Rapamycin Plays a Pivotal Role in the Potent Antifungal Activity Exhibited Against Verticillium dahliae by Streptomyces iranensis OE54 and Streptomyces lacaronensis sp. nov. Isolated from Olive Roots

Verticillium wilt, caused by Verticillium dahliae, poses a significant threat to olive trees (Olea europaea L.). The isolation of endophytic Streptomyces strains from olive roots has led to the discovery of several strains showing strong antifungal activity against V. dahliae, as demonstrated through in vitro and small-scale soil experiments. Molecular analyses confirmed that strain OE54 belongs to Streptomyces iranensis. The main antifungal compound identified in this strain was rapamycin. Rapamycin displayed potent antifungal effects, notably inhibiting conidiospore germination (IC50 = 87.36 μg/mL) and the hyphal growth of V. dahliae, with a minimum inhibitory concentration (MIC50) of 3.91 ng/mL. Additionally, a second rapamycin-producing strain, OE57T, was isolated. Phenotypic and genotypic analyses indicated that OE57T represents a new species, which is proposed to be named Streptomyces lacaronensis sp. nov., with OE57T designated as the type strain (=DSM 118741T; CECT 31164T). The discovery of two endophytic rapamycin-producing Streptomyces strains residing within olive roots is especially notable, given the rarity of rapamycin production among microorganisms. These findings highlight the potential of rapamycin-producing Streptomyces strains in developing biofertilizers to manage V. dahliae and reduce the impact of Verticillium wilt on olive trees and other crops.