Selenium and its Role in Higher Plants

The role of selenium and nano selenium on physiological responses in plant: a review

Selenium (Se), being an essential micronutrient, enhances plant growth and development in trace amounts. It also protects plants against different abiotic stresses by acting as an antioxidant or stimulator in a dose-dependent manner. Knowledge of Se uptake, translocation, and accumulation is crucial to achieving the inclusive benefits of Se in plants. Therefore, this review discusses the absorption, translocation, and signaling of Se in plants as well as proteomic and genomic investigations of Se shortage and toxicity. Furthermore, the physiological responses to Se in plants and its ability to mitigate abiotic stress have been included. In this golden age of nanotechnology, scientists are interested in nanostructured materials due to their advantages over bulk ones. Thus, the synthesis of nano-Se or Se nanoparticles (SeNP) and its impact on plants have been studied, highlighting the essential functions of Se NP in plant physiology. In this review, we survey the research literature from the perspective of the role of Se in plant metabolism. We also highlight the outstanding aspects of Se NP that enlighten the knowledge and importance of Se in the plant system.

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Physiological responses, tolerance, and remediation strategies in plants exposed to metalloids

Metalloids are a subset of particular concern to risk assessors and toxicologists because of their well-documented potential hazards to plant system. Most of the metalloids are major environmental contaminants which affect crop productivity when present in high concentrations in soil. Metalloids are coupled with carrier proteins of the plasma membrane and translocated to various organs causing changes in key metabolic processes, damages cell biomolecules, and finally inhibit its growth. Phytoremediation-based approaches help in understanding the molecular and biochemical mechanisms for prerequisite recombinant genetic approaches. Recent advancements in proteomics and plant genomics help in understanding the role of transcription factors, metabolites, and genes in plant system which confers metal tolerance. The present review summarizes our current status of knowledge in this direction related to various physiological responses, detoxification mechanisms, and remediation strategies of metalloids in crop plants in relation to plant-metalloid tolerance. Further, the role of various transcription factors and miRNAs in conferring metal tolerance is also briefed. Hence, the present review mainly focused on the alterations in the physiological activities of plants due to metalloid toxicity and the various mechanisms which get activated inside the plants to mitigate their toxic effects.

Significant Accumulation of Iodine and Selenium in Chicory (Cichorium intybus L. var. foliosum Hegi) Leaves after Foliar Spraying

The interactions between the uptake of selenium (as selenite and selenate) and iodine (as iodate and iodide) by red chicory (Cichorium intybus L. var. foliosum Hegi) and their effects on selected morphological and physiological characteristics were investigated. Seedlings were transplanted to the field, and at the onset of head formation, the plants were foliar-sprayed with the following solutions: Milli-Q water (control), Se (IV), Se (VI), I (-I), I (V), Se (IV) + I (-I), Se (IV) + I (V), Se (VI) + I (-I) and Se (VI) + I (V). The different treatments had no significant effects on the yield (39.8-51.5 t ha^-1) and mass (970-1200 g) of the chicory heads. The selenium content in Se-treated plants was up to 5.5-times greater than the control plants. The iodine content in the chicory leaves enriched with I was 3.5-times greater than the control plants. Iodide or iodate, applied together with selenite in the spray solution, increased the uptake of Se by chicory plants, while both forms of iodine, applied together with selenate, reduced the uptake of Se. Plants treated with I (V) had lower amounts of chlorophyll a and carotenoids than the control, while respiratory potential was higher than the control, which indicated the possible presence of stress in I (V)-treated plants. However, the potential photochemical efficiency of photosystem II was similar and close to the theoretical maximum (0.83) in the control and treated groups, which indicated that all of the plants were in good condition. Furthermore, the plant mass and yield were comparable in the control and treated groups. Molecular studies, like gene expression analysis, would represent a major upgrade of the present study by defining the mechanisms of Se and I uptake and their interactions and by enhancing the knowledge of the Se and I transporters.

Selenium and selenoproteins: it's role in regulation of inflammation

Abstract

Selenium is an essential immunonutrient which holds the human’s metabolic activity with its chemical bonds. The organic forms of selenium naturally present in human body are selenocysteine and selenoproteins. These forms have a unique way of synthesis and translational coding. Selenoproteins act as antioxidant warriors for thyroid regulation, male-fertility enhancement, and anti-inflammatory actions. They also participate indirectly in the mechanism of wound healing as oxidative stress reducers. Glutathione peroxidase (GPX) is the major selenoprotein present in the human body, which assists in the control of excessive production of free radical at the site of inflammation. Other than GPX, other selenoproteins include selenoprotein-S that regulates the inflammatory cytokines and selenoprotein-P that serves as an inducer of homeostasis. Previously, reports were mainly focused on the cellular and molecular mechanism of wound healing with reference to various animal models and cell lines. In this review, the role of selenium and its possible routes in translational decoding of selenocysteine, synthesis of selenoproteins, systemic action of selenoproteins and their indirect assimilation in the process of wound healing are explained in detail. Some of the selenium containing compounds which can acts as cancer preventive and therapeutics are also discussed. These compounds directly or indirectly exhibit antioxidant properties which can sustain the intracellular redox status and these activities protect the healthy cells from reactive oxygen species induced oxidative damage. Although the review covers the importance of selenium/selenoproteins in wound healing process, still some unresolved mystery persists which may be resolved in near future.

Graphic abstract

Effects of selenate and red Se-nanoparticles on the photosynthetic apparatus of Nicotiana tabacum

Selenium (Se) is a natural trace element, which shifts its action in a relatively narrow concentration range from nutritional role to toxicity. Although it has been well established that in plants chloroplasts are among the primary targets, the mechanism of toxicity on photosynthesis is not well understood. Here, we compared selenate and red-allotrope elemental selenium nanoparticles (red nanoSe) in in vitro tobacco cultures to investigate their effects on the structure and functions of the photosynthetic machinery. Selenate at 10 mg/L concentration retarded plant growth; it also led to a decreased chlorophyll content, accompanied with an increase in the carotenoid-to-chlorophyll ratio. Structural examinations of the photosynthetic machinery, using electron microscopy, small-angle neutron scattering and circular dichroism spectroscopy, revealed significant perturbation in the macro-organization of the pigment-protein complexes and sizeable shrinkage in the repeat distance of granum thylakoid membranes. As shown by chlorophyll a fluorescence transient measurements, these changes in the ultrastructure were associated with a significantly diminished photosystem II activity and a reduced performance of the photosynthetic electron transport, and an enhanced capability of non-photochemical quenching. These changes in the structure and function of the photosynthetic apparatus explain, at least in part, the retarded growth of plantlets in the presence of 10 mg/L selenate. In contrast, red nanoSe, even at 100 mg/L and selenate at 1 mg/L, exerted no negative effect on the growth of plantlets and affected only marginally the thylakoid membrane ultrastructure and the photosynthetic functions.

The essentiality of selenium for humans, animals, and plants, and the role of selenium in plant metabolism and physiology

After its discovery, selenium was most noted for its harmful effects. Selenium was the first element identified to occur in native vegetation at levels toxic to animals. Poisoning of animals can occur through consumption of plants containing toxic levels of selenium. Livestock consuming excessive amounts of selenized forages are afflicted with “alkali disease” and “blind staggers”. Typical symptoms of these diseases include loss of hair, deformed hooves, blindness, colic, diarrhoea, lethargy, increased heart and respiration rates, and eventually death. On the other hand, selenium deficiency in animal feeds can cause “white muscle disease”, a degenerative disease of the cardiac and skeletal muscles. In this regard, this review paper attempts to summarize the essentiality of selenium for humans, animals, and plants and the role of selenium in plant metabolism and physiology.

"The roots" of selenium toxicity: A new concept

Elevated levels of selenium (Se) cause toxicity in non-accumulator plant species. The primary reasons for toxic Se effect have been considered to be selenoprotein accumulation and oxidative stress. However, based on our recent paper in Plant Cell Reports and previous literature data we suggest that disturbances in the homeostasis of both reactive oxygen and nitrogen species result in selenium-induced nitro-oxidative stress, contributing to toxicity. The most characteristic symptom of Se exposure is the inhibited root elongation which is partly caused by hormonal disturbances. Our recent paper suggests the involvement of cytokinin in selenium stress sensing of the root systemAltogether, the aim of this Addendum is to present reactive nitrogen species and phytohormones as new players in plant selenium toxicity.

Nitric oxide-cytokinin interplay influences selenite sensitivity in Arabidopsis

Selenite oppositely modifies cytokinin and nitric oxide metabolism in Arabidopsis organs. A mutually negative interplay between the molecules exists in selenite-exposed roots; and their overproduction causes selenite insensitivity. Selenium-induced phytotoxicity is accompanied by developmental alterations such as primary root (PR) shortening. Growth changes are provoked by the modulation of hormone status and signalling. Cytokinin (CK) cooperates with the nitric oxide (NO) in many aspects of plant development; however, their interaction under abiotic stress has not been examined. Selenite inhibited the growth of Arabidopsis seedlings and reduced root meristem size through cell division arrest. The CK-dependent pARR5::GUS activity revealed the intensification of CK signalling in the PR tip, which may be partly responsible for the root meristem shortening. The selenite-induced alterations in the in situ expressions of cytokinin oxidases (AtCKX4::GUS, AtCKX5::GUS) are associated with selenite-triggered changes of CK signalling. In wild-type (WT) and NO-deficient nia1nia2 root, selenite led to the diminution of NO content, but CK overproducer ipt-161 and -deficient 35S:CKX2 roots did not show NO decrease. Exogenous NO (S-nitroso-N-acetyl-DL-penicillamine, SNAP) reduced the pARR5::GFP and pTCS::GFP expressions. Roots of the 35S:CKX and cyr1 plants suffered more severe selenite-triggered viability loss than the WT, while in ipt-161 and gsnor1-3 no obvious viability decrease was observed. Exogenous NO ameliorated viability loss, but benzyladenine intensified it. Based on the results, selenite impacts development by oppositely modifying CK signalling and NO level. In the root system, CK signalling intensifies which possibly contributes to the nitrate reductase-independent NO diminution. A mutually negative CK-NO interplay exists in selenite-exposed roots; however, overproduction of both molecules worsens selenite sensing. Hereby, we suggest novel regulatory interplay and role for NO and CK in abiotic stress signalling.