Endophytes: A new source for multi-target biological control agents?

Endophytes are an interesting group of plant-associated bacteria that live inside plants and show neutral or beneficial interaction with the host plants. The structure of the bacterial community in the endosphere and endorhiza of field-grown potato was analyzed in comparison to the rhizosphere and phyllosphere by a multiphasic approach over a period of two years. Results of the cultivationindependent approach revealed that T-RFLP profiles of the two endophytic microenvironments were more closely related than those from ectophytic habitats. Using cultivation, composition, diversity and richness of bacterial antagonists confirmed specificity for each microenvironment and an outstanding and promising role of the endorhiza. In an approach to measure the biocontrol potential of isolates, a total of 2,648 bacteria were evaluated for biocontrol and plant growth promotion by a hierarchical combination of assays using the soilborne plant pathogens Verticillium dahliae and Rhizoctonia solani as target. An average of 14.4% of examined bacteria expressed antifungal properties. The strains were characterized by their antagonistic mechanisms in vitro as well as their production of the plant growth hormone indole-3-acetic acid. Complementary, the plant growth promoting effect by antagonistic bacteria was determined using a newly developed microplate assay on the basis of lettuce seedlings. Seven endophytic isolates selected according to in vitro criteria were evaluated in greenhouse and field trials regarding their efficiency to control R. solani in lettuce, sugar beet, and potatoes. In addition, they were screened for their biocontrol activity against the pathogenic nematode Meloidogyne incognita. The most promising candidates Pseudomonas fluorescens 2Re2-6 and Serratia plymuthica 3Re4-18 will be commercialized as biological control agents.