Novel insights into SOS1 in managing vacuolar sodium toxicity

Salinization due to excessive water-soluble salt addition to soils is a global concern for agriculture, around 20-40% crop lossesal due to soil salinity production and food security (Tarolli et al. 2024). Most of the agricultural plants are sensitive to toxic sodium (Na + ) of soils, impairing plant growth, development and productivity that occur agricultural yield loss approximately 10-60% 20-40% (Ramakrisna et al., 2025Zheng et al. 2023). The classical approach of plant salinity tolerance is emphasized on the role of plasmamembrane-localized sodium (Na + )/proton (H + ) antiporter SALT OVERLY SENSITIVE 1 (SOS1) in extruding Na + from cytoplasm to outside (Xie et al., 2022). This model established in last two decades that improved our understanding on how plant The homeostat toxic Na + in plants. However, a novel insight into dual functions of SOS1 in vacuolar Na + sequestration has been explored using cryo-nanoscale secondary ion mass spectrometry (CryoNanoSIMS), although the role of SOS1 in Na + extrusion is well recognized in plants. grain or agricultural yield loss due to salt stress may vary according to crop genotypes and sensitivity of the crops to salinity. Salt stress and its tackling by plants is a complex issue. Thus, it is crucial to explore molecular mechanisms underlaying tackling Na + and other elements, their molecular trade-offs and interplay toward salt stress resilience. Plants have evolved several mechanisms including Na + accumulation, storing, homeostasis, elemental interdependence, and cellular adaptation are vital for exploring the mechanistic insights of salt stress responses in plants (Hualpa-Ramirez et al., 2024).Introducing of CryoNanoSIMS approach has explored new dimension of research on subcellular distribution of ions in their original and hydrated state. This mass spectrometry tool that provides direct opportunity to visualize Na + , K + and other key ions at subcellular organelle levels. The application of CryoNanoSIMS tool in salt stress response explores deeper and more dynamic picture of how root meristematic cells (RMCs) drive root growth and it manage Na + toxicity in response to different salt levels. This opinion manuscript addresses the paradigm-shifting insights specifically the double roles of SOS1 in plasmamembrane Na + export and vacuolar Na + sequestration. I argue on this novel insight and its fundamental necessity for redefining of Na + management strategies that open new opportunities designing salinity-smart future crop production. In addition, I propose to design salt-tolerant smart crop with paradigm-shifting in Na + management approaches addressing from apoplastic retention at low salt levels to vacuolar sequestration at high salt levels. Hopefully, the Na + management strategies represent an unprecedented adaptive mechanism of salinity tolerance in different plant species.3. New insights: an imaging-method for Na + tracking into root cellsTracking Na + is essential for exploring how plant manage toxic level of Na + into plant cells. Ramakrishna et al. (2025) have explored a CryoNanoSIMS-method technique by which Na + and other elements containing specimens were observed with a focus beam. The location of Na + in cytoplasm and its accumulation inside vacuole was identified in Arabidopsis root meristem cells using CryoNanoSIMS approach. Furthermore, the SOS1 location of tonoplast and its function in transporting Na + were confirmed in wild type Arabidopsis after considering a comparative study in sos1-1 mutant plants. However, the study of Arabidopsis was extended to rice, showed almost same fashion of elemental accumulation inside vacuole, while rice SOS1 transported Na + into vacuole. These consecutive findings suggesting that the CryoNanoSIMS tool is suitable for model plant Arabidopsis and other grain crops. This novel approach with gold standard of elemental imaging provides several key advantages including identification benefits of multiple target ions (Na + , K + and Ca 2+ ) at a time. This cryogenic approach keeps ion distribution almost natural state that is required for avoiding ion redistribution. This tool allows chemical fixation of sample materials, provides stability that crucial is for results accuracy and avoiding diffusible ion (Na + ) leaching of cells and tissues. Although CryoNanoSIMS is not a method for the elements that contains atomic mass higher than the K. Without this aspect, the CryoNanoSIMS method is suitable for Na + tracking, supporting the SOS1 involving Na + sequestration inside vacuole. The CryoNanoSIMS-based imaging method adds new dimension to our understanding of elements distribution, homeostasis, and cellular adaptation under salt stress.Plants roots are the first sensing organ, especially the root meristematic tissue play pivotal role in salt sensing, signalling, transport and storage of Na + ion in response to salt stress (Rahman et a., 2022;Wang et al. 2022). Ramakrishna et al. (2025) have recently explored the process of shifting Na + from the cytoplasm to vacuole of Arabidopsis roots . Plant vacuole maintains cells rigidity and store toxic Na + and other elements that help to protect enzymes present in the cytoplasm. Hints, Na + accumulation and localization pattern are different from the K + and other elements in the subcellular organelles. Thus, tracking of toxic Na + inside plant cells show the way of its accumulation pattern into plant cells, indicating the hallmark of Na + tackling processes that help to plants dealing with salt stress.There are sSeveral imaging techniques are available for investigating imaging elements and their distribution patterns into plant cells, including particle-induced X-ray emission (microPIXE), and low energy synchrotron-based X-ray fluorescence microscopy (XFM). The limitation of the fluorescent sensor or radioisotope-based elemental imaging approach is it can identify only single target ions at a time. A recent study by Ramakrishna et al. (2025) have explored a cryo nanoscale secondary ion mass spectrometry (CryoNanoSIMS) ion microprobe approach. CryoNanoSIMS is a study of biological tissues by visualizing and quantifying of metabolic processes at subcellular level (Meibom et al., 2023). This new tool provides promising breakthrough of handling toxic Na + and other elements during salt stress, opening new dimension of research in salt stress responses, and plant resilience for sustainable agricultural production. CryoNanoSIMS study is concentration dependant approach for localizing Na + patterns. For instance, at low external Na + level (≤ 2.5 mM salt) the Na + is mainly accumulated in cell walls and low presence of Na + into cytoplasm. However, moderate to high saline condition (25-100 mM salt) with wild type plant shows a significant redirection of Na + into vacuole, while sos1mutant exhibit reverse pattern that enhance Na + in cytoplasm and decline vacuolar accumulation (Figure 1). This finding suggest that plant possess distinct detoxification mechanisms that depends on the intensity of salt stress.At low Na + level, controlling Na + in the apoplast through minimizing accumulation or moderate Na + extrusion is metabolically potential. However, as the external Na + increases to significant toxic level, this strategy becomes inefficient. In this scenario, cell must require to active better mechanism of salt tolerance mechanism like vacuolar sequestration that separate Na + from cytoplasmic processes during cellular osmotic balance. It is still unclear, the molecular mechanism of triggering this shifting, specific function of increasing cytosolic Na + concentration, and the sensing mechanism that recognize apoplastic Na + levels. Recent study has recognized SOS1 does not significantly alter the Na + rather than plasmamembrane (PM) accumulation (Ramakrishna et al., 2025). In has already been recognized that SOS3 acts as Ca 2+ sensor, SOS2 is a protein kinase, are known to combinedly regulate SOS1 activity, but the SOS1 function at PM versus endomembrane (EM) compartments still a fascinating area of exploration.Recent discovery of SOS1 localization at late-endosome (LE) and perivacuolar compartment, along with PM and tonoplast, explores a mechanistic insight for advancing understanding of vacuolar Na + accumulation (Ramakrishna et al., 2025). This particular mechanistic framework is fascinating as it suggests that Na + is sequestrated into endosomal compartments (EC) and the way to vacuole, except is transported directly across the tonoplast (Figure 1). Thus, the intensity of surface-to-volume ratio of the vacuolar compartments (VC) provides a new area of understanding rapid Na + uptake and managing strategies of toxic Na + from the vacuole.SOS1 coordinates dual functions, Na + extrusion and vacuolar sequestration or compartmentation. Na + extrusion mechanism has already been explored. Recently, Ramakrishna et al., discover novel insights into SOS1role in rapid sequestration of Na + into endosomal compartment followed by vacuoles (Figure 1). In this mechanism, first line defense occurs by restricting Na + uptake. In second tier, triggers the PM SOS1-invoving extrusion of Na + that is active and efficient at low concentration of Na + (Figure 1A). In contrast, high level of Na + enhances stress the extrusion mechanisms are not able to manage the Na + toxicity. In third tier, endosomally-localized SOS1 sequestrates Na + into endosomes followed by it delivers to vacuole through vascular fusion (Figure1B). In fourth tier, tonoplast-localized SOS1 involve in tonoplast-based requestion of Na + , the process helps to manage toxic Na + level.The differential observations raise several questions including why sos1 mutant shows high defects in response to high salt stress but not in low, possibly the early tiers were partially active at low salt level. Moreover, why vacuolar NHX knockout mutants exhibit moderate phenotype, in this process the SOS1 might be compensated (Bassil et al. 2019). The above processes suggest that a coordination is crucial across all ties for optimum salt tolerance in plants.Discovering the dual roles of SOS1 and the salt concentration dependent Na + management strategies open new dimension of research on engineering salt tolerant crops. In traditional approaches, salt tolerance generally focuses on overexpressing single transporters that often with tissue specific effects (Pardo and Rubio, 2011). The recent breakthrough by Ramakrishna et al. (2025), suggest that a promising strategy of engineering crops with enhanced SOS1 trafficking to endosomal and VCs under salt stress. This strategy could potential if it can be achieved by fusing SOS1with trafficking signals or the system that regulate SOS1 localization. In addition, the engineering crops that could maintain proton gradient and regulate SOS1 activity. More specifically, the activity of SOS1 in endosomal compartments that could enhance vacuolar Na + sequestration efficiency.The movement and behavior of Na + ions within and around cells or subcellular organelle, is crucial for exploring cellular functions regulated by several factors including ion channels, exchangers, and transporters (Yin et al.2023). This type of Na + dynamics is regulated by Salt overly sensitive 1 (SOS1) that investigated in plant cells. A study suggests that despite fluctuations in salt levels is regulated by SOS1 and GIGANTEA proteins found in Arabidopsis (Cha et al.,2022). Location of SOS1 and its function is a debating issue. It is known the location of SOS1 at plasmamembrane surrounding, exporting the organelle-based Na + accumulation patterns inside root meristem cells. Ramakrishna et al. (2025) has engineered a type of SOS1-GFP fusion in Arabidopsis, the gene encoding SOS1protein driven by native promotor. This study discovers that the SOS1-GFP protein is located on tonoplast. The presence of SOS1 on tonoplast further confirmed by using antibody test with other relevant experiments. The identification of SOS1 on tonoplast is a groundbreaking insight, resolving the searching mechanism of Na + sequestration inside vacuole (Figure 1A). This finding breaks the debated issues thereby NHX1, NHX2, NHX4, and NH3 were proposed to present on tonoplast. These ion transporters were mostly involved in K + transporting where SOS1 location on tonoplast and its involvement in Na + transport or sequestration inside vacuole were not well explored. The function of SOS1 further confirmed by sos1 mutant, wherein SOS1 functional gene copy was absent (Figure 1B). Thus, sos1 mutant root cells were unable to accumulate Na + inside vacuole under moderate salt stress. Interestingly, SOS1 was detected on tonoplast of salt tolerant Salicornia plants (Salazar et al., 2024) Hints, the vacuolar sequestration of Na + inside vacuole through SOS1 is a strategy of salt stress resilience in plants that has been recently discovered by Ramakrishna et al. (2025) in ArabidopThis study explores new insights into dual functions of SOS1 in managing vacuolar Na + in cells. The application of CryoNanoSIMS to provide opportunity to understand rapid response of salt stress, subcellular Na + distribution that open new dimension of research on salt stress. However, it is important to recognize the technical constraints of CryoNanoSIMS, including limitations in spatial resolution, quantification accuracy, and potential matrix effects, which should be carefully considered when interpreting the results. Insightfully, recognition dual functions of SOS1 open new avenue of biotechnological application of SOS1 for developing smart-salt tolerant crops. The path forward shows clear understanding of cellular Na + homeostasis that link to recent finding of advance CryoNanoSIMS imaging, genetic and engineering approaches for sustainable agricultural solution. The SOS1-mediated salt tolerance using molecular breeding or genome editing approach would be useful to plant breeders for sustainable agricultural crop productions and ensuring global food security. Despite of recent progress of advanced imaging technology, there are several questions remain that need to address for its further exploration. For instance, once SOS1 is crucial for Arabidopsis root meristem, simultaneously other tonoplast-localized Na⁺/H⁺ antiporters like NHX1-4 also expresses. Thus, it needs to clarify whether NHX1-4 could play significant role in vacuolar Na + management. Alteration of cell switch, how rapidly cells switch between two different Na + management strategies. Finally, timecourse studies of salt stress and their temporal resolution could explore whether the alter to vacuolar sequestration is immediate or its opposite mechanisms are deployed. Further clarification of these above queries will show further direction and deep understanding of salt stress research.The process of Na + accumulation in root cells is controlled by the interaction of several Na + /H + antiporters or exchangers (Rahman et al., 2022). How such antiporters or exchangers-dependent elemental accumulation, and their subcellular distribution are occurred, is an interesting area that requires extensive attention. However, suitable approaches or methods that will provide support in tracking salt distribution inside plant cells or organelle, remain largely unknown. In addition, how plants manage different level of toxic Na + is an interesting area. In this scenario, a study by Ramakrishna et al. (2025) have provided CryoNanoSIMS imaging-method that resolve these points and reveals the involvement of SOS1 in accumulating Na + inside vacuole (Figure 1A). The author extended this study in rice and confirmed that the evolutionary trait conserved for the vacuolar accumulation. However, other type of protein rather than rice SOS1, was involved in Na + transportation inside vacuoles. It will be worth exploring of in vitro elemental imaging of key Na + /H + antiporters or exchangers response to Na + and other elemental distribution, and roles in regulating original state at different salt levels. Future work will be necessary to extend the CryoNanoSIMS approach for determining the elements those have atomic mass high than K, including different transitional metals. The potential solution of these issues with functional extension of elemental cryo-imaging system will support in identifying elemental distribution at large scale, will explore elemental interdependence under salt stress, and will plant resilience for sustainable agricultural production.