Ca2+ signaling in plant responses to abiotic stresses

Adverse variations of abiotic environmental cues that deviate from an optimal range impose stresses to plants. Abiotic stresses severely impede plant physiology and development. Consequently, such stresses dramatically reduce crop yield and negatively impact on ecosystem stability and composition. Physical components of abiotic stresses can be, for example, suboptimal temperature and osmotic perturbations, while representative chemical facets of abiotic stresses can be toxic ions or suboptimal nutrient availability. The sheer complexity of abiotic stresses causes a multitude of diverse components and mechanisms for their sensing and signal transduction. Ca²⁺, as a versatile second messenger, plays multifaceted roles in almost all abiotic stress responses in that, for a certain abiotic stress, Ca²⁺ is not only reciprocally connected with its perception, but also multifunctionally ensures subsequent signal transduction. Here, we will focus on salt/osmotic stress and responses to altered nutrient availability as model cases to detail novel insights into the identity of components that link stress perception to Ca²⁺ signal formation as well as on new insights into mechanisms of Ca²⁺ signal implementation. Finally, we will deduce emerging conceptual consequences of these novel insights and outline arising avenues of future research on the role of Ca²⁺ signaling in abiotic stress responses in plants.     

Gel‐like inclusions of C‐terminal fragments of TDP‐43 sequester stalled proteasomes in neurons

Aggregation of the multifunctional RNA‐binding protein TDP‐43 defines large subgroups of amyotrophic lateral sclerosis and frontotemporal dementia and correlates with neurodegeneration in both diseases. In disease, characteristic C‐terminal fragments of ~25 kDa ("TDP‐25") accumulate in cytoplasmic inclusions. Here, we analyze gain‐of‐function mechanisms of TDP‐25 combining cryo‐electron tomography, proteomics, and functional assays. In neurons, cytoplasmic TDP‐25 inclusions are amorphous, and photobleaching experiments reveal gel‐like biophysical properties that are less dynamic than nuclear TDP‐43. Compared with full‐length TDP‐43, the TDP‐25 interactome is depleted of low‐complexity domain proteins. TDP‐25 inclusions are enriched in 26S proteasomes adopting exclusively substrate‐processing conformations, suggesting that inclusions sequester proteasomes, which are largely stalled and no longer undergo the cyclic conformational changes required for proteolytic activity. Reporter assays confirm that TDP‐25 impairs proteostasis, and this inhibitory function is enhanced by ALS‐causing TDP‐43 mutations. These findings support a patho‐physiological relevance of proteasome dysfunction in ALS/FTD.     

Inactivation of the early calcium uptake and noradrenaline release evoked by potassium in cultured chromaffin cells

Upon stimulation with a 59 mM K solution (59K), 45Ca uptake into cultured bovine adrenal chromaffin cells quickly enhanced to reach a plateau within 60 sec. 45Ca transients could be clearly measured with a time resolution (10 sec) and a net Ca uptake (75 times the basal uptake) that considerably improve data reported in other recent papers; this experimental design allows the direct comparison of 45Ca transient data with electrophysiological measurements of chromaffin cell Ca currents. In addition, it is shown that upon sustained depolarization with 59K both, the rates of 45Ca uptake and 3H-noradrenaline release decline in a parallel manner, suggesting that the voltage-dependent Ca channel activity modulates the kinetics of the early secretory response.     

Design,Synthesis, Characterization and Computational Studies of Mannich Bases Oxadiazole Derivatives as New Class of Jack Bean Urease Inhibitors

Mannich bases consisting of 1,3,4-oxadiazole-2-thione ( 3 a – 3 l ) bearing various substituents were synthesized and found potent jack bean urease inhibitors. The prepared compounds showed significantly good inhibitory activities with IC₅₀ values from 9.45±0.05 to 267.42±0.23 μM. The compound 3 k containing 4-chlorophenyl (−R) and 4-hydroxyphenyl (−R′) was most active with IC₅₀ 9.45±0.05 μM followed by 3 e (IC₅₀ 22.52±0.15 μM) in which −R was phenyl and −R′ was isopropyl group. However, when both −R and −R′ were either 4-chlorophenyl groups ( 3 l ) or only −R′ was 4-nitrophenyl ( 3 i ), both compounds were found inactive. The detailed binding affinities of the produced compounds with protein were explored through molecular docking and data-supported in-vitro enzyme inhibition profiles. Drug likeness was confirmed by in silico ADME investigations and molecular orbital analysis (HOMO-LUMO) and electrostatic potential maps were got from DFT calculations. ESP maps exposed that there are two potential binding sites with the most positive and most negative parts.     

Cellular utilization of cytosolic NADPH in kidney and liver cells from rats fed a normal or a vitamin D-deficient diet

The amount of reducing equivalents from NADPH generated by glucose 6-phosphate dehydrogenase activity (G6PD) used in mixed function oxidation (pathway I) or in reductive biosynthesis (pathway II) has been determined by cytochemical methods and microdensitometry in cells from the pars recta (PR) and distal convoluted tubule (DCT) of the kidney and from centrilobular (CL) and periportal (PP) hepatocytes from rats fed a normal or a vitamin D-deficient diet. In the kidney, pathway I activity was similar to that of pathway II in PR, whereas in DCT pathway II was markedly predominant. Feeding a vitamin D-deficient diet resulted in an increase in the total amount of reducing equivalents in PR and DCT. This increase was due to a rise in pathway I activity in the PR, whereas in the DCT the increase resulted from a stimulation of pathway II activity. Pathway I activity in PR was inversely correlated with plasma calcium, and was significantly decreased when calcium (1 mM) was added in vitro. In the liver the total amount of reducing equivalents generated by G6PD and both hydrogen pathways, was higher in CL than in PP hepatocytes. In CL cells, a vitamin D-deficient diet induced a significant increase in both NADPH pathways. Furthermore, in these cells pathway I activity was inversely related to plasma calcium and was significantly lowered when 1 mM calcium was added in vitro. It is concluded that vitamin D status and calcium influence the production and utilization of cytosolic reducing equivalents both in kidney and liver.     

Differential physiological and biochemical responses of Quercus infectoria and Q. libani to drought and charcoal disease

The vast oak-dominated forests of the Zagros Mountains in southwestern Iran currently undergo large-scale dieback driven by a combination of drought and increasing incidence of charcoal disease caused by the fungal pathogens Biscogniauxia mediterranea and Obolarina persica. Here, we explore the interactive effects between drought and charcoal disease agents on the physiology and biochemistry of Quercus infectoria and Quercus libani seedlings. The combination of pathogen attack and water limitation hampered plant development, especially in Q. libani seedlings, negatively affecting growth, biomass production, photosynthetic efficiency, and leaf water potential. An increase in markers of oxidative damage together with the upregulation of the antioxidant defense revealed that drought stress and pathogen infection led to pro-oxidative conditions in both oak species, especially in Q. libani, where larger changes in malondialdehyde and hydrogen peroxide occurred. The upregulation of the antioxidant system was more prominent in Q. infectoria than in Q. libani, resulting in enhanced enzyme activity and accumulation of non-enzymatic antioxidants. Fungal infection stimulated the activity of chitinase, phenylalanine ammonia lyase and β-1,3-glucanase in Q. infectoria leaves and this response became more pronounced under water shortage. Our study highlights that drought stress greatly intensifies the effects of the charcoal disease. Moreover, our findings imply superior stress resistance of Q. infectoria conferred by a highly efficient antioxidant system, strong osmotic adjustment (through proline), and increases in resistance enzymes and secondary metabolites (phenols and flavonoids). Future investigations should focus on adult trees in their natural habitat including interactions with soil factors and other pathogens like nematodes, bacteria and other fungi. Because the present research was conducted on oak seedlings, the findings can be considered by forest nursery managers.     

Influence of hypertonic solutions on catecholamine release from intact and permeabilized cultured chromaffin cells

Chromaffin cells purified from bovine adrenal medulla and maintained in primary culture were used to study the effects of hyperosmolarity on the nicotine- and high potassium-induced secretory response. A similar study was also performed on cells permeabilized with digitonin and with alpha-toxin from Staphylococcus aureus. Hyperosmolarity does not affect the spontaneous release of catecholamines from either intact cells or permeabilized cells. The nicotine-induced secretion and high potassium-induced secretion from intact cells are inhibited by hypertonic solutions; a 100% inhibition of net release was observed at 660 mOsm (sucrose as osmotic agent). Veratridine- and the cation ionophore X537-A-induced release were both depressed under hyperosmotic conditions. Hyperosmolarity was shown to have reversible effects on the secretory response of intact cells. Finally, hyperosmolarity has intracellular effects on catecholamine release evoked by calcium from both detergent- and alpha-toxin-permeabilized cells. Our data show that hyperosmolarity has multiple effects on the cell membrane and the protein constituents associated with it, but has also a significant effect on intracellular reactions concerned with exocytosis.