Abstract

Our previous studies showed that living and heat-killed cells of the Rhizobium leguminosarum strain P.SOM induce in pea roots systemic resistance to infection by Orobanche crenata. To better understand the mechanisms of induced resistance, we focused on identifying the inducing agent. Since heat stable bacterial surface carbohydrates such as exopolysaccharides (EPS) and lipopolysaccharides (LPS) are essential for recognition in the symbiotic interaction between Rhizobium and legumes, their role in the R. leguminosarum-pea interaction was studied. EPS and LPS were extracted from bacterial cultures, applied to pea roots, and tested for activity as an inducer of plant resistance to broomrape. Whereas EPS did not affect O. crenata infection, LPS reduced Orobanche infection significantly in concentrations as low as 1 and 0.5 mg ml^-1. In contrast these compounds did not influence parasite germination induced by the artificial stimulant, GR24. Our results show that soil treatments with LPS resulted in a highly significant reduction of O. crenata infection of pea roots. The results clearly showed that LPS of R. leguminosarum P.SOM act as the inducing agent of systemic resistance in pea roots against Orobanche.

Key words: Rhizobium leguminosarum, Lipopolysaccharides, Exopolysaccharides, Induced resistance, Orobanche crenata, Pea.