Guzel Kudoyarova

High planting density is an effective strategy for increasing crop yields per unit of sown area. However, increasing the number of plants per unit area inhibits the growth of individual plants. The aim of the present work was to identify the importance of hormones in the growth response of wheat plants to the presence of neighbors and to assess possibility of reducing this growth-inhibitory effect using rhizosphere bacteria strains (Pseudomonas plecoglossicida 2,4-D and Pseudomonas veronii IB K11-1). We found that increased sowing density of wheat plants from 6 to 20 per pot elevated the level of abscisic acid (ABA) both in plants and soil solution bringing about stomatal closure and inhibition of their growth. Introduction of bacteria capable of catabolyzing ABA reduced the content of this hormone both in soil solution and plants, which led to a decrease in the degree of inhibition of wheat growth with increasing sowing density. High planting density reduced indolyl acetic acid (IAA) content in the roots accompanied by inhibition of root growth. Bacterial strains capable of producing IAA increased its concentration in the roots and their growth rate. The obtained results confirm the hypothesis about the ability of rhizosphere bacteria that destroy ABA and synthesize IAA to reduce the inhibitory effect of dense planting on plant growth.

Plant growth-promoting rhizobacteria increase leaf size. This causes plants to lose more water, requiring them to develop mechanisms to mitigate the effects of possible water deficit. Research has revealed a variety of mechanisms for adapting the water balance of these plants. Understanding the causes of these differences is crucial for the successful application of bacteria in agriculture. This mini-review discusses the effects of PGPR on stomatal conductance, transpiration, relative leaf water content and water potential, root growth, and hydraulic conductivity. We evaluate the importance of these parameters for maintaining water balance and for stimulating plant growth under the influence of bacterial inoculation. We also analyze the role of modifications of hormonal balance influenced by PGPR in regulating these processes. PGPR have varying effects on plant water balance. There is evidence of both increases and decreases in stomatal conductance and relative water content under the influence of bacteria. The effectiveness of these alternative responses is discussed in the context of soil water availability. Plant responses to PGPR can be classified by the following traits: 1) stomatal closure and reduced transpiration; 2) increased water uptake capacity by stimulating root growth to compensate for increased transpiration; 3) increased root water channel activity, leading to greater hydraulic conductivity; and 4) osmotic adaptation, which helps maintain leaf turgor during reduced leaf water content.

Lignin deposition plays a key role in the formation of Casparian bands, which act as a barrier to the uncontrolled movement of ions and water across the apoplast. Salinity, flooding, drought, and the presence of heavy metal ions in the soil increase lignification. This protective response increases plant resilience by reducing both the influx of toxic ions and the loss of oxygen by roots during flooding and loss of water - during its transport to the shoot through the upper soil layers, which dry out during drought. However, it can have negative consequences for plant growth and development due to reduced hydraulic conductivity resulting from the blockage of the apoplast water pathway. This work was aimed at analyzing data on how plants can solve the problem of maintaining hydraulic conductivity in the context of increased lignin deposition and the resulting limitation of water movement through the apoplast. The reduction in hydraulic conductivity can be avoided by increasing the level and activity of water channels aquaporins. Abcisic acid may play an important role in regulating both lignin deposition and the compensating increase in aquaporin levels.

One important mechanism underlying the promotive action of rhizosphere bacteria on plants is their ability to produce plant hormones, particularly cytokinins. Cytokinin must be delivered from the roots to stimulate plant growth and exert beneficial effects on the shoots. This work aimed to study the transport of bacterial cytokinins in barley plants. We studied the effects of cytokinin-producing bacteria on the growth, cytokinin content, and expression of the HvABCG gene, a homolog of the rice OsABCG18 and Arabidopsis AtABCG14 genes encoding transporters of these hormones. Rhizosphere of 3-days-old barley seedlings were inoculated with Bacillus subtilis IB-22, and cytokinins were immunoassayed on the sixth day after bacterial treatment. On day 11, leaf growth stimulation was detected under the bacterial treatment, indicating that bacterial cytokinins affect shoot performance. This hypothesis was confirmed by a 1.5 increase in zeatin riboside levels registered in the shoots of bacteria-treated plants. Estimating the expression of the HvABCG gene showed its upregulation in the roots of bacteria-treated plants, suggesting the involvement of this transporter in the delivery of cytokinins to the shoots.

Silver maple is a fast-growing, adaptable tree that often frequents wet places and thus can play an important ecological role in replanting schemes. For this, robust, high-quality seedlings are essential. In other tree species, improved seedling quality has been achieved by treating with a combination of humic substances (HSs) and bacterial strains capable of synthesizing auxin phytohormone; the benefit being attributed, without clear supporting evidence, to changes in phytohormone concentrations in the plant. To clarify the uncertainty, we conducted assays of hormones in silver maple seedlings treated with HSs and appropriate bacteria. We hypothesized that any positive additive effects between HSs and bacteria may be due to the ability of HSs to bind phytohormones. This hypothesis was tested and confirmed by using optical absorption spectra of auxins, humic acids, and their combination, as well as by modeling their interactions. The combination of humic substances and bacteria resulted in an approximately 1.5-fold increase in auxin content in roots, accompanied by a marked increase in root weight and length. We suggest this is likely the outcome of HSs binding to bacterial auxins and delivering them to plant roots. Concentrations of cytokinins and abscisic acid also changed under these treatments, which may help explain observed increases in photosynthesis and improved water balance.

Plants of the genus Populus are among the most economically important woody plants and are an experimental model system used to study woody species. We studied the ability of plant growth-promoting bacteria (PGPR) and humic substances (HSs) to influence the rooting of cuttings, a process that plays an important role in vegetative propagation. We used strains of bacteria from the collection of microorganisms of the Ufa Institute of Biology: Pseudomonas protegens DA1.2 and Enterobacter ludwigii BLK. For the experiment, cuttings of black poplar (Pópulus nigra L.) were used. They were placed in aqueous solution with either the addition of bacteria, HSs, or their combination. After 15 days, the number and length of adventitious roots were measured, and they were sampled for hormone immunoassay analysis and the determination of gene expression. The contents of the auxin indole-3-acetic acid (IAA) and abscisic acid (ABA) were measured, since both substances have an antagonistic effect on rooting. It was shown that the activation of root formation was due, on the one hand, to increased auxin levels under the influence of IAA-producing bacteria, resulting in increased expression of the PnRGF9 gene that encodes the root meristem growth factor. On the other hand, the activation of rooting was due to a decrease in ABA content resulting from its breakdown by bacteria of the DA1.2 strain, which catabolize this hormone. HSs, which can inhibit ABA synthesis, also played a role. As a result, the preparation for the rooting of black poplar cuttings with the greatest effectiveness was observed in the combined use of DA1.2 bacteria and HSs.

To perform their numerous functions in plants, hydrophobic substances, such as lipids and some hormones, must be transferred from sites of synthesis to their destination through an aqueous environment. This process is facilitated by proteins possessing a cavity lined with non-polar radicals that bind and retain hydrophobic compounds. These proteins are the Lipid Binding and Transfer Proteins (LBTPs). Although much is known about the structure and ligand affinity of animal LBTPs this review summarizes evidence that some classes of plant LBTPs are functionally homologous to those of animals. We describe how LBTPs contribute to building the functional structure of lipids as well as to signaling by lipids and plant hormones including the long-distance transport. We also consider LBTP involvement in the deposition of suberin and cutin, which protects plants from pathogen and abiotic stresses amongst other processes. Most proposed functions of LBTPs are based on their ability to bind various hydrophobic substance in vitro, on experimentally established co-localizations of LBTPs and their putative ligands in the cells and on the isolation from plants of complexes of these proteins with hydrophobic ligands. Although many aspects of plant LBTPs action remain unclear the present review highlights the hitherto widely overlooked significance of these plant proteins and a pressing need for their further study.

The role of ethylene in the adaptation of Arabidopsis thaliana to salt stress induced by 150 mM NaCl is investigated. The responses of wild-type (Columbia, WT) plants and ethylene-insensitive etr1-1 mutants to short-term daily salt treatments were compared. Parameters analyzed included growth, water status, chlorophyll content, and hormone levels (ABA, IAA, cytokinins) using ELISA and immunohistochemistry. The results revealed that in the WT, salt stress induced hormonal redistribution: accumulation of ABA, IAA, and zeatin in shoots, accompanied by decreased ABA in the root tips and cytokinins in the whole roots. These hormonal changes were associated with stomatal closure, maintained leaf hydration, and inhibition of root growth. The inhibition of root growth may contribute to reduced uptake of toxic ions from the environment. In contrast, etr1-1 mutants exhibited no changes in hormonal status, failed to close stomata—leading to decreased leaf water content—and showed a sharp decline in chlorophyll content accompanied by suppressed shoot growth. The conclusions emphasize that ethylene sensitivity is essential for initiating adaptive hormonal rearrangements that coordinate growth and stomatal responses to mitigate the effects of salt stress.

Drought and salinity are the main environmental factors reducing plant growth, productivity and crop yields