Background

Bacteria producing HCN have plant growth–promoting (PGP) characteristics, being included in biofertilizer production as a biocontrol agent [1]. The role of HCN is to display its toxic effect on the respiration of other microbes, where cyanide binds to the metal ions present in cytochrome oxidase. Particularly, it inhibits the electron transport chain by binding with the carrier ions, i.e., Cr⁶⁺and Fe^3+, and checks regular energy supply, resulting in the ultimate death of the pathogens [2]. Most of the bacterial species, mainly rhizobacteria, release HCN as secondary volatile metabolites under nutrient stress conditions in their idiophase. Major HCN producers are Pseudomonas, Bacillus, Alkaligenes, and Klebsiella pneumoniae [3]. Production of the HCN is catalyzed by a membrane-bound HCN synthase complex, encoded by the hcnABC operon, which converts glycine into HCN and carbon dioxide [4]. Biosynthesis is tightly regulated and induced under specific environmental conditions such as low oxygen availability, stationary growth phase, and the presence of glycine, highlighting its role as a competitive metabolite rather than a constitutive product [5]. Regulatory systems like the anaerobic regulator ANR, the quorum-sensing transcription factors LasR/RhlR, and the GacS/GacA two-component system play central roles in controlling hcnABC expression in response to environmental and population-level cues [6,7]. These regulatory networks integrate oxygen levels, cell density, and nutrient status to fine-tune HCN production, ensuring it occurs primarily under nutrient-limiting or competitive conditions typical of the rhizosphere. Despite being toxic, HCN is efficiently secreted extracellularly, allowing the producing bacteria to inhibit surrounding pathogens while avoiding self-harm. This well-regulated secondary metabolism confers a selective advantage in microbial communities, particularly in plant-associated niches.

Some of the experimental evidence now suggests that there is no consistent correlation between the amount of HCN produced by rhizobacteria in vitro and their biocontrol performance. Specifically, strains characterized as HCN-positive (HCN) did not exhibit strong inhibition against common phytopathogens due to HCN production. In vitro assays also revealed that the minimum inhibitory concentration (MIC) of cyanide required to impact microbial growth was approximately tenfold higher than what is naturally produced by HCN-producing bacteria, suggesting that its levels in the rhizosphere are unlikely to exert broad-spectrum antimicrobial activity [8].

Instead, an alternative functional role of HCN in rhizosphere ecology is emerging, one that centers on its capacity to mobilize essential nutrients through metal chelation. Notably, HCN forms stable complexes with divalent and trivalent transition metals, such as Fe³⁺, Al³⁺, and Ca²⁺, thereby impacting nutrient dynamics in the soil [9]. One of the most critical implications of this interaction is its effect on phosphate (PO₄^3-) availability. In acidic soils, phosphate readily forms insoluble complexes with iron and aluminum, limiting its accessibility to both plants and microbes [10]. In calcareous soils, similar immobilization occurs via calcium phosphate precipitation [11]. The action mechanism proposed is that HCN can bind with iron, either preventing the initial formation of insoluble Fe-PO₄ complexes or releasing phosphate from already precipitated iron-phosphate compounds. The latter mechanism appears more favorable, as lower concentrations of cyanide (particularly KCN in experimental setups) were sufficient to liberate phosphate from mineral-bound forms. In controlled in vitro studies, the addition of potassium cyanide to granite-based mineral sand led to a measurable increase in both solution conductivity and phosphate concentration, confirming HCN’s role in mineral dissolution and phosphate mobilization.

Previously, qualitative estimation was based on a color change in the picric acid, which changes from a yellow color to reddish brown or dull pink. This color change occurs due to the formation of isopurpurate compound in the presence of HCN in an alkaline medium [12], which can be estimated for the quantification of the HCN produced. The ease, low cost, and flexibility of the colorimetric assay have made it a critical tool for the study of microbial HCN production.