Silicon and nitric oxide interplay alleviates copper induced toxicity in mung bean seedlings

Silicon ameliorates cadmium (Cd) toxicity in pearl millet by inducing antioxidant defense system

Cadmium (Cd) stress is a significant environmental pollutant that can negatively impact crop yield and growth, and is a serious global issue. However, silicon (Si) has been shown to have a potential function in alleviating the effects of several abiotic stress conditions on crops, including Cd stress. This study investigated the effectiveness of applying silicon to soil as a method for reducing cadmium toxicity in pearl millet (IP14599) seedlings. Seeds of IP14599 were treated with Si + Cd element which cumulated to a combination of 9 treatments. Different Cd concentration of (0, 200, and 300 mg/kg-1) was taken and manually mixed into a sieved soil prior to planting and Si (0, 100 and 200 mg/kg-1) was selectively introduced till after attaining 12 days of seedling emergence. The physiochemical parameters of Cd stressed plants investigated includes chlorophyll, gas exchange attributes, proline, relative water contents, malondialdehyde (MDA) content and antioxidant enzymes (superoxide dismutase (SOD),catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD). Our result revealed that the metal (Cd) caused serious oxidative impairment thereby reducing photosynthetic performance, increased activity of MDA and Cd content in the roots and leaves of IP14599.In addition, Si increased the growth pattern and antioxidant defense action thereby mitigating the Cd toxicity. The results revealed that at Si 200, Si significantly increased the chlorophyll, carotenoids and plant height at 122 %, 69 % and 128 % under the Cd 200 and Cd 300 mg/kg-1 treatment, respectively. The single treatment at Si100 and Si 200 decreased ROS by 29 %, and 37 % respectively and MDA decreased by 33 % and 43 % in contrast to Cd 200 and 300 treatments, respectively. However, Si200 showed significant increase in the activities of APX 97 %, SOD by 89 %, CAT 35 % and POD 86 % as compared to single Si, Cd or combine Cd + Si treatment. Also, a gradual decline in Cd level in both the leaf and root was present when exposed to high concentrations of Si at Si200 and 300 mg/kg-1. Our findings revealed that Si might significantly increase the capacity to tolerate Cd stress in crop plants. Therefore, the study revealed that Si has the potential to alleviate Cd-induced toxicity by reducing Cd assimilation and enhancing the growth attributes of IP14599 plants.

Advances in the Involvement of Metals and Metalloids in Plant Defense Response to External Stress

Plants, as sessile organisms, uptake nutrients from the soil. Throughout their whole life cycle, they confront various external biotic and abiotic threats, encompassing harmful element toxicity, pathogen infection, and herbivore attack, posing risks to plant growth and production. Plants have evolved multifaceted mechanisms to cope with exogenous stress. The element defense hypothesis (EDH) theory elucidates that plants employ elements within their tissues to withstand various natural enemies. Notably, essential and non-essential trace metals and metalloids have been identified as active participants in plant defense mechanisms, especially in nanoparticle form. In this review, we compiled and synthetized recent advancements and robust evidence regarding the involvement of trace metals and metalloids in plant element defense against external stresses that include biotic stressors (such as drought, salinity, and heavy metal toxicity) and abiotic environmental stressors (such as pathogen invasion and herbivore attack). We discuss the mechanisms underlying the metals and metalloids involved in plant defense enhancement from physiological, biochemical, and molecular perspectives. By consolidating this information, this review enhances our understanding of how metals and metalloids contribute to plant element defense. Drawing on the current advances in plant elemental defense, we propose an application prospect of metals and metalloids in agricultural products to solve current issues, including soil pollution and production, for the sustainable development of agriculture. Although the studies focused on plant elemental defense have advanced, the precise mechanism under the plant defense response still needs further investigation.

Advances in nanoparticle and organic formulations for prolonged controlled release of auxins

Plant hormones have been well known since Charles Darwin as signaling molecules directing plant metabolism. Their action and transport pathways are at the top of scientific interest and were reviewed in many research articles. Modern agriculture applies phytohormones as supplements to achieve desired physiological plant response. Auxins are a class of plant hormones extensively used for crop management. Auxins stimulate the formation of lateral roots and shoots, seed germination, while extensively high concentrations of these chemicals act as herbicides. Natural auxins are unstable; light or enzyme action leads to their degradation. Moreover, the concentration dependant action of phytohormones denier one-shot injection of these chemicals and require constant slow additive of supplement. It obstructs the direct introduction of auxins. On the other hand, delivery systems can protect phytohormones from degradation and provide a slow release of loaded drugs. Moreover, this release can be managed by external stimuli like pH, enzymes, or temperature. The present review is focused on three auxins: indole-3-acetic, indole-3-butyric, and 1-naphthaleneacetic acids. We collected some examples of inorganic (oxides, Ag, layered double hydroxides) and organic (chitosan, organic formulations) delivery systems. The action of carriers can enhance auxin effects via protection and targeted delivery of loaded molecules. Moreover, nanoparticles can act as nano fertilizers, intensifying the phytohormone effect, providing slow controlled release. So delivery systems for auxins are extremely attractive for modern agriculture opening sustainable management of plant metabolism and morphogenesis.

ʟ-glutamic acid modulates antioxidant defense systems and nutrient homeostasis in lentil (Lens culinaris Medik.) under copper toxicity

Copper (Cu), an essential micronutrient, can generate reactive oxygen species (ROS) at its supra-optimal level in living cells as a transition metal, thus producing oxidative stress in plants. Therefore, protecting plants from Cu-induced oxidative stress via the exogenous application of chemical substances, particularly L-glutamic acid (L-Glu), could be a viable strategy for mitigating the toxicity of Cu. The aim of our present study was to investigate how ʟ-Glu protects lentil seedlings from oxidative stress produced by toxic Cu and allows them to survive under Cu toxicity. The results exhibited that when lentil seedlings were exposed to excessive Cu, their growth was inhibited and their biomass decreased due to an increase in Cu accumulation and translocation to the root, shoot, and leaves. Exposure to toxic Cu also depleted photosynthetic pigments, imbalanced water content, and other essential nutrients, increased oxidative stress, and reduced enzymatic and non-enzymatic antioxidants. However, pre-treatment of ʟ-Glu improved the phenotypic appearance of lentil seedlings, which was distinctly evidenced by higher biomass production, maintenance of water balance, and an increase in photosynthetic pigments when exposed to toxic Cu. ʟ-Glu also protected the seedlings from Cu-induced oxidative stress by reducing the oxidative stress marker, specifically by the efficient action of enzymatic and non-enzymatic antioxidants, particularly ascorbate, catalase, monodehydroascorbate, and glutathione peroxidase and maintaining redox balance. Furthermore, ʟ-Glu assisted in maintaining the homeostasis of Cu and other nutrient in the roots, shoots, and leaves of lentil. Collectively, our results provide evidence of the mechanism of ʟ-Glu-mediated protective role in lentil against Cu toxicity, thus proposed as a potential chemical for managing Cu toxicity not only in lentil but also other plants.

Silicon in action: Between iron scarcity and excess copper

Essential micronutrients belonging to the transition metals, such as Fe and Cu, are indispensable for plant growth and stress tolerance; however, when present in excess, they can become potentially dangerous producers of reactive oxygen species. Therefore, their homeostases must be strictly regulated. Both microelement deficiencies and elevated concentrations of heavy metals in the soil are global problems that reduce the nutritional value of crops and seriously affect human health. Silicon, a beneficial element known for its protective properties, has been reported to alleviate the symptoms of Cu toxicity and Fe deficiency stress in plants; however, we are still far from a comprehensive understanding of the underlying molecular mechanisms. Although Si-mediated mitigation of these stresses has been clearly demonstrated for some species, the effects of Si vary depending on plant species, growing conditions and experimental design. In this review, the proposed mechanistic models explaining the effect of Si are summarized and discussed. Iron and copper compete for the common metal transporters and share the same transport routes, hence, inadequate concentration of one element leads to disturbances of another. Silicon is reported to beneficially influence not only the distribution of the element supplied below or above the optimal concentration, but also the distribution of other microelements, as well as their molar ratios. The influence of Si on Cu immobilization and retention in the root, as well as Si-induced Fe remobilization from the source to the sink organs are of vital importance. The changes in cellular Cu and Fe localization are considered to play a crucial role in restoring homeostasis of these microelements. Silicon has been shown to stimulate the accumulation of metal chelators involved in both the mobilization of deficient elements and scavenging excess heavy metals. Research into the mechanisms of the ameliorative effects of Si is valuable for reducing mineral stress in plants and improving the nutritional value of crops. This review aims to provide a thorough and critical overview of the current state of knowledge in this field and to discuss discrepancies in the observed effects of Si and different views on its mode of action.

Could nitrogen compounds be indicators of tolerance to high doses of Cu and Fe in the cultivation of Leucaena leucocephala?

Nitrogen metabolism and the production of primary and secondary metabolites vary according to biotic and abiotic factors such as trace elements (TE) stress, and can, therefore, be considered biomarkers. The present study evaluated the effect of copper (Cu) and iron (Fe) TE, separately, on the metabolism of nitrogen compounds and biomass production, partitioned into shoot and roots of Leucaena leucocephala (Lam.) de Wit., and identified possible defense mechanisms linked to nitrogen metabolism. At 120 days of cultivation, the biomass production of L. leucocephala was higher when exposed to excess Fe than Cu. Nonetheless, the biomass gain (%) of plants exposed to Cu was higher, especially the biomass gains in roots. The tolerance and biomass production of L. leucocephala is related to the regulation of nitrogen metabolism and production of secondary metabolites. The biochemistry of plant metabolism against the excess of Cu and Fe TE manifested similarly, but with some specifics regarding the chemical nature of each metal. There was a reduction in the content of ureides and proteins and an increase in amino acids in the roots in relation to the increase in Cu and Fe concentrations. There was low accumulation of proline in the roots in treatments 400 and 500 mg/dm³ compared to the control for both TE. On the other hand, the total phenolic compounds in the roots increased. Our results indicate that the increased synthesis of amino acids and the accumulation of phenolic compounds is involved in the tolerance of L. leucocephala to Cu and Fe.

Silicon nanoparticles in higher plants: Uptake, action, stress tolerance, and crosstalk with phytohormones, antioxidants, and other signalling molecules

Silicon is absorbed as uncharged mono-silicic acid by plant roots through passive absorption of Lsi1, an influx transporter belonging to the aquaporin protein family. Lsi2 then actively effluxes silicon from root cells towards the xylem from where it is exported by Lsi6 for silicon distribution and accumulation to other parts. Recently, it was proposed that silicon nanoparticles (SiNPs) might share a similar route for their uptake and transport. SiNPs then initiate a cascade of morphophysiological adjustments that improve the plant physiology through regulating the expression of many photosynthetic genes and proteins along with photosystem I (PSI) and PSII assemblies. Subsequent improvement in photosynthetic performance and stomatal behaviour correspond to higher growth, development, and productivity. On many occasions, SiNPs have demonstrated a protective role during stressful environments by improving plant-water status, source-sink potential, reactive oxygen species (ROS) metabolism, and enzymatic profile. The present review comprehensively discusses the crop improvement potential of SiNPs stretching their role during optimal and abiotic stress conditions including salinity, drought, temperature, heavy metals, and ultraviolet (UV) radiation. Moreover, in the later section of this review, we offered the understanding that most of these upgrades can be explained by SiNPs intricate correspondence with phytohormones, antioxidants, and signalling molecules. SiNPs can modulate the endogenous phytohormones level such as abscisic acid (ABA), auxins (IAAs), cytokinins (CKs), ethylene (ET), gibberellins (GAs), and jasmonic acid (JA). Altered phytohormones level affects plant growth, development, and productivity at various organ and tissue levels. Similarly, SiNPs regulate the activities of catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and ascorbate-glutathione (AsA-GSH) cycle leading to an upgraded defence system. At the cellular and subcellular levels, SiNPs crosstalk with various signalling molecules such as Ca²⁺, K⁺, Na⁺, nitric oxide (NO), ROS, soluble sugars, and transcription factors (TFs) was also explained.