Physiological mechanisms of adaptation to copper-induced stress in two widespread legume plants, white sweet clover (Melilotus albus Merik.) and zigzag clover (Trifolium medium L.), growing in habitats differing in the man-made pollution. An antioxidant plant defense system was activated in response to 10 mM CuSO₄, which is a stress factor. Specific biochemical features related to adaptation to soil contamination with copper were observed in tested plant species. Superoxide dismutase was activated in response to stress in both species from various habitats. M. albus from the impact zone manifested the better capacity of proline accumulation as compared with plants from less polluted habitats. T. medium plants from the impact zone contained more active peroxidase. It was suggested that plants growing for a long time under stressful conditions manifest the greater tolerance to copper ions than plants, which did not experience stress or were subjected to the milder stress.

Zinnia elegans Jacq. is an ornamental plant, widely used in landscaping. Heavy-metal pollution in urban and rural areas is still increasing, which determines the actuality of studying plants’ reactions to pollutants. Zinnia was not sufficiently studied in this regard, so the aim of our research was to identify morphophysiological changes in this species under excess copper concentration in the soil. For this, we treated a growth substrate with 200 µM CuSO₄ solution for 20 days. At the end of the treatment, several morphological, biochemical, and molecular genetic traits were evaluated: the root and the shoot size; the concentration of H₂O₂ and malondialdehyde (MDA), as indicators of stress; the amount of the phenolic compounds and lignin; and the level of the expression of genes, which encoded their biosynthesis. The Cu amount in the substrate and zinnia organs was quantified using atomic-absorption spectroscopy; hydrogen peroxide, MDA, and phenolic compounds were determined spectrophotometrically, while the amount of lignin was determined according to Klason. Real-time PCR was used for estimation of the gene-transcription level. Lignin in tissues was visualized by fluorescent microscopy. In experimental plants, Cu accumulation was higher in the root than in the stem. This caused an increase in stress markers and a decrease in the root and stem lengths. For the first time for zinnia, it was shown that for several genes—4-coumarate-CoA ligase (4CL), cinnamoyl alcohol dehydrogenase (CAD), and class III peroxidase (PRX)—the level of expression increased under copper treatment. The rise of the transcripts’ amount of these genes was accompanied by a thickening and lignification of the cell walls in the metaxylem vessels. Thus, the adaptation of zinnia to the excess Cu in the growth medium was associated with the metabolic changes in the phenylpropanoid pathway. As a result, the lignification increased in the root, which led to the accumulation of Cu in this organ and limited its translocation through the xylem to the stem, which provided plant growth.

Metal tolerant plant growth-promoting (PGP) rhizobacteria are promising for enhancing plant productivity under copper (Cu) stress. Present pot scale experiment was conducted on Brassica napus L. to check the efficiency of rhizobacteria isolated from the rhizosphere of Tussilago farfara L. growing on Cu-contaminated soils. Out of fifty Cu tolerant strains, three isolates which showed multiple PGP traits such as indole-3-acetic acid (IAA) synthesis, phosphate (PS) solubilization, siderophore and ammonia production were identified preliminarily by morphological and physiological characteristics followed by 16S rRNA gene sequencing. The best Bacillus altitudinis strain TF16a which showed IAA: 15.5 mg L⁻¹, PS: 215 mg L⁻¹, siderophore halo zone ratio of 3.0 with high ammonia production was selected to prepare a biochar-based biofertilizer (BF). Seedling test showed maximum growth of B. napus shoot and root in presence of 5% of BF and this concentration was selected for further experiment. The pot experiment included four treatments: control (soil), 100Cu (100 mg Cu kg⁻¹ soil), 5%BF (v/v), and 5%BF+100Cu, which were carried out for 30 days, after which the morphological, physiological, and biochemical parameters of B. napus were studied. The Cu treatment caused its accumulation in shoot and root up to 16.9 and 30.4 mg kg⁻¹ DW, respectively, and increased malondialdehyde (MDA) content by 20%. Application of BF with copper led to the decrease in the Cu accumulation by 20% for shoot and 28% for root while MDA content was the same as in the control. Both treatments of BF with and without Cu increased chlorophyll a and b content by 1.3 times on average as well as non-enzymatic antioxidants such as soluble phenolic compounds (1.3 times) and free proline (1.6 times). Moreover, BF + Cu led to the increase in the biomass of shoot and root by 30 and 60%, respectively, while there was no significant effect on the growth characteristics of plants after the addition of BF without Cu. The study elucidates that BF based on B.altitudinis strain TF16a and biochar can be a promising bioformulation which could increase rapeseed growth under the moderate Cu concentration in soil.

The study of the metagenomes of bacterial communities in saline areas is relevant in connection with the global salinization of agricultural lands. The aim of this study was to investigate the biodiversity and structure of rhizobacterial communities associated with the halophyte S. marina from low and moderate sulfate–chloride salinity habitats. The bacterial community of bulk and rhizosphere soil was analyzed using high-throughput sequencing of the V1–V9 region of 16S rRNA by Oxford Nanopore Technologies. Alpha and beta diversity indices were calculated. A total of 55 phyla and 309 genera of bacteria were identified, among which Proteobacteria and Bacteroidetes dominated. The occurrence of Planctomycetes, Verrucomicrobia, and Acidobacteria in the rhizosphere was higher than in the bulk soil. Bacterial alpha diversity in the bulk soil decreased with increasing salinity, while it increased in the rhizosphere. The proportion of the halotolerant bacteria of Flavobacterium and Alteromonas genera significantly grew with increasing salinity both in the bulk and rhizosphere soil. In addition, in the rhizosphere, the percentage of Comamonas, Methylibium, Lysobacter, Planctomyces, Sphingomonas, Stenotrophomonas, and Lewinella genera increased. Among them, several genera included plant growth promoting rhizobacteria (PGPR). In the more saline bulk soil, the proportion of halotolerant genera Bacillus, Salinimicrobium, Marinobacter, Clostridium, Euzebya, KSA1, Marinobacter, Clostridium, Salinimicrobium, and Halorhodospira was also higher compared to the low saline site. Thus, increasing the salinity changed the taxonomic structure of the bacterial communities of both bulk soil and rhizosphere.