Hamid et al. 2021 — Bacterial plant biostimulants for crop growth, productivity, and health (MDPI Sustainability narrative review)

Narrative review of plant growth-promoting rhizobacteria (PGPR) as bacterial plant biostimulants (BPBs) covering five mechanism classes: (i) nutrient acquisition via N₂ fixation and solubilization of P, K, and Zn through organic acids and siderophores; (ii) antimicrobial metabolite and lytic-enzyme production; (iii) phytohormone modulation; (iv) abiotic stress amelioration including drought, salinity, temperature extremes, oxidative stress, and heavy metals; and (v) induction of systemic resistance (SAR, ISR, IST). The paper is primarily an agronomy/sustainable-agriculture review; heavy-metal contamination of food or supply chains is not its scope. Section 3.3.5 (“Heavy Metal Stress,” p. 12) contains the entire heavy-metals content: a single qualitative paragraph naming Hg, As, Cd, Pb, and Al as toxic metals that reduce crop productivity, and listing PGPR genera (Pseudomonas, Bacillus, Methylobacterium, Streptomyces) reported to mitigate heavy-metal toxicity through chelation, precipitation, complexation, redox reactions, adsorption, and exopolysaccharide-mediated biosorption. Table 3 (pp. 10–11) lists two heavy-metal-stress entries: Pseudomonas fluorescens on maize (IAA production) and Azotobacter chroococcum on maize (IAA, ammonia, ACC-deaminase production). No measured metal concentrations, no exposure values, no thresholds, no regulatory limits. Evidence tier C; relevant to HMI only as a leads document for the bioremediation/microbiome-and-metals angle.

Key numbers

This is a narrative review with no primary measurements. The only quantitative items in the paper are agronomic/market figures unrelated to heavy-metals contamination, plus a handful of secondary statistics restated from cited references. None of the figures below are author-derived measurements of metal concentrations in any food, ingredient, or environmental matrix.

Heavy-metals-relevant content (§3.3.5, p. 12)

• Toxic heavy metals named as crop-productivity stressors: Hg, As, Cd, Pb, Al. No concentrations stated.
• PGPR genera named as mitigators of heavy-metal toxicity in plants: Pseudomonas, Bacillus, Methylobacterium, Streptomyces. Mechanisms listed (chelation, precipitation, complexation, redox reactions, adsorption; EPS-mediated biosorption from polysaccharides, glycoprotein, lipopolysaccharide, soluble peptide). No quantitative efficiency values.
• Heavy-metal-stress entries in Table 3 (p. 11): Pseudomonas fluorescens on maize — IAA production (ref [99]); Azotobacter chroococcum on maize — IAA, ammonia, and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) production (ref [100]). Neither row reports metal concentrations or removal efficiencies.

Non-heavy-metals quantitative content (context only)

• Global biostimulants market: USD 1.74 billion in 2016; USD 2.30 billion in 2019; USD 2.53 billion projected for 2020; USD 4.14 billion projected for 2025 at 10.2% CAGR 2017–2025 (§2, p. 3, citing ref [27]).
• Less than 25% of commercial biostimulants are microbial-based (p. 3, citing ref [9]).
• Soil salinization affects more than 6% of global land, rendering 22% of cultivated and 33% of irrigated agrarian land under salinity stress; ~50% of arable area projected under salinity by 2050; annual increase rate 10% (§3.3.2, p. 11, citing refs [105–107]).
• Abiotic stresses (nutrient deficiency, salinity, drought, hypoxia, heat) responsible for 60–70% of crop yield deficit (§3.3, p. 9, citing ref [14]).
• Zinc in agricultural use: 96–99% of applied zinc converts to insoluble form (§3.1.5, p. 8, citing ref [81]).
• Global population projected at 9.7 billion by 2050 (§1, p. 2).
• Literature window for the review: 2015–2020 (§1, p. 3); ~184 references cited.

Table 1 commercial PGPR product inventory (pp. 4–5)

Table 1 lists ~20 commercial PGPR-based biostimulant products (FZB24, Rhizovital 42, Inomix, BactoFil B10/A10, Bio-Gold, Cedomon, Rhizosum N, Liquid PSA, Micosat F, Bioscrop BT16, Amase, PGA, Nitroguard, TwinN, Symbion-N/P/K, Ceres, Gmax) with manufacturer, PGPR strain composition, target crops, and target function. None of these entries address heavy-metals contamination of the products themselves; they describe biostimulant formulations applied to crops. This is agricultural-input product data, not HMI food-and-supply-chain product data, so the commercial products are not loaded into HMI frontmatter products:.

Methods (brief)

Narrative literature review with no PRISMA framework, no explicit inclusion/exclusion criteria, no quality assessment, and no quantitative synthesis. Stated literature window 2015–2020; databases consulted: Google Scholar, ScienceDirect, PubMed, Web of Science (p. 3). Structure: §1 introduction; §2 global market for PGPR-based biostimulants; §3 bacterial plant biostimulants — beneficial effects and mode of action (subsections on plant-growth promotion and nutrient acquisition, quality improvement, abiotic stress tolerance, disease suppression, induction of systemic resistance); §4 conclusions. Eleven authors across nine institutions (University of Kashmir; PSGVP Mandal’s Arts, Science, and Commerce College, Maharashtra; Sher-e-Kashmir University of Agricultural Sciences and Technology; Asian PGPR Society for Sustainable Agriculture / Auburn University; Universiti Teknologi Malaysia; City of Scientific Research and Technology Applications, Alexandria; Sinarmas Forestry Corporate R&D, Indonesia; Udayana University, Bali). Editor: Domenico Ronga. Funding: Universiti Teknologi Malaysia (projects 526 QJ130000.3609.02M43, QJ130000.3609.02M39) and All Cosmos Industries Sdn. Bhd. (project R.J130000.7344.4B200). The Academic Editor and journal are MDPI; peer-review depth on MDPI agronomy reviews is variable.

Limitations

C-tier narrative review with no primary contamination data. Relevant weaknesses for HMI use:

• The heavy-metals section (§3.3.5) is a single paragraph of qualitative narrative. It names five metals (Hg, As, Cd, Pb, Al) and four PGPR genera (Pseudomonas, Bacillus, Methylobacterium, Streptomyces) reported to mitigate heavy-metal toxicity but provides no concentrations, no removal efficiencies, no contact-time data, no pH conditions, no biomass dose, no plant uptake reduction percentages. The bioremediation mechanism list (chelation, precipitation, complexation, redox reactions, adsorption, EPS biosorption) is presented without quantitative comparison across mechanisms or strains.
• Speciation is undisciplined: Hg is named without distinguishing inorganic Hg from methylmercury; As is named without distinguishing iAs from tAs. Cr is not named as a target metal anywhere in §3.3.5 (the heavy-metals section lists only Hg, As, Cd, Pb, Al); the PGPR genera named in §3.3.5 (Pseudomonas, Bacillus) include strains documented elsewhere as Cr-resistant, but the review itself makes no Cr-stress claim.
• Table 3’s two heavy-metal-stress rows (P. fluorescens/maize and Azotobacter chroococcum/maize) report mode-of-action (IAA, ammonia, ACCD production) but no measurement of plant metal uptake, soil metal residual, or yield response under specified metal exposure. The primary references [99] and [100] would need to be consulted directly for any quantitative claim.
• The review’s heavy-metals coverage is incidental: the paper’s center of gravity is biostimulant mode-of-action under salinity and drought (§§3.3.2–3.3.3 are several times longer than §3.3.5). The paper would not be selected by a heavy-metals-focused systematic review using PICO-style inclusion criteria.
• Several mechanism statements about EPS-mediated biosorption are presented without distinguishing in-vitro lab demonstration from field-scale efficacy; the qualifying sentence in §3.3.5 (“in highly contaminated sites, the mobilization and consequent bioavailability of heavy metals in excess by siderophores, organic acids, or through bioleaching remains debatable”) acknowledges the gap but does not quantify it.
• English-language editing is uneven (e.g., “innovative exertion to fulfill the current food crisis” in the abstract, “heavy reactions” in §3.1.1 where “heat” is plausibly meant, paragraph-level subject-verb drift). This does not affect the review’s scientific content but signals limited copy-editing.
• The paper has no conflict-of-interest concerns relevant to heavy-metals findings (funded by Universiti Teknologi Malaysia and All Cosmos Industries Sdn. Bhd., a biostimulant-adjacent company; the funding is disclosed; the heavy-metals content is too thin to be commercially load-bearing for the sponsor).

Implications

This source has minimal direct value for the Heavy Metal Index. The wiki’s scope is heavy-metals occurrence in food, personal-care, and supply-chain matrices; this paper is an agronomy/sustainable-agriculture review of bacterial biostimulants whose heavy-metals content is one paragraph plus two table rows. It is retained as a leads document for two narrow purposes:

Microbiome-and-metals leads: the review names PGPR genera (Pseudomonas, Bacillus, Methylobacterium, Streptomyces) and bioremediation mechanisms (chelation, precipitation, complexation, redox reactions, adsorption, EPS biosorption) that overlap with the extremophile-bioremediation vocabulary catalogued in aishwarya2024-extremophiles-bioremediation-review. Useful background for any future metal-microbiome wiki page covering rhizosphere bacterial metal-resistance mechanisms; cite the primary references in those papers, not this review.

Agronomic-context leads: the review’s framing of heavy-metal stress as one of five abiotic stresses (alongside salinity, drought, heat, cold) addressable by PGPR biostimulants is useful context for any HMI work on soil-amendment or crop-uptake-reduction interventions (e.g., low-cadmium-uptake rice cultivars, bioremediation-pretreated soils). The paper does not provide quantitative intervention data and should not be cited for any specific reduction efficacy.

The chapter does not provide primary contamination data on any food matrix, ingredient, product, or regulation. No contamination_profile synthesis is triggered.

Wiki pages this source may touch

• lead
• cadmium
• arsenic
• arsenic-total
• mercury
• mercury-total
• aluminum

Verification notes

• Frontmatter metals: uses tAs (not iAs) and tHg (not MeHg) per Part 14 speciation discipline: the review names “As” and “Hg” as toxic metals in §3.3.5 without separating inorganic vs organic forms; the conservative reading is total. [[metals/mercury-methyl]] is intentionally not included in the wiki-pages bulleted list because the paper makes no specific mention of methylmercury (unlike Aishwarya 2024 which discusses MerB organomercurial lyase).
• Frontmatter ingredients: and products: are intentionally empty. The paper measures and recommends nothing for any HMI ingredient or product category. The commercial PGPR biostimulants in Table 1 are agricultural inputs (biofertilizers/biostimulants applied to crops in the field), not food or personal-care products in HMI’s scope, and they are not loaded into products:.
• Frontmatter matrices: uses [rhizosphere, contaminated-soil, plant-root]. rhizosphere and plant-root are descriptive matrix slugs new to this source page; they are plausible analogues to agricultural-soil and plant-tissue already in the matrices vocabulary but should be confirmed on the next taxonomy review. contaminated-soil is precedented (see aishwarya2024-extremophiles-bioremediation-review).
• jurisdictions: is empty because the review does not focus on any single country’s heavy-metals contamination; the only jurisdiction-specific reference is to EU Regulation 2019/1009 on biostimulants as fertilizing products (§1, p. 2), which is a fertilizer-regulation not a heavy-metals-contamination regulation and does not match any current HMI regulations slug.
• No brand firewall (Part 12) concerns identified. Table 1 names commercial biostimulant products with manufacturers (FZB24/ABiTEP, Inomix/Iabiotec, BactoFil/AGRO.bio Hungary, etc.), but these are agricultural-input vendors named in a product inventory, not contamination measurements attached to consumer-facing brands. The Part 12 firewall targets brand-name attribution of CONTAMINATION VALUES to sampled food/personal-care products; this paper has no such attribution. The commercial-product inventory is retained at the table-row count in Key numbers without per-row brand reproduction.
• No wiki/HMTc firewall (Part 2) concerns identified. The review proposes no certification thresholds and makes no comparison to any HMTc framework.
• This paper is borderline for HMI scope (single-paragraph heavy-metals coverage in a 24-page biostimulants review). Surfaced for Karen’s awareness as a relevance edge-case; the source page is written at C-tier with explicit leads-only framing per the Aishwarya 2024 precedent.
• Audit subagent (2026-06-02, PROMOTE verdict, all 5 checks clean) flagged one minor descriptive overreach in Limitations: an earlier draft claimed Cr was “discussed only in §3.4 contexts” — verified against the source that §3.4 (Disease Suppression) does not discuss Cr in any heavy-metals sense; the Limitations bullet was corrected to remove that overreach. No other findings.

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.