LAUSR

Microorganisms in the root system are abundant and exist in very high taxonomic diversity. Major players in this so-called rhizosphere are bacteria and fungi. Many bacterial species support plant growth, but some of the soil-born fungal species found in the rhizosphere have a necrotrophic life style and infest plants to feed on them. Bacteria have evolved many strategies to prevail over fungi, among them harmful enzyme activities and noxious secondary metabolites. Interactions between plant growth promoting rhizobacteria (PGPR) and phytopathogenic fungi are potentially valuable since the plant would benefit from fungal growth repression. In this respect, the role of volatile bacterial metabolites in fungistasis has been demonstrated, but the mechanisms of action are less understood. We used three phytopathogenic fungal species (Sclerotinia sclerotiorum, Rhizoctonia solani, and Juxtiphoma eupyrena) as well as one non-phytopathogenic species (Neurospora crassa) and the PGPR Serratia plymuthica 4Rx13 in co-cultivation assays to investigate the influence of bacterial volatile metabolites on fungi on a cellular level. As a response to the treatment, we found elevated lipid peroxidation, which indirectly reflected the loss of fungal cell membrane integrity. An increase in superoxide dismutase, catalase, and laccase activities indicated oxidative stress. Acclimation to these adverse growth conditions completely restored fungal growth. One of the bioactive bacterial volatile compounds seemed to be ammonia, which was a component of the bacterial volatile mixture. Applied as single compound in biogenic concentrations it also caused an increase in lipid peroxidation and enzyme activities, however, extent and pattern did not fully match the effect of the entire bacterial volatile mixture. Ammonia could be produced by root colonizing rhizobacteria which assimilate amino acids released by plant roots. The results suggest that ammonia plays a role in in bacterial-fungal interactions in the rhizosphere.