Competence of two Streptomyces strains to colonize lettuce seeds, roots and rhizosphere

Beneficial microorganisms with positive activity on plant growth and health offer an attractive alternative to conventional agriculture. However, the successful application of biological control agents (BCAs) in agriculture has been hindered by insufficient knowledge of mechanisms by which BCAs interact with the host plants and other microorganisms. Colonization of the plant roots and development of rhizosphere competence are widely considered as crucial steps through which BCAs could create useful interactions with plants, and also protect them against soil borne pathogens. Recently, Streptomyces species are gaining increased interest as BCAs due to their different mechanisms to inhibit plant pathogens. We evaluated one of these mechanisms, the abilities of two Streptomyces spp. strains, ZEA17I, and SW06W, to colonize lettuce seeds, rhizosphere, and roots. By conjugation, the strains were tagged with enhanced green fluorescent protein and apramycin resistance genes to study the lettuce colonization dynamics in sterilized sand. The preliminary results showed that both strains could successfully colonize the lettuce seeds in a very short time, and then develop rhizosphere competence up to six weeks. Endophytic colonization of the root tissue was also detected after six weeks. The seed, rhizospheric, and endophytic colonization ability of the two Streptomyces strains may contribute to their biocontrol activities against lettuce soil borne fungal pathogens.