Evidence that cAMP initiates nuclear division and infection structure formation in the bean rust fungus,Uromyces phaseoli

Nuclear division was efficiently induced in uredospores of the bean rust fungus ( Uromyces phaseoli ) germinated in the presence of comparatively high concentrations (10 −2 M ) of cAMP or several of its derivatives (Bt 2 cAMP and Su 2 cAMP). Synthesis of nuclear DNA was also started; however, appearance of appressoria, a cell differentiation event, was inhibited. At a lower concentration (10 −8 M ) of cAMP appressoria developed but at a low frequency (5.7%). This cell differentiation was also accompanied by nuclear division. Two inhibitors of cAMP phosphodiesterase (3-isobutyl-1-methyl xanthine, IBMX; 4-(3-butoxy)-4-methylbenzyl)-2-imidazolidinone, Ro. 20–1724), studied in anticipation that the compounds would cause the internal levels of cAMP to rise, readily induced both nuclear division and cell differentiation. Nuclei in germlings induced to differentiate by contact with inducive surfaces divided beginning about 2.5 h after the start of germination, while mitosis began after 6 and 10 h in germlings treated with Bt 2 cAMP and IBMX, respectively. Putative stimulators of adenylate cyclase (NaF, Forskolin, adenosine, 2′-deoxyadenosine) induced mitosis in 8, 9, 13, and 43% of the germlings, respectively. Adenine, 3′-deoxyadenosine (Cordycepin), and 5′-adenosine monophosphate stimulated nuclear division in 9, 52, and 4.2% of the germlings, respectively. Although clearly mitogenic, the fact that high levels of cAMP also inhibited differentiation compared to the cell differentiation induced in the presence of phosphodiesterase inhibitors suggests that cAMP metabolism is closely regulated in uredospores.