Selenium-Mediated Rhizosphere Blocking and Control Network: Multidimensional Mechanisms for Regulating Heavy Metal Bioavailability
, Shuiyuan Cheng 1,2,* and Yingtang Lu 1Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript presents a review on the interaction mechanisms between the rhizosphere and selenium in the attenuation of heavy metal bioavailability in soil. The topic is timely, relevant, and aligned with the scope of the journal. However, before recommend its publication, several aspects need to be strengthened: (1) the methodology and criteria used for the literature review are not clearly described; (2) the structure of sections and subsections should be improved through better paragraph organization, a short introduction to each section, and stronger cohesion between sections; and (3) the novelty and practical applications of the review should be reinforced. Specific comments are provided below:
Line 41. The term “biotoxic” is redundant and does not have a clear consensus in the scientific literature; in fact, it does not appear in the cited reference (3). Please replace it with a more appropriate term.
Lines 44–48. Please expand the discussion on the natural occurrence of selenium (background levels) and the regulatory threshold values established in international guidelines, as well as the co-occurrence of selenium with other metals and their potential toxic effects. For example, genotoxicity has been reported in:
Gallego, J. L., & Olivero-Verbel, J. (2021). Cytogenetic toxicity from pesticide and trace element mixtures in soils used for conventional and organic crops of Allium cepa L. Environmental Pollution, 276, 116558.
(Note: this article is a suggested reference, its use is at the author´s discretion)
Lines 61–66. Clearly define the scientific gap motivating this review and highlight its novelty. In addition, present the criteria used to conduct the literature review, including the time frame, keywords, and article selection criteria.
A specific section is also needed on the general dynamics of selenium in soil and the mechanisms related to its application and incorporation into soil systems. The new subsection should be included to provide the general context of selenium dynamics in soils, including interactions with different soil colloids. Competitive interactions with other ions, selective adsorption processes, and their implications for selenium bioavailability should be addressed more precisely.
In general, each section currently starts without sufficient contextualization or an adequate introduction. Please improve cohesion between sections and include an introductory paragraph for each section to strengthen the overall structure.
Line 98. References are required.
Lines 257–568. Please review whether bold formatting is necessary. Revise the writing and paragraph structure, and consider separating this content into one paragraph for each topic (Se-forms, application, and speciation).
Lines 313–334. Please review whether bold formatting is necessary and revise the writing and structure of this paragraph.
Finally, please add a subsection on future recommendations including: study limitations, a synthesis of current knowledge gaps, implications for future research, and soil and crop management recommendations based on the findings of this review.
Author Response
We firstly thank you for your patient review of our manuscript. It has been of great help to us. We have already revised the manuscript according to the suggestions you put forward.
- Line 41. The term “biotoxic” is redundant and does not have a clear consensus in the scientific literature; in fact, it does not appear in the cited reference (3). Please replace it with a more appropriate term.
Answer: Thank you for your reminder. We apologize for the incorrect use of the term and have replaced "biotoxic" with "toxic" (Line 41).
- Lines 44–48. Please expand the discussion on the natural occurrence of selenium (background levels) and the regulatory threshold values established in international guidelines, as well as the co-occurrence of selenium with other metals and their potential toxic effects. For example, genotoxicity has been reported in: Gallego, J. L., & Olivero-Verbel, J. (2021). Cytogenetic toxicity from pesticide and trace element mixtures in soils used for conventional and organic crops of Allium cepa L. Environmental Pollution, 276, 116558. (Note: this article is a suggested reference, its use is at the author´s discretion)
Answer: Thank you for your suggestions. We have added the following content to the "1. Introduction" section: 1. The natural occurrence (background level) of selenium and the regulatory thresholds specified in international guidelines (Lines 51-52); 2. A discussion on the co-occurrence of selenium with other metals and their potential toxic effects (Lines 58-61). The recommended references have been cited accordingly.
- Lines 61–66. Clearly define the scientific gap motivating this review and highlight its novelty. In addition, present the criteria used to conduct the literature review, including the time frame, keywords, and article selection criteria.
Answer: Thank you for your suggestions. We have added the research gaps, novelty of the focus of this review, and the criteria adopted for this literature review to the last paragraph of the "1. Introduction" section (Lines 66-80).
- A specific section is also needed on the general dynamics of selenium in soil and the mechanisms related to its application and incorporation into soil systems. The new subsection should be included to provide the general context of selenium dynamics in soils, including interactions with different soil colloids. Competitive interactions with other ions, selective adsorption processes, and their implications for selenium bioavailability should be addressed more precisely.
Answer: Thank you for your suggestions. As requested, we have added a new subsection "2.1 General Dynamics of Selenium in Soil", which addresses the kinetic background of selenium in soil, including the speciation transformation of selenium in soil, its interactions with different soil colloids, and its competitive effects with other ions (Lines 94-127).
- In general, each section currently starts without sufficient contextualization or an adequate introduction. Please improve cohesion between sections and include an introductory paragraph for each section to strengthen the overall structure.
Answer: Thank you for your suggestions. We have supplemented and revised the introductions for each primary section and its corresponding subsections, which not only provides sufficient background information but also enhances the coherence between sections, thus rendering the overall structure of the manuscript more cohesive.
- Line 98. References are required.
Answer: Thank you for your reminder. As requested, we have supplemented the missing references (Line 169).
- Lines 257–568. Please review whether bold formatting is necessary. Revise the writing and paragraph structure, and consider separating this content into one paragraph for each topic (Se-forms, application, and speciation).
Answer: Thank you for your suggestions. As requested, we have revised the subsection "4.1 Se-induced formation of root iron plaque": we removed the bold formatting, addressed the lack of clarity in the paragraph writing, and divided the content into several paragraphs based on the themes of selenium speciation, application time, and heavy metal speciation (Lines 360-375).
- Lines 313–334. Please review whether bold formatting is necessary and revise the writing and structure of this paragraph.
Answer: Thank you for your suggestions. As requested, we have revised the subsection "4.3.2 Se increases cell wall polysaccharide content": we removed the unnecessary bold formatting and rewrote the paragraph content with a corresponding structural adjustment (Lines 433-453).
- Finally, please add a subsection on future recommendations including: study limitations, a synthesis of current knowledge gaps, implications for future research, and soil and crop management recommendations based on the findings of this review.
Answer: Thank you for your suggestions. As requested, we have added Section "6. Challenges and Future Perspectives", which covers the current limitations and research gaps, as well as future insights and recommendations (including the selection of selenium fertilizer forms, selenium application methods, selenium application timing, combined application of selenium with microorganisms, and field management).
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript topic is highly relevant to agronomy, soil science, and environmental remediation. The sections available (Abstract, Introduction, Geochemical immobilization, Rhizosphere microbial mechanisms, Root regulation) are well structured and comprehensive. However, the manuscript requires improvements in clarity, originality of synthesis, and critical evaluation of cited literature.
Main comments:
- Critical analysis missing: The manuscript often summarizes studies without evaluating their limitations, experimental conditions, or contradictions.
- Balance of evidence: Positive effects of Se are emphasized, while negative or inconsistent findings (e.g., Se promoting As uptake under waterlogged conditions) are under-discussed.
-Figures: Some figures are crowded and text-heavy, reducing clarity.
-Terminology consistency: Terms like “immobilization,” “blocking,” and “stabilization” are used interchangeably without a precise definition.
-Future outlook: The abstract mentions “research gaps,” but the discussion of gaps is limited in the sections provided.
Suggestions:
- Paraphrase overlapping text: Revise sections on Se speciation, microbial reduction, and iron plaque formation to avoid similarity with prior reviews.
- Strengthen critical evaluation: Discuss limitations of cited studies (e.g., pot vs. field experiments, Se dosage ranges, soil types).
- Balance positive/negative outcomes: Include more discussion of cases where Se increases HM uptake (e.g., As under waterlogged conditions).
- Clarify terminology: Define “immobilization,” “blocking,” and “stabilization” clearly and use them consistently throughout the manuscript.
- Improve figures: Simplify diagrams, reduce text density, and ensure all abbreviations are explained in captions.
- Add quantitative comparisons: Where possible, include numerical data (e.g., % reduction in Cd uptake) to strengthen claims.
- Expand research gaps section: Provide a structured outlook on unanswered questions (e.g., long-term Se application risks, interactions with multiple HMs, field-scale validation).
- Discuss practical application strategies: Include considerations of Se fertilizer forms, application timing, and integration with microbial inoculants.
- Highlight novelty of synthesis: Emphasize how this review differs from existing Se–HM reviews (e.g., multidimensional integration, rhizosphere focus).
- Language editing: Conduct professional English proofreading to improve clarity and flow
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Author Response
We firstly thank you for your patient review of our manuscript. It has been of great help to us. We have already revised the manuscript according to the suggestions you put forward.
- Paraphrase overlapping text: Revise sections on Se speciation, microbial reduction, and iron plaque formation to avoid similarity with prior reviews.
Answer: Thank you for your suggestions. We have revised the sections on selenium speciation, microbial reduction and iron plaque formation to render the content more comprehensive, innovative and rigorous, thus eliminating overlaps with the content of previous reviews.
- Strengthen critical evaluation: Discuss limitations of cited studies (e.g., pot vs. field experiments, Se dosage ranges, soil types).
Answer: Thank you for your suggestions. As required by the manuscript, we have evaluated the limitations of the cited studies, including the differences in soil types between pot experiments and field experiments in Section 2 "Geochemical Immobilization of HMs by Se" (Lines 200–204); the limitations of existing research methods for root morphology in Section 4.2 "Se Reshapes Root Morphology to Avoid HM Pollution" (Lines 405–410). In addition, we have discussed the limitations of current research (e.g., the selection of selenium speciation, variations in soil types, and discrepancies between pot experiments and field practices) in the final Section 6 "Challenges and Future Perspectives".
- Balance positive/negative outcomes: Include more discussion of cases where Se increases HM uptake (e.g., As under waterlogged conditions).
Answer: Thank you for your suggestions. As requested, we have added the negative results to the manuscript, including the increased arsenic (As) uptake under waterlogging conditions in Subsection 2.2 "Effect of Eh on chemical precipitation of Se and HMs" (Lines 143–153); and the phenomenon that selenium application leads to an increased root surface area, which in turn enlarges the contact area with heavy metals (HMs) in Subsection 4.2 "Se Reshapes Root Morphology to Avoid HM Pollution" (Lines 397–404). In addition, we have emphasized the originally noted enhanced mobility of As and antimony (Sb) induced by elevated rhizosphere pH in Subsection 2.3 "Effect of rhizosphere pH on HM adsorption by soil colloids" (Lines 198–199).
- Clarify terminology: Define “immobilization,” “blocking,” and “stabilization” clearly and use them consistently throughout the manuscript.
Answer: Thank you for your suggestions. We have standardized the usage of "immobilization", "blocking" and "stabilization" throughout the entire manuscript, with the specific definitions clarified as follows: "Immobilization" focuses on rendering substances non-mobile, which involves confining heavy metals to specific carriers (e.g., soil, cellular structures) through physical, chemical and biological actions; "Blocking" emphasizes cutting off specific pathways or processes, entailing the interception of target substances' transport channels (e.g., transporters, diffusion pathways) or functional processes (e.g., enzymatic hydrolysis, signal transduction); as for "stabilization", it has been deleted from the main text due to inappropriate usage.
- Improve figures: Simplify diagrams, reduce text density, and ensure all abbreviations are explained in captions.
Answer: Thank you for your suggestions. We have made appropriate revisions to Figure 3.
- Add quantitative comparisons: Where possible, include numerical data (e.g., % reduction in Cd uptake) to strengthen claims.
Answer: Thank you for your suggestions. We have added specific numerical values at appropriate positions in the manuscript, specifically in Subsection 2.2 "Effect of Eh on chemical precipitation of Se and HMs" (Lines 143–145), Subsection 3.2.2 "Other microbe-mediated HM immobilization mechanisms" (Lines 321–324), and Subsection 4.2 "Se Reshapes Root Morphology to Avoid HM Pollution" (Lines 398–404).
- Expand research gaps section: Provide a structured outlook on unanswered questions (e.g., long-term Se application risks, interactions with multiple HMs, field-scale validation).
Answer: Thank you for your suggestions. As requested, we have added a new section "6. Challenges and Future Perspectives", which covers the current limitations and research gaps, including the risks of long-term Se application, the interactions between Se and multiple HMs, and field-scale validation, among others (Lines 540–551).
- Discuss practical application strategies: Include considerations of Se fertilizer forms, application timing, and integration with microbial inoculants.
Answer: Thank you for your suggestions. As requested, we have added a new section "6. Challenges and Future Perspectives", in which the selenium application strategies are discussed, including the selection of selenium fertilizer forms, selenium application methods and timing, as well as integrated considerations of selenium and microbial inoculants (Lines 552–567).
- Highlight novelty of synthesis: Emphasize how this review differs from existing Se–HM reviews (e.g., multidimensional integration, rhizosphere focus).
Answer: Thank you for your suggestions. As requested, we have emphasized the differences between this review and existing Se–HM reviews at the end of the Introduction, including multi-dimensional integration and a rhizosphere-focused perspective (Lines 66–73).
- Language editing: Conduct professional Engish proofreading to improve clarity and flow
Answer: Thank you for your suggestions. We have comprehensively polished the English expression of the manuscript and rectified numerous issues related to the clarity and fluency of the writing.
Reviewer 3 Report
Comments and Suggestions for AuthorsThis manuscript reviews how selenium establishes a multidimensional blocking and control network in the rhizosphere to mitigate heavy metal toxicity through geochemical immobilization and microbial community regulation. It further details how selenium alters physical root structures and activates specific genetic pathways to restrict the uptake and intracellular transport of toxic metals within crops.
I have the following comments:
- This manuscript mainly discussed that Se triggers a series of chain reactions in the rhizosphere; how does the geochemical fixation directly influence subsequent microbial selection or molecular signaling?
- Adding something about this would improve the introduction section significantly.
- In figures 1 and 2, what are the oxidation states of Se in the aquamarine/mint circles?
- In Table 2, some columns are misaligned, so I can’t tell which species, genes, functions and gene expressions correspond to what.
- Since Se fails to effectively induce iron plaque in high-Cd-contaminated soil, is there a threshold level where Se ceases to be effective? Is there a way to remediate this?
- Tables 2 and 3 talk about the up/down regulations of different genes, but are there any data on tissue-specific expressions?
- Using Roman numerals is very uncommon for negative oxidation states, such as in the case of Se(-II). I have so far only seen S(-II).
- Figure 3: spelling error “red/bule”.
- How stable are these metal-selenide precipitates under changing environmental conditions?
Author Response
We firstly thank you for your patient review of our manuscript. It has been of great help to us. We have already revised the manuscript according to the suggestions you put forward.
1. This manuscript mainly discussed that Se triggers a series of chain reactions in the rhizosphere; how does the geochemical fixation directly influence subsequent microbial selection or molecular signaling?Adding something about this would improve the introduction section significantly.
Answer: Thank you for your suggestions. We have added a paragraph on the effect of Eh on subsequent microbial selection to Subsection 2.2 "Effect of Eh on chemical precipitation of Se and HMs" (Lines 154–164). For Subsection 3.2.1 "HM immobilization via microbial regulation of Se speciation", we have both addressed the aforementioned effect of Eh (Lines 273–275) and provided an appropriate account of the impacts of microbially synthesized selenium nanoparticles (SeNPs) at the subsequent molecular level (Lines 280–287).
2. In figures 1 and 2, what are the oxidation states of Se in the aquamarine/mint circles?
Answer: Thank you for your reminder. The selenium in these two figures cannot be assigned to a specific oxidation state, as it merely denotes "selenium treatment"—both Se(Ⅳ) and Se(Ⅵ) are commonly applied in agricultural practices. However, regardless of the selenium speciation, their respective modes of action have been clearly elucidated in Figures 1 and 2.
3. In Table 2, some columns are misaligned, so I can’t tell which species, genes, functions and gene expressions correspond to what.
Answer: Thank you for your feedback. We have made changes to Table 2.
4. Since Se fails to effectively induce iron plaque in high-Cd-contaminated soil, is there a threshold level where Se ceases to be effective? Is there a way to remediate this?
Answer: Thank you for your suggestions. We first apologize sincerely for our inaccurate expression: it is not that "high Cd background inhibits iron plaque formation", but rather that "under high Cd background, Se cannot exceed the Cd adsorption threshold of iron plaque (53 mg/kg)". We have revised this erroneous content accordingly. To address this threshold issue, the combined application of Se and lime can restore the function of iron plaque by reducing the bioavailable Cd concentration in soil, thereby mitigating high Cd contamination (Lines 372–375).
5. Tables 2 and 3 talk about the up/down regulations of different genes, but are there any data on tissue-specific expressions?
Answer: Thank you for your suggestions. We first apologize for not making relevant revisions in this regard. At present, there are relatively few studies on tissue-specific gene expression in the research field covered by this review, making it difficult to systematically summarize the characteristics of tissue-specific expression. Hence, we have not conducted corresponding revisions.
6. Using Roman numerals is very uncommon for negative oxidation states, such as in the case of Se(-II). I have so far only seen S(-II).
Answer: Thank you for your suggestions. As requested, we have replaced Se(-II) with Se² throughout the entire manuscript.
7. Figure 3: spelling error “red/bule”.
Answer: Thank you for your suggestions. As requested, we have revised it to "red/blue".
8. How stable are these metal-selenide precipitates under changing environmental conditions?
Answer: Thank you for your suggestions. As requested, we have added a discussion on the stability of metal selenide precipitates at the end of Subsection 2.2 "Effect of Eh on chemical precipitation of Se and HMs" (Lines 172–178).
Reviewer 4 Report
Comments and Suggestions for AuthorsManuscript titled:
Selenium-Mediated Rhizosphere Blocking and Control Network: Multidimensional Mechanisms for Regulating HeavyMetal Bioavailability
The topic is interesting and, in my opinion, very important to study
Is this a mistake or not: the year 2023?
Agronomy 2023, 13, x. https://doi.org/10.3390/xxxxx
The illustrations are beautiful and informative. The authors discussed the chemical forms of selenium, which are important for understanding.
Line 287-290
leading to reduced lateral root number and length [73] (Table 2). Thus, Se treatment can both adjust the ratio of primary to lateral roots (reducing root surface area and thus HM uptake sites) and increase primary root length (guiding roots to migrate from the surface to deeper soil layers, actively avoiding HM-enriched topsoil and reducing exposure intensity) (Figure 3b).
Why are the words highlighted in red? Is this already a second-stage review?
The authors summarize current research gaps in the mechanisms of interaction among different Se species, in precise application strategies, and in long-term environmental risk assessment, providing a theoretical basis and technical outlook for the green remediation of HM-contaminated farmlands and the Se biofortification of crops.
Se enhances the HM barrier capacity of root cell walls was described.
Commonly, the regulation of HM bioavailability in the rhizosphere is a complex, multi-dimensional, and synergistic network of interactions. Geochemically, Se reduces HM mobility at the source by regulating rhizosphere redox potential (Eh) and pH, inducing precipitation of metal selenides (e.g., CdSe, HgSe), and enhancing soil colloid adsorption. Se reshapes the rhizosphere microbial community, indirectly immobilizing HMs by regulating Se speciation and transformation, and directly participating in HM sequestration via microbial adsorption and chelation.
The literary references are set in a different font. I would recommend changing the font to the standard one. But this is up to the editor.
Author Response
We firstly thank you for your careful review of this manuscript and your recognition of this review. Your comments are of great help to us, and we have revised the full text as per your suggestions.
- Is this a mistake or not: the year 2023? Agronomy 2023, 13, x. https://doi.org/10.3390/xxxxx
Answer: Thank you for your suggestion. We have corrected the error and revised 2023 to 2026.
- Why are the words highlighted in red? Is this already a second-stage review?
Answer: Thank you for your reminder. We have removed the unnecessary red highlights, while we have retained the insertion markers for tables and figures for ease of identification. Besides, this is the first round of review.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe current version of the manuscript satisfactorily incorporates the revisions and can be recommended for publication.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have accepted all suggestions, and the manuscript can be accepted for publication.
