Biological control and growth promotion activities of Streptomyces on European horticultural plants

Introduction: Yield losses caused by phytopathogens should be minimized to maintain the food quality and quantity for the demand of massively growing human population. At the same time, yield limitation due to soil fertility and nutrition deficiencies add extra constraints to plant production. Thus, searching for sustainable solutions to suppress phytopathogens, as well as to increase the yield is gaining high public interests. Plant root systems are colonized by vast amounts of microbes, some of which facilitate biological control and plant growth promotion activities [1]. Streptomyces, abundant in soil, are a group of filamentous bacteria producing a variety of beneficial secondary metabolites, gifting them the potential to be developed as bio-pesticides and bio-fertilizers [2]. We labeled two bioactive Streptomyces strains with EGFP marker to investigate their interactions with lettuce using confocal laser scanning microscopy (CLSM), and evaluated their biocontrol activities against Sclerotinia sclerotiorum on lettuce, as well as PGP activities on several economically important horticultural plants [2] [3] [4] [5]. Results: The abundant colonization of young lettuce seedling by two Streptomyces strains demonstrated their capability to interact with the host from early stages of seed germination and root development up to two weeks. Plant-strain specific PGP activity was observed; e.g., S. cyaneus ZEA17I promoted the growth of lamb lettuce but not that of tomato. When they were applied to S. sclerotiorum inoculated substrate in growth chamber, S. exfoliatus FT05W and S. cyaneus ZEA17I significantly reduced lettuce basal drop incidence by 40% and 50%, respectively, compared to the control (P < 0.05). Interestingly, under field conditions, S. exfoliatus FT05W reduced the disease incidence by 54% (P = 0.12), but we did not observe any protection of lettuce plants after S. cyaneus ZEA17I application. Our results indicate the greatly promising potential of Streptomyces for exploitations in agro-ecosystems.