Transcriptional and physiological changes in the S assimilation pathway due to single or combined S and Fe deprivation in durum wheat (Triticum durum L.) seedlings

The drought-induced plasticity of mineral nutrients contributes to drought tolerance discrimination in durum wheat

Drought is a major challenge for the cultivation of durum wheat, a crucial crop for global food security. Plants respond to drought by adjusting their mineral nutrient profiles to cope with water scarcity, showing the importance of nutrient plasticity for plant acclimation and adaptation to diverse environments. Therefore, it is essential to understand the genetic basis of mineral nutrient profile plasticity in durum wheat under drought stress to select drought-tolerant varieties. The research study investigated the responses of different durum wheat genotypes to severe drought stress at the seedling stage. The study employed an ionomic, molecular, biochemical and physiological approach to shed light on distinct behaviors among different genotypes. The drought tolerance of SVEMS16, SVEVO, and BULEL was related to their capacity of maintaining or increasing nutrient's accumulation, while the limited nutrient acquisition capability of CRESO and S.CAP likely resulted in their susceptibility to drought. The study highlighted the importance of macronutrients such as SO42-, NO3-, PO43-, and K+ in stress resilience and identified variant-containing genes potentially influencing nutritional variations under drought. These findings provide valuable insights for further field studies to assess the drought tolerance of durum wheat genotypes across various growth stages, ultimately ensuring food security and sustainable production in the face of changing environmental conditions.

The Interplay of Sulfur and Selenium Enabling Variations in Micronutrient Accumulation in Red Spinach

Aside from its importance in human and animal health, low levels of foliar-applied selenate (SeO4) can be advantageous in the presence of sulfur (S), contributing to improved growth, nutrient uptake, and crop quality. A hydroponic experiment in a growth chamber explored the interactive influence of Se and S on micronutrients and several quality indices, such as soluble sugars, organic acids, and total protein concentrations in spinach (Spinacia oleracea L.). Three levels of S (deprivation, adequate, and excessive) with varying quantities of Se (deficient, moderate, and higher) were examined in combination. Under S starvation and along with S nourishment in plant parts, Se treatments were found to cause noticeable variations in plant biomass and the concentrations of the examined elements and other quality parameters. Both Se levels promoted S accumulation in S-treated plants. Although the Se treatment had the opposite effect in shoots, it had a favorable impact on minerals (apart from Mn) in roots grown under S-limiting conditions. The S and Se relationship highlighted beneficial and/or synergistic effects for Mn and Fe in edible spinach portions. Reducing sugars were synergistically boosted by adequate S and moderate Se levels in roots, while in shoots, they were accumulated under moderate-or-higher Se and excessive S. Furthermore, the concentration of the quantified organic acids under S-deficient conditions was aided by various Se levels. In roots, moderate Se under high S application enhanced both malic acid and citric acid, while in the edible parts, higher Se under both adequate and elevated S levels were found to be advantageous in malic acid accumulation. Moreover, by elevating S levels in plant tissues, total protein concentration increased, whereas both moderate and high Se levels (Se1 and Se2) did not alter total protein accumulation in high S-applied roots and shoots. Our findings show that the high S and medium Se dose together benefit nutrient uptake; additionally, their combinations support soluble sugars and organic acids accumulation, contributing ultimately to the nutritional quality of spinach plants. Moreover, consuming 100 g of fresh red spinach shoot enriched with different Se and S levels can contribute to humans' daily micronutrients intake.

Interaction between Sulfate and Selenate in Tetraploid Wheat (Triticum turgidum L.) Genotypes

Selenium (Se) is an essential micronutrient of fundamental importance to human health and the main Se source is from plant-derived foods. Plants mainly take up Se as selenate (SeO42−), through the root sulfate transport system, because of their chemical similarity. The aims of this study were (1) to characterize the interaction between Se and S during the root uptake process, by measuring the expression of genes coding for high-affinity sulfate transporters and (2) to explore the possibility of increasing plant capability to take up Se by modulating S availability in the growth medium. We selected different tetraploid wheat genotypes as model plants, including a modern genotype, Svevo (Triticum turgidum ssp. durum), and three ancient Khorasan wheats, Kamut, Turanicum 21, and Etrusco (Triticum turgidum ssp. turanicum). The plants were cultivated hydroponically for 20 days in the presence of two sulfate levels, adequate (S = 1.2 mM) and limiting (L = 0.06 mM), and three selenate levels (0, 10, 50 μM). Our findings clearly showed the differential expression of genes encoding the two high-affinity transporters (TdSultr1.1 and TdSultr1.3), which are involved in the primary uptake of sulfate from the rhizosphere. Interestingly, Se accumulation in shoots was higher when S was limited in the nutrient solution.

Impact of Ferrous Sulfate on Thylakoidal Multiprotein Complexes, Metabolism and Defence of Brassica juncea L. under Arsenic Stress

Forty-day-old Brassica juncea (var. Pusa Jai Kisan) plants were exposed to arsenic (As, 250 µM Na2HAsO4·7H2O) stress. The ameliorative role of ferrous sulfate (2 mM, FeSO4·7H2O, herein FeSO4) was evaluated at 7 days after treatment (7 DAT) and 14 DAT. Whereas, As induced high magnitude oxidative stress, FeSO4 limited it. In general, As decreased the growth and photosynthetic parameters less when in the presence of FeSO4. Furthermore, components of the antioxidant system operated in better coordination with FeSO4. Contents of non-protein thiols and phytochelatins were higher with the supply of FeSO4. Blue-Native polyacrylamide gel electrophoresis revealed an As-induced decrease in almost every multi-protein-pigment complex (MPC), and an increase in PSII subcomplex, LHCII monomers and free proteins. FeSO4 supplication helped in the retention of a better stoichiometry of light-harvesting complexes and stabilized every MPC, including supra-molecular complexes, PSI/PSII core dimer/ATP Synthase, Cytochrome b6/f dimer and LHCII dimer. FeSO4 strengthened the plant defence, perhaps by channelizing iron (Fe) and sulfur (S) to biosynthetic and anabolic pathways. Such metabolism could improve levels of antioxidant enzymes, and the contents of glutathione, and phytochelatins. Important key support might be extended to the chloroplast through better supply of Fe-S clusters. Therefore, our results suggest the importance of both iron and sulfur to combat As-induced stress in the Indian mustard plant at biochemical and molecular levels through enhanced antioxidant potential and proteomic adjustments in the photosynthetic apparatus.

Crosstalk Between Iron and Sulfur Homeostasis Networks in Arabidopsis

The widespread deficiency of iron (Fe) and sulfur (S) is becoming a global concern. The underlying mechanisms regulating Fe and S sensing and signaling have not been well understood. We investigated the crosstalk between Fe and S using mutants impaired in Fe homeostasis, sulfate assimilation, and glutathione (GSH) biosynthesis. We showed that chlorosis symptoms induced by Fe deficiency were not directly related to the endogenous GSH levels. We found dynamic crosstalk between Fe and S networks and more interestingly observed that the upregulated expression of IRT1 and FRO2 under S deficiency in Col-0 was missing in the cad2-1 mutant background, which suggests that under S deficiency, the expression of IRT1 and FRO2 was directly or indirectly dependent on GSH. Interestingly, the bottleneck in sulfite reduction led to a constitutively higher IRT1 expression in the sir1-1 mutant. While the high-affinity sulfate transporter (Sultr1;2) was upregulated under Fe deficiency in the roots, the low-affinity sulfate transporters (Sultr2;1, and Sultr2;2) were down-regulated in the shoots of Col-0 seedlings. Moreover, the expression analysis of some of the key players in the Fe-S cluster assembly revealed that the expression of the so-called Fe donor in mitochondria (AtFH) and S mobilizer of group II cysteine desulfurase in plastids (AtNFS2) were upregulated under Fe deficiency in Col-0. Our qPCR data and ChIP-qPCR experiments suggested that the expression of AtFH is likely under the transcriptional regulation of the central transcription factor FIT.

Strategies and Bottlenecks in Hexaploid Wheat to Mobilize Soil Iron to Grains

Our knowledge of iron (Fe) uptake and mobilization in plants is mainly based on Arabidopsis and rice. Although multiple players of Fe homeostasis have been elucidated, there is a significant gap in our understanding of crop species, such as wheat. It is, therefore, imperative not only to understand the different hurdles for Fe enrichment in tissues but also to address specifically the knowns/unknowns involved in the plausible mechanism of Fe sensing, signaling, transport, and subsequent storage in plants. In the present review, a unique perspective has been described in light of recent knowledge generated in wheat, an economically important crop. The strategies to boost efficient Fe uptake, transcriptional regulation, and long-distance mobilization in grains have been discussed, emphasizing recent biotechnological routes to load Fe in grains. This article also highlights the new elements of physiological and molecular genetics that underpin the mechanistic insight for the identified Fe-related genes and discusses the bottlenecks in unloading the Fe in grains. The information presented here will provide much-needed resources and directions to overcome challenges and design efficient strategies to enhance the Fe density in wheat grains.

Maize Interveinal Chlorosis 1 links the Yang Cycle and Fe homeostasis through Nicotianamine biosynthesis

The Yang cycle is involved in many essential metabolic pathways in plant growth and development. As extended products of the Yang cycle, the function and regulation network of ethylene and polyamines are well characterized. Nicotianamine (NA) is also a product of this cycle and works as a key metal chelator for iron (Fe) homeostasis in plants. However, interactions between the Yang cycle and NA biosynthesis remain unclear. Here, we cloned maize interveinal chlorosis 1 (mic1), encoding a 5'-methylthioadenosine nucleosidase (MTN), that is essential for 5'-methylthioadenosine (MTA) salvage and NA biosynthesis in maize (Zea mays). A single base G-A transition in the fourth exon of mic1 causes a Gly to Asp change, resulting in increased MTA, reduced Fe distribution, and growth retardation of seedlings. Knockout of ZmMIC1 but not its paralog ZmMTN2 by CRISPR/Cas9 causes interveinal chlorosis, indicating ZmMIC1 is mainly responsible for MTN activity in maize. Transcriptome analysis showed a typical response of Fe deficiency. However, metabolic analysis revealed dramatically reduced NA content in mic1, suggesting NA biosynthesis was impaired in the mutant. Exogenous application of NA transiently reversed the interveinal chlorosis phenotype of mic1 seedlings. Moreover, the mic1 mutant overexpressing a NA synthase gene not only recovered from interveinal chlorosis and growth retardation but was also fertile. These findings provide a link between the Yang cycle and NA biosynthesis, which highlights an aspect of Fe homeostasis regulation in maize.

Arbuscular Mycorrhizal Symbiosis Differentially Affects the Nutritional Status of Two Durum Wheat Genotypes under Drought Conditions

Durum wheat is one of the most important agricultural crops, currently providing 18% of the daily intake of calories and 20% of daily protein intake for humans. However, being wheat that is cultivated in arid and semiarid areas, its productivity is threatened by drought stress, which is being exacerbated by climate change. Therefore, the identification of drought tolerant wheat genotypes is critical for increasing grain yield and also improving the capability of crops to uptake and assimilate nutrients, which are seriously affected by drought. This work aimed to determine the effect of arbuscular mycorrhizal fungi (AMF) on plant growth under normal and limited water availability in two durum wheat genotypes (Svevo and Etrusco). Furthermore, we investigated how the plant nutritional status responds to drought stress. We found that the response of Svevo and Etrusco to drought stress was differentially affected by AMF. Interestingly, we revealed that AMF positively affected sulfur homeostasis under drought conditions, mainly in the Svevo cultivar. The results provide a valuable indication that the identification of drought tolerant plants cannot ignore their nutrient use efficiency or the impact of other biotic soil components (i.e., AMF).

Phenotypic Acclimation of Maize Plants Grown under S Deprivation and Implications to Sulfur and Iron Allocation Dynamics

The aim of this work was to study maize root phenotype under sulfur deficiency stress towards revealing potential correlations between the altered phenotypic traits and the corresponding dry mass, sulfur, and iron allocation within plants at the whole-plant level. The dynamics of root morphological and anatomical traits were monitored. These traits were then correlated with plant foliage traits along with dry mass and sulfur and iron allocation dynamics in the shoot versus root. Plants grown under sulfate deprivation did not seem to invest in new root axes. Crown roots presented anatomical differences in all parameters studied; e.g., more and larger xylem vessels in order to maximize water and nutrient transport in the xylem sap. In the root system of S-deficient plants, a reduced concentration of sulfur was observed, whilst organic sulfur predominated over sulfates. A reduction in total iron concentration was monitored, and differences in its subcellular localization were observed. As expected, S-deprivation negatively affected the total sulfur concentration in the aerial plant part, as well as greatly impacted iron allocation in the foliage. Phenotypic adaptation to sulfur deprivation in maize presented alterations mainly in the root anatomy; towards competent handling of the initial sulfur and the induced iron deficiencies.

Polyester sulfur-coated urea (PSCU) application enhances brown rice iron concentrations in two alkaline soils

BACKGROUND

In neutral or alkaline soils, iron (Fe) easily forms insoluble complexes, which makes it difficult for plants to utilize Fe in the soil for nutrition. Polyester sulfur-coated urea (PSCU) is a novel controlled-release fertilizer widely used in China and some foreign countries, and it has been proven that sulfur film from controlled-release fertilizers can significantly improve the activation of Fe and other elements in the soil. However, few studies have focused on the effects of PSCU application on Fe accumulation in rice grain in alkaline soils.

RESULTS

Both our field and pot experiments proved that PSCU application could significantly improve rice grain yield and Fe concentration in brown rice in alkaline soil. This effect differs with different types of alkaline soils (i.e. medium-saline, sandy soil and/or silt soil). PSCU is released slowly, and the release rate is different in different alkaline soils. Rice shoot nitrogen (N) uptake was significantly enhanced with PSCU application.

CONCLUSION

The results suggested that PSCU application in alkaline soils could significantly enhance brown rice Fe concentration and production. This effect differed with different kinds of alkaline soils. The study identified some efficient fertilizers to improve the Fe status in alkaline soils. © 2021 Society of Chemical Industry.

Substrate control of sulphur utilisation and microbial stoichiometry in soil: Results of 13C, 15N, 14C, and 35S quad labelling

Global plant sulphur (S) deficiency is increasing because of a reduction in sulphate-based fertiliser application combined with continuous S withdrawal during harvest. Here, we applied ¹³C, ¹⁵N, ¹⁴C, and ³⁵S quad labelling of the S-containing amino acids cysteine (Cys) and methionine (Met) to understand S cycling and microbial S transformations in the soil. The soil microorganisms absorbed the applied Cys and Met within minutes and released SO₄^2- within hours. The SO₄^2- was reutilised by the MB within days. The initial microbial utilisation and SO₄^2- release were determined by amino acid structure. Met released 2.5-fold less SO₄^2- than Cys. The microbial biomass retained comparatively more C and S from Met than Cys. The microorganisms decomposed Cys to pyruvate and H₂S whereas they converted Met to α-ketobutyrate and S-CH₃. The microbial stoichiometries of C, N, and S derived from Cys and Met were balanced after 4 d by Cys-derived SO₄^2- uptake and Met-derived CO₂ release. The microbial C:N:S ratio dynamics showed rapid C utilisation and loss, stable N levels, and S accumulation. Thus, short-term organic S utilisation by soil microorganisms is determined by amino acid structure whilst long-term organic S utilisation by soil microorganisms is determined by microbially controlled stoichiometry.

Cross-Talks Between Macro- and Micronutrient Uptake and Signaling in Plants

In nature, land plants as sessile organisms are faced with multiple nutrient stresses that often occur simultaneously in soil. Nitrogen (N), phosphorus (P), sulfur (S), zinc (Zn), and iron (Fe) are five of the essential nutrients that affect plant growth and health. Although these minerals are relatively inaccessible to plants due to their low solubility and relative immobilization, plants have adopted coping mechanisms for survival under multiple nutrient stress conditions. The double interactions between N, Pi, S, Zn, and Fe have long been recognized in plants at the physiological level. However, the molecular mechanisms and signaling pathways underlying these cross-talks in plants remain poorly understood. This review preliminarily examined recent progress and current knowledge of the biochemical and physiological interactions between macro- and micro-mineral nutrients in plants and aimed to focus on the cross-talks between N, Pi, S, Zn, and Fe uptake and homeostasis in plants. More importantly, we further reviewed current studies on the molecular mechanisms underlying the cross-talks between N, Pi, S, Zn, and Fe homeostasis to better understand how these nutrient interactions affect the mineral uptake and signaling in plants. This review serves as a basis for further studies on multiple nutrient stress signaling in plants. Overall, the development of an integrative study of multiple nutrient signaling cross-talks in plants will be of important biological significance and crucial to sustainable agriculture.

Coordinated homeostasis of essential mineral nutrients: a focus on iron

In plants, iron (Fe) transport and homeostasis are highly regulated processes. Fe deficiency or excess dramatically limits plant and algal productivity. Interestingly, complex and unexpected interconnections between Fe and various macro- and micronutrient homeostatic networks, supposedly maintaining general ionic equilibrium and balanced nutrition, are currently being uncovered. Although these interactions have profound consequences for our understanding of Fe homeostasis and its regulation, their molecular bases and biological significance remain poorly understood. Here, we review recent knowledge gained on how Fe interacts with micronutrient (e.g. zinc, manganese) and macronutrient (e.g. sulfur, phosphate) homeostasis, and on how these interactions affect Fe uptake and trafficking. Finally, we highlight the importance of developing an improved model of how Fe signaling pathways are integrated into functional networks to control plant growth and development in response to fluctuating environments.

Interaction Between Sulfur and Iron in Plants

It is well known that S interacts with some macronutrients, such as N, P, and K, as well as with some micronutrients, such as Fe, Mo, Cu, Zn, and B. From our current understanding, such interactions could be related to the fact that: (i) S shares similar chemical properties with other elements (e.g., Mo and Se) determining competition for the acquisition/transport process (SULTR transporter family proteins); (ii) S-requiring metabolic processes need the presence of other nutrients or regulate plant responses to other nutritional deficiencies (S-containing metabolites are the precursor for the synthesis of ethylene and phytosiderophores); (iii) S directly interacts with other elements (e.g., Fe) by forming complexes and chemical bonds, such as Fe-S clusters; and (iv) S is a constituent of organic molecules, which play crucial roles in plants (glutathione, transporters, etc.). This review summarizes the current state of knowledge of the interplay between Fe and S in plants. It has been demonstrated that plant capability to take up and accumulate Fe strongly depends on S availability in the growth medium in both monocots and dicot plants. Moreover, providing S above the average nutritional need enhances the Fe content in wheat grains, this beneficial effect being particularly pronounced under severe Fe limitation. On the other hand, Fe shortage induces a significant increase in the demand for S, resulting in enhanced S uptake and assimilation rate, similar to what happens under S deficiency. The critical evaluation of the recent studies on the modulation of Fe/S interaction by integrating old and new insights gained on this topic will help to identify the main knowledge gaps. Indeed, it remains a challenge to determine how the interplay between S and Fe is regulated and how plants are able to sense environmental nutrient fluctuations and then to adapt their uptake, translocation, assimilation, and signaling. A better knowledge of the mechanisms of Fe/S interaction might considerably help in improving crop performance within a context of limited nutrient resources and a more sustainable agriculture.

Sulphur availability modulates Arabidopsis thaliana responses to iron deficiency

Among the mineral nutrients that are required for plant metabolism, iron (Fe) and sulphur (S) play a central role as both elements are needed for the activity of several proteins involved in essential cellular processes. A combination of physiological, biochemical and molecular approaches was employed to investigate how S availability influences plant response to Fe deficiency, using the model plant Arabidopsis thaliana. We first observed that chlorosis symptom induced by Fe deficiency was less pronounced when S availability was scarce. We thus found that S deficiency inhibited the Fe deficiency induced expression of several genes associated with the maintenance of Fe homeostasis. This includes structural genes involved in Fe uptake (i.e. IRT1, FRO2, PDR9, NRAMP1) and transport (i.e. FRD3, NAS4) as well as a subset of their upstream regulators, namely BTS, PYE and the four clade Ib bHLH. Last, we found that the over accumulation of manganese (Mn) in response to Fe shortage was reduced under combined Fe and S deficiencies. These data suggest that S deficiency inhibits the Fe deficiency dependent induction of the Fe uptake machinery. This in turn limits the transport into the root and the plant body of potentially toxic divalent cations such as Mn and Zn, thus limiting the deleterious effect of Fe deprivation.

Potential Implications of Interactions between Fe and S on Cereal Fe Biofortification

Iron (Fe) and sulfur (S) are two essential elements for plants, whose interrelation is indispensable for numerous physiological processes. In particular, Fe homeostasis in cereal species is profoundly connected to S nutrition because phytosiderophores, which are the metal chelators required for Fe uptake and translocation in cereals, are derived from a S-containing amino acid, methionine. To date, various biotechnological cereal Fe biofortification strategies involving modulation of genes underlying Fe homeostasis have been reported. Meanwhile, the resultant Fe-biofortified crops have been minimally characterized from the perspective of interaction between Fe and S, in spite of the significance of the crosstalk between the two elements in cereals. Here, we intend to highlight the relevance of Fe and S interrelation in cereal Fe homeostasis and illustrate the potential implications it has to offer for future cereal Fe biofortification studies.

The Role of Selective Protein Degradation in the Regulation of Iron and Sulfur Homeostasis in Plants

Plants are able to synthesize all essential metabolites from minerals, water, and light to complete their life cycle. This plasticity comes at a high energy cost, and therefore, plants need to tightly allocate resources in order to control their economy. Being sessile, plants can only adapt to fluctuating environmental conditions, relying on quality control mechanisms. The remodeling of cellular components plays a crucial role, not only in response to stress, but also in normal plant development. Dynamic protein turnover is ensured through regulated protein synthesis and degradation processes. To effectively target a wide range of proteins for degradation, plants utilize two mechanistically-distinct, but largely complementary systems: the 26S proteasome and the autophagy. As both proteasomal- and autophagy-mediated protein degradation use ubiquitin as an essential signal of substrate recognition, they share ubiquitin conjugation machinery and downstream ubiquitin recognition modules. Recent progress has been made in understanding the cellular homeostasis of iron and sulfur metabolisms individually, and growing evidence indicates that complex crosstalk exists between iron and sulfur networks. In this review, we highlight the latest publications elucidating the role of selective protein degradation in the control of iron and sulfur metabolism during plant development, as well as environmental stresses.

Keep talking: crosstalk between iron and sulfur networks fine-tunes growth and development to promote survival under iron limitation

Plants are capable of synthesizing all the molecules necessary to complete their life cycle from minerals, water, and light. This plasticity, however, comes at a high energetic cost and therefore plants need to regulate their economy and allocate resources accordingly. Iron-sulfur (Fe-S) clusters are at the center of photosynthesis, respiration, amino acid, and DNA metabolism. Fe-S clusters are extraordinary catalysts, but their main components (Fe2+ and S2-) are highly reactive and potentially toxic. To prevent toxicity, plants have evolved mechanisms to regulate the uptake, storage, and assimilation of Fe and S. Recent advances have been made in understanding the cellular economy of Fe and S metabolism individually, and growing evidence suggests that there is dynamic crosstalk between Fe and S networks. In this review, we summarize and discuss recent literature on Fe sensing, allocation, use efficiency, and, when pertinent, its relationship to S metabolism. Our future perspectives include a discussion about the open questions and challenges ahead and how the plant nutrition field can come together to approach these questions in a cohesive and more efficient way.

Disentangling the complexity and diversity of crosstalk between sulfur and other mineral nutrients in cultivated plants

A complete understanding of ionome homeostasis requires a thorough investigation of the dynamics of the nutrient networks in plants. This review focuses on the complexity of interactions occurring between S and other nutrients, and these are addressed at the level of the whole plant, the individual tissues, and the cellular compartments. With regards to macronutrients, S deficiency mainly acts by reducing plant growth, which in turn restricts the root uptake of, for example, N, K, and Mg. Conversely, deficiencies in N, K, or Mg reduce uptake of S. TOR (target of rapamycin) protein kinase, whose involvement in the co-regulation of C/N and S metabolism has recently been unravelled, provides a clue to understanding the links between S and plant growth. In legumes, the original crosstalk between N and S can be found at the level of nodules, which show high requirements for S, and hence specifically express a number of sulfate transporters. With regards to micronutrients, except for Fe, their uptake can be increased under S deficiency through various mechanisms. One of these results from the broad specificity of root sulfate transporters that are up-regulated during S deficiency, which can also take up some molybdate and selenate. A second mechanism is linked to the large accumulation of sulfate in the leaf vacuoles, with its reduced osmotic contribution under S deficiency being compensated for by an increase in Cl uptake and accumulation. A third group of broader mechanisms that can explain at least some of the interactions between S and micronutrients concerns metabolic networks where several nutrients are essential, such as the synthesis of the Mo co-factor needed by some essential enzymes, which requires S, Fe, Zn and Cu for its synthesis, and the synthesis and regulation of Fe-S clusters. Finally, we briefly review recent developments in the modelling of S responses in crops (allocation amongst plant parts and distribution of mineral versus organic forms) in order to provide perspectives on prediction-based approaches that take into account the interactions with other minerals such as N.

Winter Wheat Grain Quality, Zinc and Iron Concentration Affected by a Combined Foliar Spray of Zinc and Iron Fertilizers

Wheat (Triticum aestivum L.) is one of the main foods globally. Nutrition problems associated with Zinc and Iron deficiency affect more than two billion individuals. Biofortification is a strategy believed to be sustainable, economical and easily implemented. This study evaluated the effect of combined Zn and Fe applied as foliar fertilizer to winter wheat on grain yield, quality, Zn and Fe concentration in the grains. Results showed that treatments containing high Fe increased the yield. Grain crude fat content remained unaffected. Crude fiber was enhanced up to three-fold by 60% Zn + 40% Fe5.5 (5.5 kg ha−1 of 60% Zn + 40% Fe). Moreover, 80% Zn + 20% Fe5.5 (5.5 kg ha−1 of 80% Zn + 20% Fe) was the best combination for increasing crude protein. Zinc applied alone enhanced Zn concentration in grain. In addition, Fe was slightly improved by an application of Zn and Fe in the first year, but a greater increase was observed in the second year, where 100% Fe13 (13 kg ha−1 of 100% Fe) was the best in improving Fe in grain. Foliar application of Zn and Fe is a practical approach to increase Zn and Fe concentration, and to improve the quality of wheat grains.

Sulfur nutrition stimulates lead accumulation and alleviates its toxicity in Populus deltoides

Sulfur (S) can modulate plant responses to toxic heavy metals, but the underlying physiological and transcriptional regulation mechanisms remain largely unknown. To investigate the effects of S supply on lead (Pb)-induced toxicity in poplars, Populus deltoides monilifera (Aiton) Eckenw. saplings were exposed to 0 or 50 μM Pb together with one of the three S concentrations (0 (low S), 100 (moderate S) or 1500 (high S) μM Na2SO4). Populus deltoides roots absorbed Pb and it was partially translocated to the aerial organs, thereby decreasing the CO2 assimilation rate and leaf growth. Lead accumulation in poplars caused the overproduction of O2- and H2O2 to induce higher levels of total thiols (T-SH) and glutathione (GSH). Lead uptake by the roots and its accumulation in the aerial organs were repressed by low S application, but stimulated by high S supply. Lead-induced O2- and H2O2 production were exacerbated by S limitation, but alleviated by high S supply. Moreover, the concentrations of S-containing antioxidants including T-SH and GSH were reduced in S-deficient poplars, but increased in high S-treated plants, which corresponded well to the changes in the activities of enzymes involved in S assimilation and GSH biosynthesis. The transcript levels of both genes encoding sulfate transporters, i.e., SULTR1.1 and SULTR2.2, were elevated by low S application or high S supply in the roots, and the transcriptional upregulation of both genes was more pronounced under Pb exposure. Furthermore, the mRNA levels of several genes involved in S assimilation and the biosynthesis of GSH and phytochelatins, i.e., ATPS1, ATPS3, GSHS1, GSHS2 and PCS1, were upregulated in poplar roots with high S supply, particularly under Pb exposure. These results indicate that a high S supply can stimulate Pb accumulation and reduce its toxicity in poplars by improving S assimilation and stimulating the biosynthesis of S-containing compounds including T-SH and GSH.

Differentially Expressed MicroRNAs Link Cellular Physiology to Phenotypic Changes in Rice Under Stress Conditions

Plant microRNAs (miRNAs) and their target genes have important functional roles in nutrition deficiency and stress response. However, the underlying mechanisms relating relative expression of miRNAs and target mRNAs to morphological adjustments are not well defined. By combining miRNA expression profiles, corresponding target genes and transcription factors that bind to computationally identified over-represented cis-regulatory elements (CREs) common in miRNAs and target gene promoters, we implement a strategy that identifies a set of differentially expressed regulatory interactions which, in turn, relate underlying cellular mechanisms to some of the phenotypic changes observed. Integration of experimentally reported individual interactions with identified regulatory interactions explains how (i) during mineral deficiency osa-miR167 inhibits shoot growth but activates adventitious root growth by influencing free auxin content; (ii) during sulfur deficiency osa-miR394 is involved in adventitious root growth inhibition, sulfur and iron homeostasis, and auxin-mediated regulation of sulfur homeostasis; (iii) osa-miR399 contributes to cross-talk between cytokinin and phosphorus deficiency signaling; and (iv) a feed-forward loop involving the osa-miR166, trihelix and HD-ZIP III transcription factors may regulate leaf senescence during drought. This strategy not only identifies various regulatory interactions connecting phenotypic changes with cellular or molecular events triggered by stress, but also provides a framework to deepen our understanding of stress cellular physiology.

Molecular regulation of aluminum resistance and sulfur nutrition during root growth

Aluminum toxicity and sulfate deprivation both regulate microRNA395 expression, repressing its low-affinity sulfate transporter ( SULTR2;1 ) target. Sulfate deprivation also induces the high-affinity sulfate transporter gene ( SULTR12 ), allowing enhanced sulfate uptake. Few studies about the relationships between sulfate, a plant nutrient, and aluminum, a toxic ion, are available; hence, the molecular and physiological processes underpinning this interaction are poorly understood. The Al-sulfate interaction occurs in acidic soils, whereby relatively high concentrations of trivalent toxic aluminum (Al³⁺) may hamper root growth, limiting uptake of nutrients, including sulfur (S). On the other side, Al³⁺ may be detoxified by complexation with sulfate in the acid soil solution as well as in the root-cell vacuoles. In this review, we focus on recent insights into the mechanisms governing plant responses to Al toxicity and its relationship with sulfur nutrition, emphasizing the role of phytohormones, microRNAs, and ion transporters in higher plants. It is known that Al³⁺ disturbs gene expression and enzymes involved in biosynthesis of S-containing cysteine in root cells. On the other hand, Al³⁺ may induce ethylene biosynthesis, enhance reactive oxygen species production, alter phytohormone transport, trigger root growth inhibition and promote sulfate uptake under S deficiency. MicroRNA395, regulated by both Al toxicity and sulfate deprivation, represses its low-affinity Sulfate Transporter 2;1 (SULTR2;1) target. In addition, sulfate deprivation induces High Affinity Sulfate Transporters (HAST; SULTR1;2), improving sulfate uptake from low-sulfate soil solutions. Identification of new microRNAs and cloning of their target genes are necessary for a better understanding of the role of molecular regulation of plant resistance to Al stress and sulfate deprivation.

A Program for Iron Economy during Deficiency Targets Specific Fe Proteins

Iron (Fe) is an essential element for plants, utilized in nearly every cellular process. Because the adjustment of uptake under Fe limitation cannot satisfy all demands, plants need to acclimate their physiology and biochemistry, especially in their chloroplasts, which have a high demand for Fe. To investigate if a program exists for the utilization of Fe under deficiency, we analyzed how hydroponically grown Arabidopsis (Arabidopsis thaliana) adjusts its physiology and Fe protein composition in vegetative photosynthetic tissue during Fe deficiency. Fe deficiency first affected photosynthetic electron transport with concomitant reductions in carbon assimilation and biomass production when effects on respiration were not yet significant. Photosynthetic electron transport function and protein levels of Fe-dependent enzymes were fully recovered upon Fe resupply, indicating that the Fe depletion stress did not cause irreversible secondary damage. At the protein level, ferredoxin, the cytochrome-b₆f complex, and Fe-containing enzymes of the plastid sulfur assimilation pathway were major targets of Fe deficiency, whereas other Fe-dependent functions were relatively less affected. In coordination, SufA and SufB, two proteins of the plastid Fe-sulfur cofactor assembly pathway, were also diminished early by Fe depletion. Iron depletion reduced mRNA levels for the majority of the affected proteins, indicating that loss of enzyme was not just due to lack of Fe cofactors. SufB and ferredoxin were early targets of transcript down-regulation. The data reveal a hierarchy for Fe utilization in photosynthetic tissue and indicate that a program is in place to acclimate to impending Fe deficiency.

A Program for Iron Economy during Deficiency Targets Specific Fe Proteins

Iron (Fe) is an essential element for plants, utilized in nearly every cellular process. Because the adjustment of uptake under Fe limitation cannot satisfy all demands, plants need to acclimate their physiology and biochemistry, especially in their chloroplasts, which have a high demand for Fe. To investigate if a program exists for the utilization of Fe under deficiency, we analyzed how hydroponically grown Arabidopsis (Arabidopsis thaliana) adjusts its physiology and Fe protein composition in vegetative photosynthetic tissue during Fe deficiency. Fe deficiency first affected photosynthetic electron transport with concomitant reductions in carbon assimilation and biomass production when effects on respiration were not yet significant. Photosynthetic electron transport function and protein levels of Fe-dependent enzymes were fully recovered upon Fe resupply, indicating that the Fe depletion stress did not cause irreversible secondary damage. At the protein level, ferredoxin, the cytochrome-b₆f complex, and Fe-containing enzymes of the plastid sulfur assimilation pathway were major targets of Fe deficiency, whereas other Fe-dependent functions were relatively less affected. In coordination, SufA and SufB, two proteins of the plastid Fe-sulfur cofactor assembly pathway, were also diminished early by Fe depletion. Iron depletion reduced mRNA levels for the majority of the affected proteins, indicating that loss of enzyme was not just due to lack of Fe cofactors. SufB and ferredoxin were early targets of transcript down-regulation. The data reveal a hierarchy for Fe utilization in photosynthetic tissue and indicate that a program is in place to acclimate to impending Fe deficiency.

Transcriptional and physiological analyses of Fe deficiency response in maize reveal the presence of Strategy I components and Fe/P interactions

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.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3478-4) contains supplementary material, which is available to authorized users.

Molybdenum and iron mutually impact their homeostasis in cucumber (Cucumis sativus) plants

Molybdenum (Mo) and iron (Fe) are essential micronutrients required for crucial enzyme activities in plant metabolism. Here we investigated the existence of a mutual control of Mo and Fe homeostasis in cucumber (Cucumis sativus). Plants were grown under single or combined Mo and Fe starvation. Physiological parameters were measured, the ionomes of tissues and the ionomes and proteomes of root mitochondria were profiled, and the activities of molybdo-enzymes and the synthesis of molybdenum cofactor (Moco) were evaluated. Fe and Mo were found to affect each other's total uptake and distribution within tissues and at the mitochondrial level, with Fe nutritional status dominating over Mo homeostasis and affecting Mo availability for molybdo-enzymes in the form of Moco. Fe starvation triggered Moco biosynthesis and affected the molybdo-enzymes, with its main impact on nitrate reductase and xanthine dehydrogenase, both being involved in nitrogen assimilation and mobilization, and on the mitochondrial amidoxime reducing component. These results, together with the identification of > 100 proteins differentially expressed in root mitochondria, highlight the central role of mitochondria in the coordination of Fe and Mo homeostasis and allow us to propose the first model of the molecular interactions connecting Mo and Fe homeostasis.

Biochemistry and Physiology of Heavy Metal Resistance and Accumulation in Euglena

Free-living microorganisms may become suitable models for removal of heavy metals from polluted water bodies, sediments, and soils by using and enhancing their metal accumulating abilities. The available research data indicate that protists of the genus Euglena are a highly promising group of microorganisms to be used in bio-remediation of heavy metal-polluted aerobic and anaerobic acidic aquatic environments. This chapter analyzes the variety of biochemical mechanisms evolved in E. gracilis to resist, accumulate and remove heavy metals from the environment, being the most relevant those involving (1) adsorption to the external cell pellicle; (2) intracellular binding by glutathione and glutathione polymers, and their further compartmentalization as heavy metal-complexes into chloroplasts and mitochondria; (3) polyphosphate biosynthesis; and (4) secretion of organic acids. The available data at the transcriptional, kinetic and metabolic levels on these metabolic/cellular processes are herein reviewed and analyzed to provide mechanistic basis for developing genetically engineered Euglena cells that may have a greater removal and accumulating capacity for bioremediation and recycling of heavy metals.

Selenium-Induced Toxicity Is Counteracted by Sulfur in Broccoli (Brassica oleracea L. var. italica)

Selenium (Se) is an essential micronutrient for humans. Increasing Se content in food crops offers an effective approach to enhance the consumption of Se in human diets. A thoroughly understanding of the effects of Se on plant growth is important for Se biofortification in food crops. Given that Se is an analog of sulfur (S) and can be toxic to plants, its effect on plant growth is expected to be greatly affected by S nutrition. However, this remains to be further understood. Here, we evaluated the influence of Se treatments on broccoli (Brassica oleracea L. var. italica) growth when S was withheld from the growth nutrient solution. We found that Se was highly toxic to plants when S nutrition was poor. In contrast to Se treatments with adequate S nutrition that slightly reduced broccoli growth, the same concentration of Se treatments without S supplementation dramatically reduced plant sizes. Higher Se toxicity was observed with selenate than selenite under low S nutrition. We examined the bases underlying the toxicity. We discovered that the high Se toxicity in low S nutrition was specifically associated with an increased ratio of Se in proteins verse total Se level, enhanced generation of reactive oxygen species, elevated lipid peroxidation causing increased cell membrane damage, and reduced antioxidant enzyme activities. Se toxicity could be counteracted with increased supplementation of S, which is likely through decreasing non-specific integration of Se into proteins and altering the redox system. The present study provides information for better understanding of Se toxicity and shows that adequate S nutrition is important to prevent Se toxicity during biofortification of crops by Se fertilization.

Effect of three safeners on sulfur assimilation and iron deficiency response in barley (Hordeum vulgare) plants

BACKGROUND

Safeners are agrochemicals used in agriculture to protect crops from herbicide injuries. They act by stimulating herbicide metabolism. As graminaceous plants, to cope with iron (Fe) deficiency, activate sulfur (S) metabolism and release huge amounts of Fe-chelating compounds, or phytosiderophores (PSs), we investigated, in barley plants (Hordeum vulgare, L.) grown in Fe deficiency, the effects of three safeners on two enzymes of S assimilation, cysteine (Cys) and glutathione (GSH), and PS release. Finally, we monitored the root Fe content in plants treated with the most effective safener.

RESULTS

Generally, all the safeners activated S metabolism and increased Cys and GSH contents. In addition, the safened plants excreted higher levels of PSs. Given that mefenpyr-diethyl (Mef) was the most effective in causing these effects, we assessed the Fe concentration in Mef-treated barley and found higher Fe levels than those in untreated plants.

CONCLUSION

The three safeners, in different ways but specifically, activated S reductive metabolism and regulated Cys and GSH contents, PS release rate and Fe content (Mef-treated barley). The results of this research provide new indications of the biochemical and physiological mechanisms involved in the safening action. © 2016 Society of Chemical Industry.

The characterization of the adaptive responses of durum wheat to different Fe availability highlights an optimum Fe requirement threshold

Plant mechanisms responding to iron (Fe) deficiency have been widely described; it is well known that Strategy II plants, as durum wheat, cope with this stress by increasing both the synthesis and secretion of phytosiderophores (PS). The important contribution of the sulfate assimilatory pathway has been also demonstrated to improve Fe use efficiency in several grasses, such as maize, barley and wheat, most likely because PS are produced from nicotianamine, whose precursor is methionine. Here, the physiological response of durum wheat (T. durum L.) plants - in terms of plant ionome, PS release, thiols content and S pathway-related enzymes - was investigated by gradually decreasing Fe availability that allowed the identification of three specific limit Fe concentrations: 75 μM, 25 μM and 0 μM Fe, i.e. the complete Fe deprivation. At each limit, plants begin to induce different and specific adaptive responses to improve Fe acquisition or to reduce the damage resulting from limited Fe availability. The identification of the Fe availability level below which durum wheat plants start an expensive metabolic reorganization of S and several other elements, could be of benefit not only for an effective cultivation of the crop but also for the grain quality.

Regulation of Phytosiderophore Release and Antioxidant Defense in Roots Driven by Shoot-Based Auxin Signaling Confers Tolerance to Excess Iron in Wheat

Iron (Fe) is essential but harmful for plants at toxic level. However, how wheat plants tolerate excess Fe remains vague. This study aims at elucidating the mechanisms underlying tolerance to excess Fe in wheat. Higher Fe concentration caused morpho-physiological retardation in BR 26 (sensitive) but not in BR 27 (tolerant). Phytosiderophore and 2-deoxymugineic acid showed no changes in BR 27 but significantly increased in BR 26 due to excess Fe. Further, expression of TaSAMS. TaDMAS1, and TaYSL15 significantly downregulated in BR 27 roots, while these were upregulated in BR 26 under excess Fe. It confirms that inhibition of phytosiderophore directs less Fe accumulation in BR 27. However, phytochelatin and expression of TaPCS1 and TaMT1 showed no significant induction in response to excess Fe. Furthermore, excess Fe showed increased catalase, peroxidase, and glutathione reductase activities along with glutathione, cysteine, and proline accumulation in roots in BR 27. Interestingly, BR 27 self-grafts and plants having BR 26 rootstock attached to BR 27 scion had no Fe-toxicity induced adverse effect on morphology but showed BR 27 type expressions, confirming that shoot-derived signal triggering Fe-toxicity tolerance in roots. Finally, auxin inhibitor applied with higher Fe concentration caused a significant decline in morpho-physiological parameters along with increased TaSAMS and TaDMAS1 expression in roots of BR 27, revealing the involvement of auxin signaling in response to excess Fe. These findings propose that tolerance to excess Fe in wheat is attributed to the regulation of phytosiderophore limiting Fe acquisition along with increased antioxidant defense in roots driven by shoot-derived auxin signaling.

Impairment of Respiratory Chain under Nutrient Deficiency in Plants: Does it Play a Role in the Regulation of Iron and Sulfur Responsive Genes?

Plant production and plant product quality strongly depend on the availability of mineral nutrients. Among them, sulfur (S) and iron (Fe) play a central role, as they are needed for many proteins of the respiratory chain. Plant mitochondria play essential bioenergetic and biosynthetic functions as well as they have an important role in signaling processes into the cell. Here, by comparing several transcriptomic data sets from plants impaired in their respiratory function with the genes regulated under Fe or S deficiencies obtained from other data sets, nutrient-responsive genes potentially regulated by hypothetical mitochondrial retrograde signaling pathway are evidenced. It leads us to hypothesize that plant mitochondria could be, therefore, required for regulating the expression of key genes involved both in Fe and S metabolisms.

Gene expression and physiological responses to salinity and water stress of contrasting durum wheat genotypes

Elucidating the relationships between gene expression and the physiological mechanisms remains a bottleneck in breeding for resistance to salinity and drought. This study related the expression of key target genes with the physiological performance of durum wheat under different combinations of salinity and irrigation. The candidate genes assayed included two encoding for the DREB (dehydration responsive element binding) transcription factors TaDREB1A and TaDREB2B, another two for the cytosolic and plastidic glutamine synthetase (TaGS1 and TaGS2), and one for the specific Na(+) /H(+) vacuolar antiporter (TaNHX1). Expression of these genes was related to growth and different trait indicators of nitrogen metabolism (nitrogen content, stable nitrogen isotope composition, and glutamine synthetase and nitrate reductase activities), photosynthetic carbon metabolism (stable carbon isotope composition and different gas exchange traits) and ion accumulation. Significant interaction between genotype and growing conditions occurred for growth, nitrogen content, and the expression of most genes. In general terms, higher expression of TaGS1, TaGS2, TaDREB2B, and to a lesser extent of TaNHX1 were associated with a better genotypic performance in growth, nitrogen, and carbon photosynthetic metabolism under salinity and water stress. However, TaDREB1A was increased in expression under stress compared with control conditions, with tolerant genotypes exhibiting lower expression than susceptible ones.

The Interplay between Sulfur and Iron Nutrition in Tomato

Plant response mechanisms to deficiency of a single nutrient, such as sulfur (S) or iron (Fe), have been described at agronomic, physiological, biochemical, metabolomics, and transcriptomic levels. However, agroecosystems are often characterized by different scenarios, in which combined nutrient deficiencies are likely to occur. Soils are becoming depleted for S, whereas Fe, although highly abundant in the soil, is poorly available for uptake because of its insolubility in the soil matrix. To this end, earlier reports showed that a limited S availability reduces Fe uptake and that Fe deficiency results in the modulation of sulfate uptake and assimilation. However, the mechanistic basis of this interaction remains largely unknown. Metabolite profiling of tomato (Solanum lycopersicum) shoots and roots from plants exposed to Fe, S, and combined Fe and S deficiency was performed to improve the understanding of the S-Fe interaction through the identification of the main players in the considered pathways. Distinct changes were revealed under the different nutritional conditions. Furthermore, we investigated the development of the Fe deficiency response through the analysis of expression of ferric chelate reductase, iron-regulated transporter, and putative transcription factor genes and plant sulfate uptake and mobilization capacity by analyzing the expression of genes encoding sulfate transporters (STs) of groups 1, 2, and 4 (SlST1.1, SlST1.2, SlST2.1, SlST2.2, and SlST4.1). We identified a high degree of common and even synergistic response patterns as well as nutrient-specific responses. The results are discussed in the context of current models of nutrient deficiency responses in crop plants.

Ethylene Participates in the Regulation of Fe Deficiency Responses in Strategy I Plants and in Rice

Iron (Fe) is very abundant in most soils but its availability for plants is low, especially in calcareous soils. Plants have been divided into Strategy I and Strategy II species to acquire Fe from soils. Strategy I species apply a reduction-based uptake system which includes all higher plants except the Poaceae. Strategy II species apply a chelation-based uptake system which includes the Poaceae. To cope with Fe deficiency both type of species activate several Fe deficiency responses, mainly in their roots. These responses need to be tightly regulated to avoid Fe toxicity and to conserve energy. Their regulation is not totally understood but some hormones and signaling substances have been implicated. Several years ago it was suggested that ethylene could participate in the regulation of Fe deficiency responses in Strategy I species. In Strategy II species, the role of hormones and signaling substances has been less studied. However, in rice, traditionally considered a Strategy II species but that possesses some characteristics of Strategy I species, it has been recently shown that ethylene can also play a role in the regulation of some of its Fe deficiency responses. Here, we will review and discuss the data supporting a role for ethylene in the regulation of Fe deficiency responses in both Strategy I species and rice. In addition, we will review the data about ethylene and Fe responses related to Strategy II species. We will also discuss the results supporting the action of ethylene through different transduction pathways and its interaction with other signals, such as certain Fe-related repressive signals occurring in the phloem sap. Finally, the possible implication of ethylene in the interactions among Fe deficiency responses and the responses to other nutrient deficiencies in the plant will be addressed.

Deciphering the signaling mechanisms of the plant cell wall degradation machinery in Aspergillus oryzae

Background

The gene expression and secretion of fungal lignocellulolytic enzymes are tightly controlled at the transcription level using independent mechanisms to respond to distinct inducers from plant biomass. An advanced systems-level understanding of transcriptional regulatory networks is required to rationally engineer filamentous fungi for more efficient bioconversion of different types of biomass.

Results

In this study we focused on ten chemically defined inducers to drive expression of cellulases, hemicellulases and accessory enzymes in the model filamentous fungus Aspergillus oryzae and shed light on the complex network of transcriptional activators required. The chemical diversity analysis of the inducers, based on 186 chemical descriptors calculated from the structure, resulted into three clusters, however, the global, metabolic and extracellular protein transcription of the A. oryzae genome were only partially explained by the chemical similarity of the enzyme inducers. Genes encoding enzymes that have attracted considerable interest such as cellobiose dehydrogenases and copper-dependent polysaccharide mono-oxygenases presented a substrate-specific induction. Several homology-model structures were derived using ab-initio multiple threading alignment in our effort to elucidate the interplay of transcription factors involved in regulating plant-deconstructing enzymes and metabolites. Systematic investigation of metabolite-protein interactions, using the 814 unique reactants involved in 2360 reactions in the genome scale metabolic network of A. oryzae, was performed through a two-step molecular docking against the binding pockets of the transcription factors AoXlnR and AoAmyR. A total of six metabolites viz., sulfite (H₂SO₃), sulfate (SLF), uroporphyrinogen III (UPGIII), ethanolamine phosphate (PETHM), D-glyceraldehyde 3-phosphate (T3P1) and taurine (TAUR) were found as strong binders, whereas the genes involved in the metabolic reactions that these metabolites appear were found to be significantly differentially expressed when comparing the inducers with glucose.

Conclusions

Based on our observations, we believe that specific binding of sulfite to the regulator of the cellulase gene expression, AoXlnR, may be the molecular basis for the connection of sulfur metabolism and cellulase gene expression in filamentous fungi. Further characterization and manipulation of the regulatory network components identified in this study, will enable rational engineering of industrial strains for improved production of the sophisticated set of enzymes necessary to break-down chemically divergent plant biomass.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0224-5) contains supplementary material, which is available to authorized users.

Shoot ionome to predict the synergism and antagonism between nutrients as affected by substrate and physiological status

The elemental composition of a tissue or organism is defined as ionome. However, the combined effects on the shoot ionome determined by the taxonomic character, the nutrient status and different substrates have not been investigated. This study tests the hypothesis that phylogenetic variation of monocots and dicots grown in iron deficiency can be distinguished by the shoot ionome. We analyzed 18 elements in barley, cucumber and tomato and in two substrates (hydroponic vs soil) with different nutritional regimes. Multivariate analysis evidenced a clear separation between the species. In hydroponic conditions the main drivers separating the species are non essential-nutrients as Ti, Al, Na and Li, which were positively correlated with macro- (P, K) and micronutrients (Fe, Zn, Mo, B). The separation between species is confirmed when plants are grown on soil, but the distribution is determined especially by macronutrients (S, P, K, Ca, Mg) and micronutrients (B). A number of macro (Mg, Ca, S, P, K) and micronutrients (Fe, Mn, Zn, Cu, Mo, B) contribute to plant growth and several other important physiological and metabolic plant activities. The results reported here confirmed that the synergism and antagonism between them and other non-essential elements (Ti, Al, Si, Na) define the plant taxonomic character. The ionome profile might thus be exploited as a tool for the diagnosis of plants physiological/nutritional status but also in defining biofortification strategies to optimize both mineral enrichment of staple food crops and the nutrient input as fertilizers.

Transcriptional and metabolic alternations rebalance wheat grain storage protein accumulation under variable nitrogen and sulfur supply

Wheat (Triticum aestivum L.) grain storage proteins (GSPs) are major determinants of flour end-use value. Biological and molecular mechanisms underlying the developmental and nutritional determination of GSP accumulation in cereals are as yet poorly understood. Here we timed the accumulation of GSPs during wheat grain maturation relative to changes in metabolite and transcript pools in different conditions of nitrogen (N) and sulfur (S) availability. We found that the N/S supply ratio modulated the duration of accumulation of S-rich GSPs and the rate of accumulation of S-poor GSPs. These changes are likely to be the result of distinct relationships between N and S allocation, depending on the S content of the GSP. Most developmental and nutritional modifications in GSP synthesis correlated with the abundance of structural gene transcripts. Changes in the expression of transport and metabolism genes altered the concentrations of several free amino acids under variable conditions of N and S supply, and these amino acids seem to be essential in determining GSP expression. The comprehensive data set generated and analyzed here provides insights that will be useful in adapting fertilizer use to variable N and S supply, or for breeding new cultivars with balanced and robust GSP composition.

Sulfate influx transporters in Arabidopsis thaliana are not involved in arsenate uptake but critical for tissue nutrient status and arsenate tolerance

Arsenic, a non-nutrient metalloid is toxic to plants but many details on the physiology of plant adaptation to arsenic stress are not well understood. This work provides new insights about the role of sulfur assimilation in arsenate uptake, growth and arsenic tolerance. Research reported here indicates that two high affinity sulfate transporters in Arabidopsis thaliana are not involved in root uptake of arsenate. Further this study revealed that sulfate status influenced thiol levels, elemental nutrients, growth and arsenate tolerance. The hypothesis that arsenate may be transported via sulfate transporters, SULTR1;1 and SULTR1;2 in Arabidopsis, was tested. The double mutant of sultr1;1 sultr1;2 exhibited significantly less growth than the wild-type or the single mutants. The double mutant's sulfur content was significantly lower than the wild-type but the single mutants were similar to the wild-type confirming the redundant functions of SULTR1;1 and SULTR1;2. Gene expression analyses indicated that the double mutant's sulfate uptake could be explained by the expressions of SULTR1;3, SULTR2;1, and SULTR2;2 in its roots. Following arsenate supply to the roots, the double mutant accumulated significantly less arsenic in the roots and the shoots than did the single mutants and the wild-type. The double mutant accumulated significantly less potassium and phosphorus also. (35)S sulfate supplied to wild-type or double mutant roots showed that sulfate uptake was not inhibited by arsenate. Taken together, these results indicate that root uptake of arsenate is probably not via sulfate transporters, but the poor growth of the double mutant of sultr1;1 and sultr1;2 was due to its poor sulfate status and decreased levels of thiols, which had pleiotropic effects on the root uptake and translocation of potassium and phosphorus and arsenic tolerance.

Integration of P, S, Fe, and Zn nutrition signals in Arabidopsis thaliana: potential involvement of PHOSPHATE STARVATION RESPONSE 1 (PHR1)

Phosphate and sulfate are essential macro-elements for plant growth and development, and deficiencies in these mineral elements alter many metabolic functions. Nutritional constraints are not restricted to macro-elements. Essential metals such as zinc and iron have their homeostasis strictly genetically controlled, and deficiency or excess of these micro-elements can generate major physiological disorders, also impacting plant growth and development. Phosphate and sulfate on one hand, and zinc and iron on the other hand, are known to interact. These interactions have been partly described at the molecular and physiological levels, and are reviewed here. Furthermore the two macro-elements phosphate and sulfate not only interact between themselves but also influence zinc and iron nutrition. These intricated nutritional cross-talks are presented. The responses of plants to phosphorus, sulfur, zinc, or iron deficiencies have been widely studied considering each element separately, and some molecular actors of these regulations have been characterized in detail. Although some scarce reports have started to examine the interaction of these mineral elements two by two, a more complex analysis of the interactions and cross-talks between the signaling pathways integrating the homeostasis of these various elements is still lacking. However, a MYB-like transcription factor, PHOSPHATE STARVATION RESPONSE 1, emerges as a common regulator of phosphate, sulfate, zinc, and iron homeostasis, and its role as a potential general integrator for the control of mineral nutrition is discussed.

Critical function of a Chlamydomonas reinhardtii putative polyphosphate polymerase subunit during nutrient deprivation

Forward genetics was used to isolate Chlamydomonas reinhardtii mutants with altered abilities to acclimate to sulfur (S) deficiency. The ars76 mutant has a deletion that eliminates several genes, including VACUOLAR TRANSPORTER CHAPERONE1 (VTC1), which encodes a component of a polyphosphate polymerase complex. The ars76 mutant cannot accumulate arylsulfatase protein or mRNA and shows marked alterations in levels of many transcripts encoded by genes induced during S deprivation. The mutant also shows little acidocalcisome formation compared with wild-type, S-deprived cells and dies more rapidly than wild-type cells following exposure to S-, phosphorus-, or nitrogen (N)-deficient conditions. Furthermore, the mutant does not accumulate periplasmic L-amino acid oxidase during N deprivation. Introduction of the VTC1 gene specifically complements the ars76 phenotypes, suggesting that normal acidocalcisome formation in cells deprived of S requires VTC1. Our data also indicate that a deficiency in acidocalcisome function impacts trafficking of periplasmic proteins, which can then feed back on the transcription of the genes encoding these proteins. These results and the reported function of vacuoles in degradation processes suggest a major role of the acidocalcisome in reshaping the cell during acclimation to changing environmental conditions.

Iron deprivation results in a rapid but not sustained increase of the expression of genes involved in iron metabolism and sulfate uptake in tomato (Solanum lycopersicum L.) seedlings

Characterization of the relationship between sulfur and iron in both Strategy I and Strategy II plants, has proven that low sulfur availability often limits plant capability to cope with iron shortage. Here it was investigated whether the adaptation to iron deficiency in tomato (Solanum lycopersicum L.) plants was associated with an increased root sulfate uptake and translocation capacity, and modified dynamics of total sulfur and thiols accumulation between roots and shoots. Most of the tomato sulfate transporter genes belonging to Groups 1, 2, and 4 were significantly upregulated in iron-deficient roots, as it commonly occurs under S-deficient conditions. The upregulation of the two high affinity sulfate transporter genes, SlST1.1 and SlST1.2, by iron deprivation clearly suggests an increased root capability to take up sulfate. Furthermore, the upregulation of the two low affinity sulfate transporter genes SlST2.1 and SlST4.1 in iron-deficient roots, accompanied by a substantial accumulation of total sulfur and thiols in shoots of iron-starved plants, likely supports an increased root-to-shoot translocation of sulfate. Results suggest that tomato plants exposed to iron-deficiency are able to change sulfur metabolic balance mimicking sulfur starvation responses to meet the increased demand for methionine and its derivatives, allowing them to cope with this stress.

Toward new perspectives on the interaction of iron and sulfur metabolism in plants

The deficiency of nutrients has been extensively investigated because of its impact on plant growth and yield. So far, the effects of a combined nutrient limitation have rarely been analyzed, although such situations are likely to occur in agroecosystems. Iron (Fe) is a prerequisite for many essential cellular functions. Its availability is easily becoming limiting for plant growth and thus higher plants have evolved different strategies to cope with Fe deficiency. Sulfur (S) is an essential macro-nutrient and the responses triggered by shortage situations have been well characterized. The interaction between these two nutrients is less investigated but might be of particular importance because most of the metabolically active Fe is bound to S in Fe-S clusters. The biosynthesis of Fe-S clusters requires the provision of reduced S and chelated Fe in a defined stoichiometric ratio, strongly suggesting coordination between the metabolisms of the two nutrients. Here the available information on interactions between Fe and S nutritional status is evaluated. Experiments with Arabidopsis thaliana and crop plants indicate a co-regulation and point to a possible role of Fe-S cluster synthesis or abundance in the Fe/S network.