Involvement of cAMP and protein kinase A in conidial differentiation by Erysiphe graminis f. sp hordei

Erysiphe graminis f. sp. hordei, the causal agent of barley powdery mildew is an obligate biotroph. On arrival on the host, a primary germ tube (PGT) emerges from the conidium. An appressorial germ tube (AGT) then appears, fo rms an appressorium, and effects host penetration. Such developmental preci sion may be due to multiple, plant-derived signals and to endogenous tactil e and chemical signals. The transduction mechanism remains obscure. The iso lation of an expressed sequence tag (EST) homologue of the catalytic subuni t of cyclic AMP (cAMP)-dependent protein kinase A (PKA) enabled the corresp onding gene to be characterized and the transcript to be identified in coni dia and in PGT and AGT stage spores. cAMP-dependent PKA activity was detect ed in ungerminated conidia. These data suggest that PKA and cAMP are involv ed in conidial development. To substantiate this we exploited the responses of developing conidia to various surfaces, including exposure to the host leaf (fully inductive to AGT formation), cellulose membrane (semi-inductive ), and glass (non-inductive). Assessment of fungal development, following a pplication of exogenous cAMP or cAMP analogues, revealed that, at different concentrations and on different surfaces, cAMP either promoted or inhibite d conidial differentiation. Various PKA inhibitors were tested for their ef fect on PKA activity and conidial development. A negative correlation was e stablished between PKA inhibition in vitro and fungal development in vivo. Taken collectively, these data suggest that PKA and cAMP play a role in con idial differentiation in this obligate, plant-pathogenic fungus.