a higher capacity to take up nitrate in maize roots. Comparing the root transcriptomic profile of Nitrate - and Nitrate   +   WEHS -treated plants with Control (-N) ones, more than 2000 transcripts appeared to be modulated only in the presence of WEHS. Among these, genes involved in nitrate transport and assimilation ( NRT1s, NRT2s, NAR2.1, NR, GS, GOGAT, CNX, UPM ) were strongly modulated by WEHS. Furthermore, also some genes known to be linked to the nitrogen limitation responses were affected by WEHS, as transcripts coding for transcription factors (as LBD37, NIN-like protein, NFY-A, GRF5 ) and enzymes of hormones’ metabolism. The modulation of these transcripts might play a crucial role in coordinating the induction to nitrate, favouring its uptake and assimilation in WEHS-treated plants. The overexpression of nitrogen assimilatory genes by WEHS might led to an early feedback regulation of the high affinity nitrate transport system, as being operated by N-metabolites. Results of the present work shed further light on the contribution of the organic soil component to the nitrogen use efficiency in crops. [ABSTRACT FROM AUTHOR]

Background: Under limited iron (Fe) availability maize, a Strategy II plant, improves Fe acquisition through the release of phytosiderophores (PS) into the rhizosphere and the subsequent uptake of Fe-PS complexes into root cells. Occurrence of Strategy-I-like components and interactions with phosphorous (P) nutrition has been hypothesized based on molecular and physiological studies in grasses. Results: In this report transcriptomic analysis (NimbleGen microarray) of Fe deficiency response revealed that maize roots modulated the expression levels of 724 genes (508 up- and 216 down-regulated, respectively). As expected, roots of Fe-deficient maize plants overexpressed genes involved in the synthesis and release of 2'-deoxymugineic acid (the main PS released by maize roots). A strong modulation of genes involved in regulatory aspects, Fe translocation, root morphological modification, primary metabolic pathways and hormonal metabolism was induced by the nutritional stress. Genes encoding transporters for Fe2+ (ZmNRAMP1) and P (ZmPHT1;7 and ZmPHO1) were also up-regulated under Fe deficiency. Fe-deficient maize plants accumulated higher amounts of P than the Fe-sufficient ones, both in roots and shoots. The supply of 1 µM 59Fe, as soluble (Fe-Citrate and Fe-PS) or sparingly soluble (Ferrihydrite) sources to deficient plants, caused a rapid down-regulation of genes coding for PS and Fe(III)-PS transport, as well as of ZmNRAMP1 and ZmPHT1;7. Levels of 32P absorption essentially followed the rates of 59Fe uptake in Fe-deficient plants during Fe resupply, suggesting that P accumulation might be regulated by Fe uptake in maize plants. Conclusions: The transcriptional response to Fe-deficiency in maize roots confirmed the modulation of known genes involved in the Strategy II and revealed the presence of Strategy I components usually described in dicots. Moreover, data here presented provide evidence of a close relationship between two essential nutrients for plants, Fe and P, and highlight a key role played by Fe and P transporters to preserve the homeostasis of these two nutrients in maize plants. [ABSTRACT FROM AUTHOR]

To limit nitrogen (N) losses from the soil, it has been suggested to provide urea to crops in conjunction with the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT). However, recent studies reported that NBPT affects urea uptake and urease activity in plants. To shed light on these latter aspects, the effects of NBPT were studied analysing transcriptomic and metabolic changes occurring in urea-fed maize seedlings after a short-term exposure to the inhibitor. We provide evidence that NBPT treatment led to a wide reprogramming of plant metabolism. NBPT inhibited the activity of endogenous urease limiting the release and assimilation of ureic-ammonium, with a simultaneous accumulation of urea in plant tissues. Furthermore, NBPT determined changes in the glutamine, glutamate, and asparagine contents. Microarray data indicate that NBPT affects ureic-N assimilation and primary metabolism, such as glycolysis, TCA cycle, and electron transport chain, while activates the phenylalanine/tyrosine-derivative pathway. Moreover, the expression of genes relating to the transport and complexation of divalent metals was strongly modulated by NBPT. Data here presented suggest that when NBPT is provided in conjunction with urea an imbalance between C and N compounds might occur in plant cells. Under this condition, root cells also seem to activate a response to maintain the homeostasis of some micronutrients. [ABSTRACT FROM AUTHOR]

We aimed to identify the early stress response and plant performance of Medicago truncatula growing in axenic medium with ammonium or urea as the sole source of nitrogen, with respect to nitrate-based nutrition. Biomass measurements, auxin content analyses, root system architecture (RSA) response analyses, and physiological parameters were determined. Both ammonium and ureic nutrition severely affected the RSA, resulting in changes in the main elongation rate, lateral root development, and insert position from the root base. The auxin content decreased in both urea- and ammonium-treated roots; however, only the ammonium-treated plants were affected at the shoot level. The analysis of chlorophyll a fluorescence transients showed that ammonium affected photosystem II, but urea did not impair photosynthetic activity. Superoxide dismutase isoenzymes in the plastids were moderately affected by urea and ammonium in the roots. Overall, our results showed that low N doses from different sources had no remarkable effects on M. truncatula, with the exception of the differential phenotypic root response. High doses of both ammonium and urea caused great changes in plant length, auxin contents and physiological measurements. Interesting correlations were found between the shoot auxin pool and both plant length and the "performance index" parameter, which is obtained from measurements of the kinetics of chlorophyll a fluorescence. Taken together, these data demonstrate that both the indole-3-acetic acid pool and performance index are important components of the response of M. truncatula under ammonium or urea as the sole N source. [ABSTRACT FROM AUTHOR]

Background: It is well known that in the rhizosphere soluble Fe sources available for plants are mainly represented by a mixture of complexes between the micronutrient and organic ligands such as carboxylates and phytosiderophores (PS) released by roots, as well as fractions of humified organic matter. The use by roots of these three natural Fe sources (Fe-citrate, Fe-PS and Fe complexed to water-extractable humic substances, Fe-WEHS) have been already studied at physiological level but the knowledge about the transcriptomic aspects is still lacking. Results: The 59Fe concentration recorded after 24 h in tissues of tomato Fe-deficient plants supplied with 59Fe complexed to WEHS reached values about 2 times higher than those measured in response to the supply with Fe-citrate and Fe-PS. However, after 1 h no differences among the three Fe-chelates were observed considering the 59Fe concentration and the root Fe(III) reduction activity. A large-scale transcriptional analysis of root tissue after 1 h of Fe supply showed that Fe-WEHS modulated only two transcripts leaving the transcriptome substantially identical to Fe-deficient plants. On the other hand, Fe-citrate and Fe-PS affected 728 and 408 transcripts, respectively, having 289 a similar transcriptional behaviour in response to both Fe sources. Conclusions: The root transcriptional response to the Fe supply depends on the nature of chelating agents (WEHS, citrate and PS). The supply of Fe-citrate and Fe-PS showed not only a fast back regulation of molecular mechanisms modulated by Fe deficiency but also specific responses due to the uptake of the chelating molecule. Plants fed with Fe-WEHS did not show relevant changes in the root transcriptome with respect to the Fe-deficient plants, indicating that roots did not sense the restored cellular Fe accumulation. [ABSTRACT FROM AUTHOR]

Soilless cultivation systems represent an alternative to traditional agriculture, as they offer the possibility to reduce water use and to design nutrient formulations of the hydroponic solutions in order to maximize yield and quality of the products. Reduction in accumulation of undesired compounds, like e.g. nitrate, and enhanced nutritional value for human consumption, like e.g. Se-rich compounds, could be reasonably achieved. These aspects, together with the extension of the shelf life, would increase the market value of ready-to-cut vegetables. This review discusses how recent advances in understanding the role of plant nutrients and their interactions and the management of nutrient solution composition, including the addition of non-essential elements, micronutrients’ biofortification, biostimulants, together with the control of some crucial environmental conditions, like temperature, can help accomplishing this task. [ABSTRACT FROM AUTHOR]

There is increasing interest in the hydroponic technology to produce leafy vegetables for ready-to-eat salads. Optimisation of the growing system can lead to higher yield and/or improved nutritional value of the product. Selenium (Se) is an essential element for animal and humans, with quite a narrow range between deficiency and toxicity, whereas it is assumed beneficial for plants. In the present study, two cultivars (Gala and Baron) of corn salad (Valerianella locusta (L.) Laterr.) were used to test the possibility to increase Se content in the edible parts (leaves). Effects on yield, nitrate content, and accumulation of sulfur (S) and S-containing amino acids and Se and Se-containing amino acids were studied. Results showed that corn salad tolerates selenate (Na[sub 2]SeO[sub 4]) concentrations ranging from 10 to 40 μM in the nutrient solution, with plants accumulating Se at levels compatible with the need in human diets at 10 μM selenate. Se-treated plants showed some benefits with respect to a decrease of nitrate concentration and increase of pigment contents (chlorophylls and carotenoids). At 10 μM selenate, Se-cysteine and Se-methionine were produced, without affecting non-protein thiols or cysteine and methionine contents. At the higher Se supply, sulfate accumulated in the leaves with a parallel decrease in the amount of S-amino acids and a rise in the relative amount of Se-amino acids. Based on the chemical analyses, cv. Gala showed better tolerance than cv. Baron to moderate selenate supply (40 μM). [ABSTRACT FROM AUTHOR]

Despite the widespread use of urease inhibitors in agriculture, little information is available on their effect on nitrogen (N) uptake and assimilation. Aim of this work was to study, at physiological and transcriptional level, the effects of N-(n-butyl) thiophosphoric triamide (NBPT) on urea nutrition in hydroponically grown maize plants. Presence of NBPT in the nutrient solution limited the capacity of plants to utilize urea as a N-source; this was shown by a decrease in urea uptake rate and 15N accumulation. Noteworthy, these negative effects were evident only when plants were fed with urea, as NBPT did not alter 15N accumulation in nitrate-fed plants. NBPT also impaired the growth of Arabidopsis plants when urea was used as N-source, while having no effect on plants grown with nitrate or ammonium. This response was related, at least in part, to a direct effect of NBPT on the high affinity urea transport system. Impact of NBPT on urea uptake was further evaluated using lines of Arabidopsis overexpressing ZmDUR3 and dur3-knockout; results suggest that not only transport but also urea assimilation could be compromised by the inhibitor. This hypothesis was reinforced by an over-accumulation of urea and a decrease in ammonium concentration in NBPTtreated plants. Furthermore, transcriptional analyses showed that in maize roots NBPT treatment severely impaired the expression of genes involved in the cytosolic pathway of ureic-N assimilation and ammonium transport. NBPT also limited the expression of a gene coding for a transcription factor highly induced by urea and possibly playing a crucial role in the regulation of its acquisition. This work provides evidence that NBPT can heavily interfere with urea nutrition in maize plants, limiting influx as well as the following assimilation pathway. [ABSTRACT FROM AUTHOR]

White lupin is very often used as a model plant for root exudation studies due to its capability to release huge amounts of organic acids and flavonoids. The complex nature of these organic compounds makes not only their analytical determination difficult but also their extraction from soil samples. For these reasons simplified approaches, as hydroponic-based systems are widely used to study the root exudation. Therefore, the composition of a trap solution is crucial to limit artefacts causing over/underestimation of exudation rates and/or a biased molecular composition of the collected compounds. The present study was aimed at assessing the influence of different trap solutions and collection times on the quali- and quantitative root exudation pattern of white lupin ( Lupinus albus L.) grown under phosphorus (P) and iron (Fe) deficiency. Our results suggest that, in works aimed at studying root exudation processes, water is the most effective trap solution to collect the exudates like organic acids and flavonoids, especially in short time (e.g. 2 h). For longer times, low concentrations of Ca could be helpful to limit osmotic stress and possible passive leakage and/or diffusion. The use of bacteriostatic compounds as NaN and Micropur bias the results, due to interferences either with the metabolism or inhibition of the exudation processes, especially in the case of flavonoids such as quercetin. Also, the use of a pH buffer solution like 2-(N-morpholino)ethanesulfonic acid (MES) should be avoided for its undesired interferences with the release. [ABSTRACT FROM AUTHOR]

Iron (Fe) sources available for plants in the rhizospheric solution are mainly a mixture of complexes between Fe and organic ligands, including phytosiderophores (PS) and water-extractable humic substances (WEHS). In comparison with the other Fe sources, Fe-WEHS are more efficiently used by plants, and experimental evidences show that Fe translocation contributes to this better response. On the other hand, very little is known on the mechanisms involved in Fe allocation in leaves. In this work, physiological and molecular processes involved in Fe distribution in leaves of Fe-deficient Cucumis sativus supplied with Fe-PS or Fe-WEHS up to 5 days were studied combining different techniques, such as radiochemical experiments, synchrotron micro X-ray fluorescence, real-time reverse transcription polymerase chain reaction and in situ hybridization. In Fe-WEHS-fed plants, Fe was rapidly (1 day) allocated into the leaf veins, and after 5 days, Fe was completely transferred into interveinal cells; moreover, the amount of accumulated Fe was much higher than with Fe-PS. This redistribution in Fe-WEHS plants was associated with an upregulation of genes encoding a ferricIII-chelate reductase (FRO), a Fe2+ transporter (IRT1) and a natural resistance-associated macrophage protein (NRAMP). The localization of FRO and IRT1 transcripts next to the midveins, beside that of NRAMP in the interveinal area, may suggest a rapid and efficient response induced by the presence of Fe-WEHS in the extra-radical solution for the allocation in leaves of high amounts of Fe. In conclusion, Fe is more efficiently used when chelated to WEHS than PS and seems to involve Fe distribution and gene regulation of Fe acquisition mechanisms operating in leaves. [ABSTRACT FROM AUTHOR]

Plant growth-promoting rhizobacteria (PGPR) are soil bacteria that are able to colonize rhizosphere and to enhance plant growth by means of a wide variety of mechanisms like organic matter mineralization, biological control against soil-borne pathogens, biological nitrogen fixation, and root growth promotion. A very interesting feature of PGPR is their ability of enhancing nutrient bioavailability. Several bacterial species have been characterized as P-solubilizing microorganisms while other species have been shown to increase the solubility of micronutrients, like those that produce siderophores for Fe chelation. The enhanced amount of soluble macro- and micronutrients in the close proximity of the soil-root interface has indeed a positive effect on plant nutrition. Furthermore, several pieces of evidence highlight that the inoculation of plants with PGPR can have considerable effects on plant at both physiological and molecular levels (e.g., induction of rhizosphere acidification, up- and downregulation of genes involved in ion uptake, and translocation), suggesting the possibility that soil biota could stimulate plants being more efficient in retrieving nutrients from soil and coping with abiotic stresses. However, the molecular mechanisms underlying these phenomena, the signals involved as well as the potential applications in a sustainable agriculture approach, and the biotechnological aspects for possible rhizosphere engineering are still matters of discussion. [ABSTRACT FROM AUTHOR]

Background Despite its extensive use as a nitrogen fertilizer, the role of urea as a directly accessible nitrogen source for crop plants is still poorly understood. So far, the physiological and molecular aspects of urea acquisition have been investigated only in few plant species highlighting the importance of a high-affinity transport system. With respect to maize, a worldwide-cultivated crop requiring high amounts of nitrogen fertilizer, the mechanisms involved in the transport of urea have not yet been identified. The aim of the present work was to characterize the high-affinity urea transport system in maize roots and to identify the high affinity urea transporter. Results Kinetic characterization of urea uptake (<300 μM) demonstrated the presence in maize roots of a high-affinity and saturable transport system; this system is inducible by urea itself showing higher Vmax and Km upon induction. At molecular level, the ORF sequence coding for the urea transporter, ZmDUR3, was isolated and functionally characterized using different heterologous systems: a dur3 yeast mutant strain, tobacco protoplasts and a dur3 Arabidopsis mutant. The expression of the isolated sequence, ZmDUR3-ORF, in dur3 yeast mutant demonstrated the ability of the encoded protein to mediate urea uptake into cells. The subcellular targeting of DUR3/GFP fusion proteins in tobacco protoplasts gave results comparable to the localization of the orthologous transporters of Arabidopsis and rice, suggesting a partial localization at the plasma membrane. Moreover, the overexpression of ZmDUR3 in the atdur3-3 Arabidopsis mutant showed to complement the phenotype, since different ZmDUR3-overexpressing lines showed either comparable or enhanced 15[N]-urea influx than wild-type plants. These data provide a clear evidence in planta for a role of ZmDUR3 in urea acquisition from an extra-radical solution. Conclusions This work highlights the capability of maize plants to take up urea via an inducible and highaffinity transport system. ZmDUR3 is a high-affinity urea transporter mediating the uptake of this molecule into roots. Data may provide a key to better understand the mechanisms involved in urea acquisition and contribute to deepen the knowledge on the overall nitrogenuse efficiency in crop plants. [ABSTRACT FROM AUTHOR]

BACKGROUND Plants differ in their response to high aluminium (Al) concentrations, which typically cause toxicity in plants grown on acidic soils. The response depends on plant species and environmental conditions such as substrate and cultivation system. The present study aimed to assess Al-phosphate (P) dynamics in the rhizosphere of two bean species, Phaseolus vulgaris L. var. Red Kidney and Phaseolus lunatus L., in rhizobox experiments. RESULTS Root activity of the bean species induced up to a sevenfold increase in exchangeable Al and up to a 30-fold decrease in extractable P. High soluble Al concentrations triggered the release of plant-specific carboxylates, which differed between soil type and plant species. The results suggest that P. vulgaris L. mitigates Al stress by an internal defence mechanism and P. lunatus L. by an external one, both mechanisms involving organic acids. CONCLUSION Rhizosphere mechanisms involved in Al detoxification were found to be different for P. vulgaris L. and P. lunatus L., suggesting that these processes are plant species-specific. Phaseolus vulgaris L. accumulates Al in the shoots (internal tolerance mechanism), while P. lunatus L. prevents Al uptake by releasing organic acids (exclusion mechanism) into the growth media. © 2013 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

In the present study, five insoluble Fe-humic substance (HS) complexes (Fe-IHS) extracted from Leonardite were synthesized and characterized by Infrared (IR) spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS). The efficiency of Fe-IHS complexes in curing Fe chlorosis was compared with two soluble Fe sources (Fe complexed by a water-extractable humic fraction [Fe--WEHS] and the synthetic Fe-chelate, Fe-o,o-EDDHA]) and a poorly soluble mineral form (Ferrihydrite). The IR spectra revealed coordination of the Fe ions with the carboxylic and phenolic groups of the oxygen-rich humic substances. The SEM and DLS data suggest that the Fe-IHS complexes were large aggregated particles with diameter of about 500 to 1500 nm formed by different nanoparticles with very small size ranging from 50 to 200 nm. Chemical extractions show that the Fe pool is dominated by poorly ordered Fe phases strongly associated with IHS. The efficiency of Fe-IHS complexes in curing Fe chlorosis symptoms in Fe-deficient plants has been demonstrated, being, when used in high concentration, comparable to those of Fe-WEHS and Fe-o,oEDDHA. The capability of cucumber (Cucumis sativus L.) plants to acquire Fe from poorly soluble Fe forms was also confirmed when a poorly crystalline radiolabeled Fe-oxide (59 Ferrihydrite) source, used to represent a short-range order Fe phases associated with Fe-IHS complexes, has been used. The results of this work point out the possible use of environmental-friendly Fe insoluble high- molecular weight complexes (Fe-IHS) as an effective product to correct the Fe nutritional disorder. [ABSTRACT FROM AUTHOR]

Soil temperature has a crucial impact on physiological processes and growth of plants with important consequences for plant productivity and food safety including nitrate accumulation in leaf blades of leaf vegetables. Consumer demand for high-quality, fresh-cut vegetables has increased rapidly in the last decades, and temperature modulation can help control nitrate concentration in fresh vegetables, an important trait of product safety. Corn salad plants IValerianella Iocusta (L.) Laterr., cultivar Gala] were grown at three root temperatures (15, 20, and 25 °C) in a floating system. This experimental setup allowed to directly evaluate the effect of root temperature on yield and plant quality excluding the effect on soil processes and properties. Nutrient solution was renewed weekly and kept aerated while air temperature was maintained constant at 20 °C for all treatments during the entire time of experiments. At harvest, plants were collected, the shelf life evaluated, and the nutrient uptake [NO3-, iron (Fe) from 59Fe-o,oEDDHA, and 35SO42-] and mineral content were determined. Results showed that growing conditions at 20 °C of the nutrient solution led to the best plant performance in terms of yield, nitrate content at leaf level, root biomass, leaf area, and greenness with positive effects on postharvest quality, i.e., less rapid leaf loss of greenness and leaf fresh weight (FW) loss during conservation at 4 °C. At this temperature condition of the nutrient solution, it has also been observed an enhanced functionality of mechanisms involved in the acquisition of nutrients like NO3-, Fe, and SO42-, which are known to play an important role in nitrate level in leaf tissues of crops. Plants grown at 15 °C showed minor growth, whereas the nutrient solution at 25 °C caused stress for the plants affecting negatively the quality and yield. Overall, the results obtained showed that root temperature plays a fundamental role in several plant processes that affect yield and its quality; for hydroponic system cultivations, a level of growing-medium temperature close to that of the surrounding air seems suitable. [ABSTRACT FROM AUTHOR]

Iron chlorosis is a wide-spread limiting factor of production in agriculture. To cope with this problem, synthetic chelates (like EDTA or EDDHA) of Fe are used in foliar-spray or in soil treatments; however, these products are very expensive. Therefore paper-production byproducts, like Lignosulfonates (LS), with varying content of carboxylate and sulfonate groups, were tested with respect to their ability to maintain Fe in the solution of soils and to feed plants grown in hydroponics with Fe through foliar sprays or application to the nutrient solution. Results show that LS had a low capability to solubilize 59Fe-hydroxide and that preformed 59Fe(III)-LS complexes had poor mobility through a soil column (pH 7.5) and scarce stability when interacting with soils compared to 59Fe(III)-EDDHA. However when 59Fe(III)-LS were supplied to roots in a hydroponic system, they demonstrated an even higher capability to fed Fe-deficient tomato plants than 59Fe(III)-EDDHA. Hence, data here presented indicate that the low Fe use efficiency from Fe-LS observed in soil-applications is due to interactions of these Fe-sources with soil colloids rather than to the low capability of roots to use them. Foliar application experiments of 59Fe(III)-LS or 59Fe(III)-EDTA to Fe-deficient cucumber plants show that uptake and reduction rates of Fe were similar between all these complexes; on the other hand, when 59Fe(III)-LS were sprayed on Fe-deficient tomato leaves, they showed a lower uptake rate, but a similar reduction rate, than 59Fe(III)-EDTA did. In conclusion, Fe-LS may be a valid, eco-compatible and cheap alternative to synthetic chelates in dealing with Fe chlorosis when applied foliarly or in the nutrient solution of hydroponically grown plants. [ABSTRACT FROM AUTHOR]

White lupin ( Lupinus albus L.) is able to grow on soils with sparingly available phosphate (P) by producing specialized structures called cluster roots. To mobilize sparingly soluble P forms in soils, cluster roots release substantial amounts of carboxylates and concomitantly acidify the rhizosphere. The relationship between acidification and carboxylate exudation is still largely unknown. In the present work, we studied the linkage between organic acids (malate and citrate) and proton exudations in cluster roots of P-deficient white lupin. After the illumination started, citrate exudation increased transiently and reached a maximum after 5 h. This effect was accompanied by a strong acidification of the external medium and alkalinization of the cytosol, as evidenced by in vivo nuclear magnetic resonance (NMR) analysis. Fusicoccin, an activator of the plasma membrane (PM) H+-ATPase, stimulated citrate exudation, whereas vanadate, an inhibitor of the H+-ATPase, reduced citrate exudation. The burst of citrate exudation was associated with an increase in expression of the LHA1 PM H+-ATPase gene, an increased amount of H+-ATPase protein, a shift in pH optimum of the enzyme and post-translational modification of an H+-ATPase protein involving binding of activating 14-3-3 protein. Taken together, our results indicate a close link in cluster roots of P-deficient white lupin between the burst of citrate exudation and PM H+-ATPase-catalysed proton efflux. [ABSTRACT FROM AUTHOR]

Abstract: The impact of flavonoids released by phosphorus-deficient white lupin roots on inorganic P and soil microorganisms is largely unknown. We report that flavonoids isolated from white lupin roots mobilized inorganic phosphorus and decreased soil microbial respiration, citrate mineralization, and soil phosphohydrolase activities, but did not reduce the soil ATP content. The results suggest that white lupin''s release of flavonoids into the rhizosphere plays a significant role in its efficient P-acquisition strategy by solubilizing Fe-bound P and by limiting the microbial mineralization of citrate. [Copyright &y& Elsevier]