Spent growth medium of Pantoea agglomerans primes wheat suspension cells for augmented accumulation of hydrogen peroxide and enhanced peroxidase activity upon elicitation

Induced disease resistance in plants is based on multiple mechanisms, including cell “priming”, i.e. an enhancement of the capacity to mobilize cellular defense responses upon pathogen attack. Potent inducers of priming are, for example, salicylic acid, synthetic compounds such as a benzothiadiazole, and certain rhizosphere bacteria. While priming is well characterized for a number of dicot plants, only few cases of priming are documented in monocots. Here, we report that the spent growth medium of the Gram negative bacterium Pantoea agglomerans is capable of priming wheat cells (Triticum aestivum L. cv Prelude-Sr5) for elicitor-induced defense responses. Pre-incubation of suspension-cultured wheat cells with growth medium of P. agglomerans led to a strong enhancement of an oxidative burst that has been induced by chitin or chitosan and to an increase in extracellular peroxidase activity. Moreover, exopolysaccharides (EPS) were isolated from the spent growth medium and demonstrated to be sufficient for the induction of H₂O₂ priming. The EPS-induced priming was shown to be time- and concentration-dependent. We conclude that EPS are the or one of several priming-active component(s) in the spent growth medium of P. agglomerans. The present work is the first report of priming in a monocot plant by a specific component of bacterial origin. A comparison with known chemical inducers of resistance revealed that a benzothiadiazole was able to enhance the oxidative burst similar to the spent growth medium or the EPS of P. agglomerans, while salicylic acid was not.