Streptomyces sp. LH 4 promotes plant growth and resistance against Sclerotinia sclerotiorum in cucumber via modulation of enzymatic and defense pathways

Background: In the soil ecosystem, microbial diversity exists and these diverse organisms interact with plant roots and influence the physicochemical properties of plants. Some of these diverse microorganisms can cause diseases or can provide beneficial interactions with plants. Rhizobacteria are well-known beneficial microorganism that colonize the plant root zone (rhizosphere) and are referred to as plant growth-promoting rhizobacteria (PGPR) that contribute to the promotion of plant growth either directly or indirectly. PGPRs are also known for their biocontrol abilities. Sclerotinia sclerotiorum, an Ascomycetous soil inhabiting fungus, causes white rot disease in cucumbers. This disease results in the loss of millions of dollars annually. The current study was conducted to isolate naturally occurring soil inhabiting bacteria that may promote plant growth under diseased conditions and also antagonize the pathogen. Scope: The isolated LH4 strain was identified as Streptomyces sp. by 16S rRNA sequencing and phylogenetic analysis. The plant growth promoting effects and the antifungal antagonistic activities against Sclerotinia sclerotiorum were confirmed by measuring enzymatic activity of LH4 and demonstration in planta. In addition, Streptomyces sp. LH4 pure culture application exhibited significant growth inhibition of S. sclerotiorum in cucumber. Analysis of the major hormones related to pathogen defense; the jasmonic acid, and salicylic acid, showed that the modulation of these two hormones increased disease resistance in cucumber. Conclusion: The present study suggests a possible dual role of Streptomyces sp. LH4 as functional material for bio-fertilizer and biocontrol against pathogens.