Single and Combined Fe and S Deficiency Differentially Modulate Root Exudate Composition in Tomato: A Double Strategy for Fe Acquisition?

Root exudates enhanced 6:2 FTOH defluorination, altered metabolite profiles and shifted soil microbiome dynamics

6:2 Fluorotelomer alcohol (FTOH), one of per- and polyfluoroalkyl substances (PFAS), is widely used as a raw material in synthesizing surfactants and fluorinated polymers. However, little is known about the role of root exudates on 6:2 FTOH biodegradation in the rhizosphere. This study examined the effects of root exudates produced from dicot (Arabidopsis thaliana) and monocot (Brachypodium distachyon) grown under different nutrient conditions (nutrient-rich, sulfur-free, and potassium-free) on 6:2 FTOH biotransformation with or without bioaugmentating agent Rhodococcus jostii RHA1. All the exudates enhanced defluorination of 6:2 FTOH by glucose-grown RHA1. Amendment of dicot or monocot root exudates, regardless of the plant growth conditions, also enhanced 6:2 FTOH biotransformation in soil microcosms. Interestingly, high levels of humic-like substances in the root exudates are linked to high extents of 6:2 FTOH defluorination. Bioaugmenting strain RHA1 along with root exudates facilitated 6:2 FTOH transformation with a production of more diverse metabolites. Microbial community analysis revealed that Rhodococcus was predominant in all strain RHA1 spiked treatments. Different root exudates changed the soil microbiome dynamics. This study provided new insight into 6:2 FTOH biotransformation with different root exudates, suggesting that root exudates amendment and bioaugmentation are promising approaches to promote rhizoremediation for PFAS-contaminated soil.

Historical shifting in grain mineral density of landmark rice and wheat cultivars released over the past 50 years in India

The 'Green Revolution (GR)' has been successful in meeting food sufficiency in India, but compromising its nutritional security. In a first, we report altered grain nutrients profile of modern-bred rice and wheat cultivars diminishing their mineral dietary significance to the Indian population. To substantiate, we evaluated grain nutrients profile of historical landmark high-yielding cultivars of rice and wheat released in succeeding decades since the GR and its impacts on mineral diet quality and human health, with a prediction for decades ahead. Analysis of grain nutrients profile shows a downward trend in concentrations of essential and beneficial elements, but an upward in toxic elements in past 50 y in both rice and wheat. For example, zinc (Zn) and iron (Fe) concentration in grains of rice decreased by ~ 33.0 (P < 0.001) and 27.0% (P < 0.0001); while for wheat it decreased by ~ 30.0 (P < 0.0001) and 19.0% (P < 0.0001) in past more than 50 y, respectively. A proposed mineral-diet quality index (M-DQI) significantly (P < 0.0001) decreased ~ 57.0 and 36.0% in the reported time span (1960-2010) in rice and wheat, respectively. The impoverished M-DQI could impose hostile effects on non-communicable diseases (NCDs) like iron-deficiency anemia, respiratory, cardiovascular, and musculoskeletal among the Indian population by 2040. Our research calls for an urgency of grain nutrients profiling before releasing a cultivar of staples like rice and wheat in the future.

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.

Nicotianamine: A Key Player in Metal Homeostasis and Hyperaccumulation in Plants

Nicotianamine (NA) is a low-molecular-weight N-containing metal-binding ligand, whose accumulation in plant organs changes under metal deficiency or excess. Although NA biosynthesis can be induced in vivo by various metals, this non-proteinogenic amino acid is mainly involved in the detoxification and transport of iron, zinc, nickel, copper and manganese. This review summarizes the current knowledge on NA biosynthesis and its regulation, considers the mechanisms of NA secretion by plant roots, as well as the mechanisms of intracellular transport of NA and its complexes with metals, and its role in radial and long-distance metal transport. Its role in metal tolerance is also discussed. The NA contents in excluders, storing metals primarily in roots, and in hyperaccumulators, accumulating metals mainly in shoots, are compared. The available data suggest that NA plays an important role in maintaining metal homeostasis and hyperaccumulation mechanisms. The study of metal-binding compounds is of interdisciplinary significance, not only regarding their effects on metal toxicity in plants, but also in connection with the development of biofortification approaches to increase the metal contents, primarily of iron and zinc, in agricultural plants, since the deficiency of these elements in food crops seriously affects human health.

Role of mycorrhizas and root exudates in plant uptake of soil nutrients (calcium, iron, magnesium, and potassium): has the puzzle been completely solved?

Anthropogenic global change is driving an increase in the frequency and intensity of drought and flood events, along with associated imbalances and limitation of several soil nutrients. In the context of an increasing human population, these impacts represent a global-scale challenge for biodiversity conservation and sustainable crop production to ensure food security. Plants have evolved strategies to enhance uptake of soil nutrients under environmental stress conditions; for example, symbioses with fungi (mycorrhization) in the rhizosphere and the release of exudates from roots. Although crop cultivation is managed for the effects of limited availability of nitrogen (N) and phosphorus (P), there is increasing evidence for limitation of plant growth and fitness because of the low availability of other soil nutrients such as the metals potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe), which may become increasingly limiting for plant productivity under global change. The roles of mycorrhizas and plant exudates on N and P uptake have been studied intensively; however, our understanding of the effects on metal nutrients is less clear and still inconsistent. Here, we review the literature on the role of mycorrhizas and root exudates in plant uptake of key nutrients (N, P, K, Ca, Mg, and Fe) in the context of potential nutrient deficiencies in crop and non-crop terrestrial ecosystems, and identify knowledge gaps for future research to improve nutrient-uptake capacity in food crop plants.

The active Fe chelator proline-2'-deoxymugineic acid enhances peanut yield by improving soil Fe availability and plant Fe status

Iron (Fe) deficiency restricts crop yields in calcareous soil. Thus, a novel Fe chelator, proline-2'-deoxymugineic acid (PDMA), based on the natural phytosiderophore 2'-deoxymugineic acid (DMA), was developed to solve the Fe deficiency problem. However, the effects and mechanisms of PDMA relevant to the Fe nutrition and yield of dicots grown under field conditions require further exploration. In this study, pot and field experiments with calcareous soil were conducted to investigate the effects of PDMA on the Fe nutrition and yield of peanuts. The results demonstrated that PDMA could dissolve insoluble Fe in the rhizosphere and up-regulate the expression of the yellow stripe-like family gene AhYSL1 to improve the Fe nutrition of peanut plants. Moreover, the chlorosis and growth inhibition caused by Fe deficiency were significantly diminished. Notably, under field conditions, the peanut yield and kernel micronutrient contents were promoted by PDMA application. Our results indicate that PDMA promotes the dissolution of insoluble Fe and a rich supply of Fe in the rhizosphere, increasing yields through integrated improvements in soil-plant Fe nutrition at the molecular and ecological levels. In conclusion, the efficacy of PDMA for improving the Fe nutrition and yield of peanut indicates its outstanding potential for agricultural applications.

Speciation of metals in indigenous plants growing in post-mining areas: Dihydroxynicotianamine identified as the most abundant Cu and Zn ligand in Hypericum laricifolium

Ag, As, Cu, Pb and Zn were found to be the principal metallic contaminants of a post-mining area of Peru (Hualgayoc, Cajamarca). Study of metal distribution amongst roots, stems, and leaves of four indigenous hypertolerant plant species, Arenaria digyna, Puya sp., Hypericum laricifolium, Nicotiana thyrsiflora indicated significant translocation of Zn (0.6 < TF ≤ 10.0) and Cu (0.4 < TF ≤ 6.5) into aerial plant organs and substantial water-extractable fraction (20-60%) of these metals, except for A. digyna (root and stems). A study of the metal speciation by ultrahigh-performance size-exclusion (fast-SEC) and hydrophilic ion interaction (HILIC) liquid chromatography with dual ICP (inductively coupled plasma) and electrospray (ESI) Orbitrap MS detection revealed the presence of nicotianamine and deoxymugineic acid copper and zinc complexes in roots, stem and leaves of N. thyrsiflora and Puya sp., and nicotianamine alone in A. digyna. A previously unreported compound, dihydroxy-nicotianamine was identified as the most abundant Cu and Zn ligand in H. laricifolium. The presence of arsenobetaine and an arsenosugar was confirmed by ESI MS. Ag and Pb were hardly translocated to leaves and were found as high molecular species; one of the Pb-containing species co-eluted in fast-SEC-ICP MS with rhamnogalacturonan-II-Pb complex commonly found in in the walls of plants.

Genotypic differences in response of durum wheat (Triticum durum Desf.) to lime-induced iron chlorosis

Wheat, durum wheat, is the first cereal cultivated and consumed in Tunisia. Because the dominance of calcareous soils in its agroecological systems, known by their low availability of iron (Fe) inducing Fe chlorosis and limiting crop production, its yield remains low. Therefore, the search for tolerant genotypes is always current. In this context, the physiological behavior of six Tunisian genotypes of durum wheat (salim, karim, razek, khiar, inrat100, and maali) cultivated on calcareous and fertile soils for 2 months in a pot experiment was investigated. A greenhouse was used to conduct experiments under natural light. Plant growth, SPAD index, Fe nutrition, Fe distribution, and photosynthesis were monitored and used to evaluate and discriminate their respective physiological responses. On calcareous soil, results revealed reduced plant growth, active Fe, SPAD index, and net photosynthesis. Genotypic differences in the response of wheat to calcareous-induced Fe deficiency were observed and allowed to classify the genotypes Salim and Karim as relatively tolerant. These genotypes expressed Fe translocation capacity (FeT) up to 3 times, Fe use efficiency for photosynthesis (FeUEAn) up to 1.6 times, and chlorophyll use efficiency for photosynthesis (ChlUEAn) up to 3.5 times greater than that expressed by the other genotypes, particularly inrat100 and maali. Thus, the relative tolerance of Salim and Karim is the result of the high ability of Fe uptake and translocation to shoots to support chlorophyll biosynthesis, photosynthesis, and plant growth as well as an important Fe and chlorophyll use efficiency.

TpIRT1 from Polish wheat (Triticum polonicum L.) enhances the accumulation of Fe, Mn, Co, and Cd in Arabidopsis

Uptake and internal transport of micronutrients are essential for plant growth, development, and yield. In this regard, Iron Regulated Transporters (IRTs) from the Zinc Regulated Transporter (ZRT)/IRT-related protein (ZIP) family play an important role in transition metal uptake. Most studies have been focused on IRT1-like proteins in diploid species. Information on IRT1-like proteins in polyploids is limited. Here, we studied the function of TpIRT1A and TpIRT1B homoeologs in a tetraploid crop, Polish wheat (Triticum polonicum L.). Our results highlighted the importance of TpIRT1 in mediating the uptake and translocation of Fe, Mn, Co, and Cd with direct implications for wheat yield potential. Both TpIRT1A and TpIRT1B were located at the plasma membrane and internal vesicle-like organelle in protoplasts of Arabidopsis thaliana L. and increased Cd and Co sensitivity in yeast. The over-expression of TpIRT1B in A. thaliana increased Fe, Mn, Co, and Cd concentration in its tissues and improved plant growth under Fe, Mn, and Co deficiencies, while increased the sensitivity to Cd compared to wild type. Functional analysis of IRT1 homoeologs from tetraploid and diploid ancestral wheat species in yeast disclosed four distinct amino acid residues in TdiIRT1B (T. dicoccum L. (Schrank)) and TtuIRT1B (T. turgidum L.). Together, our results increase the knowledge of IRT1 function in a globally important crop, wheat.

A Smart and Sustainable Future for Viticulture Is Rooted in Soil: How to Face Cu Toxicity

In recent decades, agriculture has faced the fundamental challenge of needing to increase food production and quality in order to meet the requirements of a growing global population. Similarly, viticulture has also been undergoing change. Several countries are reducing their vineyard areas, and several others are increasing them. In addition, viticulture is moving towards higher altitudes and latitudes due to climate change. Furthermore, global warming is also exacerbating the incidence of fungal diseases in vineyards, forcing farmers to apply agrochemicals to preserve production yields and quality. The repeated application of copper (Cu)-based fungicides in conventional and organic farming has caused a stepwise accumulation of Cu in vineyard soils, posing environmental and toxicological threats. High Cu concentrations in soils can have multiple impacts on agricultural systems. In fact, it can (i) alter the chemical-physical properties of soils, thus compromising their fertility; (ii) induce toxicity phenomena in plants, producing detrimental effects on growth and productivity; and (iii) affect the microbial biodiversity of soils, thereby influencing some microbial-driven soil processes. However, several indirect (e.g., management of rhizosphere processes through intercropping and/or fertilization strategies) and direct (e.g., exploitation of vine resistant genotypes) strategies have been proposed to restrain Cu accumulation in soils. Furthermore, the application of precision and smart viticulture paradigms and their related technologies could allow a timely, localized and balanced distribution of agrochemicals to achieve the required goals. The present review highlights the necessity of applying multidisciplinary approaches to meet the requisites of sustainability demanded of modern viticulture.

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.

Changes in physiological activities and root exudation profile of two grapevine rootstocks reveal common and specific strategies for Fe acquisition

In several cultivation areas, grapevine can suffer from Fe chlorosis due to the calcareous and alkaline nature of soils. This plant species has been described to cope with Fe deficiency by activating Strategy I mechanisms, hence increasing root H⁺ extrusion and ferric-chelate reductase activity. The degree of tolerance exhibited by the rootstocks has been reported to depend on both reactions, but to date, little emphasis has been given to the role played by root exudate extrusion. We studied the behaviour of two hydroponically-grown, tolerant grapevine rootstocks (Ramsey and 140R) in response to Fe deficiency. Under these experimental conditions, the two varieties displayed differences in their ability to modulate morpho-physiological parameters, root acidification and ferric chelate reductase activity. The metabolic profiling of root exudates revealed common strategies for Fe acquisition, including ones targeted at reducing microbial competition for this micronutrient by limiting the exudation of amino acids and sugars and increasing instead that of Fe(III)-reducing compounds. Other modifications in exudate composition hint that the two rootstocks cope with Fe shortage via specific adjustments of their exudation patterns. Furthermore, the presence of 3-hydroxymugenic acid in these compounds suggests that the responses of grapevine to Fe availability are rather diverse and much more complex than those usually described for Strategy I plants.

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.