The impact of auxins used in assisted phytoextraction of metals from the contaminated environment on the alterations caused by lead(II) ions in the organization of model lipid membranes

Auxin homeostasis in plant responses to heavy metal stress

Expeditious industrialization and anthropogenic activities have resulted in large amounts of heavy metals (HMs) being released into the environment. These HMs affect crop yields and directly threaten global food security. Therefore, significant efforts have been made to control the toxic effects of HMs on crops. When HMs are taken up by plants, various mechanisms are stimulated to alleviate HM stress, including the biosynthesis and transport of auxin in the plant. Interestingly, researchers have noted the significant potential of auxin in mediating resistance to HM stress, primarily by reducing uptake of metals, promoting chelation and sequestration in plant tissues, and mitigating oxidative damage. Both exogenous administration of auxin and manipulation of intrinsic auxin status are effective strategies to protect plants from the negative consequences of HMs stress. Regulation of genes and transcription factors related to auxin homeostasis has been shown to be related to varying degrees to the type and concentration of HMs. Therefore, to derive the maximum benefit from auxin-mediated mechanisms to attenuate HM toxicities, it is essential to gain a comprehensive understanding of signaling pathways involved in regulatory actions. This review primarily emphases on the auxin-mediated mechanisms participating in the injurious effects of HMs in plants. Thus, it will pave the way to understanding the mechanism of auxin homeostasis in regulating HM tolerance in plants and become a tool for developing sustainable strategies for agricultural growth in the future.

Multifractal Analysis of the Influence of Indole-3-Acetic Acid on Fast-Activating Vacuolar (FV) Channels of Beta vulgaris L. Taproot Cells

In this paper, the multifractal properties of the ion current time series in the fast-activating vacuolar (FV) channels of Beta vulgaris L. taproot cells were investigated. These channels are permeable for only monovalent cations and mediate K+ at very low concentrations of cytosolic Ca2+ and large voltages of either polarity. Using the patch clamp technique, the currents of the FV channels in red beet taproot vacuoles were recorded and analysed by using the multifractal detrended fluctuation analysis (MFDFA) method. The activity of the FV channels depended on the external potential and was sensitive to the auxin. It was also shown that the singularity spectrum of the ion current in the FV channels is non-singular, and the multifractal parameters, i.e., the generalised Hurst exponent and the singularity spectrum, were modified in the presence of IAA. Taking into account the obtained results, it can be suggested that the multifractal properties of fast-activating vacuolar (FV) K+ channels, indicating the existence of long-term memory, should be taken into account in the molecular mechanism of the auxin-induced growth of plant cells.

Auxin regulates growth, photosynthetic efficiency and mitigates copper induced toxicity via modulation of nutrient status, sugar metabolism and antioxidant potential in Brassica juncea

The involvement of auxin (IAA) in growth and development of plants is well known, but its role in the mitigation of metal stress, especially copper (Cu), is not fully understood; therefore, it is time to explore its involvement in minimizing the stress. A pot experiment was conducted to assess the protective function of IAA, applied to the foliage, on photosynthetic machinery, carbohydrate metabolism, and growth of Brassica juncea, grown with Cu (30 or 60 mg kg^-1 of soil). Among the different concentrations (10^-10, 10^-8, or 10^-6 M), 10^-8 M of IAA alone enhanced the photosynthetic characteristics, sugar accumulation and vegetative growth with minimal cellular oxidative stress level. Moreover, the same concentration of auxin was most effective in decreasing the stress levels generated by Cu and maintained it nearly to that of the control in terms of photosynthetic attributes, gas exchange parameters, PSII activity, electron transport rate, and growth attributes. Auxin also maintained the membrane stability and ultrastructure of chloroplast, stomatal morphology with a reduction in malondialdehyde (MDA), electrolyte leakage (EL) and cell death in test plants even under Cu stress. IAA also improved the translocation of Cu from root to the aerial parts, thus enhanced the Cu-reclamation in metal contaminated soils. Our findings suggest that the application of 10^-8 M of IAA maintains the overall growth of plants and may be used as an effective agent to improve growth, photosynthesis and phyto-remediation potential of B. juncea in Cu contaminated soil.

Improved of growth and phytostabilization potential of lead (Pb) in Glebionis coronaria L. under the effect of IAA and GA3 alone and in combination with EDTA by altering biochemical attributes of stressed plants

This study evaluated the effect of EDTA, IAA and GA₃ alone and in combination in improving plant growth, Pb accumulation, and management of effective mechanisms associated with alleviation of Pb-induced adverse effect in Glebionis coronaria L. grown on industrial area in Alexandria. In this regards, 10-week-old plants were subjected to IAA and GA₃ as a foliar spray and EDTA was supplied in two doses 50 and 200mgkg^-1 soil. EDTA significantly reduced the plant growth and dry biomass, whereas GA₃ and IAA foliar spray increased growth significantly when compared with control (uncontaminated soil). In combined treatments of EDTA + GA₃ +IAA, the biomass was restored, which shows that GA₃ and IAA did compensate the negative effect of EDTA on plant growth and increased the Pb uptake significantly into roots. There were high GSH contents parallel with the increase of glutathione-S-transferase activity and induction of the antioxidant enzymes (SOD, CAT, APX, GR) as well as oxidized glutathione and ascorbic acid contents in leaves and roots when compared to control plants. This study suggests that G. coronaria is promising species for decontamination of Pb -contaminated soil and the application of EDTA together with IAA and GA₃ could be a useful strategy for enhancing the phytostabilization capability of Glebionis coronaria L. to eliminate Pb from contaminated soils. Novelty statement: The objective of this paper is to investigate the physiological performance of Glebionis coronaria as a new native for phytoremediation by phytostabilization mechanism of Pb after treatment with EDTA and phytohormones. Chemical additives of EDTA and GA₃ as well as IAA are promising alternatives to provide added benefits due to their individual credentials in improving the overall phytostabilization effectiveness and better immobilization efficiency in treating Pb contaminated soils by altering biochemical attributes of stressed plants. Currently these additives are not employed widely in large-scale field implementations, so field applications of these additives using Glebionis coronaria are essential for Phytoremediation of Pb-contaminated soils.

Exogenous plant growth regulator alleviate the adverse effects of U and Cd stress in sunflower (Helianthus annuus L.) and improve the efficacy of U and Cd remediation

Plant growth regulators (PGRs) are widely used in agricultural activities and have the potential to improve plant growth and plant tolerance against metal stress. PGR-assisted phytoextraction is now an effective and inexpensive method for enhancing the plant removal of toxic metals from soil. In this study, we conducted experiments to determine the effects of PGR treatments on soil uranium (U) and cadmium (Cd) removal by sunflowers as well as their stress response to U and Cd contamination. We found that the plant growth was inhibited by combined U and Cd stress in sunflowers compared with that of the control; however, the application of exogenous PGR had reduced the combined U and Cd stress by stimulating photosynthesis, decreasing the levels of active oxygen and lipid peroxidation, and enhancing the activity of the antioxidant defence systems. Exogenous PGR also increased the uptake of U and Cd by sunflowers and therefore, improved their U and Cd remediation efficiency. Moreover, indoleacetic acid (IAA) was the most effective PGR at removing U and Cd in the soil; the U and Cd removal efficiency was 484.21% and 238.85% higher in the 500 mg L^-1 IAA application compared with that of the control without PGR application, respectively. Furthermore, none of the PGR treatments significantly influenced the available U and Cd contents in soil. Our results, therefore, may provide some detailed understanding on the technologies for the sustainable remediation of U and Cd contaminated soil by the combination of PGR treatments and phytoextraction.

Effect of Auxin (IAA) on the Fast Vacuolar (FV) Channels in Red Beet (Beta vulgaris L.) Taproot Vacuoles

In contrast to the well-studied effect of auxin on the plasma membrane K⁺ channel activity, little is known about the role of this hormone in regulating the vacuolar K⁺ channels. Here, the patch-clamp technique was used to investigate the effect of auxin (IAA) on the fast-activating vacuolar (FV) channels. It was found that the macroscopic currents displayed instantaneous currents, which at the positive potentials were about three-fold greater compared to the one at the negative potentials. When auxin was added to the bath solution at a final concentration of 1 µM, it increased the outward currents by about 60%, but did not change the inward currents. The imposition of a ten-fold vacuole-to-cytosol KCl gradient stimulated the efflux of K⁺ from the vacuole into the cytosol and reduced the K⁺ current in the opposite direction. The addition of IAA to the bath solution with the 10/100 KCl gradient decreased the outward current and increased the inward current. Luminal auxin reduced both the outward and inward current by approximately 25% compared to the control. The single channel recordings demonstrated that cytosolic auxin changed the open probability of the FV channels at the positive voltages to a moderate extent, while it significantly increased the amplitudes of the single channel outward currents and the number of open channels. At the positive voltages, auxin did not change the unitary conductance of the single channels. We suggest that auxin regulates the activity of the fast-activating vacuolar (FV) channels, thereby causing changes of the K⁺ fluxes across the vacuolar membrane. This mechanism might serve to tightly adjust the volume of the vacuole during plant cell expansion.

Effects of exogenous 3-indoleacetic acid and cadmium stress on the physiological and biochemical characteristics of Cinnamomum camphora

Indoleacetic acid (IAA) is a plant growth regulator that plays an important role in plant growth and development, and participates in the regulation of abiotic stress. To explore the effect of IAA on cadmium toxicity in Cinnamomum camphora, an indoor potted experiment was conducted with one-year-old C. camphora seedlings. The influence of IAA on cadmium accumulation, net photosynthetic rates, respiration, photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll and carotenoids), osmoregulatory substances (proline, soluble sugar and soluble protein) and the malondialdehyde content in C. camphora leaves treated with 30 mg kg^-1 cadmium was analysed with or without the addition of 10 mg kg^-1 IAA. Cadmium accumulation in the leaves of C. camphora with the addition of exogenous IAA was significantly higher than accumulation during cadmium stress without additional IAA (ca 69.10% after 60 days' incubation). During the culture period, the net photosynthetic rate in C. camphora leaves subjected to cadmium stress without the addition of IAA was up to 24.31% lower than that of control plants. The net photosynthetic rate in C. camphora leaves subjected to cadmium stress and addition of IAA was up to 30.31% higher than that of leaves subjected to cadmium stress without the addition of IAA. Chlorophyll a, total chlorophyll and carotenoid contents in the cadmium-stressed leaves without the addition of IAA were lower than those in the control treatment. The presence of IAA increased the chlorophyll a, total chlorophyll and carotenoid contents relative to the cadmium stress without the addition of IAA. The respiration rate and concentrations of proline, soluble sugar, soluble protein and malondialdehyde in C. camphora leaves subjected to cadmium stress without the addition of IAA were higher than those in the control. The addition of IAA reduced the respiration rate, and the concentrations of proline, soluble sugar, soluble protein and malondialdehyde in C. camphora leaves when compared with the cadmium stress without the addition of IAA. These results indicate that exogenous IAA improves photosynthetic performance and the growth environment of C. camphora by enhancing the net photosynthetic rate, increasing concentrations of osmoregulatory substances, removing reactive oxygen radicals and eliminating potential damage, thereby reducing the toxic effects of cadmium on C. camphora.

Fungi-assisted phytoextraction of lead: tolerance, plant growth-promoting activities and phytoavailability

Lead (Pb) is known for its low mobility and persistence in soils. The main aim of the present study was to explore potential of different fungal strains to promote phytoextraction of Pb-contaminated soils. Five non-pathogenic fungal strains (Trichoderma harzianum, Penicillium simplicissimum, Aspergillus flavus, Aspergillus niger, and Mucor spp.) were tested for their ability to modify soil properties (pH and organic matter) and to increase Pb phytoavailability at varying concentrations. Lead tolerance of fungal strains followed the decreasing order as A. niger > T. harzianum > A. flavus > Mucor sp. > P. simplicissimum. Lead solubility induced by A. flavus and Mucor spp. was increased by 1.6- and 1.8-fold, respectively, as compared to the control soil (Pb added, without fungi). A. flavus and Mucor spp. lowered the soil pH by - 0.14 and - 0.13 units, in soils spiked with 2000 mg Pb kg^-1. The maximum increase in the percentage of organic matter (OM) recorded was 1.7-fold for A. flavus at 500 mg Pb kg^-1 soil. Plant growth-promoting assays confirmed the beneficial role of these fungal strains. Significantly high production of IAA (249 μg mL^-1) and siderophores (61%) was observed with A. niger, and phosphate solubilization with P. simplicissimum (58 μg mL^-1). Based on the results in Pb-contaminated soils, Pelargonium hortorum L. inoculated with Mucor spp. showed the potential to enhance phytoextraction of Pb by promoting Pb phytoavailability in soil and improving plant biomass production through plant growth-promoting activities.

The application of plant growth regulators to improve phytoremediation of contaminated soils: A review

Soil contamination is one of the most important environmental problems around the world. The transfer of organic contaminants and heavy metals to the food chain is a major threat to human health. Purging these contaminants often involves a lot of energy and complex engineering processes. Phytoremediation technology can be used in various environments, such as water, soil, and air, to reduce or eliminate different contaminants. The major mechanisms involved in phytoremediation include plant extraction, rhizofiltration, plant evaporation, plant stabilization, plant decomposition, and rhizosphere degradation. The efficiency of phytoremediation can be increased through using chelating and acidifying agents, applying electric current in the soil, using organic chemicals and fertilizers, planting transgenic plants, using bacteria, and applying plant growth regulators. Recently, the use of plant growth regulators has been investigated as a suitable method for improving the efficacy of phytoremediation. Effective plant growth regulators to improve phytoremediation include auxins, gibberellins, cytokinins, and salicylic acid. The activity of these materials depends on their concentration, environmental factors that affect their absorption, and the physiological state of the plant. Using these materials increases the biomass of the plant and reduces the negative effects of the presence of contaminants in the plant. The present study aimed to review the latest studies performed on the improvement of phytoremediation using plant growth regulators and their mechanisms.

Role of auxin (IAA) in the regulation of slow vacuolar (SV) channels and the volume of red beet taproot vacuoles

BACKGROUND

Auxin (IAA) is a central player in plant cell growth. In contrast to the well-established function of the plasma membrane in plant cell expansion, little is known about the role of the vacuolar membrane (tonoplast) in this process.

RESULTS

It was found that under symmetrical 100 mM K⁺ and 100 μM cytoplasmic Ca²⁺ the macroscopic currents showed a typical slow activation and a strong outward rectification of the steady-state currents. The addition of IAA at a final concentration of 1 μM to the bath medium stimulated the SV currents, whereas at 0.1 and 10 μM slight inhibition of SV currents was observed. The time constant, τ, decreased in the presence of this hormone. When single channels were analyzed, an increase in their activity was recorded with IAA compared to the control. The single-channel recordings that were obtained in the presence of IAA showed that auxin increased the amplitude of the single-channel currents. Interestingly, the addition of IAA to the bath medium with the same composition as the one that was used in the patch-clamp experiments showed that auxin decreased the volume of the vacuoles.

CONCLUSIONS

It is suggested that the SV channels and the volume of red beet taproot vacuoles are modulated by auxin (IAA).

Interactions between plant hormones and heavy metals responses

Heavy metals are natural non-biodegradable constituents of the Earth's crust that accumulate and persist indefinitely in the ecosystem as a result of human activities. Since the industrial revolution, the concentration of cadmium, arsenic, lead, mercury and zinc, amongst others, have increasingly contaminated soil and water resources, leading to significant yield losses in plants. These issues have become an important concern of scientific interest. Understanding the molecular and physiological responses of plants to heavy metal stress is critical in order to maximize their productivity. Recent research has extended our view of how plant hormones can regulate and integrate growth responses to various environmental cues in order to sustain life. In the present review we discuss current knowledge about the role of the plant growth hormones abscisic acid, auxin, brassinosteroid and ethylene in signaling pathways, defense mechanisms and alleviation of heavy metal toxicity.