Arsenic (As) contamination and bioaccumulation are a serious threat to agricultural plants. To address this issue, we checked the efficacy of As tolerant plant growth promoting bacteria (PGPB), zinc oxide nanoparticles (ZnO NPs) and oxalic acid (OA) in Luffa acutangula grown on As rich soil. The selected most As tolerant PGPB i.e Providencia vermicola exhibited plant growth promoting features i.e solubilzation of phosphate, potassium and siderophores production. Innovatively, we observed the synergistic effects of P. vermicola, ZnO NPs (10 ppm) and OA (100 ppm) in L. acutangula grown on As enriched soil (150 ppm). Our treatments both as alone and in combination alleviated As toxicity exhibited by better plant growth and metabolism. Results revealed significantly enhanced photosynthetic pigments, proline, relative water content, total sugars, proteins and indole acetic acid along with As amelioration in L. acutangula. Furthermore, upregulated plant resistance was manifested with marked reduction in the lipid peroxidation and electrolyte leakage and pronounced antagonism of As and zinc content in leaves under toxic conditions. These treatments also improved level of nutrients, abscisic acid and antioxidants to mitigate As toxicity. This marked improvement in plants’ defense mechanism of treated plants under As stress is confirmed by less damaged leaves cell structures observed through the scanning electron micrographs. We also found substantial decrease in the As bioaccumulation in the L. acutangula shoots and roots by 40 and 58% respectively under the co-application of P. vermicola, ZnO NPs and OA in comparison with control. Moreover, the better activity of soil phosphatase and invertase was assessed under the effect of our application. These results cast a new light on the application of P. vermicola, ZnO NPs and OA in both separate and combined form as a feasible and ecofriendly tool to alleviate As stress in L. acutangula.

Plant growth regulators (PGRs) assisted phytoextraction was investigated as a viable phytoremediation technology to increase the phytoextraction efficiency in contaminated soils. This study aimed to evaluate the cadimum (Cd)/lead (Pb)/zinc (Zn) phytoextraction efficiency by a hyperaccumulator Sedum alfredii Hance (S. alfredii) treated with 9 PGRs, including indole-3-acetic acid (IAA), gibberellin (GA₃), cytokinin (CKs), abscisic acid (ABA), ethylene (ETH), brassinosteroid (BR), salicylic acid (SA), strigolactones (SL) and jasmonic acid (JA), in slightly or heavily contaminated (SC and HC, respectively) soil. Results demonstrated that PGRs were able to improve S. alfredii biomass, the most significant increases were observed in GA₃ and SL for HC soil, while for SC soil, IAA and BR exhibited positive effects. The levels of Cd, Pb and Zn in the shoots of S. alfredii treated with ABA and SL were noticeably greater than in the CK treatment in HC soil, while the uptake of metals were increased by IAA and CKs in SC soil. Combined with the results of biomass and metal contents in shoots, we found that ABA showed the highest Cd removal efficiency and the maximum Pb and Zn removal efficiency was observed with GA₃, which was 62.99%, 269.23%, and 41.18%, respectively higher than the control in HC soil. Meanwhile, compared to control, the maximum removal efficiency of Cd by IAA treatment (52.80%), Pb by JA treatment (165.1%), and Zn by BR treatment (44.97%) in the SC soil. Overall, our results suggested that these PGRs, especially, ABA, SL, IAA, BR and GA₃ had great potential in improving phytoremediation efficiency of S. alfredii grown in contaminated soils.

Improper application of copper-based fungicides has made copper stress critical in viticulture, necessitating the need to identify substances that can mitigate it. In this study, leaves of ‘Shine Muscat’ (‘SM’) grapevine seedlings were treated with CuSO₄ solution (10 mM/L), CuSO₄ + 5-aminolevulinic acid (ALA) (50 mg/L), and distilled water to explore the mitigation effect of ALA. Physiological assays demonstrated that ALA effectively reduced malondialdehyde accumulation and increased peroxidase and superoxide dismutase activities in grapevine leaves under copper stress. Copper ion absorption, transport pathways, chlorophyll metabolism pathways, photosynthetic system, and antioxidant pathways play key roles in ALA alleviated-copper stress. Moreover, expression changes in genes, such as CHLH, ALAD, RCA, and DHAR, play vital roles in these processes. Furthermore, abscisic acid reduction caused by NCED down-regulation and decreased naringenin, leucopelargonidin, and betaine contents confirmed the alleviating effect of ALA. Taken together, these results reveal how grapevine responds to copper stress and the alleviating effects of ALA, thus providing a novel means of alleviating copper stress in viticulture.

Arsenite [As(III)] toxicity causes impeded growth, inadequate productivity of plants and toxicity through the food chain. Using various chemical residues for priming is one of the approaches in conferring arsenic tolerance in crops. We investigated the mechanism of abscisic acid (ABA)-induced As(III) tolerance in rice genotypes (cv. Swarna and Swarna Sub1) pretreated with 10 μM of ABA for 24 h and transferred into 0, 25 and 50 μM arsenic for 10 days. Plants showed a dose-dependent bioaccumulation of As(III), oxidative stress indicators like superoxide, hydrogen peroxide, thiobarbituric acid reactive substances and the activity of lipoxygenase. As(III) had disrupted cellular redox that reflecting growth indices like net assimilation rate, relative growth rate, specific leaf weight, leaf mass ratio, relative water content, proline, delta-1-pyrroline-5-carboxylate synthetase and electrolyte leakage. ABA priming was more protective in cv. Swarna Sub1 than Swarna for retrieval of total glutathione pool, non-protein thiols, cysteine, phytochelatin and glutathione reductase. Phosphate metabolisms were significantly curtailed irrespective of genotypes where ABA had moderated phosphate uptake and its metabolizing enzymes like acid phosphatase, alkaline phosphatase and H⁺/ATPase. Rice seedlings had regulated antioxidative potential with the varied polymorphic expression of those enzymes markedly with antioxidative enzymes. The results have given the possible cellular and physiological traits those may interact with ABA priming in the establishment of plant tolerance with As(III) over accumulation and, thereby, its amelioration for oxidative damages. Finally, cv. Swarna Sub1 was identified as a rice genotype as a candidate for breeding program for sustainability against As(III) stress with cellular and physiological traits serving better for selection pressure.

Heavy metal accumulation in agricultural land causes crop production losses worldwide. Metal homeostasis within cells is tightly regulated. However, homeostasis breakdown leads to accumulation of reactive oxygen species (ROS). Overall plant fitness under stressful environment is determined by coordination between roots and shoots. But little is known about organ specific responses to heavy metals, whether it depends on the metal category (redox or non-redox reactive) and if these responses are associated with heavy metal accumulation in each organ or there are driven by other signals. Maize seedlings were subjected to sub-lethal concentrations of four metals (Zn, Ni, Cd and Cu) individually, and were quantified for growth, ABA level, and redox alterations in roots, mature leaves (L1,2) and young leaves (L3,4) at 14 and 21 days after sowing (DAS). The treatments caused significant increase in endogenous metal levels in all organs but to different degrees, where roots showed the highest levels. Biomass was significantly reduced under heavy metal stress. Although old leaves accumulated less heavy metal content than root, the reduction in their biomass (FW) was more pronounced. Metal exposure triggered ABA accumulation and stomatal closure mainly in older leaves, which consequently reduced photosynthesis. Heavy metals induced oxidative stress in the maize organs, but to different degrees. Tocopherols, polyphenols and flavonoids increased specifically in the shoot under Zn, Ni and Cu, while under Cd treatment they played a minor role. Under Cu and Cd stress, superoxide dismutase (SOD) and dehydroascorbate reductase (DHAR) activities were induced in the roots, however ascorbate peroxidase (APX) activity was only increased in the older leaves. Overall, it can be concluded that root and shoot organs specific responses to heavy metal toxicity are not only associated with heavy metal accumulation and they are specialized at the level of antioxidants to cope with.

Cadmium (Cd) pollution in mangrove wetlands has received increasing attention as urbanization expands rapidly. As a dominant mangrove species, Kandelia obovata is highly tolerant to Cd toxicity. Plant hormones and superoxide dismutase (SODs) play critical roles in the response to heavy metal stress in K. obovata roots. Although theirs important influence have been reported, the regulation mechanism between SODs and plant hormones in Cd detoxification by K. obovata roots remains limited. Here, we investigated relationships among SOD, plant hormones, and Cd tolerance in K. obovata roots exposed to Cd. We found that Cd was retained in the epidermis and exodermis of roots, and the epidermis and exodermis had highest hydrogen peroxide (H2O2) content and SOD activity. Similarly, SOD isozymes also exhibited distinct activity in the different parts of root. Overexpressed KoCSD3 and KoFSD2 individually in Nicotiana benthamiana revealed that different SOD members contributed to H2O2 content regulation by promote the activity of downstream antioxidant enzymes under Cd treatment. In addition, assays on the effects of hormones showed that increased endogenous indole-3-acetic acid (IAA) was observed in the cortex and stele, whereas the abscisic acid (ABA) content was enhanced in the epidermis and exodermis in roots during Cd treatment. The results of exogenous hormones treatment indicated that KoFSD2 upregulated under ABA and IAA treatment, but KoCSD3 only induced by ABA stimulation. Taken together, our results reveal the relationship between SODs and plant hormones, which expands the knowledge base regarding KoSODs response to plant hormones and mediating H2O2 concentration under Cd stress.

Tropospheric ozone (O₃) impairs physiological processes of plants while nitrogen (N) deposition may cause imbalances in soil N and other nutrients such as phosphorus (P) suggesting an increase of P demand for plants. However, the combined effect of O₃, soil N and P on isoprene emission from leaves has never been tested. We therefore examined isoprene emission in leaves of Oxford poplar clone exposed to O₃ (ambient, AA [35.0 nmol mol⁻¹ as daily mean]; 1.5 × AA; 2.0 × AA), soil N (0 and 80 kg N ha⁻¹) and soil P (0, 40 and 80 kg P ha⁻¹) in July and September in a Free-Air Controlled Exposure (FACE) facility. We also investigated the response of isoprene emission to foliar N, P and abscisic acid (ABA) contents in September because the 2-C-methylerythritol-5-phosphate (MEP) pathway of isoprenoid biosynthesis produces ABA. We found that O₃ increased isoprene emission in July, which was associated to increased dark respiration, suggesting an activation of metabolism against O₃ stress as an initial response. However, O₃ decreased isoprene emission in September which was associated to reduced net photosynthesis. In September, isoprene emission was positively correlated with leaf N content and negatively correlated with leaf P content in AA. However, no response of isoprene emission to foliar N and P was found in elevated O₃, suggesting that the isoprene responses to foliar N and P depended on the O₃ exposure levels. Isoprene emission rate in 1.5 × AA and 2.0 × AA increased with increasing leaf ABA content, indicating accelerated senescence of injured leaves to favor new leaf growth when high O₃ and nutritional availability in the soil were combined. Even though foliar N and P usually act as a proxy for isoprene emission rate, the impact of recent abiotic factors such as O₃ should be always considered for modeling isoprene emission under climate change.

Due to the potential toxicity of polycyclic aromatic hydrocarbons (PAHs) to humans, the uptake and translocation of PAHs in food crops have gained much attention. However, it is still unclear whether phloem participates in the acropetal translocation of PAHs in plants. Herein, the evidence for acropetal translocation of phenanthrene (a model PAH) via phloem is firstly tested. Wheat (Triticum aestivum L.) new leaves contain significantly higher phenanthrene concentration than old leaves (P < 0.05), and the inhibitory effect on phenanthrene translocation is stronger in old leaves after abscisic acid and polyvinyl alcohol (two common transpiration inhibitors) application. Phenanthrene concentration in xylem sap is slightly higher than in phloem sap. Ring-girdling treatment can significantly reduce phenanthrene concentration in castor bean (Ricinus communis L.) leaves. Two-photon fluorescence microscope images indicate a xylem-to-phloem and acropetal phloem translocation of phenanthrene in castor bean stem. Therefore, phloem is involved in the acropetal translocation of phenanthrene in wheat seedlings, especially when the xylem is not mature enough in scattered vascular bundle plants. Our results provide a deeper understanding of PAH translocation in plants, which have significant implications for food safety and phytoremediation enhancement of PAH-contaminated soil and water.

Treated wastewater is increasingly used to meet agriculture's water needs; however, treated wastewater contains numerous contaminants of emerging concern (CECs). With exposure and uptake of CECs, phytotoxicity and health of crop plants is of concern, but is poorly understood. This study evaluated the effect of low-dose, chronic exposure to a mixture of 10 CECs, including 4 antibiotics, 3 anti-inflammatory drugs, 1 antiepileptic, 1 beta-blocker, and 1 antimicrobial, on lettuce (Lactuca sativa) and cucumber (Cucumis sativa L.) plants. The CEC mixture was added in nutrient media at 1 to 20X of their typical levels in treated wastewater effluents. Biological endpoints including germination, growth, phytohormone homeostasis, and CEC bioaccumulation were determined. Exposure to the CEC mixture did not affect the germination rate of lettuce seeds, but stimulated root elongation and increased the root-to-shoot biomass ratio during a 7 d cultivation. A dose-dependent decrease in biomass was observed in cucumber seedling after a 30 d exposure, with the highest rate CEC treatment resulting in decreases of 51.2 ± 20.9, 26.3 ± 34.1, and 33.2 ± 41.7% in the below-ground, above-ground, and total biomass, respectively. Levels of abscisic acid were significantly elevated (p < 0.05) in the leaves, but decreased (p < 0.05) in the roots. The dose-response of auxin was characterized by a hormesis effect. A significant 6-fold increase in the stem auxin level was observed at the 1X CEC rate, followed by a decrease to 2-fold the control at the 20X rate. Leaf auxin concentrations also significantly increased at the 1X CEC rate to 16-fold, followed by a decrease at the highest CEC rate. The results of this study suggeste that chronic exposure to low levels of CEC mixtures may compromise the fitness of plants, and the impairments are underlined by alterations in hormone balances.

Heavy metal pollution, as well as greenhouse effect, has become a serious threat today. Both heavy metal and heat stresses can arrest seed germination. What response can be expected for seed germination under both stress conditions? Here, the effects of heavy metals (Cu2+, Cd2+ and Hg2+) on maize seed germination were investigated at 20 °C and 40 °C. Compared with 20 °C, heat stress induced thermodormancy. However, this thermodormancy could be significantly alleviated by the addition of a low concentration of heavy metals. Heavy metals, as well as heat stress induced H2O2 accumulation in germinating seeds. Interestingly, this low concentration of heavy metal that promoted seed germination could be partly blocked by DMTU (a specific ROS scavenger), irrespective of temperature. Accordingly, H2O2 addition reinforced this promoting effect on seed germination, which was induced by a low concentration of heavy metal. Furthermore, we found that the NADPH oxidase derived ROS was required for seed germination promoted by the heavy metals. Subsequently, treatment of seeds with fluridone (a specific inhibitor of ABA) or ABA significantly alleviated or aggravated thermodormancy, respectively. However, this alleviation or aggravation could be partly attenuated by a low concentration of heavy metals. In addition, germination that was inhibited by high concentrations of heavy metals was also partly reversed by fluridone. The obtained results support the idea that heavy metal-mediated ROS and hormone interaction can finally affect the thermodormancy release or not.