Binucleate Rhizoctonia sp. AG-A, indigenous plant-growth promoting fungus in semi-arid Mediterranean soils

Purpose Binucleate Rhizoctonia sp. (BNR), indigenous to old olive groves of the Apulia region (Southern Italy), and its relationship with olive trees were investigated to search for beneficial soil microorganisms suitable for supporting plant growth and crop productions in degraded semi-arid soils. Methods Binucleate Rhizoctonia sp. was first isolated from five ancient olive groves using olive tree plantlets as living baits. The functional relationship of BNR with olive trees was estimated with inpot growth assays, using native soil amended with an artificial inoculum from the native BNR isolates. BNR was then quantified in the olive groves by digital PCR using a TaqMan assay specifically designed on the ITS regions of the native AG-A isolates. Results Digital PCR was able to discriminate BNR (Ceratobasidium sp.) from the phylogenetically close Basidiomycetes such as Rhizoctonia solani (Thanatephorus cucumeris). This assay detected an amount of BNR in five olive groves ranging from traces up to 105 target gene copies per mu L of DNA. When artificially added to native soil samples, BNR AG-A increased olive tree growth by 50% and 25% respectively under constant optimal water availability and repeatedly induced water stress. Indole-3-acetic acid production was demonstrated for the BNR AG-A isolates, thus, in part, explaining growth promotion. Conclusion Thanks to the absolute quantification of fungal DNA fragments in soil with digital PCR, findings suggest that new soil functional indicators, such as the growth promoting Rhizoctonia AG-A, may be identified with this research approach to be investigated as a biotic resource for enhancing soil ecosystem services.

Automated summary

The purpose of this study was to investigate the relationship between Binucleate Rhizoctonia sp. (BNR) and olive trees in order to find beneficial soil microorganisms suitable for supporting plant growth in degraded semi-arid soils. BNR was isolated from ancient olive groves and its functional relationship with olive trees was estimated using growth assays. The results showed that BNR AG-A increased olive tree growth and promoted the production of indole-3-acetic acid, suggesting its potential as a biotic resource for enhancing soil ecosystem services.