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

Cadmium (Cd) detoxification involves glutathione and phytochelatins biosynthesis: the higher need of nitrogen should require increased nitrate (NO) uptake and metabolism. We investigated inducible high-affinity NO uptake across the plasma membrane (PM) in maize seedlings roots upon short exposure (10 min to 24 h) to low Cd concentrations (0, 1 or 10 μM): the activity and gene transcript abundance of high-affinity NO transporters, NO reductases and PM H-ATPases were analyzed. Exposure to 1 mM NO led to a peak in high-affinity (0.2 mM) NO uptake rate (induction), which was markedly lowered in Cd-treated roots. Plasma membrane H-ATPase activity was also strongly limited, while internal NO accumulation and NO reductase activity in extracts of Cd treated roots were only slightly lowered. Kinetics of high- and low-affinity NO uptake showed that Cd rapidly (10 min) blocked the inducible high-affinity transport system; the constitutive high-affinity transport system appeared not vulnerable to Cd and the low-affinity transport system appeared to be less affected and only after a prolonged exposure (12 h). Cd-treatment also modified transcript levels of genes encoding high-affinity NO transporters ( ZmNTR2.1, ZmNRT2.2), PM H-ATPases ( ZmMHA3, ZmMHA4) and NO reductases ( ZmNR1, ZmNADH:NR). Despite an expectable increase in NO demand, a negative effect of Cd on NO nutrition is reported. Cd effect results in alterations at the physiological and transcriptional levels of NO uptake from the external solution and it is particularly severe on the inducible high-affinity anion transport system. Furthermore, Cd would limit the capacity of the plant to respond to changes in NO availability. [ABSTRACT FROM AUTHOR]

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

It is well known that in the rhizosphere soluble Fe sources available for plants are mainly a mixture of complexes between the micronutrient and organic ligands such as organic acids and phytosiderophores (PS) released by roots, microbial siderophores as well as fractions of humified organic matter. In the present work, mechanisms of Fe acquisition operating at the leaf level of plants fed with different Fe-complexes were investigated at the micro-analytical, physiological and molecular levels. Fe-deficient tomato plants ( Solanum Lycopersicum L., cv. ‘Marmande’) were fed for 24 h with a solution (pH 7.5) containing 1 µM Fe as Fe-PS, Fe-citrate or Fe-WEHS. Thereafter, leaf tissue was used for the visualization of Fe distribution, measurements of Fe content, reduction and uptake, and evaluation of expression of Fe-chelate reductase ( LeFRO1), Fe-transporter ( LeIRT1) and Ferritin ( Ferritin2) genes. Leaf discs isolated from Fe-deficient plants treated for 24 h with Fe-WEHS developed higher rates of translocation, Fe-chelate reduction and 59Fe uptake as compared to plants supplied with Fe-citrate or Fe-PS. Leaves of plants treated with Fe-WEHS also showed higher transcript levels of LeFRO1, LeIRT1 and Ferritin2 genes with respect to plants fed with the other Fe-sources. Data obtained support the idea that the efficient use of Fe complexed to WEHS-like humic fractions involves, at least in part, also the activation of Fe-acquisition mechanisms operating at the leaf level. [ABSTRACT FROM AUTHOR]