Modulation of cAMP and phosphodiesterase activity by flavonoids which induce spore germination of Nectria haematococca MP VI (Fusarium solani)

Flavonoids exuded from legume roots stimulate spore germination of a number of soilborne fungi which interact with these plants, and so may act as host cues to initiate interaction. Macroconidia of Nectria haematococca MPVI (anamorph Fusarium solani), a pathogen of pea (Pisum sativum), germinate in response to the same flavonoids which induce nod gene expression in pea-specific rhizobia. Pisatin, the isoflavonoid phytoalexin of pea, also induces germination. The present study found that cAMP levels in macroconidia were transiently induced by flavonoid treatment. Combined with previous pharmacological studies, this indicates that flavonoid signalling utilizes the cAMP pathway. A hypothesis that flavonoids modulate cAMP levels through direct inhibition of N. haematococca cAMP phosphodiesterase was tested. A low K-M cAMP phosphodiesterase activity from macroconidia was tested for inhibition by 10 flavonoids. Naringenin, a strong inducer of germination, was a strong inhibitor of phosphodiesterase (apparent K-i 33 mu M). There was a general correlation between strength of induction and inhibition of phosphodiesterase. Pisatin, structurally distinct from the others, appeared to be an exception to this trend. The results suggest that the ability of specific flavones and flavanones to inhibit cAMP phosphodiesterase is a potential mechanism through which they can induce cAMP levels and so promote germination. (C) 2000 Academic Press.