Biocontrol actinomycetes better protects cell membranes in celery (apium graveolens l.) under freezing stress in the presence of fungal pathogen

Abstract

Beneficial microbes can mitigate biotic or abiotic stress-induced damage to plant cell membranes. Yet, little is known about the effects of actinomycetes on cell membrane permeability in plants under joint biotic and abiotic stresses. Herein, the effects of three biocontrol actinomycetes (Streptomyces pactum Act12, S. globisporus Act7, and S. globisporus subsp. globisporus C28) on cell membrane permeability in the leaves of celery (Apium graveolens L. cv. “Hanyusiji”) were evaluated under fungal pathogen (biotic) and freezing (abiotic) stresses by using electrical conductivity measurements. Our results showed that, under freezing stress, any of three fungal pathogens alone resulted in increased cell membrane permeability. Under the single stress of freezing, medium and high concentrations of C28 respectively reduced cell membrane permeability by 37.0% and 30.6%; Act7 exerted no significant effects, whereas high concentration of Act12 increased cell membrane permeability. Under the dual stresses of fungal pathogen and freezing, these protective effects of Act12, Act7, and C28 did not differ significantly. Nonetheless, these protective effects depended on the type of pathogen infection involved: the largest reduction in cell membrane permeability occurred in the presence of F. oxysporum f. sp. vasinfectum (46.4-69.2%) followed by A. alternate (17.4-51.8%), with F. sambucinum ranked lowest (8.8-35.5%). In conclusion, inoculating an appropriate concentration of actinomycetes can mitigate freezing-induced cell membrane injury in celery plants. Importantly, the actinomycete strains better protected the cell membrane against freezing injury under fungal pathogen stress, but this benefit depends on the adverse effects of pathogens on cell membrane permeability.