PR-1 protein inhibits the differentiation of rust infection hyphae in leaves of acquired resistant broad bean

Treatment of broad bean leaves with salicylic acid (SA) or 2,6-dichloro-iso nicotinic acid (DCINA) induces resistance against the rust fungus Uromyces fabae fabae resulting In reduced rust pustule density. Light-microscopy stu dies showed that in induced resistant plants the rust fungus is inhibited i mmediately after penetration through the stomatal pore. The differentiation of infection structures growing within the intercellular space of the leaf , i.e. infection hyphae and haustorial mother cells, is inhibited. Furtherm ore, low-temperature scanning electron microscopy studies of freeze fractur es revealed protrusions at the tips of infection hyphae growing in induced resistant broad bean leaves. Treatment of in vitro-differentiating rust inf ection structures with intercellular fluids (IFs) from induced resistant pl ants confirmed that the fungus is sensitive towards an apoplastic anti-fung al activity only after having formed appressoria. Other legume rusts such a s U, vignae and U. appendiculatus were likewise inhibited in the presence o f IF from SA-treated broad bean leaves. Heterologous antibodies were used t o study changes in the extracellular pathogenesis-related (PR) protein patt ern after resistance induction. Western blots indicated that chitinases and beta-1,3-glucanases were present in both induced and control plants. In co ntrast, PR-1 proteins were newly synthesized in response to SA or DCINA app lication. The major induced PR-1 protein was purified and exhibited strong differentiation-inhibiting activity towards U. fabae infection structures. We conclude that the inhibition of rust infection hyphae in acquired resist ant broad bean plants is mainly due to the anti-fungal activity of this ind uced basic PR-1 protein.