Resistance mechanisms and their difference between the root and leaf of Microsorum pteropus - A novel potential aquatic cadmium hyperaccumulator

Autotoxicity effect of water extracts from rhizosphere soil of Elymus sibiricus in different planting years on seed germination, physiological characteristics and phytohormones of seedlings

Elymus sibiricus is a highly valuable perennial forage that is widely planted in the Qinghai-Tibet Plateau (QTP) region. However, E. sibiricus artificial grasslands have a short utilization lifespan, and reach the highest yield in the 2nd and 3rd year of plantation, then rapidly drop its productivity. We hypothesized that autotoxicity is one of the mechanisms for the reduction of the productivity. To test this hypothesis, we prepared the water extract from rhizosphere soils of E. sibiricus planted for 3, 4, 5, and 8 years and examined the effects of the extract concentrations at 0.05, 0.1, 0.2, and 0.5 g/mL on seed germination, seedling growth, physiological characteristics and phytohormones in the aboveground and roots of E. sibiricus. The results showed that the soil extract concentration, planting years, and their interaction had significant influences on the most of these indices. The soil extract inhibited the seed germination and growth of seedlings, and the inhibitory effects appeared to be stronger at the 0.5 g/mL rhizosphere soil extract for 5 and 8 years. The superoxide dismultase and peroxide activities, the free proline concentration, soluble sugar concentration were altered. The malondialdehyde concentration was, in general, increased, especially in 8 years soil extract. The indole acetic acid and gibberellic acids concentrations were lowered, while the abscisic acid concentration varied. These changes were depending on the extract concentration and the years of planting, without clear patterns in some of them in response to the extract concentration and planting years. In summary, autotoxicity can be a contributor to the retardation of the growth and development of artificial E. sibiricus grasslands. The inhibitory effects could be attribute to impaired antioxidant capacity and disturbance of osmortic-regulatory substances and plant hormones, and are more profound on the root than on the aboveground part of the seedlings.

Ornamental plants for the phytoremediation of heavy metals: Present knowledge and future perspectives

Environmental matrices are polluted with the plethora of contaminants, and among these, the concerns related to heavy metals (HMs) are also included. Due to the low cost in a long-term application and environmental friendliness, the use of biological remediation has gained significant attention in recent decades. The use of ornamental plants (OPs) in the field of phytoremediation is scarcely reported, and the impacts of HMs on OPs have also not been investigated in great depth. The OPs mediated HMs remediation can simultaneously remove contaminants and bring improvement in aesthetics of the site. The biomass of OPs produced after such activities can be used and sold as pot plants, cut flowers, essential oils, perfumes, air fresheners production, metal phytomining, and feedstock in silk production. The OPs also present a lower risk of HMs bioaccumulation compared to crop plants. This review focuses on the current knowledge of HMs toxicity to OPs, their applicability advantages, methods to improve the tolerance of OPs with incremented HMs uptake, challenges in the field, and future application perspectives. The case studies realted to practical application of OPs, from China, Iran, India, Oman, Pakistan, and Turkey, were also discussed. This work fetches the inter-disciplinary features and understanding for the sustainable treatment of HMs in a new novel way, to which no previous review has focused.

The combined effects of elevated atmospheric CO2 and cadmium exposure on flavonoids in the leaves of Robinia pseudoacacia L. seedlings

Flavonoids participate in several plant processes such as growth and physiological protection in adverse environments. In this study, we investigated the combined effects of eCO₂ and cadmium (Cd)-contaminated soils on the total flavonoid and monomer contents in the leaves of Robinia pseudoacacia L. seedlings. Elevated CO₂, Cd, and eCO₂+ Cd increased the total flavonoids in the leaves relative to the control, and eCO₂ mostly increased (p < 0.05) the total flavonoid content under Cd exposure. Elevated CO₂ increased (p < 0.05) robinin, rutin, and acacetin contents in the leaves of 45-day seedlings and decreased (p < 0.05) the content of robinin and acacetin at 90 and 135 d under Cd exposure except for robinin at day 45 under Cd1 and acacetin on day 135 under Cd1. Quercetin content decreased (p < 0.05) under the combined conditions relative to Cd alone. Kaempferol in the leaves was only detected under eCO₂ on day 135. The responses of total chlorophyll, total soluble sugars, starch, C, N, S, and the C/N ratio in the leaves to eCO₂ significantly affected the synthesis of total flavonoids and monomers under Cd exposure. Overall, rutin was more sensitive to eCO₂+ Cd than the other flavonoids. Cadmium, CO_2, and time had significant interactive effects on the synthesis of flavonoids in the leaves of R. pseudoacacia L. seedlings. Elevated CO₂ may improve the protection and defense system of seedlings grown in Cd-contaminated soils by promoting the synthesis of total flavonoids, although robinin, rutin, quercetin, and acacetin yields may reduce with time. Additionally, increased Cd in the leaves suggested that eCO₂ could improve the phytoremediation of Cd-contaminated soils.

Responses of glutathione and phytochelatins biosysthesis in a cadmium accumulator of Perilla frutescens (L.) Britt. under cadmium contaminated conditions

Screening new accumulators of heavy metal and identifying their tolerance, enrichment capacity of heavy metals are currently hot issues in phytoremediation research. A series of hydroponic experiments were conducted to analyze the effects of glutathione and phytochelatins in roots, stems, and leaves of Perilla frutescens under cadmium stress. The results showed that the non-protein thiols in roots and stems mainly existed in the form of GSH, PC₂, PC₃, and PC₄ under Cd stress condition, while in leaves they existed in the form of GSH, PC₂, and PC₃. Furthermore, the contents of GSH and PCs positively correlated with Cd, but negatively correlated with root vigor and chlorophyll content under Cd stress conditions. After 21 days of treatments, the contents of Cd in different parts of the plant were 1465.2-3092.9 mg· kg^-1 in the roots, 199.6-478.4 mg·kg^-1 in the stems and 61.3-96.9 mg· kg^-1 in the leaves at 2, 5, 10 mg·L^-1 Cd levels respectively, and the amount of Cd uptakes were up to 3547.7-5701.7 μg·plant^-1. Therefore, P. frutescens performed high capacity in Cd accumulation, and PCs played a key role in Cd tolerance. The application prospect of the plant in phytoremediation Cd polluted soil was also discussed.

Evaluation of cadmium hyperaccumulation and tolerance potential of Myriophyllum aquaticum

Enrichment of the hyperaccumulator bank is important for phytoremediation, and studying new hyperaccumulators has become a research hotspot. In this study, cadmium (Cd), the main representative factor of heavy-metal-polluted water, was the research object, and the Cd bioenrichment ability and tolerance of Myriophyllum aquaticum were studied for the first time. The experiment was conducted for 28 days by establishing experimental groups with different Cd concentrations (0, 10, 20, 40, 80, and 160 mg/L). The results show that M. aquaticum is a new Cd hyperaccumulator. There was no notable damage in the 40 mg/L Cd treatment group, and the Cd enrichment ability of M. aquaticum reached 17,970 ± 1020.01 mg/kg, while the bioconcentration factor (BCF) reached 449.25. At the same time, the antioxidant system (superoxide dismutase (SOD) and peroxidase (POD)) and proline (Pro) levels of M. aquaticum maintained normal plant physiology, but there were physiological anomalies in M. aquaticum at high concentrations and under long-term treatment. The results show that M. aquaticum has a high Cd bioenrichment ability and tolerance in water and can be used for phytoremediation of river water polluted by Cd.

Influence of Cd exposure on H+ and Cd2+ fluxes in the leaf, stem and root of a novel aquatic hyperaccumulator - Microsorum pteropus

Microsorum pteropus has been proven to be a potential novel aquatic Cd hyperaccumulator. In this study, Non-invasive Micro-test Technology (NMT) was used to observe the ion fluxes of different M. pteropus tissues under Cd exposure. M. pteropus can hyperaccumulate more than 1000 mg/kg Cd in roots and leaves and approximately 600 mg/kg Cd in stems after seven days of exposure to 500 μM Cd, showing that this plant have a great capacity for Cd enrichment and resistance. The NMT test found H⁺ fluxes increased in all tissues after Cd exposure, with the largest increases being observed in stems, followed by the leaves and roots. Cd²⁺ fluxes showed different accumulation levels in different tissues, with low-level Cd exposure leading to influxes into roots and leaves, and high-level Cd exposure resulting in effluxes from roots. No significant influxes or effluxes were observed in leaves under high-level Cd exposure, or in stems under low- and high-levels of Cd exposure. However, transient high-level Cd exposure showed long-term Cd²⁺ influxes into roots and short-term Cd²⁺ effluxes out of stems and leaves. The roots of M. pteropus had greater regulation mechanisms for Cd enrichment and resistance, with influxes occurring following low-level exposure and effluxes occurring from high-level exposure. When exposed to Cd, M. pteropus stems showed less transportation and absorption. Low-level Cd exposure resulted in individual leaves directly absorbing Cd from hydroponic solutions. Different Cd enrichment and resistance mechanisms were exhibited by different M. pteropus tissues.

Subcellular distribution of cadmium in a novel potential aquatic hyperaccumulator - Microsorum pteropus

Microsorum pteropus is a novel potential Cd (cadmium) aquatic hyperaccumulator. In the present study, hydroponic experiments were conducted to assess the accumulation and subcellular distribution of Cd in the root, stem and leaf of M. pteropus. SEM (scanning electron microscopy) - EDX (energy dispersive X-ray fluorescence spectrometer) and TEM (transmission electron microscopy) were used to observe the ultrastructure of different tissues under 500 μM Cd exposure. After exposure to 500 μM Cd for 7 days, the root, stem and leaf of M. pteropus can accumulate to be > 400 mg/kg Cd in dry mass with no significant influence on the growth. In the root and leaf of M. pteropus, the Cd was more likely to store in the cell wall fraction. However, Cd in the stem was mainly stored in both the cell wall fraction and the cytoplasm fraction. Under SEM observation and EDX detection, 1) Cd was found to be sequestrated in the epidermis or chelated in the root cells, 2) no significant deposit spots were observed in the stem, 3) Cd was found in the trichome of the leaf, and the sporangium was not damaged. TEM observations revealed 1) possible Cd precipitations in the root cell and 2) no significant ultrastructure variation in the stem, and 3) the chloroplast retained its structure and was not affected by the Cd. M. pteropus showed great capacity for Cd accumulation without influencing growth. In addition, the ultrastructure of all the tissues was not damaged by the Cd. M. pteropus showed a great potential in phytoremediation in heavy metal polluted water solutions, and may provide new directions for the study of resistance mechanisms of aquatic hyperaccumulators.

Cadmium accumulation capacity and resistance strategies of a cadmium-hypertolerant fern - Microsorum fortunei

Microsorum fortunei (M. fortunei), a close relative to the cadmium (Cd) hyperaccumulator Microsorum pteropus, is an epiphytic Polypodiaceae fern with strong antioxidant activity. The Cd-accumulation capacities and Cd-resistance mechanisms of M. fortunei were analyzed in this study by measuring metal contents (Cd, Fe, Mg, Ca, Zn, Mn, K and Na) and chlorophyll fluorescence parameters (F_v/F_m, qN, qP, Y(II), Y(NPQ) and Y(NO)) and by performing an RNA-sequencing analysis. M. fortunei could accumulate up to 2249.10 μg/g DW Cd in roots under a 15-day 1000 μmol/L Cd treatment, with little Cd translocated into the leaves (maximum 138.26 μg/g DW). The M. fortunei leaves could maintain their normal physiological functions with no phytosynthesis damage and few changes in metal contents or differentially expressed genes. M. fortunei roots showed a decrease in Zn concentration, with potential Cd-tolerance mechanisms such as heavy metal transporters, vesicle trafficking and fusion proteins, antioxidant systems, and primary metabolites like plant hormones, revealed by differentially expressed functional genes. In conclusion, M. fortunei may serve as a potential cadmium-hypertolerant fern that sequesters and detoxifies most cadmium in the roots, with a minimum root-to-shoot Cd translocation to guarantee the physiological functions in the more vulnerable leaves.

Differential expression of proteins in the leaves and roots of cadmium-stressed Microsorum pteropus, a novel potential aquatic cadmium hyperaccumulator

Microsorum pteropus is a fully or partially submerged Polypodiaceae fern that has been proven to be a potential Cd aquatic hyperaccumulator. Proteomic analysis was used in this study to investigate the resistance mechanisms of M. pteropus root and leaf tissues under Cd stress. M. pteropus plants were exposed to up to 500 μM Cd in hydroponics for 7 days. The plant can accumulate >4,000 mg/kg Cd in both root and leaf dry mass. Meanwhile, the proteins in roots and leaves in the 500 μM Cd treatment were separated and analyzed by proteomics. Eight proteins with altered expression in roots and twenty proteins with altered expression in leaves were identified using MALDI-TOF/TOF-MS (matrix-assisted laser desorption/ionization time of flight mass spectrometry) in this study. The proteins were involved in energy metabolism, antioxidant activity, cellular metabolism and protein metabolism. However, just three proteins were significantly differentially expressed in both tissues, and they were all involved in basal metabolism, indicating different resistance mechanisms between roots and leaves. Root tissues of M. pteropus mainly resist Cd damage by antioxidants and the enhancement of energy metabolism, while leaf tissues of M. pteropus mainly protect themselves by maintaining photosynthetic functions and the regulation of cellular metabolism.

Comparisons of cadmium bioaccumulation potentials and resistance physiology of Microsorum pteropus and Echinodorus grisebachii

To better monitor and remediate environments contaminated by cadmium (Cd), plants are used as hyperaccumulators or biomonitors; however, few have been identified for aquatic Cd pollution. In our study, two aquatic ornamental plants, Microsorum pteropus (Blume) Copel. and Echinodorus grisebachii Small, were studied for their Cd accumulation capacity, morphological characteristics, and leaf physiological indexes. Microsorum pteropus (Blume) Copel. leaf has the potential to hyperaccumulate Cd (166 mg/kg dry weight for 1 mg/L exposure), with no significant physiological difference under exposure. Echinodorus grisebachii Small had sensitive diagnostic responses to Cd toxicity, such as significant decreases in Chl (a + b) and Chl-a/b, increased peroxidase (POD) activity, greater malondialdehyde (MDA) content, and increased soluble sugar content. These results suggest that Microsorum pteropus (Blume) Copel. could have the potential to be a Cd hyperaccumulator, while Echinodorus grisebachii Small could serve as a biomonitor for Cd-contaminated water bodies.