Mild Osmotic Stress Research Articles

Mild osmotic stress offers photoprotection in Chlamydomonas reinhardtii under high light

The exposure of autotrophs to high light intensities significantly impacts their photosynthetic performance. When combined with unpredictable climate changes, the lethality of these effects is exacerbated and, often surpassing the organisms' threshold for tolerance. In this regard, our study centres on examining the mitigating effects of mild osmotic stress induced by 2% Polyethylene Glycol (PEG) in conjunction with high-light conditions, using Chlamydomonas reinhardtii as a model system. Cells were cultivated under low PEG-induced osmotic stress at various light intensities, and their responses were analyzed through biochemical and biophysical approaches. Remarkably, cells grown under lower PEG concentrations exhibited superior growth, increased biomass, and enhanced photosynthetic efficiency under high light compared to non-PEG-treated cells. Surprisingly, their non-photochemical quenching (NPQ) levels were lower, indicating the operation of a distinct photoprotective mechanism in PEG-grown samples. The PEG-grown cells demonstrated higher chlorophyll content but lower carotenoid content, supporting the NPQ data. Circular dichroism analysis suggested that the macro-organization of super-complexes was minimally disrupted in PEG-grown samples, even under high light. This was further supported by Blue native PAGE, which showed greater stability of the super-complexes in PEG-grown cells, implying heightened stability in pigment-protein interactions. Immunoblot analysis revealed minimal differences in core reaction center proteins between PEG-grown and non-PEG cells. Notably, this protective mechanism was absent in the cell wall-deficient mutant CC503. We propose that the partial photoprotection observed is attributed to the PEG shielding the cell wall. This result holds promise for enhancing algal biomass production under natural environmental conditions influenced by fluctuating light intensity.

Mild Osmotic Stress Alters mTOR Pathway Signaling

Increased osmotic stress reduces cell volume, thus disrupting homeostatic processes. This could impact signaling that promotes muscle growth and maintenance at rest and in response to mechanical stimuli. To date, the detrimental effect of osmotic stress on muscle protein turnover has been established in in vitro and in situ models, but the effects on muscular anabolic signaling in humans are unknown. Purpose: To examine the effect of fluid restriction (FR)-induced mild osmotic stress (plasma osmolality ≥295.0 mmol×kg−1) on muscular anabolic signaling in response to resistance exercise (RE) in trained young men. Hypothesis: We hypothesized that FR would alter the Akt/mTOR signaling. Method: RE-trained men (n=11, 21±1yr, 175.9±6.2cm, 79.2±12.3kg, 18.4±6.7% fat) completed 2 identical RE bouts either with (FR) or without (CON) fluid restriction from 24 hrs before to 3 hrs after RE. RE consisted of bilateral leg press and knee extensions (5 sets x 10 repetitions at 80% of 1 repetition maximum). The two conditions were 2-3 wks apart. Plasma osmolality (PO) was measured before (PRE), and 5 min, 15 min, 30 min, 1 hr, and 3 hrs after RE. Muscle samples were collected at PRE and 1 and 3 hrs after RE. Western blots were used to analyze contents of proteins in the mTOR signaling pathway: Akt, p-AktS473, mTOR, p-mTORS2448, p70S6K, p-p70S6KT389, rpS6, p-rpS6S240/244, eEF2, and p-eEF2T56 and normalized to Ponceau S staining (total protein). Results: Increased PO was successfully induced in FR vs. CON (condition effect, p<0.0001; 299±2 vs 286±1 mmol/kg). P-AktS473 significantly (p≤0.05) increased from PRE to 1 and 3 hrs in CON, and from PRE to 1h with a return to PRE by 3h in FR (condition x time). P-AktS473 was higher at 1h and lower at 3h in FR versus CON. P-mTORS2448 increased from PRE to 1 and 3 hrs regardless of condition (main time). P-p70S6KT389 was greater in FR than CON regardless of time point (main condition). P-rpS6S240/244 increased from PRE to 1 and 3 hrs for CON and FR, and was greater for FR than CON at PRE (condition x time). Conclusion: Baseline anabolic signaling appeared elevated when individuals were subjected to mild osmotic stress. After RE, signaling through mTOR was elevated as expected when plasma was iso-osmotic, although anticipated time effects for p-p70S6K and p-eEF2 were not detected. Upstream of mTOR, activation of Akt was stronger yet shorter in duration with mild osmotic stress. Downstream of mTOR, p70S6K and rpS6 activation were greater with mild osmotic stress. While these results suggest increased anabolic signaling with mild osmotic stress, concomitant catabolic signaling should be assessed. Together, the present results and future measures will inform how conditions that confer osmotic stress, such as dehydration, may alter intramuscular protein homeostasis at rest and in response to an anabolic stimulus. This study was funded by National Strength and Conditioning Association Foundations. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The re-establishment of desiccation tolerance in germinated tomato (Solanum lycopersicum) seeds

Abstract Desiccation tolerance (DT) of seeds, one of the plant's environmental adaptation mechanisms, allows them to survive as seeds in a quiescent state under extremely water-deficient conditions during the plant's life cycle, followed by seed germination and seedling establishment under favourable water conditions. The seed-DT is lost after radicle emergence; however, there is a developmental period called the ‘DT window’ during which the germinating seeds can re-induce DT following a cue from their ambient low water potential (i.e. mild osmotic stress). The DT re-inducibility within the DT window has been used as a model biosystem for understanding molecular mechanisms that activate/supress DT in a number of plant species. However, the characteristics of the DT window for species particularly important to the agroindustry are still largely fragmented. Here, physiological analyses were performed, aiming to elucidate the properties of the DT window in tomato, a model species for Solanaceae, holding a key strategic position for the seed industry and commercial use around the world. We revealed that (i) the DT window of tomato seeds is closed when the developing radicle reaches about 4 mm after germination, (ii) the most effective ambient water potential to re-induce DT into seeds is about −1.5 MPa and (iii) there is organ specificity of DT re-induction with hypocotyls, showing a longer DT window than cotyledons and roots in post-germination seeds.

Constitutively active B2 Raf-like kinases are required for drought-responsive gene expression upstream of ABA-activated SnRK2 kinases

Osmotic stresses, such as drought and high salinity, adversely affect plant growth and productivity. The phytohormone abscisic acid (ABA) accumulates in response to osmotic stress and enhances stress tolerance in plants by triggering multiple physiological responses through ABA signaling. Subclass III SNF1-related protein kinases 2 (SnRK2s) are key regulators of ABA signaling. Although SnRK2s have long been considered to be self-activated by autophosphorylation after release from PP2C-mediated inhibition, they were recently revealed to be activated by two independent subfamilies of group B Raf-like kinases, B2-RAFs and B3-RAFs, under osmotic stress conditions. However, the relationship between SnRK2 phosphorylation by these RAFs and SnRK2 autophosphorylation and the individual physiological roles of each RAF subfamily remain unknown. In this study, we indicated that B2-RAFs are constantly active and activate SnRK2s when released from PP2C-mediated inhibition by ABA-binding ABA receptors, whereas B3-RAFs are activated only under stress conditions in an ABA-independent manner and enhance SnRK2 activity. Autophosphorylation of subclass III SnRK2s is not sufficient for ABA responses, and B2-RAFs are needed to activate SnRK2s in an ABA-dependent manner. Using plants grown in soil, we found that B2-RAFs regulate subclass III SnRK2s at the early stage of drought stress, whereas B3-RAFs regulate SnRK2s at the later stage. Thus, B2-RAFs are essential kinases for the activation of subclass III SnRK2s in response to ABA under mild osmotic stress conditions, and B3-RAFs function as enhancers of SnRK2 activity under severe stress conditions.

Deformation of caveolae impacts global transcription and translation processes through relocalization of cavin-1

Caveolae are invaginated membrane domains that provide mechanical strength to cells in addition to being focal points for the localization of signaling molecules. Caveolae are formed through the aggregation of caveolin-1 or -3 (Cav1/3), membrane proteins that assemble into multifunctional complexes with the help of caveola-associated protein cavin-1. In addition to its role in the formation of caveolae, cavin-1, also called polymerase I and transcript release factor, is further known to promote ribosomal RNA transcription in the nucleus. However, the mechanistic link between these functions is not clear. Here, we found that deforming caveolae by subjecting cells to mild osmotic stress (150–300 mOsm) changes levels of GAPDH, Hsp90, and Ras only when Cav1/cavin-1 levels are reduced, suggesting a link between caveola deformation and global protein expression. We show that this link may be due to relocalization of cavin-1 to the nucleus upon caveola deformation. Cavin-1 relocalization is also seen when Cav1-Gαq contacts change upon stimulation. Furthermore, Cav1 and cavin-1 levels have been shown to have profound effects on cytosolic RNA levels, which in turn impact the ability of cells to form stress granules and RNA-processing bodies (p-bodies) which sequester and degrade mRNAs, respectively. Our studies here using a cavin-1-knockout cell line indicate adaptive changes in cytosolic RNA levels but a reduced ability to form stress granules. Taken together, our findings suggest that caveolae, through release of cavin-1, communicate extracellular cues to the cell interior to impact transcriptional and translational.

Caveolae deformation induces nuclear relocalization of cavin-1 to impact transcription

Caveolae are membrane domains that provide mechanical strength to cells to localize signaling molecules. These invaginations are composed of caveolin-1 or −3 (Cav1/3) molecules that assemble into domains with the help of cavin-1. Besides organizing caveolae, cavin-1 also known as polymerase I and transcript release factor (PTRF), promotes ribosomal RNA transcription into the nucleus. Here, we find that deforming caveolae by subjecting cells to mild osmotic stress (300 to 150 mOsm), changes levels of GAPDH, HSP90 AND Ras only when Cav1/cavin-1 levels are reduced suggesting link between caveolae deformation and protein expression.

Impact of Drought on Soluble Sugars and Free Proline Content in Selected Arabidopsis Mutants.

Simple SummaryDrought has severe effects on plants, negatively impacting economic, agricultural and environmental processes. Depending on the duration and the strength of water stress conditions, plants adjust a series of physiological, cellular, and molecular mechanisms aimed at providing a correct stress response and, if possible, at establishing stress tolerance. The model plant Arabidopsis thaliana is one of the best tools for analyzing the involvement of specific genes in the response to drought. Thanks to this tool, the role of two genes encoding enzymes involved in sugars metabolism, and one gene encoding an enzyme involved in proline synthesis, have been investigated. In addition to suggesting an interaction between the metabolism of proline and that of soluble sugars, results broaden our understanding of the predominant role played by the accumulation of soluble sugars in counteracting mild osmotic stress.Water shortage is an increasing problem affecting crop yield. Accumulation of compatible osmolytes is a typical plant response to overcome water stress. Sucrose synthase 1 (SUS1), and glucan, water dikinase 2 (GWD2) and δ1-pyrroline-5-carboxylate synthetase 1 (P5CS1) are members of small protein families whose role in the response of Arabidopsis thaliana plants to mild osmotic stress has been studied in this work. Comparative analysis between wild-type and single loss-of-function T-DNA plants at increasing times following exposure to drought showed no differences in the content of water-insoluble carbohydrate (i.e., transitory starch and cell wall carbohydrates) and in the total amount of amino acids. On the contrary, water-soluble sugars and proline contents were significantly reduced compared to wild-type plants regardless of the metabolic pathway affected by the mutation. The present results contribute to assigning a physiological role to GWD2, the least studied member of the GWD family; strengthening the involvement of SUS1 in the response to osmotic stress; showing a greater contribution of soluble sugars than proline in osmotic adjustment of Arabidopsis in response to drought. Finally, an interaction between proline and soluble sugars emerged, albeit its nature remains speculative and further investigations will be required for complete comprehension.

A complex metabolic rearrangement towards the accumulation of glycerol and sugars consequence of a proteome remodeling is required for the survival of Chlamydomonas reinhardtii growing under osmotic stress

During its life, freshwater microalgae need to grow under varying water availabilities such as seasonal dryness of puddles or tidal variations. A vast knowledge of the physiological and molecular responses to osmotic stress is available for land plants, however these responses have been scarcely studied in microalgae. In this work we described the acclimation of Chlamydomonas reinhardtii to a mild osmotic stress during a 24 h time-course experiment. The microalgae stress-responses were reconstructed from the generated proteometabolomic datasets using a systems biology approach. Protein and metabolite dynamics showed a Chlamydomonas tolerance to osmotic stress which seems to rely on three strategies: the accumulation of osmolyte/osmoprotective sugars and glycerol, the increase of ROS scavenging mechanisms, and the alteration of cell physiology. These responses involved specific proteome and metabolome changes, probably tuned by several signaling proteins (MAPK, PP2Cs, PKL1, GUN4) and metabolites as 2-hydroxyglutarate which accumulated under stress. Moreover, the function of the plant orthologs of these proteins suggested the potential activation of brassinosteroid- and ABA-mediated pathways in a plant-like way and the activation of a HOG-like osmorelay resembling a yeast-like response. These pathways were also probably regulating a long-term acclimation mediated by epigenetic mechanisms, being promising targets for further osmotic response characterization or exploitation towards engineering strains accumulating glycerol or other industry-demanding biomolecules.

Impact of osmotic stress on the growth and root architecture of introgression lines derived from a wild ancestor of rice and a modern cultivar.

Many modern rice varieties have been intensively selected for high-yielding performance under irrigated conditions, reducing their genetic diversity and potentially increasing their susceptibility to abiotic stresses such as drought. In this study, we tested benefits for stress tolerance of introducing DNA segments from wild ancestor Oryza rufipogon to the modern cultivar O. sativa cv Curinga (CUR) by applying a gradient of osmotic stress to both parents and seven introgressed lines. Shoot growth of O. rufipogon had a high tolerance to osmotic stress, and the number of total root tips increased under mild osmotic stress. One introgression line showed greater shoot growth, root growth, and higher number of total root tips than the parent line CUR under osmotic stress. Abscisic acid (ABA) is a key hormone mediating plant responses to abiotic stresses. Both root and shoot growth of O. rufipogon were much more sensitive to ABA than CUR. Introgression lines varied in the extent to which the sensitivity of their growth responses to ABA and some lines correlated with their sensitivity to osmotic stress. Our results suggest that rice responses to ABA and osmotic stress are genotype dependent, and growth responses of rice to ABA are not a consistent indicator of resilience to abiotic stress in introgression lines.

AtSK11 and AtSK12 Mediate the Mild Osmotic Stress-Induced Root Growth Response in Arabidopsis.

Although most osmotic stresses are harmful to plant growth and development, certain drought- or polyethylene glycol (PEG)-induced mild osmotic stresses promote plant root growth. The underlying regulatory mechanisms of this response remain elusive. Here, we report that the GLYCOGEN SYNTHASE KINASE 3 (GSK3) genes ARABIDOPSIS THALIANA SHAGGY-RELATED KINASE 11 (AtSK11) (AT5G26751) and AtSK12 (AT3G05840) are involved in the mild osmotic stress (−0.4 MPa) response in Arabidopsis thaliana. When grown on plant medium infused with different concentrations of PEG to mimic osmotic stress, both wild-type (WT) and atsk11atsk12 plants showed stimulated root growth under mild osmotic stress (−0.4 MPa) but repressed root growth under relatively strong osmotic stress (−0.5, −0.6, −0.7 MPa) as compared to the mock condition (−0.25 MPa). The root growth stimulation of atsk11atsk12 was more sensitive to −0.4 MPa treatment than was that of WT, indicating that AtSK11 and AtSK12 inhibit the mild stress-induced root growth response. RNA-seq analysis of WT and atsk11atsk12 plants under three water potentials (−0.25 MPa, −0.4 MPa, −0.6 MPa) revealed 10 differentially expressed candidate genes mainly involved in cell wall homeostasis, which were regulated by AtSK11 and AtSK12 to regulate root growth in response to the mild stress condition (−0.4 MPa). Promoter motif and transcription factor binding analyses suggested that the basic helix-loop-helix (bHLH) transcription factor bHLH69/LJRHL1-LIKE 2 (LRL2) may directly regulate the expression of most −0.4 MPa-responsive genes. These findings indicate that mild osmotic stress (−0.4 MPa) promotes plant growth and that the GSK3 family kinase genes AtSK11 and AtSK12 play a negative role in the induction of root growth in response to mild osmotic stress.

Morpho-physiological adaptation and DNA methylation of wheat seedlings under osmotic stress

The quality and yield of wheat (Triticum aestivum L.) are dramatically affected by drought. We used morphological and physiological characteristics and degree of DNA methylation to compare the responses of two wheat cultivars under osmotic stress, and found that the two cultivars behaved differently. Root development, leaf growth, and the accumulation of proline and soluble carbohydrate in wheat cv. AK58 all showed drought tolerance. Drought tolerance of wheat cv. XM13 was mainly improved by accumulation of proline and soluble carbohydrate. The degree of DNA methylation in wheat showed tissue specificity and increased significantly in leaf tissue with increasing osmotic stress, but decreased significantly in root tissue under mild osmotic stress. In addition, changes of DNA methylation differed between two wheat cultivars under osmotic stress, and this change was especially significant in AK58. Therefore, wheat AK58 may have stronger self-adjustment ability under osmotic stress compared with XM13, and might respond more rapidly to osmotic stress through the change of DNA methylation. This finding could be significant for revealing drought-tolerance mechanisms of plants.

Karrikin-KAI2 signalling provides Arabidopsis seeds with tolerance to abiotic stress and inhibits germination under conditions unfavourable to seedling establishment.

The control of seed germination in response to environmental conditions is important for plant success. We investigated the role of the karrikin receptor KARRIKIN INSENSITIVE2 (KAI2) in the response of Arabidopsis seeds to osmotic stress, salinity and high temperature. Germination of the kai2 mutant was examined in response to NaCl, mannitol and elevated temperature. The effect of karrikin on germination of wild-type seeds, hypocotyl elongation and the expression of karrikin-responsive genes was also examined in response to such stresses. The kai2 seeds germinated less readily than wild-type seeds and germination was more sensitive to inhibition by abiotic stress. Karrikin-induced KAI2 signalling stimulated germination of wild-type seeds under favourable conditions, but, surprisingly, inhibited germination in the presence of osmolytes or at elevated temperature. By contrast, GA stimulated germination of wild-type seeds and mutants under all conditions. Karrikin induced expression of DLK2 and KUF1 genes and inhibited hypocotyl elongation independently of osmotic stress. Under mild osmotic stress, karrikin enhanced expression of DREB2A, WRKY33 and ERF5 genes, but not ABA signalling genes. Thus, the karrikin-KAI2 signalling system can protect against abiotic stress, first by providing stress tolerance, and second by inhibiting germination under conditions unfavourable to seedling establishment.

Tinkering with Osmotically Controlled Transcription Allows Enhanced Production and Excretion of Ectoine and Hydroxyectoine from a Microbial Cell Factory.

Ectoine and hydroxyectoine are widely synthesized by members of the Bacteria and a few members of the Archaea as potent osmostress protectants. We have studied the salient features of the osmostress-responsive promoter directing the transcription of the ectoine/hydroxyectoine biosynthetic gene cluster from the plant-root-associated bacterium Pseudomonas stutzeri by transferring it into Escherichia coli, an enterobacterium that does not produce ectoines naturally. Using ect-lacZ reporter fusions, we found that the heterologous ect promoter reacted with exquisite sensitivity in its transcriptional profile to graded increases in sustained high salinity, responded to a true osmotic signal, and required the buildup of an osmotically effective gradient across the cytoplasmic membrane for its induction. The involvement of the -10, -35, and spacer regions of the sigma-70-type ect promoter in setting promoter strength and response to osmotic stress was assessed through site-directed mutagenesis. Moderate changes in the ect promoter sequence that increase its resemblance to housekeeping sigma-70-type promoters of E. coli afforded substantially enhanced expression, both in the absence and in the presence of osmotic stress. Building on this set of ect promoter mutants, we engineered an E. coli chassis strain for the heterologous production of ectoines. This synthetic cell factory lacks the genes for the osmostress-responsive synthesis of trehalose and the compatible solute importers ProP and ProU, and it continuously excretes ectoines into the growth medium. By combining appropriate host strains and different plasmid variants, excretion of ectoine, hydroxyectoine, or a mixture of both compounds was achieved under mild osmotic stress conditions.IMPORTANCE Ectoines are compatible solutes, organic osmolytes that are used by microorganisms to fend off the negative consequences of high environmental osmolarity on cellular physiology. An understanding of the salient features of osmostress-responsive promoters directing the expression of the ectoine/hydroxyectoine biosynthetic gene clusters is lacking. We exploited the ect promoter from an ectoine/hydroxyectoine-producing soil bacterium for such a study by transferring it into a surrogate bacterial host. Despite the fact that E. coli does not synthesize ectoines naturally, the ect promoter retained its exquisitely sensitive osmotic control, indicating that osmoregulation of ect transcription is an inherent feature of the promoter and its flanking sequences. These sequences were narrowed to a 116-bp DNA fragment. Ectoines have interesting commercial applications. Building on data from a site-directed mutagenesis study of the ect promoter, we designed a synthetic cell factory that secretes ectoine, hydroxyectoine, or a mixture of both compounds into the growth medium.

From network to phenotype: the dynamic wiring of an Arabidopsis transcriptional network induced by osmotic stress

Plants have established different mechanisms to cope with environmental fluctuations and accordingly fine‐tune their growth and development through the regulation of complex molecular networks. It is largely unknown how the network architectures change and what the key regulators in stress responses and plant growth are. Here, we investigated a complex, highly interconnected network of 20 Arabidopsis transcription factors (TFs) at the basis of leaf growth inhibition upon mild osmotic stress. We tracked the dynamic behavior of the stress‐responsive TFs over time, showing the rapid induction following stress treatment, specifically in growing leaves. The connections between the TFs were uncovered using inducible overexpression lines and were validated with transient expression assays. This study resulted in the identification of a core network, composed of ERF6, ERF8, ERF9, ERF59, and ERF98, which is responsible for most transcriptional connections. The analyses highlight the biological function of this core network in environmental adaptation and its redundancy. Finally, a phenotypic analysis of loss‐of‐function and gain‐of‐function lines of the transcription factors established multiple connections between the stress‐responsive network and leaf growth.

Over-expression of PIP2;5 aquaporin alleviates gas exchange and growth inhibition in poplars exposed to mild osmotic stress with polyethylene glycol

The effects of mild osmotic stress conditions on aquaporin-mediated water transport are not well understood. In the present study, mild osmotic stress treatments with 20 and 50 g L−1 polyethylene glycol 6000 (PEG) in Hoagland’s mineral solution were applied for 3 weeks under controlled environmental conditions to transgenic Populus tremula × Populus alba plants constitutively over-expressing a Populus PIP2;5 aquaporin and compared with the wild-type plants. The PEG treatments resulted in growth reductions and triggered changes in net photosynthesis, transpiration, stomatal conductance and root hydraulic conductivity in the wild-type plants. However, height growth, leaf area, gas exchange, and root hydraulic conductivity were less affected by the PEG treatments in PIP2;5-over-expressing poplar lines. These results suggest that water transport across the PIP2;5 aquaporin is an important process contributing to tolerance of mild osmotic stress in poplar. Greater membrane abundance of PIP2;5 was most likely the factor that was responsible for higher root hydraulic conductivity leading to improved plant water flux and, consequently, greater gas exchange and growth rates under mild osmotic stress conditions. The results also provide evidence for the functional significance of PIP2;5 aquaporin in water transport and its strong link to growth processes in poplar.

Super-resolution Visualization of Caveola Deformation in Response to Osmotic Stress

Caveolae are protein-dense plasma membrane domains structurally composed of caveolin-1 or -3 along with other proteins. Our previous studies have shown that caveolae enhance calcium signals generated through the Gαq/phospholipase Cβ signaling pathway and that subjecting cells to hypo-osmotic stress reverses this enhancement. In this study, we have used super-resolution fluorescence microscopy supplemented by fluorescence correlation studies to determine the structural factors that underlie this behavior. We find similar and significant population of Gαq and one of its receptors, bradykinin type 2 receptor (B2R), as well as a significant population of Gαi and its coupled β2-adrenergic receptor (βAR), are localized to caveola domains. Although mild osmotic stress deforms caveolae and alters interactions between the caveolae and these proteins, the general structure and the localization of caveola components remain largely unchanged. This deformation eliminates the ability of caveolae to stabilize calcium signals mediated through Gαq-B2R, but does not affect cAMP signals mediated through Gαi and βAR. Structurally, we find that mild osmotic stress corresponding roughly to a pressure of 3.82 newtons/m2 increases the domain diameter by ∼30% and increases the fluorescence intensity in the center of the domain mouth suggesting a flattening of the invagination. Approximate calculations show that caveolae in muscle tissue have the strength to handle the stress of muscle movement.

Inheritance of protection from osmotic stress.

Exposure of mother worms to mild osmotic stress induces gene expression changes in offspring that protect them from strong osmotic stress. Inheritance of protection is now shown to depend on altered insulin-like signalling in the maternal germline, which confers protection through increased expression of zygotic gpdh-2, a rate-limiting enzyme in glycerol biosynthesis.

Mild osmotic stress promotes 4-methoxy indolyl-3-methyl glucosinolate biosynthesis mediated by the MKK9-MPK3/MPK6 cascade in Arabidopsis.

MKK9-MPK3/MPK6 cascade positively regulates IGSs' biosynthetic genes. Glucosinolates (GSs), secondary metabolites well known for their roles in plant defense, have been implicated to play an important role in plant abiotic stress response; however, the exact role in these processes and the underlying regulatory mechanisms remain elusive. Mitogen-activated protein kinase (MAPK) cascades are extensively involved in plant abiotic stress response. In this study, we examined the levels of four indolic glucosinolates (IGSs) in the shoots of Arabidopsis seedlings under mild osmotic stress conditions and found that 4-methoxy indolyl-3-methyl glucosinolate (4MI3G) accumulated and that MPK3 and MPK6 were activated. Loss of MPK3 or MPK6 function led to a reduction in mild osmotic stress-induced 4MI3G. Further analyses revealed that MKK9 acts upstream of MPK3 and MPK6 to promote 4MI3G accumulation. The level of 4MI3G induced by mild osmotic stress was reduced in the mkk9 mutant. Conversely, 4MI3G increased in MKK9 DD , a constitutively activate mutant of MKK9. Gene expression analyses indicated that the activated MKK9-MPK3/MPK6 cascade upregulates the IGS biosynthetic genes. Moreover, the lack of MYB51, the transcription factor controlling biosynthetic genes responsible for synthesizing the IGS core structure, or CYP81F2, the enzyme catalyzing core structure modification to 4MI3G, significantly reduced mild osmotic stress- and MKK9 DD -induced 4MI3G. Thus, our study demonstrates that mild osmotic stress promotes 4MI3G biosynthesis and the accumulation in Arabidopsis through activation of the MKK9-MPK3/MPK6 cascade and provides an MAPK-mediated signaling pathway for the IGS response to abiotic stress in plants.

A pump/leak model of growth: the biophysics of cell elongation in higher plants revisited.

Current concepts of growth hydraulics in higher plants are critically revisited, and it is concluded that they partly fail to interpret the experimental data adequately, particularly in the case of hydroponics-grown roots. Theoretical considerations indicate that the growth rate in roots is controlled by the extensibility of the cell wall, excluding water availability (i.e. hydraulic conductance) as a major constraint. This is supported by the findings that the growth rate does not scale with turgor, and that no radial nor axial water potential gradients have been observed in the root elongation zone. Nevertheless, a water potential deficit ranging from -0.2 to -0.6MPa has repeatedly been reported for growing cells that by far exceeds the shallow trans-membrane water potential difference required for the uptake of growth water. Unexpectedly, growth was also shown to depend on the hydraulic conductance (LP) of the plasma membrane of root cells, even though LP should generally be too large to have an impact on growth. For leaves, similar observations have been reported, but the interpretation of the data is less straightforward. Inconsistencies associated with the current model of growth hydraulics prompt the author to suggest a revised model that comprises, in addition to a passive mechanism of water transport across the plasma membrane of growing cells mediated by aquaporins ('leak') a secondary active water transport ('pump'), in analogy to a mechanism previously demonstrated for mammalian epithelia and postulated for xylem parenchyma cells in roots. Water is hypothesised to be secreted against a trans-membrane water potential difference by cotransport with solutes (salts, sugars, and/or amino acids), taking advantage of the free energy released by this transport step. The solute concentration gradient is supposed to be maintained by a subsequent retrieval of the solutes from the apoplast and back-transport at the expense of metabolic energy. Water secretion tends to reduce the turgor pressure and retards growth, but turgor and, in turn, growth can be upregulated very rapidly independent from any adjustment in the osmolyte deposition rate by increasing LP and/or reducing secondary active water transport, e.g. when the root is exposed to mild osmotic stress, as confirmed by experimental studies.

Differential root transcriptomics in a polyploid non-model crop: the importance of respiration during osmotic stress

To explore the transcriptomic global response to osmotic stress in roots, 18 mRNA-seq libraries were generated from three triploid banana genotypes grown under mild osmotic stress (5% PEG) and control conditions. Illumina sequencing produced 568 million high quality reads, of which 70–84% were mapped to the banana diploid reference genome. Using different uni- and multivariate statistics, 92 genes were commonly identified as differentially expressed in the three genotypes. Using our in house workflow to analyze GO enriched and underlying biochemical pathways, we present the general processes affected by mild osmotic stress in the root and focus subsequently on the most significantly overrepresented classes associated with: respiration, glycolysis and fermentation. We hypothesize that in fast growing and oxygen demanding tissues, mild osmotic stress leads to a lower energy level, which induces a metabolic shift towards (i) a higher oxidative respiration, (ii) alternative respiration and (iii) fermentation. To confirm the mRNA-seq results, a subset of twenty up-regulated transcripts were further analysed by RT-qPCR in an independent experiment at three different time points. The identification and annotation of this set of genes provides a valuable resource to understand the importance of energy sensing during mild osmotic stress.