A cellulose synthase‐like protein is required for osmotic stress tolerance in Arabidopsis

Osmotic stress imposed by soil salinity and drought stress significantly affects plant growth and development, but osmotic stress sensing and tolerance mechanisms are not well understood. Forward genetic screens using a root‐bending assay have previously identified salt overly sensitive (sos) mutants of Arabidopsis that fall into five loci, SOS1 to SOS5. These loci are required for the regulation of ion homeostasis or cell expansion under salt stress, but do not play a major role in plant tolerance to the osmotic stress component of soil salinity or drought. Here we report an additional sos mutant, sos6‐1, which defines a locus essential for osmotic stress tolerance. sos6‐1 plants are hypersensitive to salt stress and osmotic stress imposed by mannitol or polyethylene glycol in culture media or by water deficit in the soil. SOS6 encodes a cellulose synthase‐like protein, AtCSLD5. Only modest differences in cell wall chemical composition could be detected, but we found that sos6‐1 mutant plants accumulate high levels of reactive oxygen species (ROS) under osmotic stress and are hypersensitive to the oxidative stress reagent methyl viologen. The results suggest that SOS6/AtCSLD5 is not required for normal plant growth and development but has a critical role in osmotic stress tolerance and this function likely involves its regulation of ROS under stress.     

Osmotic stress at the barley root affects expression of circadian clock genes in the shoot

The circadian clock is an important timing system that controls physiological responses to abiotic stresses in plants. However, there is little information on the effects of the clock on stress adaptation in important crops, like barley. In addition, we do not know how osmotic stress perceived at the roots affect the shoot circadian clock. Barley genotypes, carrying natural variation at the photoperiod response and clock genes Ppd‐H1 and HvELF3, were grown under control and osmotic stress conditions to record changes in the diurnal expression of clock and stress‐response genes and in physiological traits. Variation at HvELF3 affected the expression phase and shape of clock and stress‐response genes, while variation at Ppd‐H1 only affected the expression levels of stress genes. Osmotic stress up‐regulated expression of clock and stress‐response genes and advanced their expression peaks. Clock genes controlled the expression of stress‐response genes, but had minor effects on gas exchange and leaf transpiration. This study demonstrated that osmotic stress at the barley root altered clock gene expression in the shoot and acted as a spatial input signal into the clock. Unlike in Arabidopsis, barley primary assimilation was less controlled by the clock and more responsive to environmental perturbations, such as osmotic stress.     

Stress‐induced pyruvate accumulation contributes to cross protection in a fungus

It is commonly observed that microorganisms subjected to a mild stress develop tolerance not only to higher doses of the same stress but also to other stresses – a phenomenon called cross protection. The mechanisms for cross protection have not been fully revealed. Here, we report that heat shock induced cross protection against UV, oxidative and osmotic/salt stress conditions in the cosmopolitan fungus Metarhizium robertsii. Similarly, oxidative and osmotic/salt stresses also induced cross protection against multiple other stresses. We found that oxidative and osmotic/salt stresses produce an accumulation of pyruvate that scavenges stress‐induced reactive oxygen species and promotes fungal growth. Thus, stress‐induced pyruvate accumulation contributes to cross protection. RNA‐seq and qRT‐PCR analyses showed that UV, osmotic/salt and oxidative stress conditions decrease the expression level of pyruvate consumption genes in the trichloroacetic acid cycle and fermentation pathways leading to pyruvate accumulation. Our work presents a novel mechanism for cross protection in microorganisms.     

Effects of mucilage on seed germination of the desert ephemeral plant Plantago minuta Pall. under osmotic stress and cycles of wet and dry conditions

To test the role of the seed mucilage of Plantago minuta Pall. in regulating germination under osmotic stress and cycles of hydration and dehydration, two experiments were carried out using seeds with intact mucilage and mucilage‐free seeds. In Experiment 1 seeds were immersed in a range of iso‐osmotic polyethylene glycol solutions (?1.15 to 0 MPa) for 14 days; any ungerminated seeds were transferred to deionized water to investigate the recovery germination. In Experiment 2 seeds were immersed in deionized water for 24 h, and were then incubated on filter paper for an additional 13 days to ensure complete desiccation before reimbibition to test the germination recovery percentage. Under mild osmotic stress (?0.73 to 0 MPa), the intact seeds with mucilage were shown to have higher germination rates than the mucilage‐free seeds, indicating that the mucilage led to a “fast sprouting” germination strategy under mild osmotic stress. However, when seeds were exposed to high osmotic stress (?1.15 MPa), the mucilage apparently slowed the germination rate, resulting in a “risk‐balancing” germination strategy. Extreme drought induced by polyethylene glycol solution and the desiccation pretreatment accelerated germination rates compared to non‐pretreated seeds; both germination potential and recovery percentage of the mucilage seeds were significantly higher than that of the mucilage‐free seeds. Our results revealed that the seed mucilage of P. minuta plays a crucial role in regulating seed germination rates and the germination strategies adopted by controlling seed water absorption when the seeds experience different osmotic stresses or alternating wet and dry conditions.     

Differences in strategies to combat osmotic stress in Burkholderia cenocepacia elucidated by NMR-based metabolic profiling

Aims: To investigate mechanisms of osmotic tolerance in Burkholderia cenocepacia, a member of the B. cepacia complex (Bcc) of closely related strains, which is of clinical as well as environmental importance. Methods and Results: We employed NMR‐based metabolic profiling (metabolomics) to elucidate the metabolic consequences of high osmotic stress for five isolates of B. cenocepacia. The strains differed significantly in their levels of osmotic stress tolerance, and we identified three different sets of metabolic responses with the strains least impacted by osmotic stress exhibiting higher levels of the osmo‐protective metabolites glycine‐betaine and/or trehalose. Strains either increased concentrations or had constitutively high levels of these metabolites. Conclusions: Even within the small set of B. cenocepacia isolates, there was a surprising degree of variability in the metabolic responses to osmotic stress. Significance and impact of the study: The metabolic responses, and hence osmotic stress tolerance, vary between different B. cenocepacia isolates. This study provides a first look into the potentially highly diverse physiology of closely related strains of one species of the Bcc and illustrates that physiological or clinically relevant phenotypes are unlikely to be inferable from genetic relatedness within this species group.     

Photosystem I shows a higher tolerance to sorbitol‐induced osmotic stress than photosystem II in the intertidal macro‐algae Ulva prolifera (Chlorophyta)

The photosynthetic performance of the desiccation‐tolerant, intertidal macro‐algae Ulva prolifera was significantly affected by sorbitol‐induced osmotic stress. Our results showed that photosynthetic activity decreased significantly with increases in sorbitol concentration. Although the partial activity of both photosystem I (PS I) and photosystem II (PS II) was able to recover after 30 min of rehydration, the activity of PS II decreased more rapidly than PS I. At 4 M sorbitol concentration, the activity of PS II was almost 0 while that of PS I was still at about one third of normal levels. Following prolonged treatment with 1 and 2 M sorbitol, the activity of PS I and PS II decreased slowly, suggesting that the effects of moderate concentrations of sorbitol on PS I and PS II were gradual. Interestingly, an increase in non‐photochemical quenching occurred under these conditions in response to moderate osmotic stress, whereas it declined significantly under severe osmotic stress. These results suggest that photoprotection in U. prolifera could also be induced by moderate osmotic stress. In addition, the oxidation of PS I was significantly affected by osmotic stress. P700⁺ in the thalli treated with high concentrations of sorbitol could still be reduced, as PS II was inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), but it could not be fully oxidized. This observation may be caused by the higher quantum yield of non‐photochemical energy dissipation in PS I due to acceptor‐side limitation (Y(NA)) during rehydration in seawater containing DCMU.     

Maize Sep15‐like functions in endoplasmic reticulum and reactive oxygen species homeostasis to promote salt and osmotic stress resistance

In animals, the Sep15 protein participates in disease resistance, growth, and development, but the function of its plant homologues remains unclear. Here, the function of maize Sep15 was analysed by characterization of two independent Sep15‐like loss‐of‐function mutants. In the absence of ZmSep15‐like, seedling tolerance to both water and salinity stress was compromised. The mutants experienced a heightened level of endoplasmic reticulum stress, and over‐accumulated reactive oxygen species, resulting in leaf necrosis. Characterization of Arabidopsis thaliana atsep15 mutant as well as like with ectopic expression of ZmSep15‐like indicated that ZmSep15‐like contributed to tolerance of both osmotic and salinity stress. ZmSep15‐like interacted physically with UDP‐glucose: glycoprotein glucosyltransferase1 (UGGT1). When the interaction was disrupted, the response to both osmotic and salinity stresses was impaired in maize or Arabidopsis. Co‐expressing ZmUGGT1 and ZmUGGT2 enhanced the tolerance of A. thaliana to both stressors, indicating a functional interaction between them. Together, the data indicated that plants Sep15‐like proteins promote osmotic and salinity stress resistance by influencing endoplasmic reticulum stress response and reactive oxygen species level.     

Knockout of AtDjB1, a J‐domain protein from Arabidopsis thaliana,alters plant responses to osmotic stress and abscisic acid

AtDjB1 is a member of the Arabidopsis thaliana J‐protein family. AtDjB1 is targeted to the mitochondria and plays a crucial role in A. thaliana heat and oxidative stress resistance. Herein, the role of AtDjB1 in adapting to saline and drought stress was studied in A. thaliana. AtDjB1 expression was induced through salinity, dehydration and abscisic acid (ABA) in young seedlings. Reverse genetic analyses indicate that AtDjB1 is a negative regulator in plant osmotic stress tolerance. Further, AtDjB1 knockout mutant plants (atj1‐1) exhibited greater ABA sensitivity compared with the wild‐type (WT) plants and the mutant lines with a rescued AtDjB1 gene. AtDjB1 gene knockout also altered the expression of several ABA‐responsive genes, which suggests that AtDjB1 is involved in osmotic stress tolerance through its effects on ABA signaling pathways. Moreover, atj1‐1 plants exhibited higher glucose levels and greater glucose sensitivity in the post‐germination development stage. Applying glucose promoted an ABA response in seedlings, and the promotion was more evident in atj1‐1 than WT seedlings. Taken together, higher glucose levels in atj1‐1 plants are likely responsible for the greater ABA sensitivity and increased osmotic stress tolerance.     

Gene mining in halophytes: functional identification of stress tolerance genes in Lepidium crassifolium

Extremophile plants are valuable sources of genes conferring tolerance traits, which can be explored to improve stress tolerance of crops. Lepidium crassifolium is a halophytic relative of the model plant Arabidopsis thaliana, and displays tolerance to salt, osmotic and oxidative stresses. We have employed the modified Conditional cDNA Overexpression System to transfer a cDNA library from L. crassifolium to the glycophyte A. thaliana. By screening for salt, osmotic and oxidative stress tolerance through in vitro growth assays and non‐destructive chlorophyll fluorescence imaging, 20 Arabidopsis lines were identified with superior performance under restrictive conditions. Several cDNA inserts were cloned and confirmed to be responsible for the enhanced tolerance by analysing independent transgenic lines. Examples include full‐length cDNAs encoding proteins with high homologies to GDSL‐lipase/esterase or acyl CoA‐binding protein or proteins without known function, which could confer tolerance to one or several stress conditions. Our results confirm that random gene transfer from stress tolerant to sensitive plant species is a valuable tool to discover novel genes with potential for biotechnological applications.     

Phosphatidylinositol‐hydrolyzing phospholipase C4 modulates rice response to salt and drought

Phosphatidylinositol‐specific phospholipase C (PI‐PLC) is involved in stress signalling but its signalling function remains largely unknown in crop plants. Here, we report that the PI‐PLC4 from rice (Oryza sativa cv), OsPLC4, plays a positive role in osmotic stress response. Two independent knockout mutants, plc4‐1 and plc4‐2, exhibited decreased seedling growth and survival rate whereas overexpression of OsPLC4 improved survival rate under high salinity and water deficiency, compared with wild type (WT). OsPLC4 hydrolyses PI, phosphatidylinositol 4‐phosphate (PI4P), and phosphatidylinositol‐4,5‐bisphosphate (PIP₂) to generate diacylglycerol (DAG) in vitro. Knockout of OsPLC4 attenuated salt‐induced increase of phosphatidic acid (PA) whereas overexpression of OsPLC4 decreased the level of PI4P and PIP₂ under salt treatment. Applications of DAG or PA restored the growth defect of plc4‐1 to WT but DAG kinase inhibitor 1 blocked the complementary effect of DAG in plc4‐1 under salt stress. In addition, the loss of OsPLC4 compromised the increase of inositol triphosphate and free cytoplasmic Ca²⁺ ([Ca²⁺]_cyt) and inhibited the induction of genes involved in Ca²⁺ sensor and osmotic stress response to salt stress. The results indicate that OsPLC4 modulates the activity of two signalling pathways, PA and Ca²⁺, to affect rice seedling response to osmotic stress.