Toxicity and fate of cadmium in hydroponically cultivated lettuce (Lactuca sativa L.) influenced by microplastics.

Although more attention has been paid to microplastics (MPs) pollution in environment, research on the synthetic influence of microplastic and heavy metals remains limited. To help fill this information gap, we investigated the adsorption behavior of virgin polyvinyl chloride microplastics (PVCMPs) (≤450 µm white spherical powder) on cadmium (II). The effects on seed germination, seedling growth, photosynthetic system, oxidative stress indicators of lettuce, and changes in Cd bioavailability were evaluated under Cd2+ (25 µmol/L), PVCMPs (200 mg/L), and PVCMP-Cd combined (200 mg/L + 25 µmol/L) exposures in hydroponic system. The results demonstrated that the PVCMPs effectively adsorbed Cd ions, which validated by the pseudo-second-order kinetic and the Langmuir isotherm models, indicating the sorption of Cd2+ on the PVCMPs was primary chemisorption and approximates monomolecular layer sorption. Compared to MPs, Cd significantly inhibits plant seed germination and seedling growth and development. However, Surprising improvement in seed germination under PVCMPs-Cd exposure was observed. Moreover, Cd2+ and MPs alone or combined stress caused oxidative stress with reactive oxygen species (ROS) including H2O2, O2- and Malondialdehyde (MDA) accumulation in plants, and substantially damaged to photosynthesis. With the addition of PVCMPs, the content of Cd in the leaves significantly (P<0.01) decreased by 1.76-fold, and the translocation factor and Cd2+removal rate in the water substantially (P<0.01) decreased by 6.73-fold and 1.67-fold, respectively in contrast to Cd2+ stress alone. Therefore, it is concluded the PVCMP was capable of reducing Cd contents in leaves, alleviating Cd toxicity in lettuce. Notably, this study provides a scientific foundation and reference for comprehending the toxicological interactions between microplastics and heavy metals in the environment.

Photosynthetic Responses of Racomitrium japonicum L. to Strontium Stress Evaluated through Chlorophyll a Fluorescence OJIP Transient Analysis.

Nuclides pollution and its biological effects are of great concern, especially for bryophytes during their terrestrial adaptation. Understanding PSII activity and electron transport response is vital for comprehending moss abiotic stress reactions. However, little is known about the photosynthetic performance of moss under nuclide treatment. Therefore, this study aimed to evaluate the chlorophyll fluorescence of Racomitrium japonicum L. The moss was subjected to Sr2+ solutions at concentrations of 5, 50, and 500 mg/L to evaluate chlorophyll a fluorescence using the OJIP test. Moderate and high Sr2+ stress led to inner cell membrane dissolution and reduced chlorophyll content, indicating impaired light energy absorption. At 5 mg/L Sr2+, fluorescence kinetics showed increased light energy capture, energy dissipation, and total photosynthetic driving force, thus stimulating transient photosynthetic activity of PSII and improving PSI reduction. Linear electron transfer and PSII stability significantly decreased under moderate and high Sr2+ stress, indicating potential photosynthetic center damage. Cyclic electron transfer (CEF) alleviated photosynthetic stress at 5 mg/L Sr2+. Thus, low Sr2+ levels stimulated CEF, adjusting energy flux and partitioning to protect the photosynthetic apparatus. Nevertheless, significant damage occurred due to inefficient protection under high Sr2+ stress.

Combined forage grass-microbial for remediation of strontium-contaminated soil.

Enrichment plants were screened from six forage grasses in this study to establish a complete combined forage grass-microbial remediation system of strontium-contaminated soil, and microbial groups were added to the screened dominant forage grasses. The occurrence states of strontium in forage grasses were explored by the BCR sequential extraction method. The results showed that the annual removal rate of Sudan grass (Sorghum sudanense (Piper) Stapf.) reached 23.05% in soil with a strontium concentration of 500 mg·kg-1. Three dominant microbial groups: E, G and H, have shown good facilitation effects in co-remediation with Sudan grass and Gaodan grass (Sorghum bicolor × sudanense), respectively. When compared to the control, the strontium accumulation of forage grasses in kg of soil with microbial groups was increased by 0.5-4 fold. The optimal forage grass-microbial combination can theoretically repair contaminated soil in three years. The microbial group E was found to promote the transfer of the exchangeable state and the reducible state of strontium to the overground part of the forage grass. Metagenomic sequencing results showed that the addition of microbial groups increased Bacillus spp. in rhizosphere soil, enhanced the disease resistance and tolerance of forage grasses, and improved the remediation ability of forage grass-microbial combinations.

Nitric oxide promotes adventitious root formation in cucumber under cadmium stress through improving antioxidant system, regulating glycolysis pathway and polyamine homeostasis.

Cadmium (Cd) as a potentially toxic heavy metal that not only pollutes the environment but also interferes with plant growth. Nitric oxide (NO) regulates plant growth and development as well as abiotic stress response. However, the mechanism underpinning NO-induced adventitious root development under Cd stress remains unclear. In this study, cucumber (Cucumis sativus 'Xinchun No. 4') was used as the experimental material to investigate the effect of NO on the development of adventitious roots in cucumber under Cd stress. Our results revealed that, as compared to Cd stress, 10 µM SNP (a NO donor) could considerably increase the number and length of adventitious roots by 127.9% and 289.3%, respectively. Simultaneously, exogenous SNP significantly increased the level of endogenous NO in cucumber explants under Cd stress. Our results revealed that supplementation of Cd with SNP significantly increased endogenous NO content by 65.6% compared with Cd treatment at 48 h. Furthermore, our study indicated that SNP treatment could improve the antioxidant capacity of cucumber explants under Cd stress by up-regulating the gene expression level of antioxidant enzymes, as well as reducing the levels of malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide anion (  O 2    · - ) to alleviate oxidative damage and membrane lipid peroxidation. Application of NO resulted in a decrease of the  O 2    · - , MDA, and H2O2 level by 39.6%, 31.4% and 60.8% as compared to Cd-alone treatment, respectively. Besides that, SNP treatment significantly increased the expression level of related genes involved in glycolysis processes and polyamine homeostasis. However, application of NO scavenger 2-(4-carboxy -2-phenyl)-4, 4, 5, 5-tetramethy limidazoline -1-oxyl -3-oxide (cPTIO) and the inhibitor tungstate significantly reversed the positive role of NO in promoting the adventitious root formation under Cd stress. These results suggest that exogenous NO can increase the level of endogenous NO, improve antioxidation ability, promote glycolysis pathway and polyamine homeostasis to enhance the occurrence of adventitious roots in cucumber under Cd stress. In summary, NO can effectively alleviate the damage of Cd stress and significantly promote the development of adventitious root of cucumber under Cd stress.

Effects of Arbuscular Mycorrhizal Fungi on Alleviating Cadmium Stress in Medicago truncatula Gaertn.

Heavy metal contamination is a global problem for ecosystems and human health. Remediation of contaminated soils has received much attention in the last decade. Aided mitigation of heavy metal phytotoxicity by arbuscular mycorrhizal fungi (AMF) is a cost-effective and environmentally friendly strategy. This study was carried out to investigate the mitigation effect of AMF inoculation on heavy metal toxicity in Medicago truncatula under soil cadmium stress. Therefore, a pot experiment was designed to evaluate the growth, chlorophyll fluorescence, Cd uptake and distribution, malondialdehyde (MDA) content, root soil physicochemical properties, and metabolite profile analysis of M. truncatula with/without AMF inoculation in Cd (20 mg/Kg)-contaminated soil. The results showed that inoculating AMF under Cd stress might enhance photosynthetic efficiency, increase plant biomass, decrease Cd and MDA content, and improve soil physicochemical properties in M. truncatula. Non-targeted metabolite analysis revealed that inoculation with AMF under Cd stress significantly upregulated the production of various amino acids in inter-root metabolism and increase organic acid and phytohormone synthesis. This study provides information on the physiological responses of mycorrhizal plants to heavy metal stress, which could help provide deeper insight into the mechanisms of heavy metal remediation by AMF.

Genome-Wide Identification, Comprehensive Gene Feature, Evolution, and Expression Analysis of Plant Metal Tolerance Proteins in Tobacco Under Heavy Metal Toxicity.

Plant metal tolerance proteins (MTPs) comprise a family of membrane divalent cation transporters that play essential roles in plant mineral nutrition maintenance and heavy metal stresses resistance. However, the evolutionary relationships and biological functions of MTP family in tobacco remain unclear. In the present study, 26, 13, and 12 MTPs in three main Nicotiana species (N. tabacum, N. sylvestris, and N. tomentosiformis) were identified and designated, respectively. The phylogenetic relationships, gene structures, chromosome distributions, conserved motifs, and domains of NtMTPs were systematic analyzed. According to the phylogenetic features, 26 NtMTPs were classified into three major substrate-specific groups that were Zn-cation diffusion facilitators (CDFs), Zn/Fe-CDFs, and Mn-CDFs, and seven primary groups (1, 5, 6, 7, 8, 9, and 12). All of the NtMTPs contained a modified signature sequence and the cation_efflux domain, whereas some of them also harbored the ZT_dimer. Evolutionary analysis showed that NtMTP family of N. tabacum originated from its parental genome of N. sylvestris and N. tomentosiformis, and further underwent gene loss and expanded via one segmental duplication event. Moreover, the prediction of cis-acting elements (CREs) and the microRNA target sites of NtMTP genes suggested the diverse and complex regulatory mechanisms that control NtMTPs gene expression. Expression profile analysis derived from transcriptome data and quantitative real-time reverse transcription-PCR (qRT-PCR) analysis showed that the tissue expression patterns of NtMTPs in the same group were similar but varied among groups. Besides, under heavy metal toxicity, NtMTP genes exhibited various responses in either tobacco leaves or roots. 19 and 15 NtMTPs were found to response to at least one metal ion treatment in leaves and roots, respectively. In addition, NtMTP8.1, NtMTP8.4, and NtMTP11.1 exhibited Mn transport abilities in yeast cells. These results provided a perspective on the evolution of MTP genes in tobacco and were helpful for further functional characterization of NtMTP genes.

ytiB and ythA Genes Reduce the Uranium Removal Capacity of Bacillus atrophaeus.

Two Bacillus atrophaeus strains, the first being a highly stress-resistant ATCC 9372 strain and the Ua strain identified from a chromium mine by our lab, differ in their abilities to tolerate and remove Uranium (VI) from contaminated water. An increase in U(VI) concentration in growth media led to a decrease in the tolerance and bio-remedial capacity of both strains. However, under high concentrations of U(VI) in the growth media, the ATCC 9372 strain demonstrated a higher tolerance and a higher removal capacity than the Ua strain. Two approaches, transcriptome sequencing and transgenic technology, were used to elucidate the relationship between particular genes within these two strains and their U(VI) removal capacity. Sequencing confirmed the expression of two genes unique to the Ua strain, previously designated ytiB and ythA. They encode putative proteins that show the highest levels of identity to carbonic anhydrase and cytochrome bd terminal oxidase I, respectively. Using the pBE-S DNA vector, ytiB and ythA were transformed into the ATCC 9372 strain of Bacillus atrophaeus. Under a U(VI) concentration of 120 mg/L, the removal rates of the transgenic ATCC 9372-ytiB and ATCC 9372-ythA strains decreased by 7.55% and 7.43%, respectively, compared to the removal rate of the control strain transformed with empty plasmid. The results suggest that both ythA and ytiB genes have a negative influence on the uranium removing capacity of Bacillus atrophaeus. This finding will help to elucidate the molecular mechanisms of uranium removal by bacteria.

The uptake of Cs and Sr from soil to radish (Raphanus sativus L.)- potential for phytoextraction and remediation of contaminated soils.

The (133)Cs and (88)Sr uptake by plant Raphanus sativus L. was studied during cultivation in outdoor potted-soil. The distribution, accumulation of (133)Cs, (88)Sr and the antioxidant responses in plants were measured after 30 and 60 days of cultivation. The results showed that the uptake capacity of radish for (88)Sr was far higher than that for (133)Cs when the concentration of (88)Sr was as the same as that of (133)Cs in the soil (The concentration of (88)Sr or (133)Cs in the soil was from 2.5 mg kg(-1) to 40 mg kg(-1)). The highest (88)Sr accumulation was 239.18 µg g(-1) dw, otherwise, the highest (133)Cs accumulation was 151.74 µg g(-1) dw (The concentration of (88)Sr in the soil was 40 mg kg(-1)), and the lowest (88)Sr accumulation was 131.03 µg g(-1) dw, otherwise, the lowest (133)Cs accumulation was 12.85 µg g(-1) dw (The concentration of (88)Sr in the soil is 5 mg kg(-1)). The (88)Sr and (133)Cs TF values were 1.16-1.72 and 0.24-0.60, respectively. There was little influence of high concentration of (88)Sr on the total biomass of plants, so the radish is one of the ideal phytoremediation plant for Sr polluted soils. The important physiological reasons that radish had good tolerance to (88)Sr stress were that the MDA content was higher under the (88)Sr stress than that under the (133)Cs stress, and the activities of POD and CAT were lower under the (88)Sr stress than that under (133)Cs stress.

RNA interference-based suppression of phosphoenolpyruvate carboxylase results in susceptibility of rapeseed to osmotic stress.

The diverse functions of phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) in C(3) plants are not as well understood as in C(4) plants. To investigate the functions of PEPCase in C(3) plants, rapeseed (Brassica napus L.) PEPCase gene (referred to as BNPE15) was silenced by the RNA interference (RNAi) technique. Under normal growth conditions, no significant difference in lipid content and fatty acid composition were found between wild-type (WT) and transgenic rapeseed plants. However, when these plants were subjected to osmotic stress induced by osmoticum polyethylene glycol (PEG-6000), membrane permeability and membrane lipid peroxidization in roots and leaves of transgenic plants were higher than those of WT plants. It suggested that transgenic plants are more susceptible to osmotic stress than WT plants. Taken together, the results showed that the suppression of PEPCase by RNAi leads to susceptibility to osmotic stress in rapeseed, and PEPCase is involved in the response of C(3) plants to environmental stress.

Characterization of photosystem II in salt-stressed cyanobacterial Spirulina platensis cells.

PSII activity was inhibited after Spirulina platensis cells were incubated with different salt concentrations (0-0.8 M NaCl) for 12 h. Flash-induced fluorescence kinetics showed that in the absence of DCMU, the half time of the fast and slow components decreased while that of the middle component increased considerably with increasing salt concentration. In the presence of DCMU, fluorescence relaxation was dominated by a 0.6s component in control cells. After salt stress, this was partially replaced by a faster new component with half time of 20-50 ms. Thermoluminescence measurements revealed that S(2)Q(A)(-) and S(2)Q(B)(-) recombinations were shifted to higher temperatures in parallel and the intensities of the thermoluminescence emissions were significantly reduced in salt-stressed cells. The period-four oscillation of the thermoluminescence B band was highly damped. There were no significant changes in contents of CP47, CP43, cytochrome c550, and D1 proteins. However, content of the PsbO protein in thylakoid fraction decreased but increased significantly in soluble fraction. The results suggest that salt stress leads to a modification of the Q(B) niche at the acceptor side and an increase in the stability of the S(2) state at the donor side, which is associated with a dissociation of the PsbO protein.

Genetic engineering of the biosynthesis of glycinebetaine enhances thermotolerance of photosystem II in tobacco plants.

Genetically engineered tobacco (Nicotiana tabacum L.) with the ability to accumulate glycinebetaine was established. The wild type and transgenic plants were exposed to heat treatment (25-50 degrees C) for 4 h in the dark and under growth light intensity (300 mumol m(-2) s(-1)). The analyses of oxygen-evolving activity and chlorophyll fluorescence demonstrated that photosystem II (PSII) in transgenic plants showed higher thermotolerance than in wild type plants in particular when heat stress was performed in the light, suggesting that the accumulation of glycinebetaine leads to increased tolerance to heat-enhanced photoinhibition. This increased tolerance was associated with an improvement on thermostability of the oxygen-evolving complex and the reaction center of PSII. The enhanced tolerance was caused by acceleration of the repair of PSII from heat-enhanced photoinhibition. Under heat stress, there was a significant accumulation of H(2)O(2), O (2) (-) and catalytic Fe in wild type plants but this accumulation was much less in transgenic plants. Heat stress significantly decreased the activities of catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monodehydroascorbate reductase in wild type plants whereas the activities of these enzymes either decreased much less or maintained or even increased in transgenic plants. In addition, heat stress increased the activity of superoxide dismutase in wild type plants but this increase was much greater in transgenic plants. Furthermore, transgenic plants also showed higher content of ascorbate and reduced glutathione than that of wild type plants under heat stress. The results suggest that the increased thermotolerance induced by accumulation of glycinebetaine in vivo was associated with the enhancement of the repair of PSII from heat-enhanced photo inhibition, which might be due to less accumulation of reactive oxygen species in transgenic plants.

Heat stress induces an aggregation of the light-harvesting complex of photosystem II in spinach plants.

Whole spinach (Spinacia oleracea) plants were subjected to heat stress (25 degrees C-50 degrees C) in the dark for 30 min. At temperatures higher than 35 degrees C, CO2 assimilation rate decreased significantly. The maximal efficiency of photosystem II (PSII) photochemistry remained unchanged until 45 degrees C and decreased only slightly at 50 degrees C. Nonphotochemical quenching increased significantly either in the absence or presence of dithiothreitol. There was an appearance of the characteristic band at around 698 nm in 77 K fluorescence emission spectra of leaves. Native green gel of thylakoid membranes isolated immediately from heat-stressed leaves showed that many pigment-protein complexes remained aggregated in the stacking gel. The analyses of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting demonstrated that the aggregates were composed of the main light-harvesting complex of PSII (LHCIIb). To characterize the aggregates, isolated PSII core complexes were incubated at 25 degrees C to 50 degrees C in the dark for 10 min. At temperatures over 35 degrees C, many pigment-protein complexes remained aggregated in the stacking gel of native green gel, and immunoblotting analyses showed that the aggregates were composed of LHCIIb. In addition, isolated LHCII was also incubated at 25 degrees C to 50 degrees C in the dark for 10 min. LHCII remained aggregated in the stacking gel of native green gel at temperatures over 35 degrees C. Massive aggregation of LHCII was clearly observed by using microscope images, which was accompanied by a significant increase in fluorescence quenching. There was a linear relationship between the formation of LHCII aggregates and nonphotochemical quenching in vivo. The results in this study suggest that LHCII aggregates may represent a protective mechanism to dissipate excess excitation energy in heat-stressed plants.

Differential changes in degradation of chlorophyll-protein complexes of photosystem I and photosystem II during flag leaf senescence of rice.

The stability of chlorophyll-protein complexes of photosystem I (PSI) and photosystem II (PSII) was investigated by chlorophyll (Chl) fluorescence spectroscopy, absorption spectra and native green gel separation system during flag leaf senescence of two rice varieties (IIyou 129 and Shanyou 63) grown under outdoor conditions. During leaf senescence, photosynthetic CO(2) assimilation rate, carboxylase activity of Rubisco, chlorophyll and carotenoids contents, and the chlorophyll a/b ratio decreased significantly. The 77 K Chl fluorescence emission spectra of thylakoid membranes from mature leaves had two peaks at around 685 and 735 nm emitting mainly from PSII and PSI, respectively. The total Chl fluorescence yields of PSI and PSII decreased significantly with senescence progressing. However, the decrease in the Chl fluorescence yield of PSI was greater than in the yield of PSII, suggesting that the rate of degradation in chlorophyll-protein complexes of PSI was greater than in chlorophyll-protein complexes of PSII. The fluorescence yields for all chlorophyll-protein complexes decreased significantly with leaf senescence in two rice varieties but the extents of their decrease were significantly different. The greatest decrease in the Chl fluorescence yield was in PSI core, followed by LHCI, CP47, CP43, and LHCII. These results indicate that the rate of degradation for each chlorophyll-protein complex was different and the order for the stability of chlorophyll-protein complexes during leaf senescence was: LHCII>CP43>CP47>LHCI>PSI core, which was partly supported by the green gel electrophoresis of the chlorophyll-protein complexes.