Beneficial plant-microbe interactions in agriculture: reducing lettuce basal drop by application of rhizosphere competent Streptomyces

Yield losses caused by phytopathogens are major challenges to agriculture all over the world. Biological control based on plant-microbe interactions provides economically viable, environmentally sound, and socially responsible plant protection strategies. The ability of microorganisms to suppress plant pathogens and to interact with their host are fundamental prerequisites of microbial assisted plant protection. Two Streptomyces strains isolated from surface sterile crop roots, Streptomyces exfoliatus FT05W and Streptomyces cyaneus ZEA17I, showed up to 75% in vitro mycelial growth inhibition of Sclerotinia sclerotiorum, the causal agent of lettuce drop (Sardi et al. 1992; Chen et al. in press). We labeled them with the enhanced green fluorescent protein (EGFP) marker to investigate Streptomyces’ ability to colonize the lettuce root system using confocal laser scanning microscopy (CLSM) (Bonaldi et al. 2015; Erlacher et al. 2015). The well-colonized lettuce seedlings by both strains showed Streptomyces’ capability to interact with the host from early stages of seed germination and root development. We also observed colonization by EGFP-labeled Streptomyces on two-week-old roots. Additionally, we quantified S. exfoliatus FT05W and S. cyaneus ZEA17I viability and persistence in the rhizosphere and surface sterilized lettuce root tissues. After six weeks of plant 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. Moreover, we studied the biological activity of the two strains against S. sclerotiorum in vivo. When they were applied to S. sclerotiorum inoculated substrate in growth chamber, S. exfoliatus FT05W and S. cyaneus ZEA17I reduced disease incidence significantly by 40% and 50%, respectively, compared to the control (P < 0.05). Differently, 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. Further studies on the complex host-pathogen-antagonist interactions will provide additional knowledge to understand the modes of Streptomyces acting as promising biological control agents.