Exploiting Streptomyces in agro-ecosystems for biological control and plant growth promotion

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 deficiency add extra pressure to plant production. Thus, searching for sustainable solutions to suppress phytopathogens, as well as to increase the yield is gaining high interests in recent years. A diverse assemblage of plant associated microorganisms can contribute to crucial ecosystem services in agricultural landscapes, including plant growth promotion (PGP) and biological control. Question: Do Streptomyces have promising potential to be used as biological control agents (BCAs) and PGB bacteria? Materials and methods: Two Streptomyces strains isolated from surface sterilized plant roots, S. exfoliatus FT05W and S. cyaneus ZEA17I, showed up to 75% in vitro mycelial growth inhibition of Sclerotinia sclerotiorum, the causal agent of lettuce drop [1]. We labeled them with the enhanced green fluorescent protein (EGFP) marker to investigate Streptomyces’ ability to colonize the lettuce roots using confocal laser scanning microscopy (CLSM) [2,3]. Furthermore, we quantified S. exfoliatus FT05W and S. cyaneus ZEA17I viability and persistence in the rhizosphere and in surface sterilized lettuce root tissues. Additionally, we investigated the PGP activities of the two strains on the germination and growth of different plant species in growth chamber. Finally, we studied the biological control activity of the two strains against S. sclerotiorum under both growth chamber and field conditions. Results: The abundant colonization of young lettuce seedling (2 or 3 days old) by both strains demonstrated Streptomyces’ capability to interact with the host from early stages of seed germination and root development, and the two strains were detected also on two-week-old roots. On average, after six weeks of growth, we recovered 2 x 104 CFU/g dry weight (DW) from the rhizosphere and 2 x 105 CFU/g DW from the inner root tissues. 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 disease incidence by 40% and 50%, respectively, compared to the control (P < 0.05). Interestingly, under field conditions, S. exfoliatus FT05W reduced disease incidence by 54% (P = 0.12), but we did not observe protection of lettuce plants against S. sclerotiorum by S. cyaneus ZEA17I. Conclusion: Our results indicate the promising potential of Streptomyces for exploitation in agro-ecosystem applications. Further studies will provide additional knowledge to understand the mechanism of Streptomyces mediated biocontrol and PGP.