Root hydraulic conductivity and adjustments in stomatal conductance: hydraulic strategy in response to salt stress in a halotolerant species

Special Issue "Phytohormones: Important Participators in Plant Growth and Development"

The articles published in the IJMS Special Issue "Phytohormones" are devoted to various aspects of hormonal control of plant growth and development promoting adaptation to normal and stress conditions [...].

Effect of ipt Gene Induction in Transgenic Tobacco Plants on Hydraulic Conductance, Formation of Apoplastic Barriers and Aquaporin Activity under Heat Shock

Cytokinins are known to keep stomata open, which supports gas exchange and correlates with increased photosynthesis. However, keeping the stomata open can be detrimental if the increased transpiration is not compensated for by water supply to the shoots. In this study, we traced the effect of ipt (isopentenyl transferase) gene induction, which increases the concentration of cytokinins in transgenic tobacco plants, on transpiration and hydraulic conductivity. Since water flow depends on the conductivity of the apoplast, the deposition of lignin and suberin in the apoplast was studied by staining with berberine. The effect of an increased concentration of cytokinins on the flow of water through aquaporins (AQPs) was revealed by inhibition of AQPs with HgCl₂. It was shown that an elevated concentration of cytokinins in ipt-transgenic plants increases hydraulic conductivity by enhancing the activity of aquaporins and reducing the formation of apoplastic barriers. The simultaneous effect of cytokinins on both stomatal and hydraulic conductivity makes it possible to coordinate the evaporation of water from leaves and its flow from roots to leaves, thereby maintaining the water balance and leaf hydration.

MicroRNA miR1118 contributes to wheat (Triticum aestivum L.) salinity tolerance by regulating the Plasma Membrane Intrinsic Proteins1;5 (PIP1;5) gene

microRNAs (miRNAs) are important regulators of various adaptive stress responses in crops; however, many details about associations among miRNAs, their target genes and physiochemical responses of crops under salinity stress remain poorly understood. We designed this study in a systems biology context and used a collection of computational, experimental and statistical procedures to uncover some regulatory functions of miRNAs in the response of the important crop, wheat, to salinity stress. Accordingly, under salinity conditions, wheat roots' Expressed Sequence Tag (EST) libraries were computationally mined to identify the most reliable differentially expressed miRNA and its related target gene(s). Then, molecular and physiochemical evaluations were carried out in a separate salinity experiment using two contrasting wheat genotypes. Finally, the association between changes in measured characteristics and wheat salinity tolerance was determined. From the results, miR1118 was assigned as a reliable salinity-responsive miRNA in wheat roots. The expression profiles of miR1118 and its predicted target gene, Plasma Membrane Intrinsic Proteins1,5 (PIP1;5), significantly differed between wheat genotypes. Moreover, results revealed that expression profiles of miR1118 and PIP1;5 significantly correlate to Relative Water Content (RWC), root hydraulic conductance (Lp), photosynthetic activities, plasma membrane damages, osmolyte accumulation and ion homeostasis of wheat. Our results suggest a plausible regulatory node through miR1118 adjusting the wheat water status, maintaining ion homeostasis and mitigating membrane damages, mainly through the PIP1;5 gene, under salinity conditions. To our knowledge, this is the first report on the role of miR1118 and PIP1;5 in wheat salinity response.

Salinity Tolerance of Halophytic Grass Puccinellia nuttalliana Is Associated with Enhancement of Aquaporin-Mediated Water Transport by Sodium

In salt-sensitive plants, root hydraulic conductivity is severely inhibited by NaCl, rapidly leading to the loss of water balance. However, halophytic plants appear to effectively control plant water flow under salinity conditions. In this study, we tested the hypothesis that Na⁺ is the principal salt factor responsible for the enhancement of aquaporin-mediated water transport in the roots of halophytic grasses, and this enhancement plays a significant role in the maintenance of water balance, gas exchange, and the growth of halophytic plants exposed to salinity. We examined the effects of treatments with 150 mM of NaCl, KCl, and Na₂SO₄ to separate the factors that affect water relations and, consequently, physiological and growth responses in three related grass species varying in salt tolerance. The grasses included relatively salt-sensitive Poa pratensis, moderately salt-tolerant Poa juncifolia, and the salt-loving halophytic grass Puccinellia nuttalliana. Our study demonstrated that sustained growth, chlorophyll concentrations, gas exchange, and water transport in Puccinellia nuttalliana were associated with the presence of Na in the applied salt treatments. Contrary to the other examined grasses, the root cell hydraulic conductivity in Puccinellia nuttalliana was enhanced by the 150 mM NaCl and 150 mM Na₂SO₄ treatments. This enhancement was abolished by the 50 µM HgCl₂ treatment, demonstrating that Na was the factor responsible for the increase in mercury-sensitive, aquaporin-mediated water transport. The observed increases in root Ca and K concentrations likely played a role in the transcriptional and (or) posttranslational regulation of aquaporins that enhanced root water transport capacity in Puccinellia nuttalliana. The study demonstrates that Na plays a key role in the aquaporin-mediated root water transport of the halophytic grass Puccinellia nuttalliana, contributing to its salinity tolerance.

Bacillus mycoides PM35 Reinforces Photosynthetic Efficiency, Antioxidant Defense, Expression of Stress-Responsive Genes, and Ameliorates the Effects of Salinity Stress in Maize

Soil salinity is one of the abiotic constraints that imbalance nutrient acquisition, hampers plant growth, and leads to potential loss in agricultural productivity. Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can alleviate the adverse impacts of salt stress by mediating molecular, biochemical, and physiological status. In the present study, the bacterium Bacillus mycoides PM35 showed resistance up to 3 M NaCl stress and exhibited plant growth-promoting features. Under salinity stress, the halo-tolerant bacterium B. mycoides PM35 showed significant plant growth-promoting traits, such as the production of indole acetic acid, siderophore, ACC deaminase, and exopolysaccharides. Inoculation of B. mycoides PM35 alleviated salt stress in plants and enhanced shoot and root length under salinity stress (0, 300, 600, and 900 mM). The B. mycoides PM35 alleviated salinity stress by enhancing the photosynthetic pigments, carotenoids, radical scavenging capacity, soluble sugars, and protein content in inoculated maize plants compared to non-inoculated plants. In addition, B. mycoides PM35 significantly boosted antioxidant activities, relative water content, flavonoid, phenolic content, and osmolytes while reducing electrolyte leakage, H₂O₂, and MDA in maize compared to control plants. Genes conferring abiotic stress tolerance (CzcD, sfp, and srfAA genes) were amplified in B. mycoides PM35. Moreover, all reactions are accompanied by the upregulation of stress-related genes (APX and SOD). Our study reveals that B. mycoides PM35 is capable of promoting plant growth and increasing agricultural productivity.

Overexpression of an aquaporin protein from Aspergillus glaucus confers salt tolerance in transgenic soybean

Salt stress is an important abiotic factor that causes severe losses in soybean yield and quality. Therefore, breeding salt-tolerant soybean germplasm resources via genetic engineering has gained importance. Aspergillus glaucus, a halophilic fungus that exhibits significant tolerance to salt, carries the gene AgGlpF. In this study, we used the soybean cotyledonary node transformation method to transfer the AgGlpF gene into the genome of the soybean variety Williams 82 to generate salt-tolerant transgenic soybean varieties. The results of PCR, Southern blot, ddPCR, and RT-PCR indicated that AgGlpF was successfully integrated into the soybean genome and stably expressed. When subjected to salt stress conditions via treatment with 250 mM NaCl for 3 d, the transgenic soybean plants showed significant tolerance compared with wild-type plants, which exhibited withering symptoms and leaf abscission after 9 d. The results of this study indicated that the transfer of AgGlpF into the genome of soybean plants produced transgenic soybean with significantly improved salt stress tolerance.

How roots and shoots communicate through stressful times

When plants face an environmental stress such as water deficit, soil salinity, high temperature, or shade, good communication between above- and belowground organs is necessary to coordinate growth and development. Various signals including hormones, peptides, proteins, hydraulic signals, and metabolites are transported mostly through the vasculature to distant tissues. How shoots and roots synchronize their response to stress using mobile signals is an emerging field of research. We summarize recent advances on mobile signals regulating shoot stomatal movement and root development in response to highly localized environmental cues. In addition, we highlight how the vascular system is not only a conduit but is also flexible in its development in response to abiotic stress.

Root hydraulics adjustment is governed by a dominant cell-to-cell pathway in Beta vulgaris seedlings exposed to salt stress

Soil salinity reduces root hydraulic conductivity (L_pr) of several plant species. However, how cellular signaling and root hydraulic properties are linked in plants that can cope with water restriction remains unclear. In this work, we exposed the halotolerant species red beet (Beta vulgaris) to increasing concentrations of NaCl to determine the components that might be critical to sustaining the capacity to adjust root hydraulics. Our strategy was to use both hydraulic and cellular approaches in hydroponically grown seedlings during the first osmotic phase of salt stress. Interestingly, L_pr presented a bimodal profile response apart from the magnitude of the imposed salt stress. As well as L_pr, the PIP2-aquaporin profile follows an unphosphorylated/phosphorylated pattern when increasing NaCl concentration while PIP1 aquaporins remain constant. L_pr also shows high sensitivity to cycloheximide. In low NaCl concentrations, L_pr was high and 70 % of its capacity could be attributed to the CHX-inhibited cell-to-cell pathway. More interestingly, roots can maintain a constant spontaneous exudated flow that is independent of the applied NaCl concentration. In conclusion, Beta vulgaris root hydraulic adjustment completely lies in a dominant cell-to-cell pathway that contributes to satisfying plant water demands.

Salt stress reduces root water uptake in barley (Hordeum vulgare L.) through modification of the transcellular transport path

The aim of the study was to understand the hydraulic response to salt stress of the root system of the comparatively salt-tolerant crop barley (Hordeum vulgare L.). We focused on the transcellular path of water movement across the root cylinder that involves the crossing of membranes. This path allows for selective water uptake, while excluding salt ions. Hydroponically grown plants were exposed to 100 mM NaCl for 5-7 days and analysed when 15-17 days old. A range of complementary and novel approaches was used to determine hydraulic conductivity (Lp). This included analyses at cell, root and plant level and modelling of water flow. Apoplastic barrier formation and gene expression level of aquaporins (AQPs) was analysed. Salt stress reduced the Lp of root system through reducing water flow along the transcellular path. This involved changes in the activity and gene expression level of AQPs. Modelling of root-Lp showed that the reduction in root-Lp did not require added hydraulic resistances through apoplastic barriers at the endodermis. The bulk of data points to a near-perfect semi-permeability of roots of control plants (solute reflection coefficient σ ~1.0). Roots of salt-stressed plants are almost as semi-permeable (σ > 0.8).

Differential Aquaporin Response to Distinct Effects of Two Zn Concentrations after Foliar Application in Pak Choi (Brassica rapa L.) Plants

Zinc (Zn) is considered an essential element with beneficial effects on plant cells; however, as a heavy metal, it may induce adverse effects on plants if its concentration exceeds a threshold. In this work, the effects of short-term and prolonged application of low (25 µM) and high (500 µM) Zn concentrations on pak choi (Brassica rapa L.) plants were evaluated. For this, two experiments were conducted. In the first, the effects of short-term (15 h) and partial foliar application were evaluated, and in the second a long-term (15 day) foliar application was applied. The results indicate that at short-term, Zn may induce a rapid hydraulic signal from the sprayed leaves to the roots, leading to changes in root hydraulic conductance but without effects on the whole-leaf gas exchange parameters. Root accumulation of Zn may prevent leaf damage. The role of different root and leaf aquaporin isoforms in the mediation of this signal is discussed, since significant variations in PIP1 and PIP2 gene expression were observed. In the second experiment, low Zn concentration had a beneficial effect on plant growth and specific aquaporin isoforms were differentially regulated at the transcriptional level in the roots. By contrast, the high Zn concentration had a detrimental effect on growth, with reductions in the root hydraulic conductance, leaf photosynthesis rate and Ca2+ uptake in the roots. The abundance of the PIP1 isoforms was significantly increased during this response. Therefore, a 25 µM Zn dose resulted in a positive effect in pak choi growth through an increased root hydraulic conductance.

A cross population between D. kaki and D. virginiana shows high variability for saline tolerance and improved salt stress tolerance

Persimmon (Diospyros kaki Thunb.) production is facing important problems related to climate change in the Mediterranean areas. One of them is soil salinization caused by the decrease and change of the rainfall distribution. In this context, there is a need to develop cultivars adapted to the increasingly challenging soil conditions. In this study, a backcross between (D. kaki x D. virginiana) x D. kaki was conducted, to unravel the mechanism involved in salinity tolerance of persimmon. The backcross involved the two species most used as rootstock for persimmon production. Both species are clearly distinct in their level of tolerance to salinity. Variables related to growth, leaf gas exchange, leaf water relations and content of nutrients were significantly affected by saline stress in the backcross population. Water flow regulation appears as a mechanism of salt tolerance in persimmon via differences in water potential and transpiration rate, which reduces ion entrance in the plant. Genetic expression of eight putative orthologous genes involved in different mechanisms leading to salt tolerance was analyzed. Differences in expression levels among populations under saline or control treatment were found. The ‘High affinity potassium transporter’ (HKT1-like) reduced its expression levels in the roots in all studied populations. Results obtained allowed selection of tolerant rootstocks genotypes and describe the hypothesis about the mechanisms involved in salt tolerance in persimmon that will be useful for breeding salinity tolerant rootstocks.

Modeling of Cd adsorption to goethite-bacteria composites

The accurate modeling of heavy metal adsorption in complex systems is fundamental for risk assessments in soils and associated environments. Bacteria-iron (hydr)oxide associations in soils and sediments play a critical role in heavy metal immobilization. The reduced adsorption of heavy metals on these composites have been widely reported using the component additivity (CA) method. However, there is a lack of a mechanism model to account for these deviations. In this study, we established models for Cd adsorption on goethite-Pseudomonas putida composites at 1:1 and 5:1 mass ratios. Cadmium adsorption on the 5:1 composite was consistent with the additivity method. However, the CA method over predicted Cd adsorption by approximately 8% on the 1:1 composite at high Cd concentration. The deviation was corrected by adding the site blockage reactions between P. putida and goethite. Both CA and "CA-site masking" models for Cd adsorption onto the composites were in line with the ITC data. These results indicate that CA method in simulating Cd adsorption on bacteria-iron oxides composites is limited to low bacterial and Cd concentrations. Therefore the interfacial complexation reactions that occur between iron (hydr)oxides and bacteria should be taken into account when high concentrations of bacteria and heavy metals are present.

Changes in root hydraulic conductivity facilitate the overall hydraulic response of rice (Oryza sativa L.) cultivars to salt and osmotic stress

The aim of the present work was to assess the significance of changes in root AQP gene expression and hydraulic conductivity (Lp) in the regulation of water balance in two hydroponically-grown rice cultivars (Azucena, Bala) which differ in root morphology, stomatal regulation and aquaporin (AQP) isoform expression. Plants were exposed to NaCl (25 mM, 50 mM) and osmotic stress (5%, 10% PEG6000). Root Lp was determined for exuding root systems (osmotic forces driving water uptake; 'exudation Lp') and transpiring plants (hydrostatic forces dominating; 'transpiration-Lp'). Gene expression was analysed by qPCR. Stress treatments caused a consistent and significant decrease in plant growth, transpirational water loss, stomatal conductance, shoot-to-root surface area ratio and root Lp. Comparison of exudation-with transpiration-Lp supported a significant contribution of AQP-facilitated water flow to root water uptake. Changes in root Lp in response to treatments were correlated much stronger with root morphological characteristics, such as the number of main and lateral roots, surface area ratio of root to shoot and plant transpiration rate than with AQP gene expression. Changes in root Lp, involving AQP function, form an integral part of the plant hydraulic response to stress and facilitate changes in the root-to-shoot surface area ratio, transpiration and stomatal conductance.