The salivary, metal-binding peptide histatin-5 buffers extracellular copper availability

The family of human salivary histidine-rich peptides known as histatins bind zinc (Zn) and copper (Cu), but whether they contribute to nutritional immunity by influencing Zn and/or Cu availability has not been examined. We hypothesised that histatin-5 (Hst5) limits Zn availability (and promotes bacterial Zn starvation) and/or raises Cu availability (and promotes bacterial Cu poisoning). To test this hypothesis, Group A Streptococcus (GAS), which colonises the human oropharynx, was used as a model bacterium. Contrary to our hypothesis, Hst5 did not strongly influence Zn availability. This peptide did not induce expression of Zn uptake genes in GAS, nor did it suppress growth of an ΔadcAI mutant strain that is impaired in Zn uptake. Equilibrium competition measurements confirmed that Hst5 binds Zn weakly and does not compete with the high-affinity Zn uptake protein AdcAI for binding Zn. By contrast, Hst5 bound Cu with a high affinity and strongly influenced Cu availability. However, contrary to our hypothesis, Hst5 did not promote Cu toxicity. Instead, this peptide suppressed expression of Cu-inducible genes in GAS, stopped intracellular accumulation of Cu, and rescued growth of a ΔcopA mutant strain that is impaired in Cu efflux in the presence of added Cu. These findings led us to propose a new role for Hst5 and salivary histatins as major Cu buffers in saliva that reduce the potential negative effects of Cu exposure to microbes. We speculate that histatins promote oral and oropharyngeal health by contributing to microbial homeostasis in these host niches.

Lead immobilisation in mining contaminated soil using biochar and ash from sugarcane

Immobilisation of lead (Pb) and toxic elements in contaminated soils is of importance due to their persistence in the environment. Herein, we investigated the effects of sugarcane filter cake biochar (SFCB) and sugarcane bagasse ash (SBA) on the extractability of Pb and some toxic and potentially toxic elements (As, Cd, Cu, and Zn) in polluted mine soil samples from Lower Klity Creek, Thailand. The soil was equilibrated with the SFCB and SBA at the respective rates of 0, 1, and 5% (w/w) for 120 days at field capacity. The results revealed that both SFCB and SBA materials significantly (P < 0.05) decreased Pb extractability in the studied soil, and it stabilised after 56 days of incubation. At 120 days, the SFCB and SBA application at the rates of 5% SFCB, 5% SBA, 1% SFCB, and 1% SBA decreased the extractable Pb contents by 50.35, 40.81, 29.42, and 19.27%, respectively, compared to unamended soil. The SFCB and SBA materials also improved soil chemical properties by increasing the soil pH, available phosphorus, and extractable sulfur. At 5%, SFCB decreased As extractability and increased organic carbon in the studied soil. The Zn availability in the studied soil was also improved by SFCB and SBA addition. This study highlights the potential use of biochar and ash from the sugarcane industry to stabilise Pb and As in contaminated soils.

Green manure effect on the ability of native and inoculated soil bacteria to mobilize zinc for wheat uptake (Triticum aestivum L.)

Purpose Green manuring can increase the plant available fraction of zinc (Zn) in soil, making it a potential approach to increase wheat Zn concentrations and fight human Zn deficiency. We tested whether green manure increases the ability of both the native soil bacteria and inoculated Zn solubilizing bacteria (ZSB) to mobilize Zn. Methods Wheat was grown in a pot experiment with the following three factors (with or without); (i) clover addition; (ii) soil x-ray irradiation (i.e. elimination of the whole soil biota followed by re-inoculation with the native soil bacteria); and (iii) ZSB inoculation. The incorporation of clover in both the irradiated and the ZSB treatments allowed us to test green manure effects on the mobilization of Zn by indigenous soil bacteria as well as by inoculated strains. Results Inoculation with ZSB did neither increase soil Zn availability nor wheat Zn uptake. The highest soil Zn availabilities were found when clover was incorporated, particularly in the irradiated soils (containing only soil bacteria). This was partly associated with the stimulation of bacterial activity during the decomposition of the incorporated green manure. Conclusion The results support that the activity of soil bacteria is intimately involved in the mobilization of Zn following the incorporation of green manure.

Green Manure Effect on The Ability of Native and Inoculated Soil Bacteria to Mobilize Zinc for Wheat Uptake (Triticum Aestivum L.)

PurposeGreen manuring can increase the plant available fraction of zinc (Zn) in soil, making it a potential approach to increase wheat Zn concentrations and fight human Zn deficiency. We tested whether green manure increases the ability of both the native soil bacteria and inoculated Zn solubilizing bacteria (ZSB) to mobilize Zn.MethodsWheat was grown in a pot experiment with the following three factors (with or without); (i) clover addition; (ii) soil x-ray irradiation (i.e. elimination of the whole soil biota followed by re-inoculation with the native soil bacteria); and (iii) ZSB inoculation. The incorporation of clover in both the irradiated and the ZSB treatments allowed us to test green manure effects on the mobilization of Zn by indigenous soil bacteria as well as by inoculated strains.ResultsInoculation with ZSB did neither increase soil Zn availability nor wheat Zn uptake. The highest soil Zn availabilities were found when clover was incorporated, particularly in the irradiated soils (containing only soil bacteria). This was partly associated with the stimulation of bacterial activity during the decomposition of the incorporated green manure.ConclusionThe results support that the activity of soil bacteria is intimately involved in the mobilization of Zn following the incorporation of green manure.

Combining P and Zn fertilization to enhance yield and grain quality in maize grown on Mediterranean soils

The main aim of this study was to elucidate the effect of individual and joint fertilization with P and Zn on maize plants grown on typical Mediterranean soils with a limited Zn availability. For this purpose, we examined the effects of P and Zn fertilization individually and in combination on growth, yield and grain protein content in maize grown in pots filled with three different Mediterranean soils (LCV, FER and INM). Phosphorus and Zn translocation to grain was impaired, and aboveground dry matter and yield at harvest reduced by 8–85% (LCV and FER), in plants treated with Zn or P alone relative to unfertilized (control) plants. In contrast, joint fertilization with P and Zn enhanced translocation of these nutrients to grain and significantly increased aboveground dry matter (30% in LCV, 50% in FER and 250% in INM) and grain Zn availability in comparison with control plants. Also, joint application of both nutrients significantly increased grain P (LCV) and Zn (LCV and FER) use efficiency relative P and Zn, respectively, alone. Yield was increased between 31% in LCV and 121% in FER relative to control plants, albeit not significantly. Fertilization with P or Zn significantly influenced the abundance of specific proteins affecting grain quality (viz., storage, lys-rich and cell wall proteins), which were more abundant in mature grains from plants fertilized with Zn alone and, to a lesser extent, P + Zn. Sustainable strategies in agriculture should consider P–Zn interactions in maize grown on soils with a limited availability of Zn, where Zn fertilization is crucial to ensure grain quality.

Zinc Uptake by Plants as Affected by Fertilization with Zn Sulfate, Phosphorus Availability, and Soil Properties

Zinc (Zn) deficiency constrains crop yield and quality, but soil factors influencing Zn availability to plants and reactions of applied Zn fertilizer are not fully understood. This work is aimed at studying Zn availability in soil and the use efficiency of Zn fertilizers by plants as affected by soil properties and particularly by soil available P. We performed a pot experiment involving four consecutive crops fertilized with Zn sulfate using 36 soils. The cumulative Zn uptake and dry matter yield in the four crops increased with increased initial diethylenetriamine pentaacetic acid extraction of Zn (DTPA-Zn) (R2 = 0.75 and R2 = 0.61; p < 0.001). The initial DTPA-Zn increased with increased Olsen P (R2 = 0.41; p < 0.001) and with increased ratio of Fe in poorly crystalline to Fe in crystalline oxides (R2 = 0.58; p < 0.001). DTPA-Zn decreased with increased cumulative Zn uptake, but not in soils with DTPA-Zn < 0.5 mg kg−1. Overall, the available Zn is more relevant in explaining Zn uptake by plants than applied Zn sulfate. However, in Zn-deficient soils, Zn fertilizer explained most of the Zn uptake by crops. Poorly crystalline Fe oxides and P availability exerted a positive role on Zn availability to plants in soil.

Cattle manure loadings and legacy effects on Cu and Zn availability under rainfed and irrigated conditions

Long term cattle manure applications build up nutrient pools and can lead to trace element enrichments in soils. The objectives of this study were to evaluate Cu and Zn loadings in the soil during continuous annual cattle manure applications and determine the time required for soil to return to its pre-manure available Cu and Zn levels after manure is discontinued. The manure application rates were 0, 30, 60, and 90 Mg ha-1 for rainfed and 0, 60, 120, and 180 Mg ha-1 (wet weight) for irrigated plots. While manure was applied for 45 years in some plots, applications were terminated in one subset of treatments after 14 years and in another subset after 30 years to study legacy effects after 31 and 15 years, respectively. Soil samples were collected in the fall of 2003, 2008, 2013, and 2018 and analyzed for available Cu and Zn. Crops were grown in all years continuously with Cu and Zn concentrations measured in both silage and grains harvested. The regression model developed using data collected suggests long legacy effects with recovery time to pre-manure levels ranging from 10-20 years for Cu and 23-41 years for Zn at irrigated and 10-24 for Cu and 21-32 years for Zn under rainfed, respectively. Long term applications of cattle manure could lead to accumulation of Cu and Zn, creating long-lasting legacy effects in soils with the increased environmental risk of leaching to groundwater

Insights Into Histoplasma capsulatum Behavior on Zinc Deprivation

Histoplasma capsulatum is a thermodimorphic fungus that causes histoplasmosis, a mycosis of global incidence. The disease is prevalent in temperate and tropical regions such as North America, South America, Europe, and Asia. It is known that during infection macrophages restrict Zn availability to H. capsulatum as a microbicidal mechanism. In this way the present work aimed to study the response of H. capsulatum to zinc deprivation. In silico analyses showed that H. capsulatum has eight genes related to zinc homeostasis ranging from transcription factors to CDF and ZIP family transporters. The transcriptional levels of ZAP1, ZRT1, and ZRT2 were induced under zinc-limiting conditions. The decrease in Zn availability increases fungicidal macrophage activity. Proteomics analysis during zinc deprivation at 24 and 48 h showed 265 proteins differentially expressed at 24 h and 68 at 48 h. Proteins related to energy production pathways, oxidative stress, and cell wall remodeling were regulated. The data also suggested that low metal availability increases the chitin and glycan content in fungal cell wall that results in smoother cell surface. Metal restriction also induces oxidative stress triggered, at least in part, by reduction in pyridoxin synthesis.

Identification of a unique ZIP transporter involved in zinc uptake via the arbuscular mycorrhizal fungal pathway

Low soil zinc (Zn) availability is a limiting factor for crop yield, and increasing Zn content is a major target for the biofortification of major crops. Arbuscular mycorrhizal (AM) fungi associate with the roots of most terrestrial plant species and improve the host plant’s growth and nutrition through the mycorrhizal pathway of nutrient uptake. Although the physiology of Zn uptake through the mycorrhizal pathway is well established, the identity of the molecular components responsible for Zn transport in the mycorrhizal pathway are unknown.RNA-seq analysis identified the putative Zn transporter gene MtZIP14 by its marked up-regulation in Medicago truncatula roots when colonised by the AM fungus Rhizophagus irregularis under varying soil Zn supply. Expression of GFP-tagged MtZIP14 in roots revealed that it is exclusively localised to the site of plant-fungal nutrient exchange in cortical cells, the peri-arbuscular membrane. Expression of MtZIP14 in a yeast mutant lacking Zn transport function restored growth under low Zn availability. M. truncatula MtZIP14 loss-of-function mutants had reduced shoot biomass compared to the wild-type when colonised by AM fungi and grown under low Zn. Vesicular and arbuscular colonisation, but not hyphal colonisation, were also lower in mtzip14 mutant plants.Based on these results we propose that MtZIP14 plays a key role in the transport of Zn from AM fungus to plant across the peri-arbuscular membrane, and MtZIP14 function is crucial to plant competitiveness in a low Zn soil.Significance statementMajority of crop plant species associate with arbuscular mycorrhizal fungi, which can increase plant nutrient uptake. Improving our knowledge of how Zn is taken up in mycorrhizal plants will lead to improved plant and human Zn nutrition outcomes. Here, we report a novel plant transporter with a major role in Zn nutrition of mycorrhizal plants. MtZIP14 is involved in Zn transport, is exclusively localised to the specialised plant-fungal interface in roots, and impairment of MtZIP14 gene function results in negative impacts on both plant growth and Zn nutrition.