NaCl-induced CsRCI2E and CsRCI2F interact with aquaporin CsPIP2;1 to reduce water transport in Camelina sativa L

Mechanism of an animal toxin-antidote system

Toxin-antidote systems are selfish genetic elements composed of a linked toxin and antidote. The peel-1 zeel-1 toxin-antidote system in C. elegans consists of a transmembrane toxin protein PEEL-1 which acts cell autonomously to kill cells. Here we investigate the molecular mechanism of PEEL-1 toxicity. We find that PEEL-1 requires a small membrane protein, PMPL-1, for toxicity. Together, PEEL-1 and PMPL-1 are sufficient for toxicity in a heterologous system, HEK293T cells, and cause cell swelling and increased cell permeability to monovalent cations. Using purified proteins, we show that PEEL-1 and PMPL-1 allow ion flux through lipid bilayers and generate currents which resemble ion channel gating. Our work suggests that PEEL-1 kills cells by co-opting PMPL-1 and creating a cation channel.

Plant biomarkers as early detection tools in stress management in food crops: a review

MAIN CONCLUSION

Plant Biomarkers are objective indicators of a plant's cellular state in response to abiotic and biotic stress factors. They can be explored in crop breeding and engineering to produce stress-tolerant crop species. Global food production safely and sustainably remains a top priority to feed the ever-growing human population, expected to reach 10 billion by 2050. However, abiotic and biotic stress factors negatively impact food production systems, causing between 70 and 100% reduction in crop yield. Understanding the plant stress responses is critical for developing novel crops that can adapt better to various adverse environmental conditions. Using plant biomarkers as measurable indicators of a plant's cellular response to external stimuli could serve as early warning signals to detect stresses before severe damage occurs. Plant biomarkers have received considerable attention in the last decade as pre-stress indicators for various economically important food crops. This review discusses some biomarkers associated with abiotic and biotic stress conditions and highlights their importance in developing stress-resilient crops. In addition, we highlighted some factors influencing the expression of biomarkers in crop plants under stress. The information presented in this review would educate plant researchers, breeders, and agronomists on the significance of plant biomarkers in stress biology research, which is essential for improving plant growth and yield toward sustainable food production.

Overexpression of MsRCI2D and MsRCI2E Enhances Salt Tolerance in Alfalfa (Medicago sativa L.) by Stabilizing Antioxidant Activity and Regulating Ion Homeostasis

Rare cold-inducible 2 (RCI2) genes from alfalfa (Medicago sativa L.) are part of a multigene family whose members respond to a variety of abiotic stresses by regulating ion homeostasis and stabilizing membranes. In this study, salt, alkali, and ABA treatments were used to induce MsRCI2D and MsRCI2E expression in alfalfa, but the response time and the expression intensity of the MsRCI2D,-E genes were different under specific treatments. The expression intensity of the MsRCI2D gene was the highest in salt- and alkali-stressed leaves, while the MsRCI2E gene more rapidly responded to salt and ABA treatment. In addition to differences in gene expression, MsRCI2D and MsRCI2E differ in their subcellular localization. Akin to MtRCI2D from Medicago truncatula, MsRCI2D is also localized in the cell membrane, while MsRCI2E is different from MtRCI2E, localized in the cell membrane and the inner membrane. This difference might be related to an extra 20 amino acids in the C-terminal tail of MsRCI2E. We investigated the function of MsRCI2D and MsRCI2E proteins in alfalfa by generating transgenic alfalfa chimeras. Compared with the MsRCI2E-overexpressing chimera, under high-salinity stress (200 mmol·L-1 NaCl), the MsRCI2D-overexpressing chimera exhibited a better phenotype, manifested as a higher chlorophyll content and a lower MDA content. After salt treatment, the enzyme activities of SOD, POD, CAT, and GR in MsRCI2D- and -E-overexpressing roots were significantly higher than those in the control. In addition, after salt stress, the Na+ content in MsRCI2D- and -E-transformed roots was lower than that in the control; K+ was higher than that in the control; and the Na+/K+ ratio was lower than that in the control. Correspondingly, H+-ATPase, SOS1, and NHX1 genes were significantly up-regulated, and the HKT gene was significantly down-regulated after 6 h of salt treatment. MsRCI2D was also found to regulate the expression of the MsRCI2B and MsRCI2E genes, and the MsRCI2E gene could alter the expression of the MsRCI2A, MsRCI2B, and MsRCI2D genes. MsRCI2D- and -E-overexpressing alfalfa was found to have higher salt tolerance, manifested as improved activity of antioxidant enzymes, reduced content of reactive oxygen species, and sustained Na+ and K+ ion balance by regulating the expression of the H+-ATPase, SOS1, NHX1, HKT, and MsRCI2 genes.

CsRCI2D enhances high-temperature stress tolerance in Camelina sativa L. through endo-membrane trafficking from the plasma membrane

Rare Cold Inducible 2s (RCI2s) are hydrophobic proteins in cell membranes that participate in abiotic stress tolerance mechanisms. Additionally, they are used as traceable membrane trafficking markers in endocytosis studies. Plants regulate cell homeostasis through endocytosis by limiting the activity of plasma membrane transporter proteins to adapt to stressful conditions. In this study, we found high temperature (HT) stress-induced membrane trafficking of RCI2D in Camelina sativa L. The gene expression and protein synthesis were increased by HT stress at 37 °C. Moreover, rapid membrane trafficking of CsRCI2D was traced by multiple-phase membrane fractionation using sucrose density gradients and compared with CsRCI2E/F/G from the same protein family subgroup. The distribution of CsRCI2s was shown to be similar to that of the clathrin heavy chain, which is known as a major endocytosis protein. Subcellular localization of CsRCI2D was observed in the plasma membrane and endo-membranes and overlapped with membrane lipids. CsRCI2D co-localized with lipids, and its overexpression increased the intracellular lipid content compared to that of wild-type camelina. Moreover, transgenic camelina lines showed enhanced HT stress tolerance during germination and hypocotyl elongation when compared to the wild type. These results suggest that HT-induced CsRCI2D membrane trafficking enhances HT stress tolerance in camelina.

Overexpression of MsRCI2A, MsRCI2B, and MsRCI2C in Alfalfa (Medicago sativa L.) Provides Different Extents of Enhanced Alkali and Salt Tolerance Due to Functional Specialization of MsRCI2s

Rare cold-inducible 2/plasma membrane protein 3 (RCI2/PMP3) genes are ubiquitous in plants and belong to a multigene family whose members respond to a variety of abiotic stresses by regulating ion homeostasis and stabilizing membranes, thus preventing damage. In this study, the expression of MsRCI2A, MsRCI2B, and MsRCI2C under high-salinity, alkali and ABA treatments was analyzed. The results showed that the expression of MsRCI2A, MsRCI2B, and MsRCI2C in alfalfa (Medicago sativa L.) was induced by salt, alkali and ABA treatments, but there were differences between MsRCI2 gene expression under different treatments. We investigated the functional differences in the MsRCI2A, MsRCI2B, and MsRCI2C proteins in alfalfa (Medicago sativa L.) by generating transgenic alfalfa plants that ectopically expressed these MsRCI2s under the control of the CaMV35S promoter. The MsRCI2A/B/C-overexpressing plants exhibited different degrees of improved phenotypes under high-salinity stress (200 mmol.L^-1 NaCl) and weak alkali stress (100 mmol.L^-1 NaHCO₃, pH 8.5). Salinity stress had a more significant impact on alfalfa than alkali stress. Overexpression of MsRCI2s in alfalfa caused the same physiological response to salt stress. However, in response to alkali stress, the three proteins encoded by MsRCI2s exhibited functional differences, which were determined not only by their different expression regulation but also by the differences in their regulatory relationship with MsRCI2s or H⁺-ATPase.

Adaptable and Multifunctional Ion-Conducting Aquaporins

Aquaporins function as water and neutral solute channels, signaling hubs, disease virulence factors, and metabolon components. We consider plant aquaporins that transport ions compared to some animal counterparts. These are candidates for important, as yet unidentified, cation and anion channels in plasma, tonoplast, and symbiotic membranes. For those individual isoforms that transport ions, water, and gases, the permeability spans 12 orders of magnitude. This requires tight regulation of selectivity via protein interactions and posttranslational modifications. A phosphorylation-dependent switch between ion and water permeation in AtPIP2;1 might be explained by coupling between the gates of the four monomer water channels and the central pore of the tetramer. We consider the potential for coupling between ion and water fluxes that could form the basis of an electroosmotic transducer. A grand challenge in understanding the roles of ion transporting aquaporins is their multifunctional modes that are dependent on location, stress, time, and development.

Functional responses between PMP3 small membrane proteins and membrane potential

The Plasma Membrane Proteolipid 3 (PMP3, UPF0057 family in Uniprot) family consists of abundant small hydrophobic polypeptides with two predicted transmembrane helices. Plant homologues were upregulated in response to drought/salt-stresses and yeast deletion mutants exhibited conditional growth defects. We report here abundant expression of Group I PMP3 homologues (PMP3(i)hs) during normal vegetative growth in both prokaryotic and eukaryotic cells, at a level comparable to housekeeping genes, implicating the regular cellular functions. Expression of eukaryotic PMP3(i)hs was dramatically upregulated in response to membrane potential (Vm) variability (Vm_var ), whereas PMP3(i)hs deletion-knockdown led to Vm changes with conditional growth defects. Bacterial PMP3(i)h yqaE deletion led to a shift of salt sensitivity; Vm_var alternations with exogenous K⁺ addition downregulated prokaryotic PMP3(i)hs, suggesting [K⁺ ]-Vm_var axis being a significant feedback element in prokaryotic ionic homeostasis. Remarkably, the eukaryotic homologues functionally suppressed the conditional growth defects in bacterial deletion mutant, demonstrating the conserved cross-kingdom membrane functions by PMP3(i)hs. These data demonstrated a direct reciprocal relationship between PMP3(i)hs expression and Vm differentials in both prokaryotic and eukaryotic cells. Cumulative with PMP3(i)hs ubiquitous abundance, their lipid-binding selectivity and membrane protein colocalization, we propose [PMP3(i)hs]-Vm_var axis as a key element in membrane homeostasis.

Subcellular Journey of Rare Cold Inducible 2 Protein in Plant Under Stressful Condition

Rare cold inducible 2 (RCI2) proteins are small hydrophobic membrane proteins in plants, and it has been widely reported that RCI2 expressions are dramatically induced by salt, cold, and drought stresses in many species. The RCI2 proteins have been shown to regulate plasma membrane (PM) potential and enhance abiotic stress tolerance when over-expressed in plants. RCI2 protein structures contain two transmembrane domains that are thought to be PM intrinsic proteins and have been observed at the PM and endomembranes. However, cellular trafficking of RCI2s are not fully understood. In this review, we discussed (i) general properties of RCI2s characterized in many species, (ii) the uses of RCI2s as a tracer in live cell imaging analyses and when they are fused to fluorescence proteins during investigations into vesicle trafficking, and (iii) RCI2 functionalities such as their involvement in rapid diffusion, endocytosis, and protein interactions. Consequently, the connection between physiological characteristics of RCI2s and traffic of RCI2s interacting membrane proteins might be helpful to understand role of RCI2s contributing abiotic stresses tolerance.