Little information is available on how rhizosphere bacteria affect abscisic acid (ABA) levels in plants and whether these bacterial effects are associated with improved plant water status. In this study, we tested the hypothesis that the stimulation of plant growth may be associated with the ability of ABA to increase the hydraulic conductivity of roots through the up-regulation of aquaporin. To do this, we studied the effect of bacteria capable of producing ABA on a barley mutant deficient in this hormone. Measurements of hydraulic conductivity of the ABA-deficient barley mutant Az34 showed that its tissues exhibited a reduced ability to conduct water, which correlated with lower ABA content in plants. The inoculation of Bacillus subtilis IB-22 stimulated the growth of both the mutant and its parent variety. Also, under the influence of bacteria, the ABA content in plants increased, and the increase was more significant in the mutant. This effect was accompanied by an increase in hydraulic conductivity in the roots of the ABA-deficient mutant, and immunolocalization using antibodies against PIP2;1 and PIP2;2 aquaporins revealed an increase in their abundance. Thus, the results obtained support the hypothesis about the importance of a sufficiently high ABA content in plants to maintain the abundance of aquaporins, hydraulic conductivity and the growth of barley plants.

Light is vital for any plant, so there are many mechanisms for adapting to its deficiency. For example, when shading by other plants, the stems are elongated due to the resources going to other organs, in particular, roots, the branching of which may decrease. The decrease in root branching under the influence of low light is explained by a decrease in auxin transport (IAA) from shoots to roots. However, data on the effect of low light on auxins is rather contradictory. The data obtained in this work indicate a decrease in the concentration of auxins in the roots and phloem exudate of plants at low light intensity, which indicates the role of auxins in the transmission of a signal from the shoot about this external factor and its effect on the level of auxins in roots. The root growth response to low light was manifested as a decrease in branching, and since auxins are known to be required to induce lateral root growth, this low light effect appears to be related to the long-range signal from the leaves in the form of reduced delivery of auxins through the phloem.

On March 3, 2024, sad news spread among Russian plant physiologists: an outstanding scientist and wonderful person, Farida Minnikhanovna Shakirova, died. Friends and colleagues exchanged letters in an attempt to honor her memory. Members of Editorial board of the journal “Ecobiotech”, created by colleagues F.M. Shakirova, decided to write about her scientific activities and her amazing qualities: kindness, dedication to science, efficiency, talent as a researcher, willingness to share her experience with colleagues. We would like young colleagues who did not have the opportunity to know her personally to learn from her example.

Increased auxin levels in root primordia are important in controlling root branching, while their interaction with abscisic acid (ABA) likely regulates lateral root development in water-deficient plants. The role of ABA accumulation in regulating root branching was investigated using immunolocalization to detect auxin (indoleacetic acid, IAA) and ABA (abscisic acid) in root primordia of the ABA-deficient barley mutant Az34 and its parental genotype (cv. Steptoe) barley plants. Osmotic stress strongly inhibited lateral root branching in Steptoe plants, but hardly affected Az34. Root primordial cells of Steptoe plants had increased immunostaining for ABA but diminished staining for IAA. ABA did not accumulate in root primordia of the Az34, and IAA levels and distribution were unaltered. Treating Az34 plants with exogenous ABA decreased root IAA concentration, while increasing root primordial ABA accumulation and decreasing root primordial IAA concentration. Although ABA treatment of Az34 plants increased the root primordial number, it decreased the number of visible emerged lateral roots. These effects were qualitatively similar to that of osmotic stress on the number of lateral root primordia and emerged lateral roots in Steptoe. Thus ABA accumulation (and its crosstalk with auxin) in root primordia seems important in regulating lateral root branching in response to water stress.

The presence of PGP bacteria in the rhizosphere has a positive effect on plant growth and increases their productivity both under favorable and stressful conditions. Despite the fact that much attention is paid to the various mechanisms of action of growth-promoting bacteria on plants, the aspect of the effect of bacteria on changes in the formation of apoplastic barriers and their effect on water transport remains almost unexplored. Previously, it was shown that Pseudomonas mandelii IB-Ki14 accelerated the formation of Casparian bands in wheat plants, although the involvement of LTPs (Lipid Transfer Proteins) in this process has not been shown. The aim of this study was to elucidate the effect of bacterial inoculation on LTP synthesis in the roots of the pea (Pisum sativum) and its relation to the formation of apoplastic barriers and hydraulic conductance. We aimed to test the hypothesis that the decrease in hydraulic conductance due to increased formation of apoplastic barriers can be compensated by increased activity of aquaporin water channels. Activation of genes encoding PsLTP and an increase in the amount of lipid-transporting proteins in pea root phloem induced by P. mandelii IB-Ki14 treatment were found. Inoculation was accompanied by an increase in suberin deposition in the Casparian bands, while hydraulic conductivity did not decrease. The use of Fenton's reagent, which is an inhibitor of aquaporins, showed that the hydraulic conductivity of plants treated with bacteria decreased to a greater extent compared to the control group, which indicates an increase in the activity of aquaporins under the influence of bacteria. We conclude that P. mandelii IB-Ki14 stimulates the deposition of suberin, the biosynthesis of which involves lipid-transporting proteins, and increases the activity of aquaporins, which, in turn, prevents a possible decrease in hydraulic conductivity due to the formation of apoplastic barriers in pea roots.

Lipid transfer proteins (LTPs) are known to be involved in suberin deposition in the Casparian bands of pea roots, thereby reinforcing apoplast barriers. Moreover, the Pseudomonas mandelii IB-Ki14 strain accelerated formation of the Casparian bands in wheat plants, although involvement of LTPs in the process was not studied. Here, we investigated the effects of P. mandelii IB-Ki14 on LTPs, formation of the Casparian bands, hydraulic conductance and activity of aquaporins (AQPs) in pea plants. RT PCR showed a 1.6-1.9-fold up-regulation of the PsLTP-coding genes and an increase in the abundance of LTP proteins in the phloem of pea roots induced by the treatment with P. mandelii IB-Ki14. The treatment was accompanied with increased deposition of suberin in the Casparian bands. Hydraulic conductance did not decrease in association with the bacterial treatment despite strengthening of the apoplast barriers. At the same time, the Fenton reagent, serving as an AQPs inhibitor, decreased hydraulic conductance to a greater extent in treated plants relative to the control group, indicating an increase in the AQP activity by the bacteria. We hypothesize that P. mandelii IB-Ki14 stimulates deposition of suberin, in the biosynthesis of which LTPs are involved, and increases aquaporin activity, which in turn prevents a decrease in hydraulic conductance due to formation of the apoplast barriers in pea roots.

The phytoremediation of soil contaminated with petroleum depends on the association of plants with rhizosphere bacteria capable of promoting plant growth and destroying petroleum hydrocarbonates. In the present work, we studied the effects of "Lenoil" biological product containing bacteria Pseudomonas turukhanskensis IB 1.1, capable of destroying petroleum hydrocarbons on Secale cereale L plants, which previously proved to be resistant to the weak oil pollution of gray forest soil and to the composition of microorganisms in their rhizosphere. The composition of microorganisms in the rhizosphere of rye roots was studied, morphometric parameters of shoots and roots of rye plants were estimated, and hormone concentration was immunoassayed under conditions of 4% petroleum pollution of the soil. Addition of petroleum to the soil increased the content of oligonitrophilic (by 24%) and hydrocarbon-oxidizing (by 33%) microorganisms; however, the content of cellulolytic (by 12.5 times) microorganisms in the rhizosphere decreased. The use of Lenoil led to a further increase in the number of cellulolytic (by 5.6 times) and hydrocarbon-oxidizing (by 3.8 times) microorganisms and a decrease in the number of oligonitrophilic (by 22.7%) microorganisms in the rhizosphere. Under petroleum pollution, the content of auxins (IAA), zeatin riboside, zeatin nucleotide, and zeatin decreased, while the content of abscisic acid (ABA) increased in the shoots of rye plants. Lenoil treatment led to an eight-fold increase in the IAA content in the roots and a decrease in the ABA content in the aerial part and in the roots. It was shown for the first time that the treatment of petroleum-contaminated soil with "Lenoil" increased root mass due to the development of lateral roots, concurrent with high root IAA content. Petroleum pollution increased the deposition of lignin and suberin in the roots, which strengthened the apoplastic barrier and, thus, reduced the infiltration of toxic components. The deposition of suberin and lignin decreased under "Lenoil" treatment, indicating a decrease in the concentration of toxic petroleum components in the soil degraded by the bacteria. Thus, the biological preparation reduced the growth-inhibiting effect of petroleum on rye plants by increasing the content of growth-stimulating phytohormones (IAA and cytokinins) and reducing the content of ABA, justifying the importance of further study of relevant hormones for the improvement of phytoremediation.

Pseudomonas mandelii strain IB-Ki14 has recently been shown to strengthen the apoplastic barriers of salt-stressed plants, which prevents the entry of toxic sodium. It was of interest to find out whether the same effect manifests itself in the absence of salinity and how this affects the hydraulic conductivity of barley plants. Berberine staining confirmed that the bacterial treatment enhanced the deposition of lignin and suberin and formation of Casparian bands in the roots of barley plants. The calculation of hydraulic conductance by relating transpiration to leaf water potential showed that it did not decrease in bacteria-treated plants. We hypothesized that reduced apoplastic conductivity could be compensated by the higher conductivity of the water pathway across the membranes. This assumption was confirmed by the results of the immunolocalization of HvPIP2;5 aquaporins with specific antibodies, showing their increased abundance around the areas of the endodermis and exodermis of bacteria-treated plants. The immunolocalization with antibodies against auxins and abscisic acid revealed elevated levels of these hormones in the roots of plants treated with bacteria. This root accumulation of hormones is likely to be associated with the ability of Pseudomonas mandelii IB-Ki14 to synthesize these hormones. The involvement of abscisic acid in the control of aquaporin abundance and auxins-in the regulation of and formation of apoplast barriers-is discussed.

The stomatal closure of salt-stressed plants reduces transpiration bringing about the maintenance of plant tissue hydration. The aim of this work was to test for any involvement of aquaporins (AQPs) in stomatal closure under salinity. The changes in the level of aquaporins in the cells were detected with the help of an immunohistochemical technique using antibodies against HvPIP2;2. In parallel, leaf sections were stained for abscisic acid (ABA). The effects of salinity were compared to those of exogenously applied ABA on leaf HvPIP2;2 levels and the stomatal and leaf hydraulic conductance of barley plants. Salinity reduced the abundance of HvPIP2;2 in the cells of the mestome sheath due to it being the more likely hydraulic barrier due to the deposition of lignin, accompanied by a decline in the hydraulic conductivity, transpiration, and ABA accumulation. The effects of exogenous ABA differed from those of salinity. This hormone decreased transpiration but increased the shoot hydraulic conductivity and PIP2;2 abundance. The difference in the action of the exogenous hormone and salinity may be related to the difference in the ABA distribution between leaf cells, with the hormone accumulating mainly in the mesophyll of salt-stressed plants and in the cells of the bundle sheaths of ABA-treated plants. The obtained results suggest the following succession of events: salinity decreases water flow into the shoots due to the decreased abundance of PIP2;2 and hydraulic conductance, while the decline in leaf hydration leads to the production of ABA in the leaves and stomatal closure.

There are two main pathways for water transport in plants. One of them is water transport along the cell walls and xylem vessels, called apoplastic pathway, and the other is from cell to cell through the membrane water channels aquaporins. Formation of Casparian bands by suberin and lignin deposition reduces conductivity of the apoplastic pathway. So the role of membrane water transport should increase, as mentioned in many reviews. The aim of this research was to localize barley aquaporins of the HvPIP2 family in different root zones depending on the differentiation of cell walls and deposition of Casparian bands. It was shown that the intensity of berberine fluorescence was insignificant in the zone closer to the root tip (at a distance of 3–4 cm), however fluorescence increased in the endodermis and the central cylinder of the root basal zone, indicating high lignification of cell walls and deposition of lignin and suberin in developing Casparian bands. Localization of HvPIP2;1 and HvPIP2;5 aquaporins showed no differences in their level between different root zones. The use of antibodies against aquaporins HvPIP2;2 and HvPIP2;3/4 revealed a local increase in fluorescence in the endodermal region of the basal zone of the roots characterized by high level of lignification and suberinization which apparently should compensate for the expected decrease in hydraulic conductivity as a result of the formation of apoplastic barriers. It is possible that the role of different aquaporins in water transport varies depending on the growing conditions of plants.

The study of plant adaptation mechanisms during the salt stress is required to provide an increase in plant productivity under such conditions. Along with a decrease in the availability of water for plants, the NaCL-induced inhibition of plant growth is associated with the toxic effect of sodium ions. The formation of apoplastic barriers due to the deposition of suberin and lignin restricts passive ion diffusion. However, the formation of such barriers reduces the capacity of the apoplastic pathway for water movement. In these conditions the role of transmembrane water transport is increased. This process is provided by aquaporin water channels. Thus the purpose of this work was to determine the contribution of aquaporins to hydraulic conductivity of peas plants under salinity-induced apoplastic barrier formation. An only slight decrease in plants transpiration caused by mercury chloride in the absence of salinization was in accordance with the ideas the apoplast is the dominant pathway when the Casparian bands is not formed yet. Salt stress in our experiments accelerated the development of the Casparian bands formation which could be visualized as an appearance of suberin strips in root endodermis which in turn was accompanied by a decrease in hydraulic conductivity. The decrease in hydraulic conductivity in 2 times during the mercury chloride treatment under salinity confirmed that contribution of aquaporins to the total hydraulic conductivity was increased under conditions when Casparian bands have had formed.

Local effects on plant roots or shoots are accompanied by a change of plant hormones concentration providing signal transduction in plants and their adaptation to changing environmental conditions. It is known that plants respond to drought stress by increasing the concentration of abscisic acid (ABA) in xylem sap which leads to decrease in stomatal conductance to prevent plant water loss from transpirational pathways. Earlier, we found a rapid leaf ABA accumulation in barley and wheat plants under influence of the shortterm salinity. However, the mechanism of the salt stress induced rapid accumulation of abscisic acid in plant leaves remained unclear. The aim of this study was to investigate whether the salt-induced rapid accumulation of abscisic acid in the leaves was the result of its inflow from the roots (as a root signal). In our experiments the short-term salinity did not increase but decreased the concentration of abscisic acid in xylem sap of wheat plants. Thus, detected accumulation of ABA in the leaves did not result from an increase in its inflow from the roots. Apparently leaf ABA accumulation already detected in 15 min after the onset of salinity stress could be a local reaction due to its metabolism in the shoot itself. The decrease in the leaf water potential could induce the change of abscisic acid metabolism pathways in the shoot that led to the accumulation of this hormone. A small short-term accumulation of ABA in the roots did not lead to an increase in their hydraulic conductivity.

We studied participation of aquaporins in the regulation of leaf hydraulic conductivity and relationship of hydraulic conductivity with accumulation of ABA and stomatal closure during salt stress. Using the method of immunohistochemical localization we showed that salinity led to greater decline in the level of aquaporins in the region of the vascular leaf bundles of the more salt-tolerant Prairia cultivar, accompanied by a noticeable decrease in hydraulic conductivity of the leaf. In the less salt-tolerant plants of the Mikhailovsky cultivar, significant changes in the level of aquaporins under the influence of salt stress were not found. The degree of decrease in the hydraulic conductivity of the leaf in plants of two cultivars under the influence of salt stress correlated with a decrease in transpiration. Immunohistochemical localization of abscisic acid (ABA) in leaf cells showed that during salt stress this hormone accumulated in leaf mesophyll cells and stomata. The uptake of exogenous hormone from the nutrient solution and its entry into the leaf through the vascular bundles was accompanied by an increase in staining for aquaporins and the hydraulic conductivity of the leaves, which is characteristic of the ABA action. Differences in the localization of exogenous and endogenous hormones were obviously the cause of the opposite directions of changes in hydraulic conductivity: its increase under the influence of an exogenous ABA and a decrease - under the influence of salt stress. ABA concentration assessment in xylem showed the absence of its increase during salt stress, which explains the absence changes of staining for this hormone in the region of the leaf vascular bundles and indicates that accumulation of ABA in a short-term salt stress is not the result of its delivery from the roots, but the result of its synthesis in the shoot itself.

The role of reactive oxygen species (ROS) in ABA-induced increase in hydraulic conductivity was hypothesized to be dependent on an increase in aquaporin water channel (AQP) abundance. Single ABA application or its combination with ROS manipulators (ROS scavenger ascorbic acid and NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI)) were studied on detached roots of barley plants. We measured the osmotically driven flow rate of xylem sap and calculated root hydraulic conductivity. In parallel, immunolocalization of ABA and HvPIP2;2 AQPs was performed with corresponding specific antibodies. ABA treatment increased the flow rate of xylem, root hydraulic conductivity and immunostaining for ABA and HvPIP2;2, while the addition of antioxidants prevented the effects of this hormone. The obtained results confirmed the involvement of ROS in ABA effect on hydraulic conductivity, in particular, the importance of H2O2 production by ABA-treated plants for the effect of this hormone on AQP abundance.

Rhizobacteria reduced the negative effects of salinity on wheat plants. Similarities and differences in the effect of hormone-producing halotolerant bacteria on plant growth and water relations during salinity are discussed.

Although salinity inhibits plant growth, application of appropriate rhizosphere bacteria can diminish this negative effect. We studied one possible mechanism that may underlie this beneficial response. Wheat plants were inoculated with Bacillus subtilis IB-22 and Pseudomonas mandelii IB-Ki14 and their consequences for growth, water relations, and concentrations of the hormone abscisic acid (ABA) were followed in the presence of soil salinity. Salinity alone increased ABA concentration in wheat leaves and roots and this was associated with decreased stomatal conductance, but also with chlorophyll loss. Bacterial treatment raised ABA concentrations in roots, suppressed accumulation of leaf ABA, decreased chlorophyll loss, and promoted leaf area and transpiration. However, water balance was maintained due to increased water uptake by inoculated plants, brought about in part by a larger root system. The effect may be the outcome of ABA action since the hormone is known to maintain root extension in stressed plants. Root ABA concentration was highest in salt-stressed plants inoculated with B. subtilis and this contributed to greater root hydraulic conductivity. We conclude that bacteria can raise salt resistance in wheat by increasing root ABA, resulting in larger root systems that can also possess enhanced hydraulic conductivity thereby supporting better-hydrated leaves.