Biochemical mechanisms of induced resistance by rhizobial lipopolysaccharide in pea against crenate broomrape

Lipopolysaccharides (LPS) isolated from rhizobia have been demonstrated as inducer of systemic resistance against plant diseases. The role of bacterial LPS in induced resistance in plant has not been extensively reported. The capacity of Rhizobium leguminosarum Mabrouk et al. 2007 strain P.SOM to reduce parasitism of pea by Orobanche crenata Mabrouk et al. 2007 was previously demonstrated under greenhouse and field trials. The objective of this study was to examine the capacity of LPS isolated from strain P.SOM to induce resistance in pea against crenate broomrape, with more emphasis on the biochemical mechanisms of host plant defense reaction induced by LPS. Root chamber experiments using Petri dishes revealed that LPS reduce broomrape seed germination and restricted the broomrape attachment to the host root as well as retarded tubercle formation and development. In pot experiments, pea roots treatment with LPS reduced the total number of broomrape by up to 95 %. No biomass reduction of pea was observed after application of LPS. Broomrape necrosis was observed both before and after parasite attachment to treated pea roots in Petri dishes and pots experiments. Concomitantly, reduction in infection was accompanied by enhanced phenylalanine ammonia lyase, peroxidase, and polyphenoloxidase activities. As a consequence, the derived products, phenolics, and pisatin accumulated in response to LPS and conferred mechanical and chemical barriers to the invading parasite. Our findings indicate an important role for LPS as a direct inducer of defenses in pea against broomrape.