This review presents a comprehensive and systematic study of the field of plant biostimulants and considers the fundamental and innovative principles underlying this technology. The elucidation of the biological basis of biostimulant function is a prerequisite for the development of science-based biostimulant industry and sound regulations governing these compounds. The task of defining the biological basis of biostimulants as a class of compounds, however, is made more complex by the diverse sources of biostimulants present in the market, which include bacteria, fungi, seaweeds, higher plants, animals and humate-containing raw materials, and the wide diversity of industrial processes utilized in their preparation. To distinguish biostimulants from the existing legislative product categories we propose the following definition of a biostimulant as “a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds.” The definition provided here is important as it emphasizes the principle that biological function can be positively modulated through application of molecules, or mixtures of molecules, for which an explicit mode of action has not been defined. Given the difficulty in determining a “mode of action” for a biostimulant, and recognizing the need for the market in biostimulants to attain legitimacy, we suggest that the focus of biostimulant research and validation should be upon proof of efficacy and safety and the determination of a broad mechanism of action, without a requirement for the determination of a specific mode of action. While there is a clear commercial imperative to rationalize biostimulants as a discrete class of products, there is also a compelling biological case for the science-based development of, and experimentation with biostimulants in the expectation that this may lead to the identification of novel biological molecules and phenomenon, pathways and processes, that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate.

The growth regulator Stifun at all concentrations tested (0.033, 0.33, 3.3, and 33 mg/L) affected the hormonal status of wheat seedlings (Triticum aestivum L., cv. Zhnitsa) and stimulated plant growth. This was evident in activation of cell division and elongation, as well as in the increase in shoot and root length, water content, and dry weight. Effects of Stifun on roots and shoots depended on concentration. Application of Stifun at the optimal growth stimulating concentration (0.033 mg/L) elevated the levels of zeatin, zeatin riboside, dihydrozeatin riboside, isopentenyladenosine, and IAA in roots of 2 day old seedlings, but reduced the ABA content. The levels of ABA, IAA, dihydrozeatin, and dihydrozeatin riboside in shoots increased, while the levels of zeatin riboside, isopentenyl adenine, and isopentenyladenosine decreased. The results indicate that the hormonal system plays a part in the plant response to growth stimulating action of Stifun.

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In the process of obtaining transgenic plants suitable for phytoremediation of heavy metal con taminated soils, a synthetic pseudophytochelatin gene coding for a phytochelatin analog Met(GluCys) 6 Gly was de novo designed and cloned. Contrary to natural enzymatically synthesized phytochelatins, this peptide can be made by a template synthesis. A construct carrying the gene in question under the control of the viral constitutive 35S promoter was made on the basis of a binary vector pCAMBIA 1305.1. This construct was used to express the pseudophytochelatin gene in the model transgenic tobacco plants (Nicotiana tabacum L.), and the above plants acquired additional resistance to various cadmium concentrations.

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Plant sugar transporters play an essential role in the organism’s productivity by carrying out carbohydrate transportation from source cells in the leaves to sink cells in the cortex. In addition, they aid in the regulation of a substantial part of the exchange of nutrients with microorganisms in the rhizosphere (bacteria and fungi), an activity essential to the formation of symbiotic relationships. This review pays special attention to carbohydrate nutrition during the development of arbuscular mycorrhiza (AM), a symbiosis of plants with fungi from the Glomeromycotina subdivision. This relationship results in the host plant receiving micronutrients from the mycosymbiont, mainly phosphorus, and the fungus receiving carbon assimilation products in return. While the efficient nutrient transport pathways in AM symbiosis are yet to be discovered, SWEET sugar transporters are one of the three key families of plant carbohydrate transporters. Specific AM symbiosis transporters can be identified among the SWEET proteins. The survey provides data on the study history, structure and localization, phylogeny and functions of the SWEET proteins. A high variability of both the SWEET proteins themselves and their functions is noted along with the fact that the same proteins may perform different functions in different plants. A special role is given to the SWEET transporters in AM development. SWEET transporters can also play a key role in abiotic stress tolerance, thus allowing plants to adapt to adverse environmental conditions. The development of knowledge about symbiotic systems will contribute to the creation of microbial preparations for use in agriculture in the Russian Federation.

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