The hidden network of biocrust successional stages in the High Arctic: Revealing abiotic and biotic factors shaping microbial and metazoan communities.

Despite the important role that biocrust communities play in maintaining ecosystem structure and functioning in deglaciated barren soil, few studies have been conducted on the dynamics of biotic communities and the impact of physicochemical characteristics in shaping the different successional stages. In this study an integrated approach encompassing physicochemical parameters and molecular taxonomy was used for identifying the indicator taxa and the presence of intra- and inter-kingdom interactions in five different crust/biocrust successional stages: i) physical crust, ii) cyanobacteria-dominated biocrust, iii) cyanobacteria/moss-dominated biocrust, iv) moss-dominated biocrust and v) bryophyte carpet. The phylum Gemmatimonadota was the bacterial indicator taxon in the early stage, promoting both inter- and intra-kingdom interactions, while Cyanobacteria and Nematoda phyla played a pivotal role in formation and dynamics of cyanobacteria-dominated biocrusts. A multitrophic community, characterized by a shift from oligotrophic to copiotrophic bacteria and the presence of saproxylic arthropod and herbivore insects was found in the cyanobacteria/moss-dominated biocrust, while a more complex biota, characterized by an increased fungal abundance (classes Sordariomycetes, Leotiomycetes, and Dothideomycetes, phylum Ascomycota), associated with highly trophic consumer invertebrates (phyla Arthropoda, Rotifera, Tardigrada), was observed in moss-dominated biocrusts. The class Bdelloidea and the family Hypsibiidae (phyla Rotifera and Tardigrada, respectively) were metazoan indicator taxon in bryophyte carpet, suggesting their potential role in shaping structure and function of this late successional stage. Nitrogen and phosphorus were the main physicochemical limiting factors driving the shift among different crust/biocrust successional stages. Identification and characterization of indicator taxa, biological intra- and inter-kingdom interactions and abiotic factors driving the shift among different crust/biocrust successional stages provide a detailed picture on crust/biocrust dynamics, revealing a strong interconnection among micro- and macrobiota systems. These findings enhance our understanding of biocrust ecosystems in High Arctic, providing valuable insights for their conservation and management in response to environmental shifts due to climate change.

Unraveling plant adaptation to single and combined nutrient deficiencies in a dicotyledonous and a monocotyledonous plant species.

Nutrient deficiencies considerably limit agricultural production worldwide. However, while single deficiencies are widely studied, combined deficiencies are poorly addressed. Hence, the aim of this paper was to study single and combined deficiencies of iron (Fe) and phosphorus (P) in barley (Hordeum vulgare) and tomato (Solanum lycopersicum). Plants were grown in hydroponics and root exudation was measured over the growing period. At harvest, root morphology and root and shoot ionome was assessed. Shoot-to-root-ratio decreased in both species and in all nutrient deficiencies, besides in -Fe tomato. Barley root growth was enhanced in plants subjected to double deficiency behaving similarly to -P, while tomato reduced root morphology parameters in all treatments. To cope with the nutrient deficiency barley exuded mostly chelants, while tomato relied on organic acids. Moreover, tomato exhibited a slight exudation increase over time not detected in barley. Overall, in none of the species the double deficiency caused a substantial increase in root exudation. Multivariate statistics emphasized that all the treatments were significantly different from each other in tomato, while in barley only -Fe was statistically different from the other treatments. Our findings highlight that the response of the studied plants in double deficiencies is not additive but plant specific.

δ13C as a tool for iron and phosphorus deficiency prediction in crops.

Many studies proposed the use of stable carbon isotope ratio (δ13C) as a predictor of abiotic stresses in plants, considering only drought and nitrogen deficiency without further investigating the impact of other nutrient deficiencies, that is, phosphorus (P) and/or iron (Fe) deficiencies. To fill this knowledge gap, we assessed the δ13C of barley (Hordeum vulgare L.), cucumber (Cucumis sativus L.), maize (Zea mays L.), and tomato (Solanum lycopersicon L.) plants suffering from P, Fe, and combined P/Fe deficiencies during a two-week period using an isotope-ratio mass spectrometer. Simultaneously, plant physiological status was monitored with an infra-red gas analyzer. Results show clear contrasting time-, treatment-, species-, and tissue-specific variations. Furthermore, physiological parameters showed limited correlation with δ13C shifts, highlighting that the plants' δ13C, does not depend solely on photosynthetic carbon isotope fractionation/discrimination (Δ). Hence, the use of δ13C as a predictor is highly discouraged due to its inability to detect and discern different nutrient stresses, especially when combined stresses are present.

Fire-induced effects on the bioavailability of potentially toxic elements in a polluted agricultural soil: implications for Cr uptake by durum wheat plants.

Fire events can modify the distribution and speciation of potentially toxic elements (PTEs) in soil, especially if they are associated to organic matter (OM). In fact, OM can undergo substantial structural modifications at high temperatures, up to the complete mineralization. The present study aims to investigate the changes of PTEs' bioavailability to durum wheat (Triticum durum Desf.) plants after simulating fire events (up to 300 °C and 500 °C) in an agricultural soil polluted by Cr, Zn, Cu, and Pb. The PTEs' uptake and allocation in plant tissues were assessed using the RHIZOtest system. After the fire simulations, no evident risk of accumulation and translocation in plants was observed for Zn, Pb, and Cu. Conversely, a high accumulation in roots and a significant translocation to shoots were observed for Cr, which reached concentrations of 829 mg kg-1 in roots and 52 mg kg-1 in shoots at 500 °C. Additional experimental evidence suggested that Cr was taken up by plants grown on heated soils as Cr(VI). Once acquired by roots, only a small part of Cr (up to 6%) was translocated to shoots where it was likely present as mobile forms, as evidenced by micro X-ray fluorescence (µ-XRF) analyses. Overall, the results obtained provide evidence that the high temperatures occurring during fire events can increase the mobility and bioavailability of certain PTEs transforming apparently safe environments into potentially dangerous sources of pollution. These processes can ultimately affect the human health through the food chain transfer of PTEs or their migration into surface water and groundwater.

Plant species-specific impact of polyethylene microspheres on seedling growth and the metabolome.

Microplastics (MPs) are ubiquitous contaminants. In recent decades, the hazardous impacts of MPs on the environment have raised significant concern. However, little attention has been focused on the interaction between MPs and plants in terrestrial agroecosystems. This study aims to investigate the effects of polyethylene microspheres (PE-MS) on the germination, morphology, and metabolism of barley (Hordeum vulgare L.), cucumber (Cucumis sativus L.), and tomato (Solanum lycopersicum L.). Specifically, seeds were soaked in PE-MS solutions at three concentrations (10, 100, and 1000 mg L-1), while control seeds were treated with distilled water. After five days, the morphological parameters of barley (i.e., shoot and root biomass, length, and average diameter) were significantly affected by PE-MS treatment, even at the lowest concentration, without a dose dependency. On the other hand, the effect of PE-MS on the morphological parameters of cucumber and tomato was evident only at the highest concentration (1000 mg L-1). PE-MS also induced metabolomic reprogramming of shoots and roots in all three plant species. There was a downregulation of fatty acids and secondary metabolites (except in tomato shoots). In addition, the response of amino acids and hormones was highly heterogeneous among species and plant parts. In particular, the response of metabolites changed within species among different plant parts. In conclusion, we found a strong influence of MS-PE on the metabolic profile of the three plant species and a positive priming of seedling growth, especially in barley, where all the morphological parameters considered were significantly improved. Further investigations are needed to fully understand the mechanisms underlying MP-plant interactions, especially in the long term.

Can Inoculation With the Bacterial Biostimulant Enterobacter sp. Strain 15S Be an Approach for the Smarter P Fertilization of Maize and Cucumber Plants?

Phosphorus (P) is an essential nutrient for plants. The use of plant growth-promoting bacteria (PGPB) may also improve plant development and enhance nutrient availability, thus providing a promising alternative or supplement to chemical fertilizers. This study aimed to evaluate the effectiveness of Enterobacter sp. strain 15S in improving the growth and P acquisition of maize (monocot) and cucumber (dicot) plants under P-deficient hydroponic conditions, either by itself or by solubilizing an external source of inorganic phosphate (Pi) [Ca3(PO4)2]. The inoculation with Enterobacter 15S elicited different effects on the root architecture and biomass of cucumber and maize depending on the P supply. Under sufficient P, the bacterium induced a positive effect on the whole root system architecture of both plants. However, under P deficiency, the bacterium in combination with Ca3(PO4)2 induced a more remarkable effect on cucumber, while the bacterium alone was better in improving the root system of maize compared to non-inoculated plants. In P-deficient plants, bacterial inoculation also led to a chlorophyll content [soil-plant analysis development (SPAD) index] like that in P-sufficient plants (p < 0.05). Regarding P nutrition, the ionomic analysis indicated that inoculation with Enterobacter 15S increased the allocation of P in roots (+31%) and shoots (+53%) of cucumber plants grown in a P-free nutrient solution (NS) supplemented with the external insoluble phosphate, whereas maize plants inoculated with the bacterium alone showed a higher content of P only in roots (36%) but not in shoots. Furthermore, in P-deficient cucumber plants, all Pi transporter genes (CsPT1.3, CsPT1.4, CsPT1.9, and Cucsa383630.1) were upregulated by the bacterium inoculation, whereas, in P-deficient maize plants, the expression of ZmPT1 and ZmPT5 was downregulated by the bacterial inoculation. Taken together, these results suggest that, in its interaction with P-deficient cucumber plants, Enterobacter strain 15S might have solubilized the Ca3(PO4)2 to help the plants overcome P deficiency, while the association of maize plants with the bacterium might have triggered a different mechanism affecting plant metabolism. Thus, the mechanisms by which Enterobacter 15S improves plant growth and P nutrition are dependent on crop and nutrient status.

The hidden effects of agrochemicals on plant metabolism and root-associated microorganisms.

Agrochemicals are commonly used in agriculture to protect crops and ensure yields. Several of them are mobile within the plant and, being perceived as xenobiotics regardless of their protective/curative roles, they induce a reprogramming of secondary metabolism linked to the detoxification processes even in the absence of phenotype symptoms. Moreover, it is well documented that plants are able to shape the microbial population at the rhizosphere and to significantly affect the processes occurring therein thanks to the root exudation of different metabolites. Here we show that plant metabolic response to foliarly-applied pesticides is much broader than what previously thought and includes diverse and compound-specific hidden processes. Among others, stress-related metabolism and phytohormones profile underwent a considerable reorganization. Moreover, a distinctive microbial rearrangement of the rhizosphere was recorded following foliar application of pesticides. Such effects have unavoidably energetic and metabolic costs for the plant paving the way to both positive and negative aspects. The understanding of these effects is crucial for an increasingly sustainable use of pesticides in agriculture.

Corrigendum: Root Handling Affects Carboxylates Exudation and Phosphate Uptake of White Lupin Roots.

[This corrects the article DOI: 10.3389/fpls.2020.584568.].

Millimetre-resolution mapping of citrate exuded from soil-grown roots using a novel, low-invasive sampling technique.

The reliable sampling of root exudates in soil-grown plants is experimentally challenging. This study aimed at developing a citrate sampling and mapping technique with millimetre-resolution using DGT (diffusive gradients in thin films) ZrOH-binding gels. Citrate adsorption kinetics, DGT capacity, and stability of ZrOH gels were evaluated. ZrOH gels were applied to generate 2D maps of citrate exuded by white lupin roots grown in a rhizotron in a phosphorus-deficient soil. Citrate was adsorbed quantitatively and rapidly by the ZrOH gels; these gels can be stored after sampling for several weeks prior to analysis. The DGT capacity of the ZrOH gel for citrate depends on the ionic strength and the pH of the soil solution, but was suitable for citrate sampling. We generated for the first time 2D citrate maps of rhizotron-grown plants at a millimetre resolution to measure an illustrated plant response to phosphorus fertilization, demonstrating that DGT-based citrate sampling is suitable for studying root exudation in soil environments, at high spatial resolution. The change of binding material would also allow sampling of other exudate classes and exudation profiles of entire root systems. These aspects are crucial in cultivar breeding and selection.

The effect of earthworms on plant response in metal contaminated soil focusing on belowground-aboveground relationships.

Contaminated soils are lands in Europe deemed less favourable for conventional agriculture. To overcome the problem of their poor fertility, bio-fertilization could be a promising approach. Soil inoculation with a choice of biological species (e.g. earthworm, mycorrhizal fungi, diazotroph bacteria) can be performed in order to improve soil properties and promote nutrients recycling. However, questions arise concerning the dynamics of the contaminants in an inoculated soil. The aim of this study was to highlight the soil-plant-earthworm interactions in the case of a slightly contaminated soil. For this purpose, a pot experiment in controlled conditions was carried out during 2 months with a Cd, Zn, and Cu contaminated sandy soil, including conditions with or without earthworms (Aporrectodea caliginosa) and with or without plants (Lolium perenne). The three components of the trace element bioavailability were studied to understand the belowground-aboveground relationships and were quantified as followed: i) environmental availability in soils by measuring trace element concentrations in soil solution, ii) environmental bioavailability for organisms by measuring trace element concentrations in depurated whole earthworms bodies and in the plant aerial biomass, and iii) toxicological bioavailability, by measuring survival rate and body weight changes for earthworms and biomass for plants. The results showed that earthworm inoculation increased the content of all studied TE in soil solution. Moreover, lower concentrations of Cd and Zn were found in plants in the presence of earthworms while the bioavailability decreased when compared to the condition without plants. The trace element bioaccumulation in earthworms did not produce a direct toxicity, according to the earthworm survival rate and body weight results. Finally, our pot experiment confirmed that even in contaminated soils, the presence of A. caliginosa promotes plant adaptation and improves biomass production, reducing trace element uptake.

Root Handling Affects Carboxylates Exudation and Phosphate Uptake of White Lupin Roots.

The reliable quantification of root exudation and nutrient uptake is a very challenging task, especially when considering single root segments. Most methods used necessitate root handling e.g. root dissecting/cutting. However, there is a knowledge gap on how much these techniques affect root physiology. Thus, this study aimed at assessing the effect of different root handling techniques on the phosphate (Pi) uptake and carboxylate exudation of white lupin roots. White lupin plants were grown hydroponically in a full and Pi-deficient nutrient solution for 60 days. Phosphate uptake and carboxylate exudation of cluster and non-cluster roots were measured using custom made cells 1, 4, and 8 h after the onset of light. Three different experimental set-ups were used: i) without cutting the root apparatus from the shoots, nor dissecting the root into smaller root sections - named intact plant (IP); ii) separating the roots from the shoots, without dissecting the root into smaller sections - named intact root (IR); iii) separating the roots form the shoots and dissecting the roots in different sections-named dissected roots (DR). The sampling at 8 h led to the most significant alterations of the root Pi uptake induced by the sampling method. Generally, roots were mainly affected by the DR sampling method, indicating that results of studies in which roots are cut/dissected should be interpreted carefully. Additionally, the study revealed that the root tip showed a very high Pi uptake rate, suggesting that the tip could act as a Pi sensor. Citrate, malate and lactate could be detected in juvenile, mature and senescent cluster root exudation. We observed a significant effect of the handling method on carboxylate exudation only at sampling hours 1 and 8, although no clear and distinctive trend could be observed. Results here presented reveal that the root handling as well as the sampling time point can greatly influence root physiology and therefore should not be neglected when interpreting rhizosphere dynamics.

Copper toxicity affects phosphorus uptake mechanisms at molecular and physiological levels in Cucumis sativus plants.

Due to the deliberate use of cupric fungicides in the last century for crop-defence programs, copper (Cu) has considerably accumulated in the soil. The concentrations of Cu often exceed the safety limits of risk assessment for Cu in soil and this may cause toxicity in plants. Copper toxicity induces nutritional imbalances in plants and constraints to plants growth. These aspects might be of paramount importance in the case of phosphorus (P), which is an essential plant macronutrient. In this work, hydroponically grown cucumber plants were used to investigate the influence of the exposure to different Cu concentrations (0.2, 5, 25 and 50 µM) on i) the phenotypic traits of plants, particularly at root level, ii) the nutrient content in both roots and shoots, and iii) the P uptake mechanisms, considering both the biochemical and molecular aspects. At high Cu concentrations (i.e. above 25 µM), the shoot and root growth resulted stunted and the P influx rate diminished. Furthermore, two P transporter genes (i.e. CsPT1.4 and CsPT1.9) were upregulated at the highest Cu concentration, albeit with different induction kinetics. Overall, these results confirm that high Cu concentrations can limit the root acquisition of P, most likely via a direct action on the uptake mechanisms (e.g. transporters). However, the alteration of root plasma membrane permeability induced by Cu toxicity might also play a pivotal role in the observed phenomenon.

Phosphorus deficiency changes carbon isotope fractionation and triggers exudate reacquisition in tomato plants.

Plant roots are able to exude vast amounts of metabolites into the rhizosphere in response to phosphorus (P) deficiency. Causing noteworthy costs in terms of energy and carbon (C) for the plants. Therefore, it is suggested that exudates reacquisition by roots could represent an energy saving strategy of plants. This study aimed at investigating the effect of P deficiency on the ability of hydroponically grown tomato plants to re-acquire specific compounds generally present in root exudates by using 13C-labelled molecules. Results showed that P deficient tomato plants were able to take up citrate (+ 37%) and malate (+ 37%), particularly when compared to controls. While glycine (+ 42%) and fructose (+ 49%) uptake was enhanced in P shortage, glucose acquisition was not affected by the nutritional status. Unexpectedly, results also showed that P deficiency leads to a 13C enrichment in both tomato roots and shoots over time (shoots-+ 2.66‰, roots-+ 2.64‰, compared to control plants), probably due to stomata closure triggered by P deficiency. These findings highlight that tomato plants are able to take up a wide range of metabolites belonging to root exudates, thus maximizing C trade off. This trait is particularly evident when plants grew in P deficiency.

Selenium Biofortification in Fragaria × ananassa: Implications on Strawberry Fruits Quality, Content of Bioactive Health Beneficial Compounds and Metabolomic Profile.

Selenium (Se) is an essential nutrient for humans, due to its antioxidant properties, whereas, to date, its essentiality to plants still remains to be demonstrated. Nevertheless, if added to the cultivation substrate, plants growth resulted enhanced. However, the concentration of Se in agricultural soils is very variable, ranging from 0.01 mg kg-1 up to 10 mg kg-1 in seleniferous areas. Therefore several studies have been performed aimed at bio-fortifying crops with Se and the approaches exploited were mainly based on the application of Se fertilizers. The aim of the present research was to assess the biofortification potential of Se in hydroponically grown strawberry fruits and its effects on qualitative parameters and nutraceutical compounds. The supplementation with Se did not negatively affect the growth and the yield of strawberries, and induced an accumulation of Se in fruits. Furthermore, the metabolomic analyses highlighted an increase in flavonoid and polyphenol compounds, which contributes to the organoleptic features and antioxidant capacity of fruits; in addition, an increase in the fruits sweetness also was detected in biofortified strawberries. In conclusion, based on our observations, strawberry plants seem a good target for Se biofortification, thus allowing the increase in the human intake of this essential micronutrient.