Iron allocation in leaves of Fe-deficient cucumber plants fed with natural Fe complexes

Using Mathematical Optimization Models to Improve the Efficiency of Duckweeds (Wolffia arrhiza and Lemna minor) Micropropagation

The method of plant micropropagation is widely used to obtain genetically homogeneous and infection-free plants for the needs of various industries and agriculture. Optimization of plant growth and development conditions plays a key role in economically successful micropropagation. Computer technologies have provided researchers with new approaches for modeling and a better understanding of the role of the factors involved in plant growth in vitro. To develop new models for optimizing growth conditions, we used plants with a high speed of vegetative in vitro reproduction, such as duckweed (Wolffia arrhiza and Lemna minor). Using the development of the optimal modeling of the biological processes, we have obtained the prescriptions for an individually balanced culture medium that enabled us to obtain 1.5-2.0 times more duckweed biomass with a 1.5 times higher protein concentration in the dry mass. Thus, we have demonstrated that the method of optimization modeling of the biological processes based on solving multinomial tasks from the series of quadratic equations can be used for the optimization of trophic needs of plants, specifically for micropropagation of duckweeds in vitro.

Separate Effects of Foliar Applied Selenate and Zinc Oxide on the Accumulation of Macrominerals, Macronutrients and Bioactive Compounds in Two Pea (Pisum sativum L.) Seed Varieties

Selenium (Se) and zinc (Zn) are important cofactors for antioxidant enzymes. Foliar Se/Zn application is a highly efficient strategy of plant biofortification. However, its effects on the accumulation of macrominerals, macronutrients and bioactive compounds in the pea plant (Pisum sativum L.) have been poorly investigated. A two-year pot experiment was performed to study responses of two pea varieties (Ambassador, Premium) to foliar-applied sodium selenate (0/50/100 g Se/ha) and zinc oxide (0/375/750 g Zn/ha) at the flowering stage. Concentrations of Ca, Mg, K, Na, soluble solids (SSC), protein, chlorophyll a and b, total chlorophyll, total carotenoids and total condensed tannins (TCT) were determined in seeds. Mg concentration in Ambassador and chlorophyll a concentration in Premium were positively affected, in part, by selenate and zinc oxide, respectively. Selenate and zinc oxide increased, in part, protein concentration in Premium. Highest protein concentration was found in Premium treated with 375 g Zn/ha (27.6% DW) vs. the control (26.6% DW). Significant (all p < 0.001) positive correlations were found, among others, between concentrations of Zn and Mg (r2 = 0.735) and between Zn and protein (r2 = 0.437) for Ambassador, and between Mg and protein (r2 = 0.682), between Zn and Mg (r2 = 0.807), as well as between Zn and protein (r2 = 0.884) for Premium. TCT significantly (all p < 0.05) and positively correlated with SSC (r2 = 0.131), chlorophyll b (r2 = 0.128) and total chlorophyll (r2 = 0.109) for Ambassador. This study provides new nutritional data on Se/Zn biofortified peas, important for improving agronomic biofortification of pea plants.

Short Term Elevated CO2 Interacts with Iron Deficiency, Further Repressing Growth, Photosynthesis and Mineral Accumulation in Soybean (Glycine max L.) and Common Bean (Phaseolus vulgaris L.)

Elevated CO2 (eCO2) has been reported to cause mineral losses in several important food crops such as soybean (Glycine max L.) and common bean (Phaseolus vulgaris L.). In addition, more than 30% of the world’s arable land is calcareous, leading to iron (Fe) deficiency chlorosis and lower Fe levels in plant tissues. We hypothesize that there will be combinatorial effects of eCO2 and Fe deficiency on the mineral dynamics of these crops at a morphological, biochemical and physiological level. To test this hypothesis, plants were grown hydroponically under Fe sufficiency (20 μM Fe-EDDHA) or deficiency (0 μM Fe-EDDHA) at ambient CO2 (aCO2, 400 ppm) or eCO2 (800 ppm). Plants of both species exposed to eCO2 and Fe deficiency showed the lowest biomass accumulation and the lowest root: shoot ratio. Soybean at eCO2 had significantly higher chlorophyll levels (81%, p < 0.0001) and common bean had significantly higher photosynthetic rates (60%, p < 0.05) but only under Fe sufficiency. In addition, eCO2 increased ferric chelate reductase acivity (FCR) in Fe-sufficient soybean by 4-fold (p < 0.1) and in Fe-deficient common bean plants by 10-fold (p < 0.0001). In common bean, an interactive effect of both environmental factors was observed, resulting in the lowest root Fe levels. The lowering of Fe accumulation in both crops under eCO2 may be linked to the low root citrate accumulation in these plants when grown with unrestricted Fe supply. No changes were observed for malate in soybean, but in common bean, shoot levels were significantly lower under Fe deficiency (77%, p < 0.05) and Fe sufficiency (98%, p < 0.001). These results suggest that the mechanisms involved in reduced Fe accumulation caused by eCO2 and Fe deficiency may not be independent, and an interaction of these factors may lead to further reduced Fe levels.

Biostimulant Substances for Sustainable Agriculture: Origin, Operating Mechanisms and Effects on Cucurbits, Leafy Greens, and Nightshade Vegetables Species

Climate change is a pressing matter of anthropogenic nature to which agriculture contributes by abusing production inputs such as inorganic fertilizers and fertigation water, thus degrading land and water sources. Moreover, as the increase in the demand of food in 2050 is estimated to be 25 to 70% more than what is currently produced today, a sustainable intensification of agriculture is needed. Biostimulant substances are products that the EU states work by promoting growth, resistance to plant abiotic stress, and increasing produce quality, and may be a valid strategy to enhance sustainable agricultural practice. Presented in this review is a comprehensive look at the scientific literature regarding the widely used and EU-sanctioned biostimulant substances categories of silicon, seaweed extracts, protein hydrolysates, and humic substances. Starting from their origin, the modulation of plants' hormonal networks, physiology, and stress defense systems, their in vivo effects are discussed on some of the most prominent vegetable species of the popular plant groupings of cucurbits, leafy greens, and nightshades. The review concludes by identifying several research areas relevant to biostimulant substances to exploit and enhance the biostimulant action of these substances and signaling molecules in horticulture.

Chemical Structure and Biological Activity of Humic Substances Define Their Role as Plant Growth Promoters

Humic substances (HS) are dominant components of soil organic matter and are recognized as natural, effective growth promoters to be used in sustainable agriculture. In recent years, many efforts have been made to get insights on the relationship between HS chemical structure and their biological activity in plants using combinatory approaches. Relevant results highlight the existence of key functional groups in HS that might trigger positive local and systemic physiological responses via a complex network of hormone-like signaling pathways. The biological activity of HS finely relies on their dosage, origin, molecular size, degree of hydrophobicity and aromaticity, and spatial distribution of hydrophilic and hydrophobic domains. The molecular size of HS also impacts their mode of action in plants, as low molecular size HS can enter the root cells and directly elicit intracellular signals, while high molecular size HS bind to external cell receptors to induce molecular responses. Main targets of HS in plants are nutrient transporters, plasma membrane H⁺-ATPases, hormone routes, genes/enzymes involved in nitrogen assimilation, cell division, and development. This review aims to give a detailed survey of the mechanisms associated to the growth regulatory functions of HS in view of their use in sustainable technologies.

Plasmopara viticola infection affects mineral elements allocation and distribution in Vitis vinifera leaves

Plasmopara viticola is one of the most important pathogens infecting Vitis vinifera plants. The interactions among P. viticola and both susceptible and resistant grapevine plants have been extensively characterised, at transcriptomic, proteomic and metabolomic levels. However, the involvement of plants ionome in the response against the pathogen has been completely neglected so far. Therefore, this study was aimed at investigating the possible role of leaf ionomic modulation during compatible and incompatible interactions between P. viticola and grapevine plants. In susceptible cultivars, a dramatic redistribution of mineral elements has been observed, thus uncovering a possible role for mineral nutrients in the response against pathogens. On the contrary, the resistant cultivars did not present substantial rearrangement of mineral elements at leaf level, except for manganese (Mn) and iron (Fe). This might demonstrate that, resistant cultivars, albeit expressing the resistance gene, still exploit a pathogen response mechanism based on the local increase in the concentration of microelements, which are involved in the synthesis of secondary metabolites and reactive oxygen species. Moreover, these data also highlight the link between the mineral nutrition and plants' response to pathogens, further stressing that appropriate fertilization strategies can be fundamental for the expression of response mechanisms against pathogens.

Root-shoot-root Fe translocation in cucumber plants grown in a heterogeneous Fe provision

Iron (Fe) is an essential micronutrient for plant life and development. However, in soil, Fe bioavailability is often limited and variable in space and time, thus different regions of the same root system might be exposed to different nutrient provisions. Few studies showed that the response to variable Fe provision is controlled at local and systemic levels, albeit the identity of the signals involved is still elusive. Iron itself was suggested as local mediator, whilst hormones were proposed for the long-distance signalling pathway. Therefore, the aim of this work was to assess whether Fe, when localized in a restricted area of the root system, might be involved in both local and systemic signaling. The combination of resupply experiments in a split-root system, the use of ⁵⁷Fe isotope and chemical imaging techniques allowed tracing Fe movement within cucumber plants. Soon after the resupply, Fe is distributed to the whole plant, likely to overcome a minimum Fe concentration threshold aimed at repressing the deficiency response. Iron was then preferentially translocated to leaves and, only afterwards, the root system was completely resupplied. Collectively, these observations might thus highlight a root-to-shoot-to-root Fe translocation route in cucumber plants grown on a patchy nutrient substrate.

Biostimulant Action of Dissolved Humic Substances From a Conventionally and an Organically Managed Soil on Nitrate Acquisition in Maize Plants

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.

Common and specific responses to iron and phosphorus deficiencies in roots of apple tree (Malus × domestica)

Iron and phosphorus are abundant elements in soils but poorly available for plant nutrition. The availability of these two nutrients represents a major constraint for fruit tree cultivation such as apple (Malus × domestica) leading very often to a decrease of fruit productivity and quality worsening. Aim of this study was to characterize common and specific features of plant response to Fe and P deficiencies by ionomic, transcriptomic and exudation profiling of apple roots. Under P deficiency, the root release of oxalate and flavonoids increased. Genes encoding for transcription factors and transporters involved in the synthesis and release of root exudates were upregulated by P-deficient roots, as well as those directly related to P acquisition. In Fe-deficiency, plants showed an over-accumulation of P, Zn, Cu and Mn and induced the transcription of those genes involved in the mechanisms for the release of Fe-chelating compounds and Fe mobilization inside the plants. The intriguing modulation in roots of some transcription factors, might indicate that, in this condition, Fe homeostasis is regulated by a FIT-independent pathway. In the present work common and specific features of apple response to Fe and P deficiency has been reported. In particular, data indicate similar modulation of a. 230 genes, suggesting the occurrence of a crosstalk between the two nutritional responses involving the transcriptional regulation, shikimate pathway, and the root release of exudates.

Physiological and transcriptomic data highlight common features between iron and phosphorus acquisition mechanisms in white lupin roots

In agricultural soil, the bioavailability of iron (Fe) and phosphorus (P) is often below the plant's requirement causing nutritional deficiency in crops. Under P-limiting conditions, white lupin (Lupinus albus L.) activates mechanisms that promote P solubility in the soil through morphological, physiological and molecular adaptations. Similar changes occur also in Fe-deficient white lupin roots; however, no information is available on the molecular bases of the response. In the present work, responses to Fe and P deficiency and their reciprocal interactions were studied. Transcriptomic analyses indicated that white lupin roots upregulated Fe-responsive genes ascribable to Strategy-I response, this behaviour was mainly evident in cluster roots. The upregulation of some components of Fe-acquisition mechanism occurred also in P-deficient cluster roots. Concerning P acquisition, some P-responsive genes (as phosphate transporters and transcription factors) were upregulated by P deficiency as well by Fe deficiency. These data indicate a strong cross-connection between the responses activated under Fe or P deficiency in white lupin. The activation of Fe- and P-acquisition mechanisms might play a crucial role to enhance the plant's capability to mobilize both nutrients in the rhizosphere, especially P from its associated metal cations.

Humic Substances Contribute to Plant Iron Nutrition Acting as Chelators and Biostimulants

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.

Chloroplasts preferentially take up ferric-citrate over iron-nicotianamine complexes in Brassica napus

Fe uptake machinery of chloroplasts prefers to utilise Fe(III)-citrate over Fe-nicotianamine complexes. Iron uptake in chloroplasts is a process of prime importance. Although a few members of their iron transport machinery were identified, the substrate preference of the system is still unknown. Intact chloroplasts of oilseed rape (Brassica napus) were purified and subjected to iron uptake studies using natural and artificial iron complexes. Fe-nicotianamine (NA) complexes were characterised by 5 K, 5 T Mössbauer spectrometry. Expression of components of the chloroplast Fe uptake machinery was also studied. Fe(III)-NA contained a minor paramagnetic Fe(II) component (ca. 9%), a paramagnetic Fe(III) component exhibiting dimeric or oligomeric structure (ca. 20%), and a Fe(III) complex, likely being a monomeric structure, which undergoes slow electronic relaxation at 5 K (ca. 61%). Fe(II)-NA contained more than one similar chemical Fe(II) environment with no sign of Fe(III) components. Chloroplasts preferred Fe(III)-citrate compared to Fe(III)-NA and Fe(II)-NA, but also to Fe(III)-EDTA and Fe(III)-o,o'EDDHA, and the K_m value was lower for Fe(III)-citrate than for the Fe-NA complexes. Only the uptake of Fe(III)-citrate was light-dependent. Regarding the components of the chloroplast Fe uptake system, only genes of the reduction-based Fe uptake system showed high expression. Chloroplasts more effectively utilize Fe(III)-citrate, but hardly Fe-NA complexes in Fe uptake.

Synergism and antagonisms between nutrients induced by copper toxicity in grapevine rootstocks: Monocropping vs. intercropping

The long-term use of Cu-containing fungicides contaminates vineyards soils, which can induce Cu toxicity and nutrient imbalances in several plant species. The aim of this work was to evaluate the effect of Cu toxicity on two grapevine rootstocks, Fercal and 196.17, and to elucidate if intercropping with oat can alleviate grapevine Cu toxicity. Plants were hydroponically-cultivated and treated with different Cu concentrations. At harvest the biomass accumulation, the SPAD index and the symplastic and apoplastic root and leaves ionome were measured to evaluate possible synergistic and/or antagonistic effects on other micro- and macronutrients. The root exudation analysis was correlated with genes expression (VvPEZ-like), whereas PCA analysis performed on the grapevine and oat ionome revealed that both mono- and intercropped 196.17 rootstock display a positive effect on Zn and Mn in the root tissues at high Cu concentrations. An increase of Zn and Mn in roots was also reported for the intercropped Fercal rootstock at high Cu concentrations while an antagonistic relation was reported for root Zn concentration in the monocropped Fercal rootstock. Our results showed that grapevine and oat compete for nutrient uptake and that this phenomenon can possibly alleviate grapevine Cu toxicity. However, Fercal rootstock is able to take advantage from oat, while 196.17 is disadvantaged by the intercropping system. Even though intercropping system seems to be a valuable tool to counteract grapevine Cu toxicity, the application of this agricultural practice has shown to be species dependent and should be evaluated for each rootstock.

Early transcriptomic response to Fe supply in Fe-deficient tomato plants is strongly influenced by the nature of the chelating agent

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 (59)Fe concentration recorded after 24 h in tissues of tomato Fe-deficient plants supplied with (59)Fe 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 (59)Fe 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.