Initiation of appressorial germ tube differentiation and appressorial hooking: distinct morphological events regulated by cAMP signalling in Blumeria graminis f.sp. hordei

Abstract

Appressorial differentiation by conidia of the barley powdery mildew fungus, Blumeria graminis f.sp. hordei, is dependent on perception of multiple leaf-derived signals. Recently, we have demonstrated that cAMP signalling and PKA play an important, but complex, role during early B. graminis conidial development. Here, we demonstrate that a rise in cAMP levels correlates with conidial differentiation on the barley leaf surface. No change in cAMP levels is observed when conidia fail to differentiate on a non-inductive surface. Moreover, the cAMP levels appear to both increase and decrease during conidial development on barley leaves, suggesting that appressorial differentiation requires differential activation of the cAMP-signalling pathway. In addition, we have dissected the time periods over which cAMP, the cAMP analogue 8-Br-cAMP, and the PKA inhibitor H89, are able to affect conidial differentiation. This reveals that H89 is only active prior to 4 h post-inoculation, corresponding to the initiation of appressorial germ tube development, whereas only cAMP and 8-Br-cAMP inhibit the later process of appressorial hooking, at 4–8 h post-inoculation. Thus, we provide data that supports a model in which cAMP signalling is required to be active to trigger initiation of appressorial germ tube development and inactive to allow subsequent appressorial germ tube hooking.