Assessment of phytoremediation capacity of three halophytes: Suaeda monoica, Tamarix indica and Cressa critica

Salicylic acid foliar application meliorates Portulaca oleraceae L. growth responses under Pb and Ni over-availability while keeping reliable phytoremediation potential

The efficacy of SA foliar use on Pb and Ni-induced stress tolerance and phytoremediation potential by Portulaca oleraceae L. were assayed as a factorial trial based on a completely randomized design with four repetitions. The factors included; SA foliar application (0 and 100 µM) and HMs application of Pb [0, 150, and 225 mg kg-1 soil Lead (II) nitrate] and Ni [0, 220, and 330 mg kg-1 soil Nickel (II) nitrate]. Plant height, stem diameter, shoot and root fresh and dry weight, photosynthetic pigments, total soluble proteins, palmitic acid, stearic acid, arachidic acid, and some macro- and micro-elements contents were reduced facing the HMs stress, but SA foliar application ameliorated these traits. HMs stress increased malondialdehyde content, total antioxidant activity, total flavonoids, phenolics, and linolenic acid content, while SA foliar application declined the mentioned parameters. Moreover, shoot and root Pb and Ni content enhanced in the purslane plants supplemented by SA under the HMs stress. The results propose SA foliar application as a reliable methodology to recover purslane growth characters and fatty acid profiles in the soil contaminated with the HMs. The idea is that SA would be potentially effective in alleviating HMs contamination while keeping reasonable phytoremediation potential.

The complete chloroplast genome of the halophyte flowering plant Suaeda monoica from Jeddah, Saudi Arabia

The complete chloroplast genome (plastome) of the annual flowering halophyte herb Suaeda monoica Forssk. ex J. F. Gmel. family (Amaranthaceae) that grows in Jeddah, Saudi Arabia, was identified for the first time in this study. Suaeda monoica is a medicinal plant species whose taxonomic classification remains controversial. Further, studying the species is useful for current conservation and management efforts. In the current study, the full chloroplast genome S. monoica was reassembled using whole-genome next-generation sequencing and compared with the previously published chloroplast genomes of Suaeda species. The chloroplast genome size of Suaeda monoica was 151,789 bp, with a single large copy of 83,404 bp, a small single copy of 18,007 bp and two inverted repeats regions of 25,189 bp. GC content in the whole genome was 36.4%. The cp genome included 87 genes that coded for proteins, 37 genes coding for tRNA, 8 genes coding for rRNA and one non-coding pseudogene. Five chloroplast genome features were compared between S. monoica and S. japonica, S. glauca, S. salsa, S. malacosperma and S. physophora. Among Suaeda genus and equal to most angiosperms chloroplast genomes, the RSCU values were conservative. Two pseudogenes (accD and ycf1), rpl16 intron and ndhF-rpl32 intergenic spacer, were highlighted as suitable DNA barcodes for different Suaeda species. Phylogenetic analyses show Suaeda cluster into three main groups; one in which S. monoica was closer to S. salsa. The obtained result provided valuable information on the characteristics of the S. monoica chloroplast genome and the phylogenetic relationships.

Microbial community assembly and functional profiles along the soil-root continuum of salt-tolerant Suaeda glauca and Suaeda salsa

Developing and planting salt-tolerant plants has become a promising way to utilize saline-alkali land resources and ensure food security. Root-associated microbes of salt-tolerant plants have been shown to promote plant growth and alleviate high salt stress, yet very little is known about the salt resistance mechanisms of core microbes in different niches. This study characterized the microbial community structures, assembly processes, and functional profiles in four root-related compartments of two salt-tolerant plants by amplicon and shotgun metagenomic sequencing. The results showed that both plants significantly altered the microbial community structure of saline soils, with greater microbial alpha diversity in the rhizosphere or rhizoplane compared with bulk soils. Stochastic process dominated the microbial assembly processes, and the impact was stronger in Suaeda salsa than in S. glauca, indicating that S. salsa may have stronger resistance abilities to changing soil properties. Keystone species, such as Pseudomonas in the endosphere of S. glauca and Sphingomonas in the endosphere of S. salsa, which may play key roles in helping plants alleviate salt stress, were identified by using microbial co-occurrence network analysis. Furthermore, the microbiomes in the rhizoplane soils had more abundant genes involved in promoting growth of plants and defending against salt stress than those in bulk soils, especially in salt-tolerant S. salsa. Moreover, microbes in the rhizoplane of S. salsa exhibited higher functional diversities, with notable enrichment of genes involved in carbon fixation, dissimilar nitrate reduction to ammonium, and sulfite oxidation. These findings revealed differences and similarities in the microbial community assembly, functional profiles and keystone species closely related to salt alleviation of the two salt-tolerant plants. Overall, our study provides new insights into the ecological functions and varied strategies of rhizosphere microbes in different plants under salt stress and highlights the potential use of keystone microbes for enhancing salt resistance of plants.

Evaluation of the potential of two halophytes to extract Cd and Zn from contaminated saltwater

The contamination of saltwater by toxic heavy metals has become a worldwide problem. The application of phytoextraction to remove these pollutants seems to be more efficient and cheaper compared to physicochemical methods. In this work, we evaluated the potential of two halophyte species to accumulate cadmium and zinc from contaminated water. Seedlings of Carpobrotus edulis L. and Sesuvium portulacastrum L. were cultivated during 1 month on pots filled with saltwater (200 mM NaCl) containing different concentrations of Cd2+ (0, 50, 100 μM) and of Zn2+ (0, 200, and 400 μM) applied separately. Results showed that both halophytes were more resistant to Zn2+ than Cd2+ and that Sesuvium better tolerates the two metals. Zn2+ and Cd2+ concentrations in the shoot as well as the values of translocation factors suggest that these species are able to absorb and to concentrate Cd2+ and Zn2+ in their roots and shoots. Hence, after 1 month of culture on 50 μM Cd2+, plants were able to extract 31% and 21% of Cd, respectively, in S. portulacastrum and C. edulis. The Zn-extraction efficiency was less important and reached 18 and 19%, respectively, in S. portulacastrum and C. edulis cultivated under 200 μM Zn2+. Given together, data demonstrate the efficiency of the use of halophytes, especially S. portulacastrum, to extract Zn2+ and Cd2+ from salt wastewater.

Comparative transcriptomic and metabolomic analyses provide insights into the responses to NaCl and Cd stress in Tamarix hispida

Salinity and heavy metal pollution seriously affect plant growth. Tamarix hispida (T. hispida) has the potential to remediate soil saline-alkali and heavy metal pollution. In this study, the response mechanisms of T. hispida under NaCl, CdCl₂ (Cd) and combined CdCl₂ and NaCl (Cd-NaCl) stresses were explored. Overall, the antioxidant system showed changes under the three stresses. The addition of NaCl inhibited the absorption of Cd²⁺. However, there were obvious differences in the transcripts and metabolites identified among the three stress responses. Interestingly, the number of DEGs was greatest under NaCl stress (929), but the number of differentially expressed metabolites (DEMs) was lowest (48), with 143 and 187 DEMs identified under Cd and Cd-NaCl stress, respectively. It is worth noting that both DEGs and DEMs were enriched in the linoleic acid metabolism pathway under Cd stress. In particular, the content of lipids changed significantly under Cd and Cd-NaCl stress, suggesting that maintaining normal lipid synthesis and metabolism may be an important way to improve the Cd tolerance of T. hispida. Flavonoids may also play an important role in the response to NaCl and Cd stress. These results provide a theoretical basis for cultivating plants with improved salt and cadmium repair abilities.

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.

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

In recent years, heavy metal pollution in saline soil is increasingly severe due to the rapid development of industry and agriculture. Halophytes can survive at higher concentrations of salt and heavy metal, which make them suitable candidates for the phytoremediation of heavy metals in saline soils. In the present study, the halophyte plant Kochia scoparia (L.) Schrad. seedlings were exposed to different doses of Cd (0, 5, 10, 30 mg/kg) and NaCl (0, 200, 400, 800 mM) to explore its tolerance and phytoremediation ability for Cd. There was no significant toxic effect of Cd on the K. scoparia seedlings. NaCl reduced the biomass of K. scoparia compared with the control, but did not show any visible toxic symptom. Furthermore, Cd accumulation in K. scoparia is mainly distributed in the shoot; especially when exposed to low-Cd (5 mg/kg) treatment, the accumulation of Cd in the shoots was up to 5.42-22.25 mg/kg, which was 3.18-53.4 times of that in the roots. Moreover, the contents of glutathione and oxalate in plants increased gradually with the increase of NaCl concentration. Under the treatment of 800 mM NaCl without Cd, the content of glutathione reached the highest 51.21 μg/g, and the proportion of oxalate reached the highest 28.76% under the treatment of 30 mg/kg Cd with 400 Mm NaCl. Finally, we also found the significant alterations of cadmium chemical forms in rhizosphere soil with the addition of NaCl. Overall, K. scoparia could be an efficient and valuable candidate for the phytoextraction of low-Cd (5 mg/kg)-contaminated saline soil.

Nanofertilizer Possibilities for Healthy Soil, Water, and Food in Future: An Overview

Conventional fertilizers and pesticides are not sustainable for multiple reasons, including high delivery and usage inefficiency, considerable energy, and water inputs with adverse impact on the agroecosystem. Achieving and maintaining optimal food security is a global task that initiates agricultural approaches to be revolutionized effectively on time, as adversities in climate change, population growth, and loss of arable land may increase. Recent approaches based on nanotechnology may improve in vivo nutrient delivery to ensure the distribution of nutrients precisely, as nanoengineered particles may improve crop growth and productivity. The underlying mechanistic processes are yet to be unlayered because in coming years, the major task may be to develop novel and efficient nutrient uses in agriculture with nutrient use efficiency (NUE) to acquire optimal crop yield with ecological biodiversity, sustainable agricultural production, and agricultural socio-economy. This study highlights the potential of nanofertilizers in agricultural crops for improved plant performance productivity in case subjected to abiotic stress conditions.

Halophytes for phytoremediation of hazardous metal(loid)s: A terse review on metal tolerance, bio-indication and hyperaccumulation

Phytoremediation is a cost-effective and environment friendly method for cleaning metal(loid)s from contaminated soils. Species with exceptionally higher shoot metal concentrations (hyperaccumulators) seem ideal for phytoremediation, though some metal tolerant species with 'above normal' values with higher translocation factor (TF) may also serve the purpose. Halophytes not only remove salts and metalloids from soils but may also be cultivated as non-conventional crops. Nurturing halophytes requires precise understanding of their nature and efficient management for sustainable use. Species with low metal concentrations in their edible parts (especially leaves) may be grown as forage and fodder, but those with metal hyperaccumulation could prove fatal due to their serious health hazards. Like other metallophytes, redundant use of the term 'metal hyperaccumulation' among halophytes needs to be revisited for its ambiguity and potential pitfalls. Similarly, understanding of metal tolerance and shoot accumulation nature of halophytes is needed prior to their use. This review is an attempt to compare halophytes with potential of metal bioindication, phytostabilization and hyperaccumulation (as per definition) as well as their 'obligate' and 'facultative' nature for appropriate uses.