Fluorescent pseudomonads, in particular, have been extensively studied and most often implicated in biocontrol of plant pathogens, due to their ability to produce several antimicrobial compounds, including HCN (reviewed in Haas and Défago, 2005). bacteria colonize competitor niches, produce iron-chelating and antibiotic compounds, and excrete volatiles, which induce plant systemic resistance (reviewed in Santoyo et al., 2012). To promote plant growth, Pseudomonas spp. and the plant rhizosphere has been well-documented ( Dutta and Podile, 2010) and bacteria from this genus are currently used as model organisms for studies on root colonization ( Lugtenberg et al., 2001). In this context, the association between Pseudomonas spp. These bacteria regulate plant developmental processes, inhibit the growth of phytopathogens, increase nutrient availability and fix nitrogen (reviewed in Lugtenberg and Kamilova, 2009). 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.Ī beneficial rhizobacterial community is essential for growth and development of the plant host ( Morgan et al., 2005) and includes plant growth promoting (PGP) rhizobacteria. 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. No such effect could be observed for maize, where plantlets depend on the nutrients stored in the endosperm. 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. 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. The level of HCN produced by the rhizobacteria in vitro does not correlate with the observed biocontrol effects, thus disproving the biocontrol hypothesis. 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. Among them, HCN is recognized as a biocontrol agent, based on its ascribed toxicity against plant pathogens. Plant growth promoting rhizobacteria produce chemical compounds with different benefits for the plant. 3Remote Controlled Theranostic Systems Lab, Saratov State University, Saratov, Russia.2Department of Environmental Sciences, Jožef Stefan Institute, Ljubljana, Slovenia.1Institute of Metagenomics and Microbial Technologies, Ljubljana, Slovenia. Tomaž Rijavec 1,2 and Aleš Lapanje 1,2,3 *
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