Anna Dmitrievna Kozhevnikova

Strontium (Sr) is a wide-spread alkaline earth metal that is similar to calcium (Ca) in its chemical properties, although it does not play the physiological role inherent to the latter. At low doses, Sr can stimulate plant growth and development. However, at elevated levels in the environment, it has a negative effect on many processes occurring in plant cells, leading to growth inhibition and reduced productivity. The majority of studies have been focused on the effects of radioactive Sr isotopes which produce additional toxic effects due to ionizing radiation, but the effects of stable Sr have not received enough attention. The current scoping review summarizes the data on the uptake mechanisms and transport pathways of stable Sr and examines the characteristic patterns of its distribution in plant tissues and organs. Available data on the effects of stable Sr on enzyme activity, mineral nutrition, photosynthesis, plant growth, and morphogenesis are analyzed. Understanding the transport pathways of Sr, as well as the factors determining its dual effects on plant growth and development, underlies the development of approaches used in biofortification and phytoremediation technologies. The review may be useful for researchers working on plant mineral nutrition, metal stress, and phytoremediation.

Nickel (Ni) is a ubiquitous trace metal and an essential ultra micronutrient. It is a constituent of several enzymes and is currently known to be involved in a number of physiological processes in plants, including nitrogen metabolism, growth and seed germination. However, at elevated concentrations, Ni exerts multiple toxic effects on various physiological processes, which leads to impaired plant growth and morphogenesis, and reduced productivity. This review summarizes the current knowledge of the mechanisms of Ni toxicity, analyzing its effects on enzyme activity, membrane structure and functioning, antioxidant and glyoxalase systems, mineral nutrition, water regime, photosynthesis, plant growth and morphogenesis. Nickel effects on excluder plants, which accumulate Ni primarily in their roots, and hyperaccumulator plants, which accumulate Ni primarily in their shoots, are compared. Studying the mechanisms of Ni toxicity may substantially contribute to an integral modern approach to investigating plants under constantly changing environmental conditions. It is essential for understanding the full spectrum of physiological and biochemical reactions in plants that are involved in plant tolerance to Ni. Furthermore, understanding of the mechanisms of Ni growth inhibitory effects is important for the enhancement of agricultural techniques aimed at increasing crop productivity worldwide. It is also crucial for the development of approaches used in phytoremediation and phytomining technologies.

Some metals and metalloids such as copper (Cu), manganese (Mn), nickel (Ni), zinc (Zn), iron (Fe), boron (B), and molybdenum (Mo) are essential for most plants but toxic when supplied in supraoptimal quantities; meanwhile, the biological role of cadmium (Cd), mercury (Hg), lead (Pb), and arsenic (As), with rare exceptions, is unknown, and they are toxic for plants even at fairly low concentrations

Low-molecular-weight organic acids (LMWOAs) are essential O-containing metal-binding ligands involved in maintaining metal homeostasis, various metabolic processes, and plant responses to biotic and abiotic stress. Malate, citrate, and oxalate play a crucial role in metal detoxification and transport throughout the plant. This review provides a comparative analysis of the accumulation of LMWOAs in excluders, which store metals mainly in roots, and hyperaccumulators, which accumulate metals mainly in shoots. Modern concepts of the mechanisms of LMWOA secretion by the roots of excluders and hyperaccumulators are summarized, and the formation of various metal complexes with LMWOAs in the vacuole and conducting tissues, playing an important role in the mechanisms of metal detoxification and transport, is discussed. Molecular mechanisms of transport of LMWOAs and their complexes with metals across cell membranes are reviewed. It is discussed whether different endogenous levels of LMWOAs in plants determine their metal tolerance. While playing an important role in maintaining metal homeostasis, LMWOAs apparently make a minor contribution to the mechanisms of metal hyperaccumulation, which is associated mainly with root exudates increasing metal bioavailability and enhanced xylem loading of LMWOAs. The studies of metal-binding compounds may also contribute to the development of approaches used in biofortification, phytoremediation, and phytomining.

The effects of high-intensity light on the pigment content, photosynthetic rate, and fluorescence parameters of photosystem II in high-pigment tomato mutants (hp 3005) and low-pigment mutants (lp 3617) were investigated. This study also evaluated the dry weight percentage of low molecular weight antioxidant capacity, expression patterns of some photoreceptor-regulated genes, and structural aspects of leaf mesophyll cells. The 3005 mutant displayed increased levels of photosynthetic pigments and anthocyanins, whereas the 3617 mutant demonstrated a heightened content of ultraviolet-absorbing pigments. The photosynthetic rate, photosystem II activity, antioxidant capacity, and carotenoid content were most pronounced in the high-pigment mutant after 72 h exposure to intense light. This mutant also exhibited an increase in leaf thickness and water content when exposed to high-intensity light, suggesting superior physiological adaptability and reduced photoinhibition. Our findings indicate that the enhanced adaptability of the high-pigment mutant might be attributed to increased flavonoid and carotenoid contents, leading to augmented expression of key genes associated with pigment synthesis and light regulation.

Nicotianamine (NA) is a low-molecular-weight N-containing metal-binding ligand, whose accumulation in plant organs changes under metal deficiency or excess. Although NA biosynthesis can be induced in vivo by various metals, this non-proteinogenic amino acid is mainly involved in the detoxification and transport of iron, zinc, nickel, copper and manganese. This review summarizes the current knowledge on NA biosynthesis and its regulation, considers the mechanisms of NA secretion by plant roots, as well as the mechanisms of intracellular transport of NA and its complexes with metals, and its role in radial and long-distance metal transport. Its role in metal tolerance is also discussed. The NA contents in excluders, storing metals primarily in roots, and in hyperaccumulators, accumulating metals mainly in shoots, are compared. The available data suggest that NA plays an important role in maintaining metal homeostasis and hyperaccumulation mechanisms. The study of metal-binding compounds is of interdisciplinary significance, not only regarding their effects on metal toxicity in plants, but also in connection with the development of biofortification approaches to increase the metal contents, primarily of iron and zinc, in agricultural plants, since the deficiency of these elements in food crops seriously affects human health.

Phytochelatins (PCs) are small cysteine-rich peptides capable of binding metal(loid)s via SH-groups. Although the biosynthesis of PCs can be induced in vivo by various metal(loid)s, PCs are mainly involved in the detoxification of cadmium and arsenic (III), as well as mercury, zinc, lead, and copper ions, which have high affinities for S-containing ligands. The present review provides a comprehensive account of the recent data on PC biosynthesis, structure, and role in metal(loid) transport and sequestration in the vacuoles of plant cells. A comparative analysis of PC accumulation in hyperaccumulator plants, which accumulate metal(loid)s in their shoots, and in the excluders, which accumulate metal(loid)s in their roots, investigates the question of whether the endogenous PC concentration determines a plant’s tolerance to metal(loid)s. Summarizing the available data, it can be concluded that PCs are not involved in metal(loid) hyperaccumulation machinery, though they play a key role in metal(loid) homeostasis. Unraveling the physiological role of metal(loid)-binding ligands is a fundamental problem of modern molecular biology, plant physiology, ionomics, and toxicology, and is important for the development of technologies used in phytoremediation, biofortification, and phytomining.