Endocytic regulation of alkali metal transport proteins in mammals, yeast and plants

The Biostimulant, Potassium Humate Ameliorates Abiotic Stress in Arabidopsis thaliana by Increasing Starch Availability

Potassium humate is a widely used biostimulant known for its ability to enhance growth and improve tolerance to abiotic stress. However, the molecular mechanisms explaining its effects remain poorly understood. In this study, we investigated the mechanism of action of potassium humate using the model plant Arabidopsis thaliana. We demonstrated that a formulation of potassium humate effectively increased the fresh weight accumulation of Arabidopsis plants under normal conditions, salt stress (sodium or lithium chloride), and particularly under osmotic stress (mannitol). Interestingly, plants treated with potassium humate exhibited a reduced antioxidant response and lower proline accumulation, while maintaining photosynthetic activity under stress conditions. The observed sodium and osmotic tolerance induced by humate was not accompanied by increased potassium accumulation. Additionally, metabolomic analysis revealed that potassium humate increased maltose levels under control conditions but decreased levels of fructose. However, under stress, both maltose and glucose levels decreased, suggesting changes in starch utilization and an increase in glycolysis. Starch concentration measurements in leaves showed that plants treated with potassium humate accumulated less starch under control conditions, while under stress, they accumulated starch to levels similar to or higher than control plants. Taken together, our findings suggest that the molecular mechanism underlying the abiotic stress tolerance conferred by potassium humate involves its ability to alter starch content under normal growth conditions and under salt or osmotic stress.

Exomer Is Part of a Hub Where Polarized Secretion and Ionic Stress Connect

Plasma membrane and membranous organelles contribute to the physiology of the Eukaryotic cell by participating in vesicle trafficking and the maintenance of ion homeostasis. Exomer is a protein complex that facilitates vesicle transport from the trans-Golgi network to the plasma membrane, and its absence leads to the retention of a set of selected cargoes in this organelle. However, this retention does not explain all phenotypes observed in exomer mutants. The Schizosaccharomyces pombe exomer is composed of Cfr1 and Bch1, and cfr1Δ and bch1Δ were sensitive to high concentrations of potassium salts but not sorbitol, which showed sensitivity to ionic but not osmotic stress. Additionally, the activity of the plasma membrane ATPase was higher in exomer mutants than in the wild-type, pointing to membrane hyperpolarization, which caused an increase in intracellular K⁺ content and mild sensitivity to Na⁺, Ca²⁺, and the aminoglycoside antibiotic hygromycin B. Moreover, in response to K⁺ shock, the intracellular Ca²⁺ level of cfr1Δ cells increased significantly more than in the wild-type, likely due to the larger Ca²⁺ spikes in the mutant. Microscopy analyses showed a defective endosomal morphology in the mutants. This was accompanied by an increase in the intracellular pools of the K⁺ exporting P-type ATPase Cta3 and the plasma membrane Transient Receptor Potential (TRP)-like Ca²⁺ channel Pkd2, which were partially diverted from the trans-Golgi network to the prevacuolar endosome. Despite this, most Cta3 and Pkd2 were delivered to the plasma membrane at the cell growing sites, showing that their transport from the trans-Golgi network to the cell surface occurred in the absence of exomer. Nevertheless, shortly after gene expression in the presence of KCl, the polarized distribution of Cta3 and Pkd2 in the plasma membrane was disturbed in the mutants. Finally, the use of fluorescent probes suggested that the distribution and dynamics of association of some lipids to the plasma membrane in the presence of KCl were altered in the mutants. Thus, exomer participation in the response to K⁺ stress was multifaceted. These results supported the notion that exomer plays a general role in protein sorting at the trans-Golgi network and in polarized secretion, which is not always related to a function as a selective cargo adaptor.

Could vesicular transport of Na+ and Cl- be a feature of salt tolerance in halophytes?

Background

Halophytes tolerate external salt concentrations of 200 mm and more, accumulating salt concentrations of 500 mm and more in their shoots; some, recretohalophytes, excrete salt through glands on their leaves. Ions are accumulated in central vacuoles, but the pathway taken by these ions from the outside of the roots to the vacuoles inside the cells is poorly understood. Do the ions cross membranes through ion channels and transporters or move in vesicles, or both? Vesicular transport from the plasma membrane to the vacuole would explain how halophytes avoid the toxicity of high salt concentrations on metabolism. There is also a role for vesicles in the export of ions via salt glands.

Scope and Methods

We have collected data on the fluxes of sodium and chloride ions in halophytes, based on the weight of the transporting organs and on the membrane area across which the flux occurs; the latter range from 17 nmol m-2 s-1 to 4.2 μmol m-2 s-1 and values up to 1 μmol m-2 s-1 need to be consistent with whatever transport system is in operation. We have summarized the sizes and rates of turnover of vesicles in plants, where clathrin-independent vesicles are 100 nm or more in diameter and can merge with the plasma membrane at rates of 100 s-1. We gathered evidence for vesicular transport of ions in halophytes and evaluated whether vesicular transport could account for the observable fluxes.

Conclusions

There is strong evidence in favour of vesicular transport in plants and circumstantial evidence in favour of the movement of ions in vesicles. Estimated rates of vesicle turnover could account for ion transport at the lower reported fluxes (around 20 nmol m-2 s-1), but the higher fluxes may require vesicles of the order of 1 μm or more in diameter. The very high fluxes reported in some salt glands might be an artefact of the way they were measured.

Distinctive physiological and molecular responses to cold stress among cold-tolerant and cold-sensitive Pinus halepensis seed sources

BACKGROUND

Forest species ranges are confined by environmental limitations such as cold stress. The natural range shifts of pine forests due to climate change and proactive-assisted population migration may each be constrained by the ability of pine species to tolerate low temperatures, especially in northern latitudes or in high altitudes. The aim of this study is to characterize the response of cold-tolerant versus cold-sensitive Pinus halepensis (P. halepensis) seedlings at the physiological and the molecular level under controlled cold conditions to identify distinctive features which allow us to explain the phenotypic difference. With this objective gas-exchange and water potential was determined and the photosynthetic pigments, soluble sugars, glutathione and free amino acids content were measured in seedlings of different provenances under control and cold stress conditions.

RESULTS

Glucose and fructose content can be highlighted as a potential distinctive trait for cold-tolerant P. halepensis seedlings. At the amino acid level, there was a significant increase and accumulation of glutathione, proline, glutamic acid, histidine, arginine and tryptophan along with a significant decrease of glycine.

CONCLUSION

Our results established that the main difference between cold-tolerant and cold-sensitive seedlings of P. halepensis is the ability to accumulate the antioxidant glutathione and osmolytes such as glucose and fructose, proline and arginine.

Low-affinity iron transport protein Uvt3277 is important for pathogenesis in the rice false smut fungus Ustilaginoidea virens

Ustilaginoidea virens is the causal agent of rice false smut disease resulting in quantitative and qualitative losses in rice. To gain insights into the pathogenic mechanisms of U. virens, we established a T-DNA insertion mutant library of U. virens through Agrobacterium tumefaciens-mediated transformation and selected an enhanced pathogenicity mutant (i.e., B3277). We analyzed the biological characteristics of the wild-type P1 and B3277. The growth rate and sporulation of B3277 were decreased compared with those of P1; the ferrous iron could be utilized by B3277, but inhibited the growth of P1. Southern blot analysis was performed to verify the copy number of the foreign gene inserted in the genomic DNA and only one copy of the T-DNA was found. The combined hiTAIL-PCR with RACE-PCR analysis showed the successful cloning of full length of the T-DNA flanking gene associated with pathogenicity, named Uvt3277. Gene expression was analyzed using real-time PCR. Results revealed that Uvt3277 was expressed at lower levels in B3277 than in P1. This gene was then subjected to bioinformatics analysis. The encoded protein of Uvt3277 exhibited high homology with low-affinity iron transporter proteins in some fungi. Transformation of the RNAi vector by constructing the hairpin RNA of the target gene was confirmed as successful. The pathogenicity of the transformant also increased. These results suggested that Uvt3277 may have an important function associated with the pathogenesis of U. virens. This study provides insights into the pathogenic mechanism of U. virens and a molecular target of disease control.

Regulation of the Yeast Hxt6 Hexose Transporter by the Rod1 α-Arrestin, the Snf1 Protein Kinase, and the Bmh2 14-3-3 Protein

Cell viability requires adaptation to changing environmental conditions. Ubiquitin-mediated endocytosis plays a crucial role in this process, because it provides a mechanism to remove transport proteins from the membrane. Arrestin-related trafficking proteins are important regulators of the endocytic pathway in yeast, facilitating selective ubiquitylation of target proteins by the E3 ubiquitin ligase, Rsp5. Specifically, Rod1 (Art4) has been reported to regulate the endocytosis of both the Hxt1, Hxt3, and Hxt6 glucose transporters and the Jen1 lactate transporter. Also, the AMP kinase homologue, Snf1, and 14-3-3 proteins have been shown to regulate Jen1 via Rod1. Here, we further characterized the role of Rod1, Snf1, and 14-3-3 in the signal transduction route involved in the endocytic regulation of the Hxt6 high affinity glucose transporter by showing that Snf1 interacts specifically with Rod1 and Rog3 (Art7), that the interaction between the Bmh2 and several arrestin-related trafficking proteins may be modulated by carbon source, and that both the 14-3-3 protein Bmh2 and the Snf1 regulatory domain interact with the arrestin-like domain containing the N-terminal half of Rod1 (amino acids 1-395). Finally, using both co-immunoprecipitation and bimolecular fluorescence complementation, we demonstrated the interaction of Rod1 with Hxt6 and showed that the localization of the Rod1-Hxt6 complex at the plasma membrane is affected by carbon source and is reduced upon overexpression of SNF1 and BMH2.

A functional Rim101 complex is required for proper accumulation of the Ena1 Na+-ATPase protein in response to salt stress in Saccharomyces cerevisiae

The maintenance of ionic homeostasis is essential for cell viability, thus the activity of plasma membrane ion transporters must be tightly controlled. Previous studies in Saccharomyces cerevisiae revealed that the proper trafficking of several nutrient permeases requires the E3 ubiquitin ligase Rsp5 and, in many cases, the presence of specific adaptor proteins needed for Rsp5 substrate recognition. Among these adaptor proteins are nine members of the arrestin-related trafficking adaptor (ART) family. We studied the possible role of the ART family in the regulation of monovalent cation transporters. We show here that the salt sensitivity phenotype of the rim8/art9 mutant is due to severe defects in Ena1 protein accumulation, which is not attributable to transcriptional defects. Many components of the Rim pathway are required for correct Ena1 accumulation, but not for the accumulation of other nutrient permeases. Moreover, we observe that strains lacking components of the endosomal sorting complexes required for transport (ESCRT) pathway previously described to play a role in Rim complex formation present similar defects in Ena1 accumulation. Our results show that, in response to salt stress, a functional Rim complex via specific ESCRT interactions is required for the proper accumulation of the Ena1 protein, but not induction of the ENA1 gene.

Vcx1 and ESCRT components regulate intracellular pH homeostasis in the response of yeast cells to calcium stress

Endosomal sorting complexes required for transport (ESCRTs) are involved in the formation of multivesicular bodies and sorting of targeted proteins to the yeast vacuole. The deletion of seven genes encoding components of the ESCRT machinery render Saccharomyces cerevisiae cells sensitive to high extracellular CaCl2 concentrations as well as to low pH in media. In this work, we focused on intracellular pH (pHin) homeostasis of these mutants. None of the studied ESCRT mutants exhibited an altered pHin level compared to the wild type under standard growth conditions. Nevertheless, 60 min of CaCl2 treatment resulted in a more significant drop in pHin levels in these mutants than in the wild type, suggesting that pHin homeostasis is affected in ESCRT mutants upon the addition of calcium. Similarly, CaCl2 treatment caused a bigger pHin decrease in cells lacking the vacuolar Ca(2+)/H(+) antiporter Vcx1 which indicates a role for this protein in the maintenance of proper pHin homeostasis when cells need to cope with a high CaCl2 concentration in media. Importantly, ESCRT gene deletions in the vcx1Δ strain did not result in an increase in the CaCl2-invoked drop in the pHin levels of cells, which demonstrates a genetic interaction between VCX1 and studied ESCRT genes.

Systems biology of monovalent cation homeostasis in yeast: the translucent contribution

Maintenance of monovalent cation homeostasis (mainly K(+) and Na(+)) is vital for cell survival, and cation toxicity is at the basis of a myriad of relevant phenomena, such as salt stress in crops and diverse human diseases. Full understanding of the importance of monovalent cations in the biology of the cell can only be achieved from a systemic perspective. Translucent is a multinational project developed within the context of the SysMO (System Biology of Microorganisms) initiative and focussed in the study of cation homeostasis using the well-known yeast Saccharomyces cerevisiae as a model. The present review summarize how the combination of biochemical, genetic, genomic and computational approaches has boosted our knowledge in this field, providing the basis for a more comprehensive and coherent vision of the role of monovalent cations in the biology of the cell.

Ubiquitination in plant nutrient utilization

Ubiquitin (Ub) is well-established as a major modifier of signaling in eukaryotes. However, the extent to which plants rely on Ub for regulating nutrient uptake is still in its infancy. The main characteristic of ubiquitination is the conjugation of Ub onto lysine residues of acceptor proteins. In most cases the targeted protein is rapidly degraded by the 26S proteasome, the major proteolysis machinery in eukaryotic cells. The Ub-proteasome system is responsible for removing most abnormal peptides and short-lived cellular regulators, which, in turn, control many processes. This allows cells to respond rapidly to intracellular signals and changing environmental conditions. This perspective will discuss how plants utilize Ub conjugation for sensing environmental nutrient levels. We will highlight recent advances in understanding how Ub aids nutrient homeostasis by affecting the trafficking of membrane bound transporters. Given the overrepresentation of genes encoding Ub-metabolizing enzymes in plants, intracellular signaling events regulated by Ub that lead to transcriptional responses due to nutrient starvation is an under explored area ripe for new discoveries. We provide new insight into how Ub based biochemical tools can be exploited to reveal new molecular components that affect nutrient signaling. The mechanistic nature of Ub signaling indicates that dominant form of any new molecular components can be readily generated and are likely shed new light on how plants cope with nutrient limiting conditions. Finally as part of future challenges in this research area we introduce the newly discovered roles of Ub-like proteins in nutrient homeostasis.