Unravelling plant-microbe interactions: successful colonization of lettuce by tagged biocontrol streptomyces

The ability of the Biological Control Agents (BCAs) to colonize plant tissues is an important feature involved in microbe-assisted plant protection. Plant-microbe interaction research increased especially in the last decade thanks to technological revolution. Molecular methods and the development of advanced microscopic techniques allow researchers to explore gene expression and localization of beneficial microorganisms within plants. The Green Fluorescent Protein (GFP) and its modified version, Enhanced GFP (EGFP), more adapt for expression in mammalian cells and GC-rich actinomycetes like Streptomyces, have been widely used as markers to study gene expression, as well as plant-microbe interactions. We transformed five Streptomyces strains which showed strong inhibition activity against Sclerotinia sclerotiorum, with the EGFP construct to study their interactions with lettuce. The fitness of transformed strains was similar to wild-type; the transformants maintained similar sporulation, mycelium growth rate, and the ability to produce important secondary metabolites and lytic enzymes. Two strains, Streptomyces cyaneus ZEA17I and Streptomyces sp. SW06W, were used to study lettuce colonization dynamics by seed coating method. Streptomycete colonies were visualized in three-day-old seedlings. In addition, the colonization and spatio-temporal dynamics were studied in sterile and in non-sterile substrates. The strains were recovered from rhizosphere and inner