Kochia scoparia L., a newfound candidate halophyte, for phytoremediation of cadmium-contaminated saline soils

Unravelling the halophyte Suaeda maritima as an efficient candidate for phytostabilization of cadmium and lead: Implications from physiological, ionomic, and metabolomic responses

Cadmium (Cd) and lead (Pb) are among the most toxic heavy metals affecting human health and crop yield. Suaeda maritima (L.) Dumort is an obligate halophyte that is well adapted to saline soil. The inbuilt salinity tolerance mechanisms of halophytes help them to survive in heavy metal-contaminated rhizospheric soil. In the present study, growth and ionomic responses, reactive oxygen species (ROS) accumulation, modulations of phytochelatins, antioxidative defense, and metabolomic responses were studied in S. maritima imposed to Cd and Pb stresses with an aim to elucidate Cd and Pb tolerance mechanisms and phytoremediation potential of this halophyte. Our results showed a reduction of biomass in S. maritima, which may serve as an energy conservation strategy for survival under heavy metal stress. The increased accumulation of ROS with concomitant higher expression of various antioxidative enzymes suggests the efficient scavenging of ROS. The metabolite profiling revealed significant up-regulation of sugars, sugar alcohols, amino acids, polyphenols, and organic acids under Cd and Pb stresses suggesting their possible role in osmotic balance, ionic homeostasis, ROS scavenging, and signal transduction for stress tolerance. In S. maritima, the translocation factors (Tf) are <1 in both Cd and Pb treatments, which indicates that this halophyte has high phytostabilization potential for Cd and Pb in roots and through restricted translocation of heavy metal ions to the aboveground part. The findings of this study offer comprehensive information on Cd and Pb tolerance mechanisms in S. maritima and suggest that this halophyte can detoxify the HMs through physiological, ionic, antioxidative, and metabolic regulations.

Pervasive influence of heavy metals on metabolic pathways is potentially relieved by hesperidin to enhance the phytoremediation efficiency of Bassia scoparia

Hesperidin (HSP), a flavonoid, is a potent antioxidant, metal chelator, mediator of signaling pathways, and regulator of metal uptake in plants. The study examined the ameliorative effects of HSP (100 μM) on Bassia scoparia grown under excessive levels of heavy metals (zinc (500 mg kg-1), copper (400 mg kg-1), cadmium (100 mg kg-1), and chromium (100 mg kg-1)). The study clarifies the underlying mechanisms by which HSP lessens metabolic mayhem to enhance metal stress tolerance and phytoremediation efficiency of Bassia scoparia. Plants manifested diminished growth because of a drop in chlorophyll content and nutrient acquisition, along with exacerbated deterioration of cellular membranes reflected in elevated reactive oxygen species (ROS) production, lipid peroxidation, and relative membrane permeability. Besides the colossal production of cytotoxic methylglyoxal, the activity of lipoxygenase was also higher in plants under metal toxicity. Conversely, hesperidin suppressed the production of cytotoxic ROS and methylglyoxal. Hesperidin improved oxidative defense that protected membrane integrity. Hesperidin caused a more significant accumulation of osmolytes, non-protein thiols, and phytochelatins, thereby rendering metal ions non-toxic. Hydrogen sulfide and nitric oxide endogenous levels were intricately maintained higher in plants treated with HSP. Hesperidin increased metal accumulation in Bassia scoparia and thereby had the potential to promote the reclamation of metal-contaminated soils.

Effect of brackish water irrigation on cadmium migration in a soil-maize system

Phytoremediation is an effective way to reduce heavy metal content in agricultural soil. The effects of brackish water irrigation on phytoremediation efficiency of plants have not yet been completely understood. In this study, the effects of brackish water irrigation on cadmium (Cd) uptake by maize as the phytoremediator were investigated. In a pot experiment, maize seedlings were grown in soil with exogenously added Cd (0, 5, 10, or 15 mg kg-1) and irrigated with deionized water (T1), natural brackish water (T2), or water with NaCl with salinity equal to that of natural brackish water (T3). Salt stress and cation antagonism caused by brackish water affected maize plant growth and Cd uptake. Under 5, 10, and 15 mg kg-1 Cd, Cd accumulation in maize shoots was 5.55, 7.08, and 5.71 μg plant-1; 4.08, 3.04, and 5.38 μg plant-1; and 2.48, 3.44, and 5.33 μg plant-1 under the T1, T2, and T3 treatments, respectively. Cd accumulation in the shoots was significantly lower under the T2 and T3 treatments than under the T1 treatment at 5 and 10 mg kg-1 Cd; however, no significant differences were observed among all treatments at 15 mg kg-1 Cd. These findings indicated that phytoremediation efficiency decreased in response to both salt stress and cation antagonism caused by brackish water under low soil-Cd concentrations; however, this effect was negligible under high soil-Cd concentration. Therefore, brackish water irrigation can be considered for the phytoremediation of soils contaminated with high Cd levels to save freshwater resources.

Synergistic mechanisms of bioorganic fertilizer and AMF driving rhizosphere bacterial community to improve phytoremediation efficiency of multiple HMs-contaminated saline soil

The addition of Arbuscular mycorrhizal fungi (AMF) or bioorganic fertilizer (BOF) alone has been reported to enhance plant tolerance to heavy metals and salt stress and promote plant growth, while their synergistic effects on plant growth and rhizosphere microorganism are largely unknown. This study explored the effects of AMF (Rhizophagus intraradices), BOF and BOF + RI assisted phytoremediation on heavy metals contaminated saline soil improvement and revealed the microbial mechanism. For this purpose, a pot trial consisting of four treatments (CK, RI, BOF and BOF + RI) was carried out. The results showed that the biomass, nutrient element contents, the accumulation of heavy metals and Na of Astragalus adsurgens and soil properties were most significantly improved by BOF + RI. BOF + RI significantly impacted rhizosphere microbial diversity, abundance and community composition. Chloroflexi and Patescibacteria at the phylum level and Actinomadura, Iamia, and Desulfosporosinus at the genus level were significantly enriched in BOF + RI. Network analysis revealed that BOF + RI significantly changed the keystone and enhanced complexity and interaction. Most of the keystones had roles in promoting plant growth and stress resistance. This study suggested that phytoremediation assisted by BOF and AMF is an attractive approach to ameliorate heavy metals contaminated saline soil.

Interactive Temperature and CO2 Rise, Salinity, Drought, and Bacterial Inoculation Alter the Content of Fatty Acids, Total Phenols, and Oxalates in the Edible Halophyte Salicornia ramosissima

In this work, we studied the combined effect of increased temperature and atmospheric CO₂, salt and drought stress, and inoculation with plant-growth-promoting rhizobacteria (PGPR) on the growth and some nutritional parameters of the edible halophyte Salicornia ramosissima. We found that the increase in temperature and atmospheric CO₂, combined with salt and drought stresses, led to important changes in S. ramosissima fatty acids (FA), phenols, and oxalate contents, which are compounds of great importance for human health. Our results suggest that the S. ramosissima lipid profile will change in a future climate change scenario, and that levels of oxalate and phenolic compounds may change in response to salt and drought stress. The effect of inoculation with PGPR depended on the strains used. Some strains induced the accumulation of phenols in S. ramosissima leaves at higher temperature and CO₂ while not altering FA profile but also led to an accumulation of oxalate under salt stress. In a climate change scenario, a combination of stressors (temperature, salinity, drought) and environmental conditions (atmospheric CO_2, PGPR) will lead to important changes in the nutritional profiles of edible plants. These results may open new perspectives for the nutritional and economical valorization of S. ramosissima.

Halophytes for the sustainable remediation of heavy metal-contaminated sites: Recent developments and future perspectives

Increasing land degradation by high level of metal wastes is of prime concern for the global research communities. In this respect, halophytes having specific features like salt glands, exclusion of excess ions, heavy metals (HMs) compartmentalization, large pool of antioxidants, and associations with metal-tolerant microbes are of great promise in the sustainable clean-up of contaminated sites. However, sustainable clean-up of HMs by a particular halophyte plant species is governed considerably by physico-chemical characteristics of soil and associated microbial communities. The present review has shed light on the superiority of halophytes over non-halophytes, mechanisms of metal-remediation, recent developments and future perspectives pertaining to the utilization of halophytes in management of HM-contaminated sites with the aid of bibliometric analysis. The results revealed that the research field is receiving considerable attention in the last 5-10 years by publishing ∼50-90% documents with an annual growth rate of 15.41% and citations per document of 29.72. Asian (viz., China, India, and Pakistan) and European (viz., Spain, Portugal, Belgium, Argentina) countries have been emerged as the major regions conducting and publishing extensive research on this topic. The investigations conducted both under in vitro and field conditions have reflected the inherent potential of halophyte as sustainable research tool for successfully restoring the HM-contaminated sites. The findings revealed that the microbial association with halophytes under different challenging conditions is a win-win approach for metal remediation. Therefore, exploration of new halophyte species and associated microorganisms (endophytic and rhizospheric) from different geographical locations, and identification of genes conferring tolerance and phytoremediation of metal contaminants would further advance the intervention of halophytes for sustainable ecological restoration.