Hydrogen Cyanide in the Rhizosphere: Not Suppressing Plant Pathogens, but Rather Regulating Availability of Phosphate

Abstract

Plant growth promoting rhizobacteria produce chemical compounds with different benefits for the plant. Among them, HCN is recognized as a biocontrol agent, based on its ascribed toxicity against plant pathogens. Based on several past studies questioning the validity of this hypothesis, we have re-addressed the issue by designing a new set of in vitro experiments, to test if HCN-producing rhizobacteria could inhibit the growth of phytopathogens. The level of HCN produced by the rhizobacteria in vitro does not correlate with the observed biocontrol effects, thus disproving the biocontrol hypothesis. We developed a new concept, in which HCN does not act as a biocontrol agent, but rather is involved in geochemical processes in the substrate (e.g., chelation of metals), indirectly increasing the availability of phosphate. Since this scenario can be important for the pioneer plants living in oligotrophic alpine environments, we inoculated HCN producing bacteria into sterile mineral sand together with germinating plants and showed that the growth of the pioneer plant French sorrel was increased on granite-based substrate. No such effect could be observed for maize, where plantlets depend on the nutrients stored in the endosperm. To support our concept, we used KCN and mineral sand and showed that mineral mobilization and phosphate release could be caused by cyanide in vitro. We propose that in oligotrophic alpine environments, and possibly elsewhere, the main contribution of HCN is in the sequestration of metals and the consequential indirect increase of nutrient availability, which is beneficial for the rhizobacteria and their plant hosts.

Functional Diversity of Soil Bacteria from Organic Agro Ecosystem

“…In a recent study by Rijavec and Lapanje (2016) have shown that there is no correlation between the amount of HCN produced by a particular strain and its ability to inhibit the growth of phytopathogenic bacteria or fungi. These workers proposed a new role for HCN production by rhizospheric bacteria.…”

Effects of discrete bioactive microbial volatiles on plants and fungi

“…The inhibition of phytopathogenic fungi was correlated with plant growth promotions, and subsequently, HCN producer strains seemed to be very attractive biocontrol agents (Voisard et al ). A very recent study, however, showed in vitro that rhizobacterial HCN did not correlate with the biocontrol effect but instead acted via sequestration of metal ions and indirectly caused an increase of phosphate availability (Rijavec and Lapanje ).…”

Wars between microbes on roots and fruits

“…The cyanide ion is a potent inhibitor of many metalloenzymes, especially copper-containing cytochrome c oxidases 85 . In addition to a role in the control of plant diseases 6 , cyanide may play a role in increasing the availability of nutrients for plants through its ability to sequestrate metal ions 86 . Dimethyl sulfide, produced by the rhizospheric bacteria P. fluorescens and S. plymuthica , has bacteriostatic effects on the plant pathogens Agrobacterium tumefaciens and Agrobacterium vitis 87 .…”