Submitted to: Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/4/1995
Publication Date: N/A
Citation: Interpretive Summary: The fungus Cochliobolus carbonum causes leaf spot of corn. The most destructive strains of this pathogen cause damage to the plant by producing a toxin that is active only against susceptible corn varieties. The toxin is important for the pathogen to overcome mechanisms of host plant resistance. However, it is not known when the toxin is produced during infection and what factors influence its synthesis and release. We analyze the chemical compounds released from spores of the fungus germinating under conditions that promoted the formation of appressoria, which are the structures produced during leaf infection. Very small quantities of the toxin were detected and quantified by plasma desorption mass spectrometry. Spores that were induced to form appressoria synthesized and released the toxin, but spores incubated under conditions that did not allow appressorium formation failed to produce the toxin. The results indicate that the development of infection structures and production of the toxin are coordinated events that determine the success of the fungal pathogen.
Technical Abstract: The fungus Cochliobolus carbonum causes leaf spot of maize. Highly virulent isolates produce a host-selective, peptide toxin that is active against susceptible maize genotypes. Prior to infection, spores must germinate and differentiate appressoria, structures specialized for leaf penetration. Plasma desorption mass spectrometry detected as little as 0.5 ng toxin and revealed that spores induced to form appressoria in vitro synthesized and released the toxin at a time coincident with maturation of appressoria. Spores incubated under non-inductive conditions failed to produce toxin, indicating that toxin synthesis is regulated by infection-related morphogenesis. These observations confirm that the fungus can negate host defenses upon penetration.